Sample Page design

Irina Angel

*Corresponding author
Daniel Gandia, MD
Cancer Medicine Advisor at INFIP, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina; E-mail: drgandia@gmail.com

Article information

Received: September 14th, 2022; Revised: October 3rd, 2022; Accepted: October 10th, 2022; Published: October 12th, 2022

Cite this article

Gandia D, Suarez C. Cancer disease-oriented-drug development examples. Cancer Stud Mol Med Open J. 2022; 7(1): 1-2. doi: 10.17140/CSMMOJ-7-132

Licence

cc Copyright 2022 by Gandia D. This is an open-access article distributed under Creative Commons Attribution 4.0 International License (CC BY 4.0), which
allows to copy, redistribute, remix, transform, and reproduce in any medium or format, even commercially, provided the original work is properly cited.

Background: Studies have shown that fatigue is a suggested factor in the causation of injury during soccer play (Greig, 2009), and has been associated with a decrease in hip range of movement (ROM) (Bradley et al., 2007). The prevalence of fatigue in the lower extremities is one of the main factors for hamstring and rectus femoris injuries (Woods et al., 2004). This may be a result of reduced ROM of the hip flexor (HF) muscle group. Reduction in ROM in these muscles may lead to suboptimal positioning of the pelvis through an increase in anterior pelvic tilt (APT). This could reduce lower body power output (LBPO) and increase tension placed on the hamstrings.
Aim: The aim of this study was to investigate the effect of fatigue during a 90-minute soccer-specific protocol on lower extremity range of movement, hamstring flexibility and lower body power output in soccer players.
Method: Eleven male amateur soccer players from the University of Hull men’s football team (Mean ± SD: Age: 20.9 ± 3.0. years) performed a multi-directional, aerobic multi-stage fitness simulation known as the SAFT 90 protocol (Small, McNaughton, Greig & Lovell, 2010). The SAFT 90 protocol was developed to replicate the physiological and mechanical demands made during soccer match-play. Data was collected with each participant partaking in four lab-based sessions.  Four performance tests were conducted at four intervals, pre SAFT 90 (t=0), interval one (t=15), interval 2 (t=30) and post SAFT 90 (t=45). The four performance tests used were the modified thomas test (MTT) (Harvey, D. 1998), Active Straight Leg Raise (Cook et al, 2006b), Standing Horizontal Jump to measure lower body power output (Harman & Garhammer, 2008). The study adopted a repeated measures design with all participants undergoing the following conditions: foam rolling (FR) of the HF, static stretching (SS) of the HF, a combination of both and a control condition in which none of the interventions were performed.  Each condition was performed after the first two intervals of the SAFT 90. Condition order was randomised for each participant.
Results: Analysis of the data showed that pre SAFT 90 average hip flexor ROM was 178° for the iliopsoas and 130° for the rectus femoris, with an average of 1.83m on the horizontal jump (HJ), and 70.7° on hamstring flexibility, compared to  post SAFT 90 measurements, each average measurement had increased to 191° for the iliopsoas and decreased to 128 ° for the rectus femoris, increasing 2.08m on the HJ and 75.7° on hamstring flexibility, thus resulting in an increase in performance . The post SAFT 90 measurements show an increase in hip and hamstring ROM, lower body power output and hamstring flexibility. Interventions of static hip flexor stretching and single sessions of foam rolling the hip flexor muscles show slight improvements, however the combination method showed the highest increase in performance measures.
Discussion and Conclusions: The study findings show that a combination of FR followed by static HF stretching increases hip ROM, hamstring ROM, hamstring flexibility and lower body power output after 45 minutes of intermittent soccer specific simulation. A decrease in lower body power output, ROM and flexibility was found as a result of fatigue, in the latter stages of the protocol following the control session. Behm, D. ECSS 2017, states that FR 5-10 (s) increase ROM, however FR 30-60 s increases ROM to a greater degree. Intermittent rolling after static stretching has been shown to maintain ROM gains to a significantly greater degree than single sessions of stretching or without rolling.

Introduction

Background

Due to the sub-maximal nature of soccer the muscles being worked rely heavily on the activation of the aerobic energy system (Bangsbo, 1994). Soccer matches last for 90 minutes which is split into two 45-minute halves. Although the physiological demands of soccer may vary according to the system of play or tactics employed, there are some consistencies in the movement patterns during the course of match play. Soccer-specific endurance has been found to be related to high-intensity running (KRUSTRUP et al., 2003).  In English league first division matches, players change activity every 5-6 s on average, have brief pauses averaging only 3 s every 2 minutes, although rest breaks tend to last longer and happen more frequently at lower levels of competition. Sprints average around 15m and occur every 90 s whilst players run at a cruise or sprint once every 30 s (Reilly & Thomas, 1976; Reilly 1994). Sprinting occurs every 90–120 seconds and high-intensity efforts every 40–70 seconds (Bradley, Di Mascio, Peart, Olsen & Sheldon, 2010; Reilly, T, 1994). Several studies have looked at the work rate and distance covered by elite soccer players, and by using time – motion analysis, it has been demonstrated that elite players typically cover a total distance of 9 – 12 km during a game (Bangsbo 1994; Bangsbo, Nørregaard, & Thorsøe, 1991; Reilly & Thomas, 1976; Rienzi, Drust, Reilly, Carter, & Martin, 1998; Van Gool, Van Gerven, & Boutmans, 1988; Mohr, Krustrup, & Bangsbo, 2003a; Mohr, Ellingsgaard, Andersson, Bangsbo, & Krustrup, 2003b), however a study by Reilly and Williams (2003) suggested that soccer players cover between 8-13km on average during soccer match play. This distance is covered by intermittent patterns. The majority of the activity performed in a soccer match is completed by low intensity activities such as walking and jogging (Rienzi et al, 2000).

One study using the ProZone3® system found that players tend to cover shorter distances performing high intensity running and sprinting during the second half of matches rather than during the first half (Di Salvo et al., 2009). Several authors have examined low-standard and amateur players (Saltin, 1973; Whitehead, 1975; Van Gool et al., 1988; Ohashi et al., 1988); others have observed players at the top of their national league (Bangsbo et al; 1991; Bangsbo, 1994). These studies have suggested that the amount of high intensity exercise is a valid measure of physical performance in soccer. It has also been observed that players at higher standard of competition perform significantly more high-intensity running than those at a lower standard (Ekblom, 1986; Bangsbo et al., 1991). Further research has shown that players at a higher standard of play perform more high-intensity running than peers at lower standards (Andersson, Randers, Heiner-Moller, Krustrup, & Mohr, 2010; Bangsbo et al., 1991; Ekblom, 1986; Mohr, Krustrup, Andersson, Kirkendal, & Bangsbo, 2008). 

On average, the overall distance covered by outfield players during a match consists of 25% walking, 37% jogging, 20% cruising sub maximally, 11% sprinting and 7% moving backwards (Reilly and Thomas, 1976; Reilly, 1994). It has been estimated that approximately 80-90% of performance is spent in low to moderate intensity activity whereas the remaining 10-20% are high intensity activities (Bangsbo, 1994a1997; O’Donoghue, 1998; Reilly and Thomas, 1976; Rienzi et al., 2000).

During a 90-minute game, top elite soccer players will on average cover 10km per game (Di Mascio & Bradley, 2013; Bangsbo, Mohr & Krustrup, 2006).  Of this distance, studies have recognised that the total distance in the second half and intensity of activities are impaired as opposed to the first half due to the physiological and biomechanical demands within soccer (Mohr, Krustrup & Bangsbo, 2003). Furthmore, Mohr et al. (2003) showed that the amount of high-intensity running decreased after the most intense periods (suggesting a temporary form of fatigue) and decreased markedly towards the end of the game (suggesting a more permanent form of fatigue). These high-intensity efforts constitute the anaerobic components of soccer match play, and it is this period that may have the potential to affect the outcome of a match or induce fatigue (Bradley et al., 2009).

It has been shown that high-intensity running decreases after the most intense period of the game and at the end of the game, indicating temporary and permanent forms of fatigue (Bradley et al., 2009; (Harley et al., 2010; Hill-Haas, Coutts, Rowsell & Dawson, 2009).

At high standards of soccer match play, the effect of fatigue on performance is noticeable in the second half due to a drop in the work rate (Thomas & Reilly, 1976; Reilly, 1994). It is normally considered that the quadricep muscle group plays a key role in activities that rely on explosive strength such as jumping, and kicking the ball, while the hamstring muscle controls the running activities and helps stabilize the knee during change of direction and tackling in soccer (Fried & Lloyd, 1992). Strength in the lower limbs is a concern in soccer: the quadriceps, hamstrings and triceps surae groups must generate high levels of forces for explosive actions such as kicking, tackling, jumping, changing direction and changing of pace. For outfield players, the lower part of the trunk, hip flexors as well as the plantar flexors and dorsi-flexors of the ankle are used most frequently. The vertical and standing long jumps have been notably employed as measures of explosive leg strength. Several authors have investigated the relationship between hamstring flexibility and hamstring injury (BURKETT, 1970; Ekstrand & Gillquist, 1982; Liemohn, 1978; Worrell et al., 1991). Worrell et al. (1991) and Liemohn (1978) reported hamstring-injured subjects were less flexible than non-injured subjects.

Hip ROM has been shown to decrease immediately during soccer training and up to 24 hours post training as a result of fatigue (Bradley et al., 2007; Moller, Oberg & Gillquist, 1985). Muscle tightness, more notably in the hamstring group, has been linked with increased risk of muscular injury in Swedish professionals (Ekstrand, 1982). Flexibility work is advocated both as a warm-up and in its own right for long-term improvement for range of movement. Flexibility work is usually performed at the beginning of a training session. Stretching exercises have been recommended immediately after training (Ekblom, 1986). Flexibility is quantified by the ROM available at a joint, (Alter, 2004).  Previous studies have found that soccer training reduces ROM in players immediately, up to post 24 hours training and progressively over time (Möller, Öberg & Gillquist, J, 1985), and soccer players were found to have a lower ROM than non-players, (Ekstrand, J., & Gillquist, J, 1982; Hattori, & OHTA, Sohta,1986). Ekstrand (1986) has shown that soccer players tend to be less flexible than non-athletes and that inflexibility, particularly in the hamstring muscle group, predisposes them to injury. Elite English League players are particular susceptible to hamstring strains (Woods, 2004).

Increasing flexibility through passive static stretching is common among athletes and recreationally active people. It is reported that static stretching results in greater flexibility and range of motion (Ross, 2007; Ford, et al 2005) and reduces risk of injury (Jamtvedt et al., 2010). An increasingly popular technique that may be incorporated with static stretching is self-myofascial release (SMR). It is suggested that SMR improves ROM through autogenic inhibition where the massage also increases blood flow and reduces adhesions and scar tissue (Robertson, M. I. K. E, 2008).

Time motion analysis is useful for examining the activity pattern and physical aspects of soccer (Reilly & Thomas, 1976; Mayhew & Wenger, 1985; Bansgbo et al., 1991; Bangsbo, 1994).

Several authors have examined low-standard and amateur players (Saltin, 1973; Whitehead, 1975; Van Gool et al., 1988; Ohashi et al., 1988); others have observed players at the top of their national league (Bangsbo et al., 1991; Bangsbo, 1994). These studies have suggested that the amount of high intensity exercise is a valid measure of physical performance in soccer. It has also been observed that players at higher standard of competition perform significantly more high-intensity running than those at a lower standard (Ekblom, 1986; Bangsbo et al., 1991).  Studies have shown that the frequency of sprinting, high-intensity running and overall distance covered are greater in the first half than in the second half of a soccer match (Bangsbo, 1994; Bangsbo et al., 1991; Mohr et al., 2003a; Reilly & Thomas, 1976). This suggests that performance is decreased in the second half and that fatigue occurs in the latter stages of the match.

Aim & initial hypothesis

As current literature suggests that static stretching and self-myofascial release techniques singularly increase flexibility and range of motion, that a combination of both might show more beneficial effects, therefore the aim and purpose of this study was to look at the effect static stretching of the hip flexor muscles, self-myofascial release technique in foam rolling of the hip flexor muscles and a combination of hip flexor static stretches and foam rolling has on hip flexor ROM, hamstring ROM, hamstring flexibility and lower body power output in soccer players. 

As research suggests, single sessions of static stretching and single sessions of myofascial release techniques improves flexibility and ROM, therefore the hypothesis would suggest that combining the two methods would have a more influential effect on improving lower body power and ROM of the hip flexors and hamstrings.

LITERATURE REVIEW

Fatigue and effects of fatigue on performance

Fatigue can be classified as a reduction of maximal force or power that is associated

with sustained exercise and reflected in a decline in performance (Rahnama et al., 2003;

Reilly, 1994; Taylor, Bulter, & Gandevia, 2000). Enoka and Stuart (1992) defined muscular fatigue as the maximal force exerted because of central and/or peripheral mechanisms. In sports where performance must be sustained for a prolonged period, fatigue is represented by the inability to sustain the required work-rate. This decline in physical capability coincident with the onset of fatigue has been defined as a reduced capacity to generate the required level of force (Hawley, J. A., & Reilly, T, 1997). Muscular fatigue is often identified as a risk factor linked to the high incidence of hamstring strain injuries observed across all levels of soccer (Steffen et al., 2008; Petersen, Thorborg, Nielsen, Budtz-Jørgensen & Hölmich, 2011), with injury typically observed in the latter stages of training and each half of competitive matches.

During the second half, the total distance and high-intensity running has been shown to decline as a consequence of the fatigue induced by the first half (Bangsbo, J., Nørregaard, L., & Thorsoe, F, 1991; MOHR, KRUSTRUP & BANGSBO, 2003; Reilly, T, 1976; Van Gool, D., Van Gerven, D., & Boutmans, J,1988). Reilly, (1996) found that players cover less distances in the latter stages of both the first half and second half of a soccer match. A study on Danish soccer players by Bangsbo et al. (1991), noted a 5% drop in distance covered in the second half of a soccer match than the distance covered in the first half.  Mohr et al. (2003a) observed that for both top class players and professional players of a lower standard, the amount of high-intensity running was reduced in the last 15 min of a game. A study on Belgian university soccer players by Van Gool et al. (1988) found the corresponding difference in distance covered was 450 m. This highlights that fatigue occurs in the latter stages of soccer matches.  Aerobic fitness does seem to allow players to continue at a high work rate, as an inverse relationship between maximal aerobic power (V0 ² max) and decrement in work rate has been noted (Reilly & Thomas, 1976). Players who are aerobically well trained are better able to maintain their work rates towards the end of the game than those of poorer aerobic fitness (Smaros, 1980; Reilly, 1994).

Reilly and Thomas (1976) found that the number of goals scored in the latter stages of games were higher than predicted goals scored however, this cannot account for the decrease in work rate, which effects both teams equally. It could, however, be a result of increased risk taken by the losing team, a change in tactics or mental fatigue causing lapses in concentration (Reilly and Thomas, 1976). Fatigue is most pronounced in centre-backs and strikers, and less apparent in midfield players and full- backs, who tend to have the higher (V0 ²max) values. Although midfield players cover the greatest distances among players in outfield roles, their superior aerobic fitness levels enable them to maintain a high exercise intensity throughout the game. Studies by Reilly and Thomas (1976), Bangsbo (1994) and Mohr et al. (2003) found that the amount of high-intensity exercise decreases towards the end of soccer matches. Studies have demonstrated that the amount of sprinting, high-intensity running and distance covered are lower in the second half than in the first half of a game (Bangsbo, 1994; Bangsbo et al., 1991; Mohr et al., 2003a,b; Reilly & Thomas, 1976). This may indicate that performance is inhibited in the second half and fatigue occurs towards the end of a game. Mohr et al. (2003) found that high intensity running was reduced in the last fifteen minutes of a game in top-class and professional players. Mohr et al. (2005) reviewed fatigue in soccer players through time-motion analysis and performance measures during match play. They suggested that fatigue occurs at three different stages: (1) after high-intensity bouts; (2) initial phase of second half; and (3) the latter stages of the game. Soccer is an intermittent sport that relies primarily on the dependency of the aerobic energy system (Krustrup, Zebis, Jensen & Mohr, 2010; Bangsbo, 1994).  A soccer players heart rate during match play, very rarely drops below 65% of their maximum heart rate, which suggests that blood flow is relatively always high in the working leg muscles then when at rest resulting in high oxygen delivery (Bangsbo, 2014; Helgerud, Engen, Wisloff, & Hoff, 2001). As soccer is a high intensity sport, soccer players require periods of rest or low intensity activity to help aid the removal of lactate that has built up, and during this process, oxygen becomes available and the lactate returns to pyruvate, allowing for continued aerobic metabolism and energy to help the body recover a high intensity bout Bangsbo et al., 2007).

 

Hip Flexors

The main muscles of the hip flexors consist of the rectus femoris, Sartorius and iliopsoas, which attach to the joint of the hip to allow the femur to flex onto the pelvis, allowing the knee to pull up (Brophy et al., 2009). Also located around the hip are the gluteal muscles (gluteus maximus, medius and minimus). These muscles are important in allowing rotation of the hip and quadriceps produce both vertical and horizontal force (Tortora & Derrickson, 2006).  In relation to body mechanics, increased forward lean and anterior pelvic tilt may increase the relative length of the hamstrings due to their biartiucualr nature, and therefore increase predisposition to strain injury (Small et al, 2009).

The most predominantly used muscles in soccer constitute of the hamstring, knee flexors and hip flexor muscle groups. The hip flexors are located on the frontal side of the hip on the upper thigh and is responsible for kicking of the ball (Mognomi, Narici, Sirtori & Lorenzelli, 1994). The hamstrings consist of the bicep femoris, semimembranosus and semitendinosus used to bend the knees and move the hip backwards. This is used in soccer during the phase of pulling the hip back to exert power through the ball (Small et al., 2009) Furthermore, the knee flexor muscle group (rectus femoris, vastus lateralises, vastus medius and the vastus intermedius) provide flexion and extension at the knee as well as providing stability (Greig, 2008).

Studies by Kellis, Katis and Vrabas (2006), and Mohr et al. (2003) have considered fatigue to be a performance constrain that affects motor and perceptual processing throughout sport. Mohr, Krustup and Bangsbo (2005) took this further to suggest that it is expressed in the reduced ability for a player to be able to perform match specific actions as coordination and muscular strength capacity as a result of metabolic changes.

Range of Motion

Flexibility is defined as the range of motion available in a joint or group of joints (de Vries 1986; Hebbelinck 1998; Hubley-Kozey 1991; Liemohn 1988; Stone and Kroll 1991). Previous studies have found that soccer training reduces range of motion in players immediately, up to 24 hours post training and gradually over time (Möller, Öberg & Gillquist, 1985). A very strong relationship between preseason ROM in the hip and knee flexors and incidence of muscle strain injury in these muscle groups. Typically, players who injured the knee or hip flexor muscles during the season had a preseason ROM approximately 3° less than that of the uninjured players. Ekstrand and Gillquist (1982) also found that muscle tightness in the hamstrings or knee flexors of soccer players led to increased incidence of hamstring injury, and Witvrouw et al, (2003) observed that low preseason hamstring flexibility significantly correlated with risk of injury during a competitive season. In terms of the intrinsic risk factors, lack of muscle flexibility is one of the most commonly postulated risk factors for the development of muscle injuries (Garrett, J. W, 1996; Gleim & McHugh, 1997; van Mechelen, Hlobil & Kemper, 1992; Worrell, 1994). Despite this, a review of the literature shows that information concerning muscle flexibility as an intrinsic risk factor for musculoskeletal injuries in soccer players is incomplete, and prospective studies are scarce. Lack of flexibility may produce early muscle fatigue or alter normal movement patterns (Krivickas et al., 1996).

Lower Body Power Output

Current research such as Small et al. (2010) and Rahnama, Reilly, Lees and Graham-Smith (2003) have examined peak torque as a measurement for lower body power which has solely focused on fatigue of the hamstring muscles. As a result of focusing on the hamstrings, potential relationships with further lower extremity muscles such as the quadriceps, and hip flexors have been ignored. In a study by Rahnama et al, (200) a total of thirteen male amateur soccer players were assessed by participating in a 90-minute soccer related simulation on a treadmill. The subjects performed maximal knee extension and flexion movements in a seated position at different intervals. The study found that knee extensor and flexor muscles used to exert force decreased with fatigue as the protocol carried on. This suggests that as a result of the repetitive nature of soccer muscle contractions of the quadriceps, hip flexors and hamstrings when jumping, and striking the ball, fatigue will impact on the amount of force exerted and decrease the amount of force generated (Rahnama et al., 2003). This however, will vary across different playing positions as for example, strikers and central defenders engage more in actions like jumping to win headers (Reilly, 2003; Bangsbo, 1994). Restricted length of the hip flexor muscles is identified as a potential causation for musculoskeletal injuries to the lower extremities such as hamstring injuries (Gabbe, Bennell & Finch, 2006). Specifically, to soccer, reciprocal inhibition of the gluteus maximus, secondary to over-activity of the hip flexor muscle group, has been implicated to occur and lead to a decline in lower body power output and to increase risk of injury (Mills et al; 2015; Liebenson, 2017). With an increased dependency placed on the hamstrings and hip abductors to produce hip extension torque is due to reciprocal inhibition (Wagner et al., 2010, Lieberman, 2006). In relation to soccer, players would now be using secondary hip flexor muscles such as the hamstrings, when performing technical skills and biomechanical movement patterns when kicking and jumping for the ball. As a result of the secondary hip extensors taking over to provide power output, increased tension is then placed on the hamstrings abductors, and with soccer being repetitive and high intensity, the strain placed on the hamstrings will increase as more movements are repeated. As a result of fatigue activating secondary hip extensors to take over the movement, Mills et al (2005), this allows an increase of anterior cruciate ligament (ACL) loading and put soccer players with hip flexor tightness at increased risk of injury. Mills et al (2015) states that soccer players with tightness of the hip flexor muscles are at a higher risk of sustaining hamstring injuries than soccer players who do not have tight hip flexor muscles. The findings from this study indicate that hip flexor muscle tightness needs to be considered and addressed in training programmes, as the result of tight hip flexors could have a negative impact and reduce lower body power and increase risk of injury to muscles of the lower extremities.

Methods

 

Ethical approval

Ethical approval was granted prior to study by the Sport, Health and Exercise Science Ethics Committee at the University of Hull (see appendices A & B). All procedures were conducted within the bounds of the approved ethics.

Experimental Design

The study adopted a repeated measures design, with participants attending the Sport, Health and Exercise Science Biomechanics laboratory on five separate occasions, a familiarisation session where participants were measured using the modified Thomas test, Active Straight Leg Raise and standing long jump, before completing one 15-minute interval of the SAFT 90 soccer simulation, then performed three minutes of foam rolling the quadriceps, hip flexors and Soas muscles. Each muscle was foam rolled for 30s on both sides of the body, followed by static stretching of the hip flexor muscles lasting 30s on each stretch on both legs totalling two minutes.

The order of each session was randomised to avoid any interaction and/or learning effect. However, the familiarisation and baseline measurements were not randomised and were carried out in the same order for every lab session.

 

Participants

Eleven male amateur soccer players from the University of Hull men’s football club (n=11; 20.9 ± 3.0 yrs.;) took part in, and completed the study. Before any formal testing took place, all participants completed an informed consent and pre-exercise medical questionnaires before participating. All subjects that took part did not have any current or long-term injuries to their lower extremities or were undergoing any rehabilitation at any point during the testing. It should be noted that two participants were withdrawn from testing due to injury. The participants were referred to as test participant number X throughout the whole study to ensure participant privacy and anonymity were upheld. The playing positions of the participants varied from four full backs (37%), three centre backs (27%), two centre midfielders (18%) and two wingers (18%).

 

Familiarisation

All eleven participants visited the Sport, Health and Exercise Science Biomechanics laboratory for a familiarisation trial, replicating each protocol so they knew what each of the four full testing sessions would consist of. To increase validity and reliability of the tests and results, participants were given a demonstration on how each performance tests were carried out. To further help the participants understand the study, the researcher conducted the selected performance tests on the participants.  The participants completed one 15-minute interval of the SAFT 90 to help them understand the protocol. Participants also completed 3 minutes of foam rolling, split into six 30s bouts, with 30s for both left and right quadriceps, left and right hip flexors and both left and right soas muscle groups. Participants also performed the static hip flexor stretches for 30s on each side.

 

EXPERIMENTAL PROTOCOLS

Modified Thomas Test

The Modified Thomas test (MTT) (figure 1), was used to obtain measures of flexibility for the iliopsoas and quadriceps muscles (Harvey, 1998). Refer to Table 1 for MTT protocol.

 

 

 

 

Figure 1 – Modified Thomas Test Harvey, D. (1998)

Table 1

Modified Thomas Test Harvey, D. (1998)

Instructions

Description

Participants laid on the treatment bed supine

Laid on their back, flat on the treatment table

Participants bring non tested knee to the chest

lumbar spine was flat on the treatment table and that the pelvis was held in posterior rotation

Participants lower tested leg to the floor

Participant held the contralateral hip in maximal flexion while the limb being tested was lowered to the floor

 

First measurement was taken

Angle of hip flexion – passive length of iliopsoas

Second measurement was taken

 

Angle of knee flexion – passive length of quadriceps

 

Standing Long Jump

The standing long jump was used to assess lower extremity functional strength and dynamic power (Harman & Garhammer, 2008; Reiman, M. P., & Manske, R. C, 2009). The participants stood so that their feet were bilateral, shoulder width apart and feet behind the starting line. The participant performed a standing long jump and the distance was marked from the landing position of the back of the heel. All participants started at the same starting point to get an accurate measure of the distance jumped.


Active Straight Leg Raise

The active straight leg raise tests the ability to dis-associate the lower extremity from the trunk while maintaining stability in the torso (Cooke et al, 2006b). The active straight leg raise test assesses active hamstring and gastro-soleus flexibility while maintaining a stable pelvis and active extension of the opposite leg (Cooke et al, 2006b).   For the purpose of this study, angle of the non-tested leg and the tested leg (raised leg) was measured to determine the flexibility of the hamstring in the dominant leg.

 

Table 2 Active Straight Leg Raise Protocol (Cook et al, 1998b)

Active Straight Leg Raise

Instructions

Participant assumes the starting position by laying supine with both arms in an anatomical position and head flat on the floor.

The midpoint between the anterior superior iliac spine ASIS) and midpoint of the patella is identified, where a dowel is then placed.

The participant will raise their dominant leg with a dorsiflexed ankle and an extended knee.

The opposite knee should remain in contact with the floor.

Once the end range position is achieved, and the malleolus is located past the dowel, then the angle is measured.

 

 

 

 

 

 

 

 

 

 

 

Figure 2 Active Straight Leg Raise (Cook et al. (1998b)

Multi-Directional Soccer-Specific Fatigue Protocol (Small et al, 2009)

Participants performed a multi-directional 90-minute intermittent exercise known as the SAFT 90 (Small, McNaughton, Grieg, & Lovell, 2009), see Figure 3.

 The SAFT 90 was designed to replicate movement patterns and similar fatigue levels associated with soccer match play. The SAFT 90 covers a 20 m distance and is guided by an audio CD. See Table 3 for SAFT 90 verbal instructions. There was no warm up included in the testing prior to any lab testing.

 

Figure 3 SAFT 90 Soccer Simulation (Small et al, 2009)

 

 

Table 3 SAFT 90 Protocol (Small et al, 2009)

SAFT 90

List of instructed movements

Up Jog

Participant will jog up and around the first cone, then jog through the course

Up Stride

Participant will jog up and around the first cone, then sprint through the course

Side Jog

Side step around the first cone then jog through the course

Side Stride

Side step around the first cone then spring through the course

Stand

Stop at the cone and wait for further verbal instructions

Walk

Walk from the end cone back to the starting cone

Jog

Jog from the end cone back to the starting cone

Stride

Spring from the end cone back to the starting cone

 

Self-Myofascial Release

Self-myofascial release (SMR) also known as self-massage can be done using a foam roller. Athletes use a foam roller to apply pressure to the sensitive areas of the muscle (Boyle, 2010). Boyle (2010) states that foam rollers are now used to apply sweeping strokes to the long muscle groups like the calves, adductors and quadriceps along with small directed force to areas like the hip rotators and glute medius. Boyle (2010) likens a muscle to a rubber band with a knot in it, and the FR is what unties the knot. This is what allows us to create tissue length and allows us to stretch the muscle. McDonald (2013) suggested that SMR has been more frequently used to increase flexibility and ROM.

 

 

Foam rolling hip flexor muscles

Participant placed a cylinder-shaped foam roller on the floor, and placed their quadriceps muscle on top of the foam roller and rolled the quadriceps muscle for a total of 30s. The participants then foam rolled over the hip for the same duration in the same way they foam rolled the quadriceps. They then swapped legs and foam rolled the opposite leg. Figure 4 shows participant R foam rolling the hip flexor muscles. Figure 5 shows the foam roller used to foam roll the hip flexor muscles.

 

 

Figure 4                                                                             Figure 5

 

Foam Rolling Soas Muscle

The participant placed a spherical shaped foam roller on the floor and laid on top of the foam roller ball, with the foam roller ball placed on the Soas muscle. The participants rolled up and down and side to side for a duration of 30s, then swapped to the opposite side. Figure 6 shows participant X foam rolling the Soas muscle on the right-hand side. Figure 7 shows the foam rolling ball that was used to foam roll the Soas muscle.2

                                                                                                      

 

 

 

 

 

 

 

 

 

 

Figure 6                                                                                      Figure 7

Hip Flexor Stretches

DePino et al., (2000) investigated an acute stretching protocol and found that after a bout of static stretching there was an enhancement in hamstring flexibility. Ayala and Andujar (2010) noted that even though static stretching increases muscle flexibility, the duration of 15 s, 30 s and 15 s of static stretching are all as equally effective.

The couch stretch (refer to figure 8 & 9) were held for 30s in a static position.

                                                                   


Figure 8 Couch Stretch                                                          Figure 9 Couch Stretch

 

Combination of foam rolling and stretching

Referring back to what Boyle (2010) suggests as a knot in a muscle, to consider stretching before rolling and untying the knot, as you stretch the muscle you are pulling both ends of the muscle and effectively tightening the knot. The foam roller unties the knot before stretching can begin. Boyle (2010) states that foam rolling decreases muscle density and also that foam rolling should be used to decrease knots and trigger points in muscles, followed by bouts of static stretching to work on increasing flexibility.

DATA ANALYSIS

Descriptive statistics of outcome measures used included means, averages and standard deviations of test scores, employing a repeated measures analysis of variance (ANOVA). The scores recorded from the results, adopted a repeated measures design, with the dependant variables being the performance fitness tests. This statistical analysis was used to identify if the results showed any significant difference. Statistical analysis was performed using IBM SPSS Statistics v24.0 (IBM Corp, New York, USA). Significance was set at p<0.05, meaning if any data was lower than (P <0.05) then it shows a significant difference, where as anything above would not show any significant difference.  Microsoft Excel 2013 (Microsoft, Washington, USA) was used to produce all graphs displaying the data in the results section.

RESULTS

Modified Thomas Test

Mean performance scores achieved from the modified Thomas test pre-test, after 15 minutes, 30 minutes and post SAFT 90 protocols for all four intervention methods are shown on the graphs in figure 5 below. Results show an increase of 2.61%, for the combination of foam rolling and static stretching, 4.86% on static hip flexor stretching, a decrease of 1.06% for the foam rolling method and no change from the control method from pre performance testing at 0 minutes and post measurements at 45 minutes. While the graphs show the highest increase in hip ROM from the single session of static stretching, testing for some of the participants came post 24 hours of a competitive soccer match which could have some effect on the results. Results from statistical analysis show that there was no significant difference found between conditions (P< 0.13), but significance was found between time and condition (P<0.01).

Lowest average measurements were recorded at 185 º at the 30-minute interval (T=30) for the control method, 185 º at post SAFT 90 (T=45) for the foam rolling condition, 185 º pre SAFT 90 (T=0) for static stretching and 191 º at pre SAFT 90 (T=0) and 15 minute intervals (T=15) for the combination intervention. There is not a clear trend from the lowest average measurements. The highest average measurements of 192 º at the 30-minute interval (T=30) for the control, 187 º after 15 minutes (T=15) for foam rolling, 194 º post SAFT 90 (T=45) for static stretching and 197 º after 30 minutes (T=30) for the combined condition. There is no clear trend shown from the highest average measurements of hip ROM from this data.

 

 

 

 

 

              Figure 10 Hip ROM data

Data from figure 11 below, shows the pre-test average score of 132º to the post test average score of 126 º for the combination of static hip flexor stretching and foam rolling, an increase in hamstring ROM by 4.54%. The data further shows an increase of 5.18% from an average pre-test score of 135 º to a post-test average score of 128 º showing an increase in hamstring ROM by 5.18%. The pre-test average score 121 º to a post-test average score of 127 º, showing a decrease of 4.95% on hamstring ROM, with a decrease of 0.75% shown by pre-test average 133 º and post-test average 134 º from the control testing. When data was analysed, significance was found for condition (P < 0.04). There wasn’t any significance found for time (P < 1.87) nor was any significance found for time and condition (P <0.74).

Lowest average measurements of 134 º were recorded at post SAFT 90 (t=45), 127 º post SAFT 90 (T=45), 135 º pre SAFT 90 (T=0) and 132 º pre SAFT 90 (T=0) for the control, foam rolling, hip flexor static stretching and the combination interventions respectively. There is not a clear trend from this data. The highest average measurements of 129 º after 15 minutes (T=15) for the control condition, 117 º after 15 minutes (T=15) for foam rolling, 128 º post SAFT 90 (T=45) for static stretching and 126 º after 30 minutes (T=30) and post SAFT 90 (T=45) for the combination condition. For the highest average measurements there is no clear trend shown from the results.

Figure 11 Hamstring ROM data

Active Straight Leg Raise

Figure.12 shows the mean angle (degrees) of the Active Straight Leg Raise participants achieved on each of the four performance measurements for each intervention.  The results of each individual at each measurement interval, was calculated to give a group mean rather than individual means for all participants. The graph shows an average pre-test measurement of 71 º while showing an average post-test average of 78 º, highlighting an increase of 9.85% for hamstring flexibility from combining foam rolling with static hip flexor stretching.

With the pre-test average of 67 º and a post-test average of 77 º this shows an incline of 14.9% from static hip flexor stretching, while also showing an increase of 2.6% for foam rolling. No increase nor decline is shown with the pre-test average score and post-test average score both measuring 71 º for the control testing. No significance was found for condition (P <0.07); however, significance was found for time (P <0.05). There was no significance found for condition and time (P < 5.73).

Lowest average measurements of 71 º pre and post SAFT 90 (T=0) (T=45) for the control condition, 75 º pre SAFT 90 (T=0) for the foam rolling condition, 67 º pre SAFT 90 (T=0) for static stretching and 71 º pre SAFT 90 (T=0) for the combination condition. The trend from the Active Straight Leg Raise test shows the lowest average scores for all four conditions at pre SAFT 90 measurements. Highest average measurements of 72 º were recorded after 15 minutes (T=15) and 30 minutes (T=30) for control, 79 º after 30 minutes (T=30) for foam rolling, 78 º after 30 minutes (T=30) for static stretching and 78 º post SAFT 90 (T=45) for the combination condition. The trend for this set of data on hamstring flexibility shows improvements after 30 minutes of the protocol.

Figure 12 Hamstring Flexibility Data

Lower Body Power Output

Figure.13 shows an improvement of 36.7% from the combination intervention method as the pre-test average jump distance increased from 1.88 m to 2.57 m. Static hip flexor stretching showing improvement on lower body power output by 5.64% by an increase of 10 cm from 1.77 m to 1.87 m. Foam rolling showed an improvement of 9.18%, with the jump distance increasing from 1.85 m pre-testing to 2.02 m post-testing. The control intervention also showed improvement of lower body power output by an increase of 4.39% from 1.82 m pre-testing to 1.90 m post testing. There was no evidence of significance from the data for condition (P <0.58). There was no significance found for either time (P <0.06) or condition and time (P <9.93). The combination intervention peaked at the 45-minute interval and was the highest showing a group average score of 2.57m. The highest jump distance average peaked after 45-minutes for foam rolling, static hip flexor stretching and the combination interventions, in contrast to the second lowest jump distance at 45-minutes for the control group.

The lowest averages for each intervention were recorded at pre SAFT 90 (t=0) for the control with an average of 1.82 m, 1.85 m for foam rolling recorded pre SAFT 90 (T=0), 1.77 m for static stretching recorded pre-SAFT 90 (T=0) and 1.88 m for the combination intervention also recorded pre SAFT 90 (t=0).  The clear trend for the lowest average measurements is all recorded pre-SAFT 90. The highest average measurements of 1.94 m after 15 minutes (T=15) for the control condition, 2.02 m post-SAFT 90 (T=45) for the foam rolling condition, 1.87 m post-SAFT 90 (T=45) for static stretching and 2.57 m post-SAFT 90 (T=45) for the combination condition. A clear trend of improvements for static stretching, foam rolling and a combination of both static stretching and foam rolling are all recorded with peak measurements recorded post-SAFT 90. The control has the highest average recorded after 15-minutes.

Figure 13 Lower Body Power Output Data

 

DISCUSSION

The aim of this study was to investigate the effect of fatigue during a 90-minute soccer-specific protocol on lower extremity range of movement, hamstring flexibility and lower body power output in soccer players. The findings from this study found that they are no significant differences on fatigue in relation to range of motion of the lower extremities, hamstring flexibility and lower body power output. The mean data trends were able to highlight a decrease in performance from the control group from pre-SAFT 90 to post-SAFT 90. The trends from the results graphs do show a small percentage increase in hip and hamstring range of motion, hamstring flexibility and lower body power output for each of the three interventions of static hip flexor stretching, foam rolling the hip flexor muscles and a combination of both interventions. However, although an increase in these measurements, they increases were not be found significant following statistical analysis.

Effects on hip and hamstring ROM

The results from the current study show increases in hip and hamstring ROM from static hip flexor stretching, self-myofascial release in form rolling of the hip flexor muscles and a combination of the two interventions. However, the increase in ROM is relatively small, thus meaning there is no significant difference from the intervention methods. The control testing results would suggest that with fatigue the hamstring length would shorten during the soccer simulation protocol, more notably during the sprinting phases in the latter stages of the protocol as the muscle is working eccentrically (Small et al, 2009). This was found only during the control session as the results show a reduction in hamstring flexibility on the Active Straight Leg Raise, hamstring and hip flexor angle was reduced in the latter stages of the SAFT 90 simulation.

Effects on Hamstring Flexibility

Research by Bradley and Portas (2007) identified that 24% of all muscular injuries recorded were strains of the hip flexor muscles, which highlights a decrease in range of movement. Players that they identified with a low hip flexor and knee flexor range of movement were more susceptible and at risk to injury. In relation to the results from this study, this suggests that players are more susceptible to injury at the latter stages of the first half if the player has omitted either static hip flexor stretching, foam rolling the hip flexor muscles or a combination of both. This is shown by the decrease of hip and hamstring ROM after the 45-minute interval of this study; however, it should be considered that even though the single session of stretching, single bout of foam rolling and a combination of the two do show hip and knee flexor range of movement, the increase is small and not significant, meaning the range of movement after 45-minutes of soccer simulation players are still at risk of injury.

Lower Body Power Output

Data from figure 13 shows that without any intervention, lower body power decreases during soccer simulation as participants become more prone to fatigue, showing a decline in power output from the lower extremities. However, with interventions of static stretching, foam rolling and a combination of both, fatigue is reduced and as a result lower body power output is increased. The highest amount of lower body power output was recorded after 45-minutes of the SAFT 90 when participants combined foam rolling of the hip flexor muscles for a duration of 3-minutes and static stretching of the hip flexors for a duration of 3-minutes in that order. The improvement of lower body power is shown between 15-minutes and 30-minutes after foam rolling and static stretching as the average distance at the 15-minute interval is 1.97 m which increases to an average measurement of 2.51 m, showing an increase of 27.4% in that 15-minute period of the protocol.

Quadriceps Muscular Fatigue

The rectus femoris is a biartiucualr muscle, which is located on the anterior side of the quadriceps. The rectus femoris is important for the technique of kicking with a majority of the work load performed eccentrically, (Woods et al., 2004: Mendiguchia et al., 2012). To this date they have been limited study focusing on the rectus femoris muscle specifically, with a huge focus on the quadriceps muscle group as a whole, rather than individual muscles of the quadriceps. The results shown in figure 11 show that there was decline in the participants ability to perform the rectus femoris assessment in the control group, however the results show an increase in ability to perform the rectus femoris assessment after 45-minutes of the three intervention methods of foam rolling, static hip flexor stretching and a combination of both these methods. Although there was a general increase in hamstring ROM in the latter three measurements, there was no significant difference found (P <0.07) for condition.

CONCLUSION

The findings from this study agree with the hypothesis of a combination of foam rolling the hip flexor muscles, followed by static hip flexor stretching increases hip and hamstring range of movement, hamstring flexibility and lower body power output in soccer players. The results from the study also strengthen current literature that single bouts of static stretching and single sessions of myofascial release techniques in foam rolling improve lower extremities range of movement, flexibility and lower body power output. Despite there being no significant difference in the performance measurements, the increase in range of movement, flexibility and lower body power output is still improved from combining foam rolling and static stretching of the hip flexor muscle groups. This study also shows that without interventions of static stretching and foam rolling, fatigue has a negative impact on hip flexor range of movement, hamstring range of movement, hamstring flexibility and lower body power output after 45 minutes of the SAFT 90 intermittent soccer simulation. The findings from this study have taken into consideration the possible effects of participants being pre fatigued from competitive soccer matches and training the day or two days prior to testing, which could have some impact on the results gathered from this study.

LIMITATIONS

The total number of participants at the start of the testing was greater than the final number of participants (n=11) that completed the study due to injury from external factors beyond the control of the study.

Prior to the study, the SAFT 90 soccer simulation was going to be performed for the full 90-minute duration, however due to the protocol being based on elite premier league data, and the participant sample being amateur university soccer players, the technical ability levels, fitness levels and biomechanical ability being. As a result of this the total SAFT 90 protocol time was reduced in half to 45 minutes to inhabit the physical and fitness levels of the participants. Furthermore, as the participant sample consisted of amateur university soccer players they were involved in physical activity in the form of training and/or competitive games 24-hours prior to testing. This factor could have affected the results to some extent, limiting players to a lower rate of their physical and physiological capabilities, thus having more control of participants physical participation would be required in order to minimise potential affecting variables.

This study was unable to take into account some biomechanical and technical movement patterns that occur during competitive soccer as the protocol used was a laboratory-based soccer simulation. The protocol omitted movements such as shooting, tackling, passing and jumping, which if included could give a deeper insight into the extent of fatigue from competitive soccer matches.

A further limitation of this study was that participants playing positions varied from four full backs, three centre backs, two centre midfielders and two wingers, and as current literature suggests, and physiological demands vary from different playing positions. This is a limitation of the study as the partisans of the study was made up of different playing positions.

Practical Implications

The key findings from this study would still be beneficial to coaches and sport scientists of both amateur, semi-professional and professional soccer clubs as it gives them some indication of interventions to improve conditions of the lower extremities in soccer players, and possibly highlights when players are more prone to fatigue and when fatigue has the most detrimental impact on physiological aspect, technical and biomechanical aspect of sports performance.  Furthermore, results from this study could go some way in reducing the risk of injuries to the lower extremities, with specific focus on hip flexor, quadriceps, and hamstring muscle groups, as athletes’ fatigue, coaches and sport scientists can adapt or implement specific training programmes to facilities a player’s levels of fatigue and aid players’ recovery. The results from this study could still add some knowledge into the effects fatigue has lower extremity range of movement, flexibility and lower body power output in soccer players. The results could also provide some insight on the effect combining foam rolling of the hip flexor muscles followed by static hip flexor stretches in that order have on improving hip flexor range of movement, hamstring range of movement, hamstring flexibility and lower body power output in soccer players. Furthermore, the findings from this study can give soccer coaches a better understanding of when amateur soccer players begin to fatigue during soccer match play.

FUTURE RESEARCH RECOMMENDATIONS

While the current findings from this study and some of the current literature, that of Huange, (2010) strengthen the underpinning theory of single bouts of musculotendinous massage improve hip ROM, Ayala & Andujar, 2010), further strengthen the use of static stretching to increase hamstring ROM, future research should look at combining self-myofascial release in the form of foam rolling the hip flexors at the half time interval during competitive soccer matches. This would look at improving lower extremity ROM, hamstring flexibility and lower body power to see if this prolongs the time fatigue hits soccer players in the second half of soccer matches, mainly focusing on the latter stages of the second half. This could be measured by focusing on lower extremities ROM, hamstring flexibility and lower body power post game after interventions at the half time interval and post-game measurements without any half time interval. Progressive research should also look at dynamic hip flexor stretches, and combining this method with static hip flexor stretches and further with foam rolling of the hip flexors. Further research should look at the impact anterior pelvic tilt has on the lower extremities in soccer players.

  1. Alter, M., & Alter, M. (2004). Science of flexibility. Champaign, IL: Human Kinetics.
  2. Agre, J. (1985). Hamstring Injuries. Sports Medicine2(1), 21-33. http://dx.doi.org/10.2165/00007256-198502010-00003
  3. Arnason, A., Andersen, T., Holme, I., Engebretsen, L., & Bahr, R. (2007). Prevention of hamstring strains in elite soccer: an intervention study. Scandinavian Journal Of Medicine & Science In Sports18(1), 40-48. http://dx.doi.org/10.1111/j.1600-0838.2006.00634.x
  4. Bahr, R. (2003). Risk factors for sports injuries — a methodological approach. British Journal Of Sports Medicine37(5), 384-392. http://dx.doi.org/10.1136/bjsm.37.5.384
  5. Bangsbo, J., Nørregaard, L., & Thorsoe, F. (1991). Activity profile of competition soccer. Canadian journal of sport sciences= Journal canadien des sciences du sport16(2), 110-116.
  6. Bennell, K., Wajswelner, H., Lew, P., Schall-Riaucour, A., Leslie, S., Plant, D., & Cirone, J. (1998). Isokinetic strength testing does not predict hamstring injury in Australian Rules footballers. British Journal Of Sports Medicine32(4), 309-314. http://dx.doi.org/10.1136/bjsm.32.4.309
  7. Bradley, P., & Portas, M. (2007). The Relationship between Preseason Range of Motion and Muscle Strain Injury in Elite Soccer Players. The Journal of Strength and Conditioning Research21(4), 1155. http://dx.doi.org/10.1519/r-20416.1
  8. Brooks, J., Fuller, C., Kemp, S., & Reddin, D. (2006). Incidence, Risk, and Prevention of Hamstring Muscle Injuries in Professional Rugby Union. The American Journal Of Sports Medicine34(8), 1297-1306. http://dx.doi.org/10.1177/0363546505286022
  9. BURKETT, L. (1970). Causative factors in hamstring strains. Medicine & Science In Sports & Exercise2(1), 39???42. http://dx.doi.org/10.1249/00005768-197002010-00010
  10. Croisier, J., Forthomme, B., Namurois, M., Vanderthommen, M., & Crielaard, J. (2002). Hamstring Muscle Strain Recurrence and Strength Performance Disorders. The American Journal Of Sports Medicine30(2), 199-203. http://dx.doi.org/10.1177/03635465020300020901
  11. Di Salvo, V., Gregson, W., Atkinson, G., Tordoff, P., & Drust, B. (2009). Analysis of High Intensity Activity in Premier League Soccer. International Journal Of Sports Medicine30(03), 205-212. http://dx.doi.org/10.1055/s-0028-1105950
  12. Ekstrand, J., & Gillquist, J. (1982). The frequency of muscle tightness and injuries in soccer players. The American Journal Of Sports Medicine10(2), 75-78. http://dx.doi.org/10.1177/036354658201000202
  13. Ekstrand, J., Hagglund, M., & Walden, M. (2009). Injury incidence and injury patterns in professional football: the UEFA injury study. British Journal Of Sports Medicine45(7), 553-558. http://dx.doi.org/10.1136/bjsm.2009.060582
  14. Ekstrand, J., Hägglund, M., & Waldén, M. (2011). Epidemiology of Muscle Injuries in Professional Football (Soccer). The American Journal Of Sports Medicine39(6), 1226-1232. http://dx.doi.org/10.1177/0363546510395879
  15. Ford, G. S., Mazzone, M. A., & Taylor, K. (2005). The effect of 4 different durations of static hamstring stretching on passive knee-extension range of motion. Journal of Sport Rehabilitation14(2), 95-107.
  16. Fried, T., & Lloyd, G. (1992). An Overview of Common Soccer Injuries. Sports Medicine14(4), 269-275. http://dx.doi.org/10.2165/00007256-199214040-00005
  17. Garrett, J. W. (1996). Muscle strain injuries. The American journal of sports medicine24(6 Suppl), S2-8.
  18. Gleim, G., & McHugh, M. (1997). Flexibility and Its Effects on Sports Injury and Performance. Sports Medicine24(5), 289-299. http://dx.doi.org/10.2165/00007256-199724050-00001
  19. Godges, J., MacRae, P., & Engelke, K. (1993). Effects of Exercise on Hip Range of Motion, Trunk Muscle Performance, and Gait Economy. Physical Therapy73(7), 468-477. http://dx.doi.org/10.1093/ptj/73.7.468
  20. Hartig, D., & Henderson, J. (1999). Increasing Hamstring Flexibility Decreases Lower Extremity Overuse Injuries in Military Basic Trainees. The American Journal Of Sports Medicine27(2), 173-176. http://dx.doi.org/10.1177/03635465990270021001
  21. HATTORI, K., & OHTA, S. (1986). Ankle joint flexibility in college soccer players. Journal of human ergology15(1), 85-89.
  22. Hawkins, R., & Fuller, C. (1999). A prospective epidemiological study of injuries in four English professional football clubs. British Journal Of Sports Medicine33(3), 196-203. http://dx.doi.org/10.1136/bjsm.33.3.196
  23. Hawley, J. A., & Reilly, T. (1997). Fatigue revisited. Journal of sports sciences15(3), 245.
  24. Heiser, T., Weber, J., Sullivan, G., Clare, P., & Jacobs, R. (1984). Prophylaxis and management of hamstring muscle injuries in intercollegiate football players. The American Journal Of Sports Medicine12(5), 368-370. http://dx.doi.org/10.1177/036354658401200506
  25. Henderson, G., Barnes, C., & Portas, M. (2010). Factors associated with increased propensity for hamstring injury in English Premier League soccer players. Journal Of Science And Medicine In Sport13(4), 397-402. http://dx.doi.org/10.1016/j.jsams.2009.08.003
  26. Jamtvedt, G., Herbert, R., Flottorp, S., Odgaard-Jensen, J., Havelsrud, K., Barratt, A., Mathieu, E., Burls, A. and Oxman, A. (2010). A pragmatic randomised trial of stretching before and after physical activity to prevent injury and soreness. Journal of Science and Medicine in Sport, 12, p.e180.
  27. Kujala, U., Orava, S., & Järvinen, M. (1997). Hamstring Injuries. Sports Medicine23(6), 397-404. http://dx.doi.org/10.2165/00007256-199723060-00005
  28. Liebenson, C. (2007).Rehabilitation of the spine (1st ed.). Philadelphia [etc.]: Lippincott Williams & Wilkins.
  29. Lieberman, D. (2006). The human gluteus maximus and its role in running.Journal Of Experimental Biology209(11), 2143-2155. http://dx.doi.org/10.1242/jeb.02255
  30. Liemohn, W. (1978). Factors related to hamstring strains. The Journal of sports medicine and physical fitness18(1), 71-76.
  31. Mair, S., Seaber, A., Glisson, R., & Garrett, W. (1996). The Role of Fatigue in Susceptibility to Acute Muscle Strain Injury. The American Journal Of Sports Medicine24(2), 137-143. http://dx.doi.org/10.1177/036354659602400203
  32. Mills, M., Franks, B., Goto, S., Blackburn, T., Cates, S., & Clark, M. et al. (2015). Effect of restricted hip flexor muscle length on hip extensor muscle activity and Lower extremity biomechanics in college-aged female soccer players.International Journal Of Sports Physical Therapy10(7), 946-954.
  33. Möller, M., Öberg, B., & Gillquist, J. (1985). Stretching Exercise and Soccer: Effect of Stretching on Range of Motion in the Lower Extremity in Connection with Soccer Training. International Journal Of Sports Medicine06(01), 50-52. http://dx.doi.org/10.1055/s-2008-1025813
  34. Orchard, J., Marsden, J., Lord, S., & Garlick, D. (1997). Preseason Hamstring Muscle Weakness Associated with Hamstring Muscle Injury in Australian Footballers. The American Journal Of Sports Medicine25(1), 81-85. http://dx.doi.org/10.1177/036354659702500116
  35. Petersen, J., Thorborg, K., Nielsen, M., Budtz-Jørgensen, E., & Hölmich, P. (2011). Preventive Effect of Eccentric Training on Acute Hamstring Injuries in Men’s Soccer. The American Journal Of Sports Medicine39(11), 2296-2303. http://dx.doi.org/10.1177/0363546511419277
  36. Reilly, T. (1994). Physiological profile of the player. Football (soccer), 371-425.
  37. Rienzi, E., Drust, B., Reilly, T., Carter, J. E. X. L., & Martin, A. (2000). Investigation of anthropometric and work-rate profiles of elite South American international soccer players. Journal of Sports Medicine and Physical Fitness40(2), 162.
  38. Robertson, M. I. K. E. (2008). Self-myofascial release purpose, methods and techniques. Robertson training systems.
  39. Small, K., McNaughton, L., Greig, M., & Lovell, R. (2010). The effects of multidirectional soccer-specific fatigue on markers of hamstring injury risk. Journal Of Science And Medicine In Sport13(1), 120-125. http://dx.doi.org/10.1016/j.jsams.2008.08.005
  40. Small, K., McNaughton, L., Greig, M., Lohkamp, M., & Lovell, R. (2009). Soccer Fatigue, Sprinting and Hamstring Injury Risk. International Journal Of Sports Medicine30(08), 573-578. http://dx.doi.org/10.1055/s-0029-1202822
  41. Steffen, K., Myklebust, G., Olsen, O. E., Holme, I., & Bahr, R. (2008). Preventing injuries in female youth football–a cluster‐randomized controlled trial. Scandinavian journal of medicine & science in sports18(5), 605-614.
  42. Turl, S., & George, K. (1998). Adverse Neural Tension: A Factor in Repetitive Hamstring Strain?. Journal Of Orthopaedic & Sports Physical Therapy27(1), 16-21. http://dx.doi.org/10.2519/jospt.1998.27.1.16
  43. TW, W., DH, P., BM, G., & JH, G. (1991). Comparison of isokinetic strength and flexibility measures between hamstring injured and noninjured athletes. Clinical Journal Of Sport Medicine1(3), 213. http://dx.doi.org/10.1097/00042752-199107000-00024
  44. van Mechelen, W., Hlobil, H., & Kemper, H. (1992). Incidence, Severity, Aetiology and Prevention of Sports Injuries. Sports Medicine14(2), 82-99. http://dx.doi.org/10.2165/00007256-199214020-00002
  45. Verrall, G. (2001). Clinical risk factors for hamstring muscle strain injury: a prospective study with correlation of injury by magnetic resonance imaging. British Journal Of Sports Medicine35(6), 435-439. http://dx.doi.org/10.1136/bjsm.35.6.435
  46. Wagner, T., Behnia, N., Ancheta, W., Shen, R., Farrokhi, S., & Powers, C. (2010). Strengthening and Neuromuscular Reeducation of the Gluteus Maximus in a Triathlete with Exercise-Associated Cramping of the Hamstrings.Journal Of Orthopaedic & Sports Physical Therapy40(2), 112-119. http://dx.doi.org/10.2519/jospt.2010.3110
  47. Witvrouw, E., Danneels, L., Asselman, P., D’Have, T., & Cambier, D. (2003). Muscle Flexibility as a Risk Factor for Developing Muscle Injuries in Male Professional Soccer Players. The American Journal Of Sports Medicine31(1), 41-46. http://dx.doi.org/10.1177/03635465030310011801
  48. Woods, C. (2004). The Football Association Medical Research Programme: an audit of injuries in professional football–analysis of hamstring injuries. British Journal Of Sports Medicine38(1), 36-41. http://dx.doi.org/10.1136/bjsm.2002.002352
  49. Worrell, T. (1994). Factors Associated with Hamstring Injuries. Sports Medicine17(5), 338-345. http://dx.doi.org/10.2165/00007256-199417050-00006
  50. Worrell, T., & Perrin, D. (1992). Hamstring Muscle Injury: The Influence of Strength, Flexibility, Warm-Up, and Fatigue. Journal Of Orthopaedic & Sports Physical Therapy16(1), 12-18. http://dx.doi.org/10.2519/jospt.1992.16.1.12