Numerous methods exist for evaluating athletic performance and assessing risk factors. One of these is the hamstrings-to-quadriceps (H:Q) ratio, which can be beneficial in both athletic and rehabilitation settings alike. The H:Q ratio is a measure and comparison of the strength of both the hamstrings and quadriceps muscles (14,17). This ratio can be used to assess the strength of the hamstrings and quadriceps as a unit, evaluate the risk of injury athletes may face, and can be useful for rehabilitation settings in deeming athletes' readiness to return to play. An average ratio for athletes is typically between 50 and 80% depending on the velocity tested, and in a rehabilitation setting a ratio of 60% is generally desired (29,32). Therefore, this ratio serves as an important physical measurement for both athletes and recreationally active individuals.
Risk of injury is an important aspect of sports performance, which coaches and athletes attempt to reduce. Using the H:Q ratio, allied health professionals are able to examine the injury risk of not only the hamstrings muscle, but also the knee joint as well because it often acts as a unit within the individual (15). Anterior cruciate ligament (ACL) injury is common in many different sports and activities for both the athlete and recreational individual. Many studies suggest that ACL injuries are caused by actions of the larger and stronger quadriceps, which can sometimes overpower the hamstrings muscle group (2). In addition, studies have found that ACL injuries are 2–8 times more common in females compared with a male athlete (2). However, if properly trained, the hamstrings may assist the ACL in reducing the anterior force on the ligament and possibly prevent an injury from occurring (2). If the hamstrings are analyzed and found to be weak compared with the quadriceps, proper strategies may be made to decrease the differences between the 2 muscles. Having a greater H:Q ratio could potentially result in better coactivation of hamstrings and quadriceps actions and potentially help decrease the risk of injury in athletes susceptible to ACL tears and other injuries that athletes may face in the sports setting.
Previously, stretching has been used before athletic events and practice to increase range of motion (ROM) in joints and muscles (6,38,40). In addition, stretching has been found to be beneficial in reducing imbalances in the body due to decreased flexibility within the muscle and fascia (15). Muscular fascia can be restricted for various reasons, including a response to injury, disease, inactivity, or inflammation, all of which can cause a decrease in elasticity of the fascia and muscle and can be detrimental to performance (30). However, recent advances in the field have shown that there may be few benefits of stretching to prevent injuries during competition or practice (13,15,17,38,42). In recent years, there have been contradictory recommendations regarding the use of stretching immediately before competition and athletic performance events, as well as regarding which form of stretching to use without risking decreased performance. Collectively, recent literature has reported acute decreases in performance in activities requiring power, high force, or economy, following a stretching protocol (15,34,40). Other studies have noted that static stretching specifically has resulted in a decrease in performance if performed immediately before an activity (3,16). Aware of the studies that static stretching may decrease performance if performed immediately before sports and exercise, many professionals seek to find a method of increasing ROM without hampering performance.
Foam rolling (FR) or self-myofascial release has recently gained popularity in the field for its use of increasing ROM and applying a massage-like effect on the muscle and fascia. Fascia acts as a support, stability, and cushion mechanism for bones and muscles and surrounds many different organs and muscles (5,37). In its role as connective tissue, fascia moves in a thixotropic fashion, where the more it is moved, the softer and more malleable it comes; however, when it is not disturbed, it tends to take on a more solid consistency (35). Muscular fascia has been found to help with the mobility of the muscle, cellular circulation, and the elasticity of muscle; and it is vital that the fascia be loose and malleable (1). Foam rolling has allowed athletes and individuals to achieve a way of increasing the mobility of the fascia to gain the benefits it may have on performance. Foam rolling is used by applying the body weight of an individual on a foam roller to exert pressure onto the targeted tissues, which allows the isolation of specific muscles to be rolled over (25). Foam rolling involves rolling over the muscle starting at either the proximal or distal end and rolling to the opposite end of the muscle, which can also act as a form of self-massage for the individual while on the foam roller (30,39). This rolling motion can stimulate both physiological and mechanical properties within the muscle (28). Foam rolling has also been believed to correct muscle imbalance, reduce muscle soreness, improve ROM and coordination, improve neuromuscular efficiency, and help alleviate stress on the joint; however, few studies have been conducted to confirm or refute these purported effects of FR (5,21,30). Despite the lack of conclusive evidence, FR continues to gain acceptance in the athletic and rehabilitation field, as well as with recreationally active individuals who desire a massage-like effect from the manipulation.
Although there have been a few studies, current research suggests positive benefits of FR on flexibility (8,10,30,38). The H:Q ratio is also a vital component for athletes in gauging performance and the risk of injury in their activity. By assessing the athlete's risk of injury, better prevention and training can be achieved and the risk might perhaps decrease. Therefore, the purpose of this study was to examine the effects of FR on ROM, peak torque (PT), H:Q ratios, and muscle activation. It was hypothesized that FR would increase ROM after FR, with no accompanying changes in PT, muscle activation, or the H:Q ratios.
Experimental Approach to the Problem
The experiment consisted of 3 visits to the laboratory on a within-subjects design (Figure 1). The first visit was a familiarization session where anthropometric measurements were collected. The second and third visits were randomly ordered, one being the experimental and the other being the control. During the familiarization session, participants signed a written informed consent and filled out a health status questionnaire. Height was measured using a stadiometer (SECA stadiometer, Chino, CA, USA), and body mass was measured using an electric scale (Ohaus ES Series scale, Parsippany, NJ, USA). The participants then were familiarized with the ROM and FR protocols. The subjects were next familiarized with the isokinetic dynamometer, participating in the same warm-up and protocol they would perform on the second and third visits. While introducing the isokinetic dynamometer, electromyography (EMG) locations were shown to the participants and the process was explained. The familiarization visit lasted approximately 45.64 ± 0.81 minutes.
On the experimental or control day, participants followed the same general protocol but sessions were randomized. Visits were separated by at least 48 hours, but no more than 72 hours, and were conducted approximately during the same time of the day (morning, afternoon, or evening). Participants were instructed to maintain similar hydration levels, food consumption on the day of the 2 testing periods, and to complete similar exercise routines the day before the testing visits. On the experimental day, the participants were tested for isokinetic PT, muscle activation, and flexibility, before and after FR the hamstrings muscle group. The experimental session lasted approximately for 56.71 ± 1.34 minutes. The control session was identical to the experimental visit, except that the subjects sat still for 3 minutes rather than receiving FR. The control visit lasted approximately for 57.29 ± 1.13 minutes.
Twenty-two recreationally active women (mean age ± SD = 21.55 ± 1.82 years [age range: 20-27 years old], height 161.91 ± 6.58 cm, body mass 61.47 ± 10.54 kg, body mass index [BMI] 23.32 ± 2.82 kg·m−2) volunteered for this study. This number was determined by an a priori power analysis using an alpha level of 0.05 and power of 0.80 using G*Power software, which resulted in a minimum of 9 participants (3.1, Dusseldorf, Germany) (7,23). Twenty-one women reported engaging in 5.36 ± 4.56 hours per week of aerobic exercise, 14 reported 3.45 ± 4.62 hours per week of resistance training, and 13 reported 1.66 ± 2.32 hours of recreational sports. All participants stated some form of regular, weekly exercise. These participants were classified as recreationally active, which was defined as participating in physical activity at least 3 days per week, for at least 30 minutes each session, for at least 6 months. All participants were free from lower-body injury within the previous 6 months of their participation in the study or any previous injury that would preclude them from performing the tests. Participants also had a BMI under 30 to ensure lower variability among participants and potential confounding effects from obesity. BMI was measured using the height and weight, which was collected during the familiarization visit. Before beginning the experiment, written informed consent was obtained from all individual participants included in the study. The study was approved by California State University.
Foam Rolling Protocol
The FR protocol was designed to target the hamstrings muscle group and was developed based on adaptations from previous studies (24,30) and extensive piloting. For the hamstrings protocol, participants foam rolled only the dominant side. Subjects were told to sit on a mat with the foam roller under the top of the hamstrings muscle and hands placed behind their back for balance (Figure 2). Limbs were extended in front of the body and crossed at the ankles to obtain additional pressure on the hamstrings. Subjects were instructed to roll from the ischial tuberosity down to the popliteal fossa using small kneading motions on the way down. Subjects were instructed to roll the proximal one-third of the hamstrings for 10 seconds (Figure 2A), working their way distally down the back of the hamstrings, divided into 3 sections for a total of 30 seconds.
Participants then rested for 10 seconds and repeated the procedure. This was performed for a total of 3 times. Participants were told to place as much pressure on the foam roller as possible. The foam roller used in the study was a 36-inch Power Systems, high density foam roller (Power Systems, Knoxville, TN, USA). The foam roller used was made of closed cell polyethylene foam for maximal firmness and maximum effectiveness (33).
Range of Motion Protocol
Range of motion for the hamstrings muscles was conducted using the straight leg raise on the dominant limb. For this test, the participant laid on the mat with both limbs fully extended. The subject was then instructed to lift the ipsilateral limb and maintain full knee extension, whereas the opposite limb remained extended on the mat. The hip joint was flexed passively until resistance was met or the ipsilateral limb could no longer be fully extended with the hips remaining on the mat (22). The ROM measurement of the hip joint angle was determined using a digital goniometer (Halo Medical Devices, Subiaco, WA, Australia), by lining up the goniometer with the midline of the trunk, the greater trochanter of the hip, and the lateral epicondyle of the knee, with the instrument placed in the middle of the thigh (Figure 3). Once a maximal stretch was achieved, the number was recorded.
While on the isokinetic dynamometer, electrodes for muscle activation were placed on the participant's dominant thigh. Two preamplified bipolar surface electrodes (EL254S; Biopac Systems Inc., Goleta, CA, USA) were placed over the biceps femoris and rectus femoris muscles (13,15). The biceps femoris electrode was placed at the mid-point of the ischial tuberosity and the lateral epicondyle, whereas the rectus femoris electrode was placed at 50% of the distance between the anterior superior iliac spine and the superior part of the patella (13,15). Both EMG locations were marked on the second visit to ensure identical location for the third visit. A reference electrode was placed over the spinous process of the seventh cervical vertebrae and consisted of a pregelled, single-use electrode (EL501; Biopac, Goleta, CA, USA). All electrodes were placed over shaved, slightly abraded skin that was cleaned with isopropyl alcohol. Raw EMG scores recorded muscle activation using a Biopac data system (MP150WSW; Biopac Systems Inc.) while on the dynamometer during all measured tests. Data were displayed on a laptop computer (Inspiron 8200; Dell Inc., Round Rock, TX, USA) and analyzed offline using software (AcqKnowledge 5.0; Goleta, CA, USA). Sampling frequency was set at 1,000 Hz, with signals bandpass filtered at 10–500 Hz, and all values recorded were expressed as root mean square. Muscle activation of the repetition with the highest PT was analyzed at each velocity and all values were normalized to the maximum recorded value. The EMG signal within the load range (9) was visually inspected and analyzed.
Isokinetic Dynamometer Protocol
An isokinetic dynamometer (Humac Norm CSMi, Stoughton, MA, USA) was used to analyze PT at 3 different velocities; 60, 180, and 300°·s−1 for concentric PT and 60 and 180°·s−1 for eccentric PT (13,17). Subjects were in a seated position, with their dominant limb secured to the machine while they held onto the seat handles. Straps were place over their shoulders and across their lap to isolate the dominant limb. The contralateral leg was placed behind the stabilization bar. The axis of the dynamometer was aligned with the axis of rotation of the knee, and the shin was secured to a pad on the lever arm. A warm-up was conducted in both experimental and control conditions, before each pre- and posttest. The warm-up consisted of 4 kicks at increasing intensity, approximately 25, 50, 75, and 100% of their perceived maximum effort at each velocity (13–17). The participant then completed 3 maximal repetitions at each speed. Each participant was given verbal cues as to what to do such as “kick,” “pull-back,” “drop,” and “resist.” The conventional H:Q ratios were calculated at each velocity by diving the highest concentric hamstrings PT by the highest concentric quadriceps PT, and the functional H:Q ratios by dividing by the highest eccentric hamstrings PT by the highest concentric quadriceps PT (13,17).
Hamstrings flexibility was analyzed using a 2-way repeated measures analysis of variance (ANOVA) (time [pre-vs. post] × condition [experimental vs. control]). Concentric quadriceps PT, concentric hamstrings PT, eccentric hamstrings PT, conventional H:Q ratios, biceps femoris EMG, and rectus femoris EMG were analyzed separately using three-way repeated measures ANOVAs (velocity [60 °·s−1 vs. 180°·s−1 vs. 300°·s−1] × time [pre-vs. post] × condition [experimental vs. control]). Hamstrings eccentric PT and functional H:Q ratios were analyzed using three-way repeated measures ANOVA (velocity [60°·s−1 vs. 180°·s−1] × time [pre-vs. post] × condition [experimental vs. control]). Paired t-tests and post hoc tests with a Bonferroni correction were used when appropriate. All data were expressed as mean ± SE and analyzed using Statistical Package for the Social Sciences 24 (SPSS, Inc., Chicago, IL, USA). An α level at p ≤ 0.05 was considered statistically significant.
Range of Motion
There was a two-way interaction for time × condition (p < 0.001). Range of motion increased from pre- to posttest (Table 1) under the FR condition (p < 0.001), whereas ROM decreased under the control condition (p = 0.013).
Concentric Hamstrings Peak Torque
There was no three-way interaction for velocity × time × condition (p = 0.778) and no 2-way interactions for time × condition (p = 0.506), velocity × condition (p = 0.748), or velocity × time (p = 0.334). In addition, there was no main effect for condition (p = 0.273). However, there were main effects for time (p = 0.022) and velocity (p < 0.001). PT decreased from pre- to posttesting and decreased as angular velocity increased (Table 2).
Eccentric Hamstrings Peak Torque
There was no three-way interaction for velocity × time × condition (p = 0.131) and no two-way interactions for time × condition (p = 0.075), velocity × condition (p = 0.523), or velocity × time (p = 0.369) (Table 2). In addition, there were no main effects for condition (p = 0.803), time (p = 0.193), or velocity (p = 0.343).
Concentric Quadriceps Peak Torque
There was no three-way interaction for velocity × time × condition (p = 0.708) and no two-way interactions for time × condition (p = 0.258), velocity × condition (p = 0.655), or velocity × time (p = 0.771). In addition, there were no main effects for condition (p = 0.065) or time (p = 0.893). However, there was a main effect for velocity (p < 0.001). PT decreased as angular velocity increased (table 2).
Conventional H:Q Ratio
There was no three-way interaction for velocity × time × condition (p = 0.693) and no 2-way interactions for time × condition (p = 0.136), velocity × condition (p = 0.648), or velocity × time (p = 0.918). There were also no main effects for condition (p = 0.570) or velocity (p = 0.572). However, there was a main effect for time (p = 0.010). Conventional H:Q ratios decreased from pre- to posttesting (Table 3).
Functional H:Q Ratio
There was no three-way interaction for velocity × time × condition (p = 0.388) and no two-way interactions for time × condition (p = 0.647) or velocity × time (p = 0.948). In addition, there were no main effects for condition (p = 0.200) or time (p = 0.099). However, there was an interaction for velocity × condition (p = 0.040). Functional H:Q increased as angular velocity increased under both FR (p < 0.001) and control conditions (p < 0.001) (Table 3).
Concentric Biceps Femoris Muscle Activation
There was no 3-way interaction for velocity × time × condition (p = 0.724) and no 2-way interactions for time × velocity (p = 0.807), condition × velocity (p = 0.555), or condition × time (p = 0.192). In addition, there were no main effects for condition (p = 0.134) or velocity (p = 0.558). However, there was a main effect for time (p = 0.028). Muscle activation decreased from pre- to posttest (Figure 4).
Eccentric Biceps Femoris Muscle Activation
There was no three-way interaction for velocity × time × condition (p = 0.511) and no two-way interactions for time × velocity (p = 0.498), condition × velocity (p = 0.985), or condition × time (p = 0.625) (Figure 4). In addition, there were no main effects for condition (p = 0.901), velocity (p = 0.561), or time (p = 0.835).
Concentric Rectus Femoris Muscle Activation
There was no 3-way interaction for velocity × time × condition (p = 0.112) and no 2-way interactions for time × velocity (p = 0.602), condition × velocity (p = 0.860), or condition × time (p = 0.583). In addition, there were no main effects for condition (p = 0.963) or time (p = 0.321). However, there was a main effect for velocity (p < 0.001). There was a significant difference between 60° s−1 and 180° s−1 as well as 60° s−1 and 300° s−1 and 180-300°s−1 (p ≤ 0.05) (Figure 4).
The primary results of the study indicated that FR of the hamstrings did not cause an altered PT of the lower limb when compared with a control. Range of motion improved in the FR condition, yet decreased in the control condition. Concentric hamstrings PT also decreased from pre- to posttesting for both conditions. Concentric biceps femoris muscle activation decreased from pre- to posttest, whereas the concentric rectus femoris muscle activation resulted in a difference between certain velocities. Although conventional H:Q ratios decreased from pre- to posttesting, functional ratios increased as velocity increased. Quadriceps and concentric hamstrings PT decreased as angular velocity increased.
The results of the FR intervention on ROM were consistent with previous research reporting an increase in ROM as a result of a FR protocol on specific muscle groups (3,8,27,30,38,39). In a study comparing the effects of FR and proprioceptive neuromuscular facilitation stretching, researchers reported significant increases in flexibility with both experimental conditions (27). MacDonald et al. (30), also examined flexibility of the quadriceps muscle group and found an increase in ROM after a bout of FR. This study also reported that the increase in ROM did not decrease force production, nor did it affect muscle activation. Similarly, Sullivan et al. (39) also found an increase in flexibility without a decrease in performance following an FR protocol. In a recent study, researchers compared multiple types of stretching modalities and found that FR, when compared with static and dynamic stretching, caused significant increases in flexibility without impairing performance compared with the other methods of increasing flexibility (38). In comparison, Arazi et al. (3) reported that although FR did increase flexibility of the joints, performance measures decreased. By contrast, Behara and Jacobson (8) found that FR did increase flexibility, but did not alter measures of performance.
Although the current FR protocol did not result in large increases in ROM, other studies have found no significant increases in ROM after a bout of FR (11,18,31,41). In a study by Miller and Rockey (31), researchers reported no significant changes in ROM between an FR group and a control group after an eight-week FR protocol. In this study, those in the FR group completed an FR protocol 3 times per week for 2 months, and researchers found no significant differences between the 2 groups. In a more recent study, researchers tested bouts of hamstrings muscle FR that were both 2 minutes and 4 minutes in duration and reported no significant increases in flexibility compared with baseline ROM tests (18). Other research by Casanova et al. (11) found no significant increase in ROM following a roller massage protocol on the ankle plantar flexor, whereas Vigotsky et al. (41), reported that a bout of self-myofascial release did not improve hip flexibility.
When examining the results of the present study, concentric hamstrings PT decreased from pre- to posttest under both conditions. Two previous studies reported no significant changes after a bout of FR and dynamic stretching on knee extension torque or static and dynamic stretching on the hamstrings PT, respectively (4,8). Discrepancies in the results between previous studies and the present investigation could perhaps have been because of differences in testing interventions and protocols, as well as the use of more highly trained populations. For instance, Behara and Jacobson (8) used highly trained Division I NCAA football players as participants. Similarly, a study by Serefoglu et al. (36) reported that when stretching either the hamstrings or quadriceps muscles, it resulted in no change in PT of the opposite nonstretched muscle. In addition, there were no significant changes in eccentric hamstrings PT found in this study (36). To our knowledge, the present study was the first to investigate FR and its effect on eccentric PT. However, our results contrast with a previous study by Costa et al. (16), where after static stretching, researchers found a decrease in knee flexion PT following a static stretching protocol. Nevertheless, the present results are similar to those of Cramer et al. (20), who reported that after a bout of static stretching, there were no significant changes in eccentric PT. Another study by Cramer et al. (19) found that after a bout of static stretching, there were no significant changes in eccentric PT compared with the pretest. PT and muscle activation in the present study decreased from pre- to posttest, presumably as a result of fatigue. Overall, the results of the current study align with previous literature, while differing with other research, showing that more exploration is needed on the topic.
Hamstrings-to-quadriceps ratio can be an important aspect when examining the strength of lower limb muscles. The present research found that the conventional H:Q ratios decreased from pre- to posttest after both conditions. We anticipated the conventional H:Q ratios to decrease from pre- to posttest as there was a decrease in muscle activation, which was likely the cause of the decrease in muscle PT and subsequent change in ratios. However, our study did not attain the same results as previous studies. In a study by Costa et al. (17), following a hamstrings and quadriceps stretching protocol, investigators reported that participants had a decrease in the functional H:Q ratio. However, for the same study, only the conventional ratio decreased when stretching only the hamstrings (17). Similarly, Ayala et al. (4) reported no change in either of the H:Q ratios after a bout of static and dynamic stretching. In another study examining the combined effects of static stretching and fatigue, there were no significant changes found for either of the H:Q ratios compared with a nonstretching group (14). Although we found that the conventional H:Q ratios decreased, it decreased across both groups from pre- to posttest. Therefore, it may not have been caused by the FR intervention that was applied only in one group. To summarize, this is a positive effect for the future of FR, whereas the results from our study may underscore the hypothesis that FR may not affect or provide a decrease in the H:Q ratio, when compared with a control condition.
Our results varied compared with current literature involving an FR intervention and muscle activation. The present research study demonstrated that muscle activation of the biceps femoris muscle decreased from pre- to posttest, although there was no significant change in eccentric EMG activation of the biceps femoris. Similarly, one study reported that there was no significant change in muscle activation following a static stretching protocol when examining knee extension or flexion (15). Likewise, the biceps femoris did not have any significant effects from the FR as it occurred in both interventions, which is similar to the results found in the previous study (15). Despite the current findings, this disagrees with a previous study where Cavanaugh et al. (12) reported that after following an FR protocol on the hamstrings, researchers found no significant decrease in quadriceps activation. MacDonald et al. (30) also found that following an FR protocol, there was an increase in ROM, but no decrease in force production or muscle activation when compared with the control condition. Similar to MacDonald et al., Serefoglu et al. (36) reported that stretching antagonist muscles resulted in no change in muscle activation. In another form of self-myofascial release, Huang et al. (26) found that a massage of 30 seconds, while increasing hamstrings flexibility, did not result in a significant change in muscle activation. Overall, the results of this study showed a decrease in biceps femoris activation following both the FR and control group, which was different from previous studies that did not report a change in muscle activation following a stretching protocol (12,30,36). As the current study did find a decrease in muscle activation following the protocol, this finding could be attributed to fatigue the individual may have had or the relatively short time between the pre- and posttests, which may not have been enough time for the participant to recover.
In future investigations, researchers could use this research design to investigate differences between men and women, as well as examining more highly trained individuals versus recreationally active women. This change could possibly elicit different results than the current ones based on differences between sex or force output for the individuals. Another potential future study could examine the chronic effects of a training study using FR as there is little research in this area.
In conclusion, hamstrings ROM increased after following an FR protocol for the hamstrings muscles. There was also no decrease in performance measures after the FR, when compared with a control condition. Overall, the results of this study show that FR may be beneficial in increasing flexibility without resulting in a decrease in functional H:Q ratios and may be useful for increasing ROM in an athletic or rehab setting when comparing with other methods of increasing flexibility. Strength and conditioning coaches and practitioners may wish to incorporate FR as a useful tool for increasing ROM in the lower limbs without resulting in negative performance measures or increases in injury risk when compared with a non-FR control condition.
The authors have no conflicts of interest to disclose.
1. Aboodarda SJ, Spence AJ, Button DC. Pain pressure threshold of a muscle tender spot increases following local and non-local rolling massage. BMC Musculoskelet Disord 16: 265, 2015.
2. Ahmad CS, Clark AM, Heilmann N, Schoeb JS, Gardner TR, Levine WN. Effect of gender and maturity on quadriceps-to-hamstring strength ratio and anterior cruciate ligament laxity. Am J Sports Med 34: 370–374, 2006.
3. Arazi H, Aside A, Hosseini K. Comparison of two different warm-ups (static-stretching and massage): Effects on flexibility and explosive power. Acta Kinesiologica 6: 55–59, 2012.
4. Ayala F, De Ste Croix M, Sainz De Baranda P, Santonja F. Acute effects of static and dynamic stretching on hamstring eccentric isokinetic strength and unilateral hamstring to quadriceps strength ratios. J Sports Sci 31: 831–839, 2013.
5. Barnes MF. The basic science of myofascial release: Morphologic change in connective tissue. JM Ther 1: 231–238, 1997.
6. Baxter C, Mc Naughton LR, Sparks A, Norton L, Bentley D. Impact of stretching on the performance and injury risk
of long-distance runners. Res Sports Med 25: 78–90, 2017.
7. Beck TW. The importance of a priori sample size estimation in strength and conditioning research. J Strength Cond Res 27: 2323–2337, 2013.
8. Behara B, Jacobson BH. Acute effects of deep tissue foam rolling and dynamic stretching on muscular strength, power, and flexibility in division I linemen. J Strength Cond Res 31: 888–892, 2017.
9. Brown LE, Weir JP. ASEP procedures recommendation I: Accurate assessment of muscular strength and power. J Exerc Physiol Online 4: 1–21, 2001.
10. Bushong JR. Foam rolling as a warm-up: The effect on lower extremity flexibility to aerobic and stretching. Theses and Dissertations 105, 2011.
11. Casanova N, Reis JF, Vaz JR, Machado R, Mendes B, Button DC, Pezarat-Correia P, Freitas SR. Effects of roller massager on muscle recovery after exercise-induced muscle damage. J Sports Sci 36: 1–8, 2017.
12. Cavanaugh MT, Aboodarda SJ, Hodgson D, Behm DG. Foam rolling of quadriceps decreases biceps femoris activation. J Strength Cond Res 31: 2238–2245, 2016.
13. Costa PB, Herda TJ, Herda AA, Cramer JT. Effects of dynamic stretching on strength, muscle imbalance, and muscle activation. Med Sci Sports Exerc 46: 586–593, 2014.
14. Costa PB, Ruas CV, Smith CM. Effects of stretching and fatigue on peak torque, muscle imbalance and stability. J Sports Med Phys Fitness, 2017. Epub ahead of print.
15. Costa PB, Ryan ED, Herda TJ, Defreitas JM, Beck TW, Cramer JT. Effects of static stretching on the hamstrings-to-quadriceps ratio and electromyographic amplitude in men. J Sports Med Phys Fitness 49: 401–409, 2009.
16. Costa PB, Ryan ED, Herda TJ, DeFreitas JM, Beck TW, Cramer JT. Effects of stretching on peak torque and the H: Q ratio. Int J Sports Med 30: 60–65, 2009.
17. Costa PB, Ryan ED, Herda TJ, Walter AA, Defreitas JM, Stout JR, Cramer JT. Acute effects of static stretching on peak torque and the hamstrings-to-quadriceps conventional and functional ratios. Scand J Med Sci Sports 23: 38–45, 2013.
18. Couture G, Karlik D, Glass SC, Hatzel BM. The effect of foam rolling duration on hamstring range of motion. Open Orthop J 9: 450–455, 2015.
19. Cramer JT, Housh TJ, Coburn JW, Beck TW, Johnson GO. Acute effects of static stretching on maximal eccentric torque production in women. J Strength Cond Res 20: 354–358, 2006.
20. Cramer JT, Housh TJ, Johnson GO, Weir JP, Beck TW, Coburn JW. An acute bout of static stretching does not affect maximal eccentric isokinetic peak torque, the joint angle at peak torque, mean power, electromyography, or mechanomyography. J Orthop Sports Phys Ther 37: 130–139, 2007.
21. Curran PF, Fiore RD, Crisco JJ. A comparison of the pressure exerted on soft tissue by 2 myofascial rollers. J Sport Rehabil 17: 432–442, 2008.
22. Davis DS, Quinn RO, Whiteman CT, Williams JD, Young CR. Concurrent validity of four clinical tests used to measure hamstring flexibility. J Strength Cond Res 22: 583–588, 2008.
23. Faul F, Erdfelder E, Lang AG, Buchner A. G*Power 3: A flexible statistical power analysis program for the social, behavioral, and biomedical sciences. Behav Res Methods 39: 175–191, 2007.
24. Halperin I, Aboodarda SJ, Button DC, Andersen LL, Behm DG. Roller massager improves range of motion of plantar flexor muscles without subsequent decreases in force parameters. Int J Sports Phys Ther 9: 92–102, 2014.
25. Healey KC, Hatfield DL, Blanpied P, Dorfman LR, Riebe D. The effects of myofascial release with foam rolling on performance. J Strength Cond Res 28: 61–68, 2014.
26. Huang SY, Di Santo M, Wadden KP, Cappa DF, Alkanani T, Behm DG. Short-duration massage at the hamstrings musculotendinous junction induces greater range of motion. J Strength Cond Res 24: 1917–1924, 2010.
27. Junker DH, Stöggl TL. The foam roll as a tool to improve hamstring flexibility. J Strength Cond Res 29: 3480–3485, 2015.
28. Kelly S, Beardsley C. Specific and cross-over effects of foam rolling on ankle dorsiflexion range of motion. Int J Sports Phys Ther 11: 544–551, 2016.
29. Kong PW, Burns SF. Bilateral difference in hamstrings to quadriceps ratio in healthy males and females. Phys Ther Sport 11: 12–17, 2010.
30. MacDonald GZ, Penney MD, Mullaley ME, Cuconato AL, Drake CD, Behm DG, Button DC. An acute bout of self-myofascial release increases range of motion without a subsequent decrease in muscle activation or force. J Strength Cond Res 27: 812–821, 2013.
31. Miller JK, Rockey AM. Foam rollers show no increase in the flexibility of the hamstring muscle group. J Undergraduate Res 9, 2006.
32. Orchard J, Marsden J, Lord S, Garlick D. Preseason hamstring muscle weakness associated with hamstring muscle injury in Australian footballers. Am J Sports Med 25: 81–85, 1997.
33. Peacock CA, Krein DD, Silver TA, Sanders GJ, Carlowitz VON. KA. An acute bout of self-myofascial release in the form of foam rolling improves performance testing. Int J Exerc Sci 7: 202–211, 2014.
34. Rubini EC, Costa AL, Gomes PS. The effects of stretching on strength performance. Sports Med 37: 213–224, 2007.
35. Sefton J. Myofascial release for athletic trainers, part 1: Theory and session guidelines. Athl Ther Today 9: 48–49, 2004.
36. Serefoglu A, Sekir U, Gür H, Akova B. Effects of static and dynamic stretching on the isokinetic peak torques and electromyographic activities of the antagonist muscles. J Sports Sci Med 16: 6–13, 2017.
37. Stecco C, Schleip R. A fascia and the fascial system. J Bodyw Mov Ther 20: 139–140, 2016.
38. Su H, Chang NJ, Wu WL, Guo LY, Chu IH. Acute effects of foam rolling, static stretching, and dynamic stretching during warm-ups on muscular flexibility and strength in young adults. J Sport Rehabil 26: 1–24, 2016.
39. Sullivan KM, Silvey DB, Button DC, Behm DG. Roller-massager application to the hamstrings increases sit-and-reach range of motion within five to ten seconds without performance impairments. Int J Sports Phys Ther 8: 228–236, 2013.
40. Trehearn TL, Buresh RJ. Sit-and-reach flexibility and running economy of men and women collegiate distance runners. J Strength Cond Res 23: 158–162, 2009.
41. Vigotsky AD, Lehman GJ, Contreras B, Beardsley C, Chung B, Feser EH. Acute effects of anterior thigh foam rolling on hip angle, knee angle, and rectus femoris length in the modified Thomas test. PeerJ 3: e1281, 2015.
42. Woods K, Bishop P, Jones E. Warm-up and stretching in the prevention of muscular injury. Sports Med 37: 1089–1099, 2007.