Improvements in Acute Explosive Power without a Subsequent Decrease in the Range of Motion of Passive Hip-Flexion Muscles in Taekwondo Players Using Foam Rollers

Authors:  Ani AGOPYAN^1*, Tugce KAHRAMAN², Meral KUCUK YETGIN¹ and Demet TEKIN³

Affiliations: ¹ Department of Coaching Education, Faculty of Sport Sciences, Marmara University, Istanbul, Turkey; ² National Education Foundation, Secondary School, Istanbul, Turkey; ³ Department of Physiotherapy and Rehabilitation, School of Health Sciences, Fenerbahce University, Istanbul, Turkey.

Corresponding Author:
Ani AGOPYAN, Assoc.Prof.
Department of Coaching Education, Faculty of Sport Sciences, Marmara University,
Marmara Universitesi Anadoluhisarı Yerleşkesi  Spor
Bilimleri Fakultesi  Goksu Mah. Cuma Yolu Cad. No:1PK.
34815 – Beykoz / Istanbul – TURKEY
+90 216 308 56 61; Mobile +90 532 714 17 51; Fax: +90 216 332 16 20
Email: aniagopyan@marmara.edu.tr

Ani Agopyan is an associate professor at Department of Coaching Education, Faculty of Sport Sciences in Marmara University, Istanbul-Turkey.

Improvements in Acute Explosive Power without a Subsequent Decrease in the Range of Motion of Passive Hip-Flexion Muscles in Taekwondo Players Using Foam Rollers

ABSTRACT

Background: Foam rolling (FR) has been developed as a popular intervention, however the acute effect of muscular and range of motion (ROM) function using a FR is unknown in young taekwondo players.

Objective: This study examined the acute effects of multiple (lower extremity and back part of the body) FR exercises on joint ROM and vertical jump performance in taekwondo athletes.

Methods: Nineteen (age: 13.21 ± 0.85 years) black-belt taekwondo athletes (n_male = 13; n_female = 6), volunteered to participate of this study. Anthropometric measurements, Passive Straight-Leg Raise Test (PSLR) on each limb and vertical jump tests were performed on the first measurement day. In the second day, following a common warm-up model, participants performed FR exercises on five areas (erector spinae, iliotibial band, hamstrings, quadriceps, and calves) for three minutes (30 seconds/1 set for each area). The tests were administered to all participants again by swapping over on the third day. A Two-Way and Three-Way ANOVA Repeated Measures tests were performed.

Results: It was not observed any significant interaction between the three factors (time, side, or gender) and PSLR-ROM degrees of dominant and non-dominant legs (p>0.05). There were observed significant differences between the pre-and post-test measurements in the vertical jump height (+13.02%) and jump power (+5.23%) performance after FR exercises. Conclusion: Multiple FR exercises did not acute affect PSLR-ROM in taekwondo athletes. FR exercises may be effective within warm-up protocols and seems to be an efficient strategy to promote acute improvements in vertical jump performance in a short time period.

Key words: myofascial release, range of motion, hamstrings muscles, power performance, taekwondo

INTRODUCTION

Taekwondo requires the development of certain physical characteristics: aerobic and anaerobic strength, muscular strength, muscular endurance, flexibility, and speed (8). This sport features striking and pushing movements that require a multi-dimensional and high-degree of joint range of motion (ROM) around the hip area (17). Stiffness of antagonist muscles can create passive resistance, and therefore it can affect the performance of these striking movements. In addition, studies report that the strength of lower extremities is important for optimal taekwondo performance, kicking, jumping, and preserving postural stabilization, and more studies on this subject are required in adolescent athletes (18).

Taekwondo athletes use different stretching methods, such as dynamic, static, and proprioceptive neuromuscular facilitation (PNF) methods in warm-ups along with additional flexibility training to increase ROM in the complicated hip structure (20). However, in recent years, there have been contradictory recommendations regarding the use of stretching immediately before competition and athletic performance events. Therefore, there have been recommendations that different exercise models should be used in the warm-up process (30) which is an essential component for optimizing performance before an exercise session.

In addition to the classic relaxation methods, a myofascial relaxation (MFR) method can be used to increase the extensibility of the muscle tendon unit (23) and it is one type of warmup regimen. Another MFR technique is the self-induced myofascial release (SMR) method used to treat limitations in soft tissues. A cylindrical compressed foam roller (FR) can be used by applying pressure through rolling their bodyweight over certain areas, and it can be evaluated under the SMR method (34). The popularity of FRs is related to their low price, practicability and time-efficient structures and FRs have different dimensions, different densities and different types of massage sticks (40). Exercises performed with FRs have been widely used in sports and rehabilitation fields to change muscular tonus, to renew tissue flexibility and to increase ROM (13, 29). It has been hypothesized that FRs can decrease the rate of fasciation conjoined muscle cases and scar tissues, decrease the duration of recovery and deliver increased muscular performance (23) and reduce arterial stiffness in certain tissues and repair the endothelial function (32) concluded that FR exercises acutely increased in knee joint ROM at 2 minutes post foam rolling (12.7%) and 10 minutes post foam rolling (10.3%) of the quadriceps muscles with no concomitant detrimental effects on neuromuscular force production. Other investigators have found that foam rolling can facilitate acute increases in ankle, knee, and hip passive ROM, reduce (21) or have no effect (6) in maximal isometric strength or velocity and maximum repetitions performance, and reduce delayed-onset muscle soreness (29, 33). It has been found that different results created by FR practices can be affected by the pressure, the duration, the rhythm, the bodyweight of participants, and the type of FR (12, 38).

Although there has been increased focus on the effects of foam rolling and its applications to physical activity, there is discrepancy and lack of evidence-based practice concerning the multiple type exercises that can be used for the most beneficial outcomes. Therefore, further investigations are needed. To our knowledge, there is lack of investigations regarding the effects of foam rolling particularly in warm up exercises in taekwondo athletes which strength and flexibility are important components.

Within this context, the main goals of this study were as follows: (a) to verify the acute effects of multiple FR exercises (lower extremity and back part of the body) on PSLR-ROM, vertical jump height (VJH) and vertical jump power (VJP), conducted as a warm-up model of 12- to 14-year-old female and male taekwondo athletes; (b) to compare these effects of multiple FR exercises according to gender. This study aimed to contribute to the formation of a new training model for increasing muscular performance and ROM in taekwondo athletes with FR practices used during the warm-up stage.

METHODS

Participants

A total of 19 (n_male = 13; n_female = 6), 12 to 14 years of age, taekwondo athletes volunteered to participate in the study. According to the power analysis for two-way repeated analysis of variance (ANOVA) measurements from our previous pilot study, we arrived at an estimated sample size of 19 participants – for conditions of 80% power – to get an eta squared value (i.e., effect size) of 0.72 to 0.89. The pilot study lacked sufficient power to measure the effects of statistical significance.

The participants of the study were taekwondo athletes who had been licensed for at least three years and who had the same training program (3 days/week; min 90 min/day) and were of the same grade (black belt). This study was initiated after permission (Protocol number: 250-28032016-17) was obtained from the ethics committee of local university. In addition, participants’ families were informed about the study and their written permission was obtained as the participants were younger than 18. None of the participants had previous experience of using a foam roller before. All participants were asked about the leg they use to kick during matches, and the dominant leg was found to be the right one.

Participant’s inclusion criteria consisted of no major lower and upper extremities, muscle or bone injuries, or tendon or ligament injuries in the last year, free from any current injury or disease that could affect strength, power, or flexibility, and free of any circulation problems. All participants were free of injury and illness at the time of testing.

Study Design

A randomized within-subject design was used to explore the acute effects of multiple warm-up exercises with FRs to the muscular performance and PSLR-ROM of taekwondo athletes. Participants visited to the training facility three different occasions. In the first occasion, they had a familiarization session that reproduced all experimental procedures. On the other each visit, the subjects performed one of the two interventions (e.g., with FR or without FR). The sessions were at least 3 days apart and were presented in a balanced order (Table 1).

All tests were conducted on the same days (Tuesday and Thursday) and hours (18:30 to 20:30) of the week during the competition preparation period. The tests were instructed and supervised in a sports hall by two test leaders with previously experience of physical strength and power testing. The participants were requested not to attend any activity and to be in a resting condition on test days and were instructed not to eat or drink anything for at least 3 hours before the experiment.

At the first visit, all participants were fully familiarized with the FR exercises and VJ test in order to reduce the effects of learning. Familiarization consisted of giving the subjects specific instructions on how to perform foam rolling and a demonstration by the second author to ensure correct technique. On the second visit, the parents completed a questionnaire regarding their children’s personal information (age of training, daily and weekly taekwondo training hours) and medical histories. In addition, PSLR–ROM and VJ tests were respectively performed after anthropometric measurements (body weight and height) and a common warm-up model had been conducted. Studies indicated that PSLR-ROM can differ before and after the warm-up exercise (7). Therefore, a warm-up model (Table 2) that was similar to the one used by the athletes in their own training, and that contained the dynamic stretching exercises (2 sets and 15 minutes in total), was used.

Table 1. Study Model

Day 1	Day 2	Day 3
	Group 1 (randomized)	Group 1 (randomized)
Anthropometric tests Warm-up (15 minutes) ↓ Passive Straight-Leg Raise Test Range of Motion (PSLR-ROM) ↓ Vertical Jump Test	Warm-up (15 minutes) ↓ Foam Roller Exercises (3 minutes) ↓ Passive Straight-Leg Raise Test Range of Motion (PSLR-ROM)	Warm-up (15 minutes) ↓ Foam Roller Exercises (3 minutes) ↓ Vertical Jump Test
	Group 2 (randomized)	Group 2 (randomized)
	Warm-up (15 minutes) ↓ Foam Roller Exercises (3 minutes) ↓ Vertical Jump Test	Warm-up (15 minutes) ↓ Foam Roller Exercises (3 minutes) ↓ Passive Straight-Leg Raise Test Range of Motion (PSLR-ROM)

On the last two visits, the order of the two experimental trials (e.g., with FR or without FR) was randomized and counterbalanced for each subject. All participants performed exercises with FR and then half of the participants underwent PSLR–ROM test while the other participants performed the VJ test. On the last day, the participants who had been placed in the PSLR–ROM test condition on the first day were placed in the VJ test condition and vice versa. The tests were performed according to group based on Table 2.

Table 2. Warm-up drills.

Warm Up Exercises (2 sets) / Total 15 minutes

Walking lunge: Lunge forward with alternating legs while keeping torso vertical. (each leg 5 times)

Walking with high-knee: Begin exercise by raising the left knee up towards the chest as high as the participant can. Step forward and lower leg back down. Progress to a high-knee walking march by performing this drill while marching forward. (each leg 5 times)

Squat: Stand with the feet a little more than shoulder-width apart. Assume a semi squat position, knees at roughly a 45-degree angle, and walk sideways by keeping the arms outstretched in front of the chest to provide balance during the exercise. (each side 5 times)

Walking butt kicks: Subject kicks heels towards buttocks while moving forward. (each leg 5 times)

Frankenstein walks: Participants walk with both hands extended in front of the body, palms down. Extend one leg by kicking up towards the outstretched hands and then return to starting position before repeating with other leg. (each leg 5 times)

Jumping jack: Stand with feet together and hands at the side. In one action, jump and spread the legs and arms apart on a horizontal plane and clap hands above the head. Once these movements have been performed, return immediately to the starting position. (10 times)

Performance Assessment

Anthropometric Measurements. The heights and weights of the participants were measured while they were wearing shorts and T-shirt with no footwear. Height was measured to the nearest 0.1 cm using standardized measuring equipment (Holtain Height Stadiometer) and bodyweight was determined to the nearest 0.05 kg with a digital scale (WB-110A, Tanita, and Tokyo, Japan).

Passive Straight-Leg Raise Test. The criterion measure for hamstrings muscles flexibility was determined by executing a maximum Passive Straight-Leg Raise Test (PSLR) on each limb. This test is characterized by a passive hip flexion movement, with the knee in full extension, which is related to hamstrings extensibility (19). This test was selected due to its widespread acceptance as the criterion measure for hamstring flexibility and high-test retest reliability of PSLR test (ICC) = 0.94) has been reported previously (9). ROM measurements for the PSLR were taken using a digital inclinometer (Saunders Baseline Digital Inclinometer, Model# 12–1057, Fabrication Enterprises, White Plains, NY; Accuracy ± 0.09 kg). The reliability and validity of the inclinometer was found to range between 0.83 and 0.97 (28). The amount of movement through a ROM is measured in degrees. The inclinometer was zeroed on a flat and level surface before the tests. Inclinometer placement for the hamstrings muscles flexibility assessment was on the medial tibial shaft distal to the tibial tuberosity (1).

The PSLR test was performed in the supine position on an exercise bench. The participant’s leg was lifted passively by the tester into hip flexion with the knee in an extended position. The ankle was fixed of the tested leg in a relaxed position to avoid the possible influence of gastrocnemius shortness on the scores (3).

A trained examiner kept the contralateral leg straight to avoid external rotation and fixed the pelvis to prevent posterior pelvic tilt. The endpoint for straight-leg raising was determined by 1 or more of these 3 criteria: (a) the examiner’s perception of firm resistance, (b) palpable onset of pelvic rotation, and (c) the participant feeling a strong but tolerable stretch, slightly before the occurrence of pain (14). The leg was kept at the maximum point for two seconds. The score criterion of hip flexion ROM was the maximum angle read from the inclinometer at the point of maximum hip flexion.

The measurement was repeated three times, and the best result was recorded in degrees. The measurement was repeated by repositioning the device after the participant returned to the initial position (1). Measurements were performed on the right and left legs, respectively. Time intervals between the measurements were 30 seconds. The cutoff score of the PSLR test was used 80 degrees for the detection of short hamstring muscle flexibility (26).

PSLR tests were performed with 10 athletes (n=5 female, n=5 male) on 2 non-subsequent days to determine the reliability of the inclinometer before the initiation of the present study. Cronbach’s alpha reliability analysis was performed to determine the reliability between the measurements. Accordingly, the following results were found: PSLR (right) α= 0.989 and PSLR (left) α= 0.987

Vertical Jump Test. Maximal vertical jump height (VJH-cm) and vertical jump power (VJP- watts) were determined from a squat jumping (SJ) position. All measurements were taken using the Swift Performance Speed Mat (Swift Performance, Brisbane, Australia) and recorded on the Swift Performance SpeedLight iPad application version 493 (Swift Performance, Brisbane, Australia). This system was found to be highly reliable for the VJH and VJP measurements (flight time r = 0.95; contact time r = 0.99) (27). The SJ test was performed while knees were bent at 90º and hands were held on the hips without performing prior movements. Participants were asked to jump as high as possible without moving their hands from their waists. Then, after jumping, they returned to the starting position. Three measurements were performed, and participants rested for 30 seconds between each measurement (27). The best jump height and power were evaluated by the analyses.

VJ measurements were performed with 10 athletes (n=5 female, n=5 male) on 2 non-subsequent days to determine the reliability of the VJ device before the initiation of the present study. Cronbach’s alpha reliability analysis was performed to determine the reliability between the measurements. Accordingly, the following results were found: VJH α= 0.988 and VJP α= 0.988.

Foam Roller Exercises

The exercises performed with the FR (Busso Bs52 Foam Roller Pvc; 15-cm x 60-cm, material type EVA) in this study were conducted by the participants themselves timed by a digital metronome (Samsung Note 3) set at 40 beats in 30 seconds to maintain a consistent speed. Movements were performed on each sagittal plane by returning from the top of the beginning point to the end point and then returning to the beginning point. Participants were asked to move from the beginning point to the end point using the FR on each beat. They were told to support their bodyweight on the FR as much as possible during the exercises. All participants used FR exercises on five areas (from the back and lower extremities) including the erector spinae, the iliotibial (IT) band, the hamstrings, the quadriceps femoris and the calf muscle groups. Exercises performed on the IT band were performed separately on the right and left legs. Exercise duration was determined to be 30 seconds per trial for each muscle group. Transitions during the movements were recorded at 5 to 6 seconds approximately. The total duration of exercises was set to be 205 to 210 seconds (180 seconds of practice and 25 to 30 seconds of transition). Right and left legs were measured for PSLR–ROM degree, and angular differences were evaluated later. Athletes performed all FR exercises wearing a dobok, the taekwondo uniform, to ensure consistency with the practices in this field of martial arts.

FR practices performed are listed in order below:

a) Erector spinae:Participants were asked to lay on their backs with their knees bent. The FR was positioned under the back. The participants were asked to roll the FR up and down the erector spinae muscles while using arms wide open on both sides and the feet for support (Figure 1-A).

 b) Iliotibial band:Participants were asked to lie on their sides with their knees bent, and to position the FR on the iliotibial band close to the patella. The leg used in the exercise was held straight while the other was bent, the sole of the foot was kept on the floor, and support was provided by the arms and feet on the floor. Participants were asked to roll the FR using force starting from a point close to the pelvis, the starting point of the iliotibial band, to the patella (Figure 1-B). Movements were performed on both right and left legs, respectively.

c) Hamstrings:Participants sat on the floor with their knees bent. Then they were asked to put their hands on the floor to support themselves and to position the FR on the hamstring muscle groups of both legs in a way to perform the exercise on both legs simultaneously. Participants were asked to roll the FR from an area close to pelvis to the popliteal area with force (Figure 1-C).

 d) Quadriceps femoris (QF):Participants were asked to lie face down with their elbows on the ground in front of them. Then they moved the FR to a position in the midpoint of the QF using one hand. They were then asked to use support from their arms to hold their legs out in a stretched position. They were also requested to move the FR over all the QF muscle between the proximal end of the pelvis and the patella, the beginning and end points (Figure 1-D).

e) Calf: Participants sat on the floor with their legs straight, and were told to position the FR on the upper section of the Achilles tendon in a way to perform the exercise over the calf groups of both legs. They were then told to put their hands on the floor and roll the FR over the calf muscle from the Achilles tendon to the popliteal area (Figure 1-E).

Statistical Analysis

Data are presented as mean ± standard deviation, minimum, maximum and median values. Shapiro-Wilk test was used to analyze normality. The Student t test was used for the variables with normal distribution, while the Mann-Whitney U test was used for the ones that did not display normal distribution in the inter-group comparisons. Regarding the intra-group comparisons, a paired sample t-test was used for the variables displaying normal distribution while the Wilcoxon Signed Ranks test was used for the variables that did not display normal distribution. The distribution in percentage between the groups was evaluated using the chi-square test. A Two-Way ANOVA Repeated Measures test was performed for the dominant and non-dominant sides, for the pre-test and post-test measurements for PSLR–ROM in degrees, and the gender factor was evaluated using a Three-Way ANOVA Repeated Measures test. The relationships between the variables were evaluated with Spearman’s Correlation Coefficient. Results were considered statistically significant at an alpha level of p ≤ 0.05.

In addition, an impact size analysis was also performed in this study. Eta square (η2) was used to measure the impact size, and the impact size was determined to be small if 0.01 < η2 < 0.06, moderate if 0.06 ≤ η2 < 0.14, and great if 0.14 ≤ η2 (11). The SPSS version 22 (IBM-SPSS, Inc., Armonk, NY, USA) software statistics program was used for the statistical analyses.

RESULTS

Results for the statistical evaluations were presented according to group and gender. Statistical analyses for the demographic characteristics of the participants in this study are presented in Table 3. No statistical differences were found between any characteristics (age, height, athlete age, daily training duration) of the males and females other than the bodyweight (p < 0.05) (Table 3) Body weights for males were found to be higher than females.

Table 3. Statistical analysis for characteristics and training experience of all participants and comparison of parameters by gender.

Variable		Total (n=19)	Male (n=13)	Female (n=6)	a p value
					Between groups
Age (years)	Min–Max	12–14	12–14	12–14	0.298
	(Median)	(13)	(13)	(14)
	Mean ± SD	13.21 ± 0.85	13.08 ± 0.86	13.50 ± 0.84
Height (cm)	Min–Max	144.8–180	144.8–180	152.1–168.8	0.661
	(Median)	(163)	(161.5)	(165)
	Mean ± SD	161.91 ± 9.22	161.28 ± 10.43	163.27 ± 6.46
Body weight (kg)	Min–Max	37.1–58.4	37.1–57.5	41.1–58.4	0.035*
	(Median)	(50.7)	(47.9)	(55.1)
	Mean ± SD	48.36 ± 7.16	46.18 ± 6.60	53.08 ± 6.37
Training age (years)	Min–Max	3–7	3–7	3–7	0.752
	(Median)	(4)	(4)	(4.5)
	Mean ± SD	4.68 ± 1.34	4.62 ± 1.33	4.83 ± 1.47
Daily training duration (min)	Min–Max	90–120	90–120	90–120	0.374
	(Median)	(110)	(110)	(110)
	Mean ± SD	110.05 ± 10.02	110.92 ± 10.80	110.0 ± 15.49

^aMann Whitney U Test
* p< 0.05

Evaluation of Passive Hip Flexion Range of Motion Measurements

The analysis performed for the entire group indicated that the difference in percentage between the degrees of pre-test and post-test of PSLR–ROM (dominant side-right: −2.59%; non-dominant side-left:  3.56%) was not statistically significant for the dominant and non-dominant sides (p > 0.05) (Table 4).

The time and side interaction for all groups indicated that only the effect on the PSLR –ROM degree was statistically significant for p < 0.05 (Table 5). The difference between the pre-test and post-test PSLR–ROM degrees and percentage changes (dominant side: −2.59%; non-dominant side: −3.56%) was not statistically significant for the dominant and non-dominant sides (p > 0.05). Regarding the evaluation by gender, the gender and side × gender interaction was statistically significant for PSLR–ROM degree (p <0.05).

Female PSLR–ROM degrees for dominant and non-dominant legs were significantly higher than those of males (p < 0.01) (Table 6).Regarding the intra-group comparison (Table 6), the difference and percentage changes between the pre-test and post-test PSLR–ROM degrees for male and female athletes (dominant side female: −2.80%; male: −2.50%; non-dominant side- female −5.25%; male: −2.78%) were not statistically significant for p > 0.05.

Table 4. Statistical analysis regarding all participants for passive straight-leg raise test range of motion (PSLR-ROM) values by side.

PSLR-ROM (°)		Passive Straight-Leg Raise Test (PSLR)-Range of Motion (ROM) (°)		bp value Between groups
		Dominant side (n=19)	Non-dominant side (n=19)
Pre-test	Min–Max	82.7–179.1	84.9–180	0.099b
	(Median)	(119.8)	(117.3)
	Mean ± SD	121.92 ± 29.38	116.62 ± 25.91
Post-test	Min–Max	74.2–180	81.9–180	0.035b*
	(Median)	(124.4)	(110.4)
	Mean ± SD	118.57 ± 31.25	112.14 ± 25.82
bP (within group)		0.326	0.066
Difference (%) (Pre-test to Post-test)	Min–Max	−24.2 to 45.3	−24.2 to14.1	0.809c
	(Median)	(−2.7)	(−2.6)
	Mean ± SD	−2.59 ± 14.16	−3.56 ± 8.75

Abbreviations:  ^bPaire Samples t Test; ^cWilcoxon Signed Ranks Test
*p < 0.05

Table 5. Repeated measures analysis of variance results for passive straight-leg raise test range of motion (PSLR-ROM), vertical jump height and vertical jump power values by side and gender.

Evaluation of Pre-test and Post-test Passive Straight-Leg Raise Test (PSLR)- Range of Motion (ROM) Values (°) by Side
Factor	Test value	ap value
Time*Side	F=0.094	0.763
Time	F=3.228	0.089
Side	F=6.579	0.019*
Evaluation of Pre-test and Post-test Passive Straight-Leg Raise Test (PSLR)-ROM Values (°) by Gender and Side
Factor	Test value		dp value
Time*Side*Gender	F=0.093		0.764
Time*Side	F=0.152		0.702
Time*Gender	F=0.449		0.512
Side*Gender	F=5.971		0.026*
Time	F=3.578		0.076
Side	F=12.916		0.002**
Gender	F=34.996		0.001**
Evaluation of Pre-test and Post-test Vertical Jump Height (cm) Values by Gender
Factor	Test value	ap value
Time*Gender	F=0.158	0.692
Time	F=6.271	0.023*
Gender	F=1.662	0.215
Evaluation of Pre-test and Post-test Vertical Jump Power (W) Values by Gender
Factor	Test value	apvalue
Time*Gender	F=0.158	0.696
Time	F=6.271	0.023*
Gender	F=1.662	0.215

Abbreviations:  ^aTwo Way ANOVA Repeated Measures; ^dThree Way ANOVA Repeated Measures
* p < 0.05. **p < 0.01

Table 6. Statistical analyses of pre-test and post-test Passive Straight-Leg Raise Test-Range of Motion (PSLR-ROM) values by gender and side.

PHF-ROM (°)		Passive Straight-Leg Raise Test (PSLR)-Range of Motion (ROM) (°)						bp value (Male)	bp value (Female)
		Dominant side			Non-dominant side
		Male (n=13)	Female (n=6)	ep (Male – Female)	Male (n=13)	Female (n=6)	ep (Male – Female)	Within groups	Within groups
				Between groups			Between groups
Pretest	Min–Max	82.7–135	133.2–179.1	0.001**	84.9–126.6	124.4–180	0.001**	0.555	0.037*
	(Median)	(101.8)	(159.5)		(102.2)	(142.4)
	Mean ± SD	105.26 ± 15.27	158.00 ± 16.22		102.93 ± 14.69	146.27 ± 18.88
Posttest	Min–Max	74.2–135.1	134.9–180	0.001**	81.9–131	114.7–180	0.001**	0.414	0.020*
	(Median)	(94.0)	(150.2)		(91.4)	(136.6)
	Mean ± SD	102.53 ± 21.96	153.32 ± 15.46		99.85 ± 16.63	138.77 ± 22.22
bP (within group)		.568	.243		.309	.094
Difference (%) (Pre-test to Post-test)	Min–Max	−24.2 to 45.3	−10 to 1.3	0.539 f	−24.2 to 14.1	−16.3 to 0.8	0.599f	0.463c	0.173c
	(Median)	(−4.6)	(−0.1)		(−2.6)	(−4)
	Mean ± SD	−2.50 ± 17.00	−2.80 ± 5.29		−2.78 ± 9.78	−5.25 ± 6.43

Abbreviations: ^bPaired Samples t Test; ^eStudent t Test; ^cWilcoxon Signed Ranks Test; ^fMann Whitney U Test
*p < 0.05. **p < 0.01.

No statistically significant difference was found between the pre-test and post-test dominant and non-dominant PSLR–ROM degrees in the analysis separately performed for female and male athletes (p > 0.05). Independently of the sides, the impact size of the changes in the post-test PSLR–ROM degrees was moderate for male athletes (p < 0.01, d = 0.081) and great for female athletes (p < 0.01, d = 0.403; Table 6).

Evaluation of VJH and VJP

The effects of time and gender variables on VJH (p = 0.023; p < 0.05; Table 5) and VJP (p = 0.023; p < 0.05; Table 5) indicated that the main effect of time is significant. However, gender had no statistically significant effect on VJH or VJP (p > 0.05). The analysis performed for all athletes indicated that the difference between the pre-test and post-test was significant for VJH (13.02%; p = 0.012; p < 0.05) and VJP (5.23%; p = 0.012; p < 0.05; Table 7).

Table 7. Statistical analyses of pre-test and post-test vertical jump height (VJH) and vertical jump power (VJP) values by all participants and gender.

VJH (cm)		Vertical Jump Height (cm)				fp (Male–Female)
		Total (n=25)	Male (n=13)	Female (n=6)
Pre-test	Min–Max	11.5–37	11.5–37	16.2–27.7		0.254
	(Median)	(25.4)	(27.5)	(24.4)
	Mean ± SD	25.42 ± 5.70	26.26 ± 6.31	23.58 ± ±3.93
Post-test	Min–Max	14.9–37	14.9–37	22.7–27.4		0.016*
	(Median)	(27.4)	(28.4)	(26.1)
	Mean ± SD	27.97 ± 4.77	29.06 ± 5.38	25.62 ± 1.65
cp (within group)		0.012*	0.019*	0.345
Difference (%) (Pre-test to Post-test)	Min–Max	−6.3 to 69.6	−6.3 to 69.6	−3.4 to 60.5		0.430
	(Median)	(9.9)	(10.5)	(2.4)
	Mean ± SD	13.02 ± 20.77	13.59 ± 19.91	11.77 ± 24.47
VJP (W)		Vertical Jump Power (W)				fp (Male–Female)
		Total (n=25)	Male (n=13)		Female (n=6)
Pre-test	Min–Max	2164.5–4066.9	2164.5–4066.9		2936.1–3768.5	0.661
	(Median)	(3352.7)	(3294.1)		(3400.9)
	Mean ± SD	3316.13 ± 448.03	3290.06 ± 508.45		3372.63 ± 310.77
Post-test	Min–Max	2374.9–4066.9	2374.9–4066.9		2886.6–3749.9	0.726
	(Median)	(3532.2)	(3465.6)		(3562.1)
	Mean ± SD	3474.47 ± 415.41	3463.38 ± 466.1		3498.49 ± 314.36
cp (within group)		0.012*	0.019*		0.345
Difference (%) (Pre-test to Post-test)	Min–Max	−3.3 to 26.9	−3.3 to 26.9		−1.7 to 19.5	0.335
	(Median)	(4.6)	(6.1)		(1.1)
	Mean ± SD	5.23 ± 7.71	5.82 ± 7.83		3.96 ± 7.99

Abbreviations:  ^cWilcoxon Signed Ranks Test; ^fMann Whitney U Test
*p < 0.05

Regarding the percentage values reflecting the differences between the pre-test and post-test figures for gender, no statistically significant difference was found between the VJH and VJP data for the female (VJH: 11.77%; VJP: 3.96%) and male (VJH: 13.59%; VJP: 5.82%) athletes (p > 0.05; Table 7). The increase between the pre-test and post-test VJH figures for male athletes (p = 0.019; p < 0.05; Table 7) and VJP (p = 0.019; p < 0.05; Table 5) was statistically significant (p = 0.019; p < 0.05; Table 7) while the difference between the same measured values for female athletes was not significant (p > 0.05; Table 7)

Results of Correlation Analysis

The findings showed that there was no statistically significant correlation (p > 0.05) between the values of PSLR (dominant and non-dominant sides), VJH and VJP (data not shown).

DISCUSSION

This study examined the acute effects of FR exercises in PSLR–ROM, VJH and VJP test values performed on the back extensor (erector spinae)  and lower extremity muscles (erector spinae, hamstrings, calves, quadriceps femoris and iliotibial band), each for 30 seconds with 1 repetition at 40-beat metronome rhythm per minute and an exercise duration of 3 minutes in total. Results of this study could not be compared to any other study as there are no studies with similar participants that examined the acute effects of FRs.

The main result of this study was that FR exercises performed on the back-extensor muscles and lower extremity muscles of female and male black-belt taekwondo athletes between the ages of 12 and 14 positively increased VJH (13.02%) and VJP (5.23%) test values. In addition, there were no significant acute effect of FR exercises to the PSLR test values on the dominant (−2.59%) and non-dominant (−3.56%) legs, which indicated that FR practice did not affect an increase of ROM for hamstring flexibility.

Regarding the evaluation of our study model by genders, FR practice created a difference in (pre-test and post-test) VJH and VJP by time, and this positive difference was significantly higher for male athletes (VJH:13.59%; VJP: 5.82%) compared to females (VJH: 11.77%; VJP: 3.96%). However, gender was not a significant factor for hamstring flexibility (PSLR test), or VJH and VJP.

Stiff antagonist muscles of taekwondo athletes can create a passive resistance. As this would affect the performance of striking movements in taekwondo, taekwondo athletes should have a high degree of ROM over their hips and knees (17). Hamstring muscle activity lower than 80 degrees indicates a sufficiency in this muscle’s flexibility (26). The fact that PSLR–ROM values of the athletes in this study were at 118.6º degrees indicates that the hamstring muscle had sufficient flexibility.

Exercises performed with the FR used in the study model created no significant differences between the right and left PSLR–ROM degrees of male and female athletes. No significant interaction was presented with regard to time, side, or gender. However, the fact that right and left PSLR–ROM degrees were higher for females compared to males indicated that the hamstring flexibility of females was higher. In addition, PSLR–ROM degrees regarding the right leg were significantly higher compared to those of the left leg, which can be related to the training models used.

In addition to its increasing popularity in the last ten years, FRs are regarded as one of the 20 best fitness trends (10) which has increased its use in sports fields. Accordingly, there are different ideas regarding the effects of FRs on muscular function and ROM (12,22,37,38) which has increased the necessity to perform more detailed studies in this field. This study indicated no acute FR effect on producing change in the PSLR–ROM on dominant and non-dominant legs. Despite the methodological differences, these results are similar to the study by Vigotsky et al. (39) which indicated no effect from FR exercises performed for 2 sets over 60 seconds over the right frontal femur area on knee flexion or rectus femoris extensibility.

Our results did not mirror the studies reporting that FR exercises increased mobility in shoulders (15), the lumbopelvic area (38), the hips (10,12,37) or the knees and ankles (25). It is difficult to present clearly the differences between the results of this study and others. The differences in these results may have arisen from various mechanisms such as test protocols, practice methods and the experiences of participants (30). In addition to the differences in the duration and rhythm of FR practices, pressure, the participants’ weight, the stiffness, and intensity of FRs, brands, sizes and variety of materials were all among the effective factors (12).

Sullivan et al. (38) found that the exercises performed on hamstring muscles using a roller massager increased hamstrings’ flexibility 4.3%, which is different to the results of this study. Sullivan et al.(38) used a roller massager made of rigid thermoplastic rather than a traditional FR. These roller massagers have higher density and smaller diameters compared to the FR used in this study. Curran et al.(13) indicated that a cylinder with high density applied higher pressure per square inch compared to a FR with less density. In addition, our practice model may not have sufficiently activated the thixotropic characteristics of fascia. The friction generated from the undulations causes warming of the fascia, promoting the fascia to take on a more fluid-like form (known as the thixotropic property of the fascia), breaking up fibrous adhesions between the layers of fascia and restoring soft-tissue extensibility (36). The evidence that soft FRs could not contribute sufficiently to the acute development of flexibility is also supported by Miller and Rockey (31). The FR exercises in this study were performed as a multiple-practice model rather than being conducted on a single area, contrary to the study of Sullivan et al. (38), which might have caused different results to be obtained.

In addition, the study model was applied for a short duration on different muscular groups (5 areas/30 seconds/1 set for each area at a 40-beat metronome rhythm) in back area and lower extremities, which might have limited any acute increase in the PSLR–ROM.

Formation and duration of mechanical stress is thought to be the most significant factor in soft-tissue compatibility (29). In addition to the recommendation to use FRs for up to 5 minutes as a general guide (29), a roller massager or an FR applied for 20 seconds, 30 seconds, 1 minute or 2 minutes might have been an active factor affecting ROM results (36). Sullivan et al.(38) suggested increasing the practice duration to achieve a statistical increase in ROM. Couture et al.(12) reported that FR practices performed as long as 2 minutes were not sufficient for developing flexibility in the knee joint, and the amount of pressure might be an effective factor.

It can be hypothesized that our duration of practice (30 seconds on each area and 3 minutes in total) does not have an effect on muscular stiffness and therefore it may not increase the ROM at the desired rate. However, various ideas indicating that 1 to 2 minutes of practice may create significant increases, reflecting the presence of uncertainties regarding the effects of dose rate over flexibility increases (5). Therefore, the fact that there is no consensus regarding what the active mechanisms could be and how they could deliver these results increases the necessity to perform research on this subject (5).

MacDonald et al. (29) implied that the high force mechanical stress application (i.e. a combination of body mass and high-pressure foam rolling) performed was enough to induce a gel-like state in the fascia leading to increased soft-tissue compliance and subsequently greater knee joint ROM. The fact that age groups and bodyweights of the participants in this study were young and low, and wearing a dobok while performing exercises, may have been other effective factors preventing the expected increase in degrees of PSLR–ROM. Efforts were made to preserve the genuine structure of the training. Moreover, an active position was considered for the ROM measurements. The hip flexion movement was passively performed in this present study, and ROM was brought to its limits. FR exercises might not have displayed the potential extensibility of the muscles at great angles. More studies can be performed using a dynamometer, the elastography technique, or similar tools for measuring the active or passive joint or muscular stiffness before and after the FR protocol.

The effective mechanism of FR use seen in ROM increases can be explained with the friction in the rotational movement performed during practice, the increase in the temperature caused by such friction, and the increase in the liquidity in that area (34). Studies report that the adhesion between the tendons is divided into different tissue layers and it can repair the tissue for extension (36). If the effectiveness of FRs is a result of the increase in tissue extension, it is hypothesized that it can increase the extensibility in muscular tendon units. However, considering the results of this study, the exercise model performed with FRs for 30 seconds on each muscle in 5 different areas, and for 180 seconds in total, did not create an effect that could activate this mechanism (34) to increase PSLR– ROM levels. However, we believe this model can be used practically in the warm-up period, as it can increase muscular power.

This study examined the acute effects of FR use on ROM and evaluated the changes in power performance. The positive effects of FR exercises in the entire group and for males’ VJ performance (13.02% for the entire group; 13.6 for the males) and power (5.23% for the entire group; 5.8% for males) parameters are presented in this study. Although the impact factor was smaller, changes in VJH and VJP were positive, which is important for the authors of the study. The difference between the jumping performances of males compared to females might have arisen from the evidence that males’ bodyweights are higher than those of females. Muscles with a greater physiological section include more sarcomere contributing to the muscular contraction, which causes the formation of more cross bridges and generation of greater power (35). Despite our evaluation in this study, the fact that the muscular mass is greater in males might have created this difference.

Although the method and duration of application of differs, the performance findings of present study are in agreement with those of Halperin et al. (21) who reported that the self-myofascial training performed on the calf muscle for ten minutes using a roller massager increased maximal force output. Contrary to that, our findings do not agree with the results of many studies, indicating that FR practices have no acute effects on muscular performance (4,16,22,29) or just affected it slightly (24). Forexample, Healey et al.(22) reported no acute effect with FRs or plank exercises on vertical jump height and power, isometric squat force and agility performance. Similarly, MacDonald et al. (29) found no FR effect on maximum voluntary contraction force, evoked force and activation of the quadriceps. Fama and Bueti (16) compared FRs with a dynamic warm up protocol and indicated that FRs had acute negative effects on bilateral depth jump, countermovement jumps, and squat jump performance. Healey et al. (22),Sullivan et al. (38) and MacDonald et al. (29)indicated an increase in ROM despite the absence of a positive and negative effect on the muscular performance with SMR. Although vertical jump performance did not change in many studies (4,16,22,29) or just affected it slightly (24) an increase in the vertical jump performance was obtained after our applications in this study. Several mechanisms might lie behind the improvement on muscular performance by foam rolling in this study. Various studies indicated that FR exercises performed for 5 to15 minutes (2) or 20 minutes (33) negatively affected the change in muscular performance and that 6-minute practices did not display an effect (4). On the contrary, in the present study, 3-minute practices did not cause a negative acute impact on muscular performance but increased the VJ performance positively. The potential theory that the exercise model in this study did not change ROM but increased the muscular performance may be explained by the fact that the FR practices in this study do not allow a negative impact on the relationship between the muscles and tendons, and they might have caused the generation of power (21).

Although the FR model used in this study did not display a significant change in PSLR–ROM, no acute negative effect was seen in the muscular performance, and a positive increase was seen in VJ performance within three minutes. It is important for knee extension and muscle flexion to be active to avoid injuries during the striking methods used in attacks at high levels in taekwondo (18). Therefore, we believe that the exercises presented by using FRs can be short and effective alternatives warm-up model that can be used during the warm-up period, and the development of explosive power can be acutely supported for taekwondo athletes.

While providing useful information to published literature, this study has certain limitations, the main methodological limitation of this study was the small sample size. Our findings were limited to young male and female taekwondo players. Overall, the present results should be considered as preliminary, and they need to be confirmed in prospective studies. Future studies should extend these observations to the different age and gender groups at the other levels of athletic population with larger sample size. Another limitation of this study would be to investigate the weight-bearing function. Although the activities were performed using a metronome, optimum efforts were made to determine to what degree the athletes reflected their own weights using controlled feedback.

The results of this study support to use FR during warm-up section in trainings. FR exercises did not cause an effect on the right and left PSLR–ROM degrees in terms of time, but a positive acute increase was found in VJH (13.02%) and VJP (5.23%) with regard to time. However, gender was not an effective factor for PSLR–ROM, or VJH and VJP.

CONCLUSIONS

The data from our study show that even though there were no differences in hamstring flexibility (maybe due to the short duration of the protocol used), the FRs exercises seems to be an efficient strategy to promote changes just boosting muscular performance in vertical jump and power generation in a short time period. Therefore, this study indicates that self-administered foam rolling exercises model on five areas (erector spinae, iliotibial band, hamstrings, quadriceps and calves) for three minutes (30 seconds/1 set for each area at a 40-beat metronome rhythm) can be used in practice as a warm-up by young athletes, particularly those doing taekwondo –a sport requiring flexibility and power. Moreover, it can be suggested that as FR becomes increasingly popular in the fitness industry, this multiple-practice model with FRs rather than being conducted on a single area can be used by athletes, trainers, athletic trainers, physiotherapists, strength and conditioning trainers and other medical professional for rapidly boosting vertical jump height and power. Studies to determine the load rates in FR practices through biomechanical analyses can be performed by increasing the number of participants. In addition, the acute and chronic effects of FR use can be presented with models with different stiffness rates. The authors of this study believe that more studies are needed to determine the FR practice method, which is to be used while warming up, to suit athletes at different levels and branches with regard to repetitions, set numbers and total duration.

APPLICATIONS IN SPORT

The present results of multiple-practice model with FRs could be used to improve vertical jump height and power performance by maintaining baseline levels of hamstring flexibility for young athletic population in a warm-up routine.

This study indicates that self-administered FRs exercises could be a new warm-up model to shorten the duration of the warm-up period by creating individual fitness and athletic performance programs.

ACKNOWLEDGMENTS

The authors thank to the taekwondo athletes, trainers, and parents who participated in this study. This study was conducted by under The Ethics Committee of Marmara University Faculty of Medicine (Protocol number: 250-28032016-17), and financially supported by the Marmara University Scientific Research Projects Committee (Project No: SAG-C-YLP-250816-0413). This study is part of a master’s thesis. None of the authors has any potential conflicts of interest associated with this research.

REFERENCES

1. Aguilar, A.J., DiStefano, L.J., Brown, C.N., Herman, D.C., Guskiewicz, K.M., & Padua, D.A. (2012). A dynamic warm-up model increases quadriceps strength and hamstring flexibility. Journal of Strength and Conditioning Research, 26(4), 1130-1141.
2. Arabaci, R. (2008). Acute effects of pre-event lower limb massage on explosive and high-speed motor capacities and flexibility. Journal of Sports Science and Medicine, 7, 549-555.
3. Ayala, F., de Baranda, P. S., Croix, M. D. S., & Santonja, F. (2013). Comparison of active stretching technique in males with normal and limited hamstring flexibility. Physical Therapy in Sport, 14(2), 98-104.
4. Bailey, U. (2014). Effect of vibration foam rolling and non-vibration foam rolling in the lower extremities on jump height. (Unpublished master‘s thesis Eastern Washington University, USA).
5. Beardsley, C., & Škarabot, J. (2015). Effects of self-myofascial release: A systematic review. Journal of Bodywork and Movement Therapies, 19(4), 747-758
6. Behara, B., & Jacobson, B. H. (2017). Acute effects of deep tissue foam rolling and dynamic stretching on muscular strength, power, and flexibility in division I linemen. Journal of Strength & Conditioning Research, 31(4), 888-892.
7. Bird, H.A. (2004). Rheumatological aspects of dance. Journal of Rheumatology 31(1), 12–13.
8. Bridge, C.A., da Silva Santos, J.F., Chaabene, H., Pieter, W., & Franchini, E. (2014). Physical and physiological profiles of taekwondo athletes. Sports Medicine, 44, 713-733.  
9. Cejudo, A., de Baranda, P. S., Ayala, F., & Santonja, F. (2015). Test-retest reliability of seven common clinical tests for assessing lower extremity muscle flexibility in futsal and handball players. Physical Therapy in Sport, 16(2), 107-113.
10. Cheatham, S.W.,  & Kolber, M.J. (2018). Does roller massage with a foam roll change pressure pain threshold of the ipsilateral lower extremity antagonist and contralateral muscle groups? An exploratory study.  Journal of Sport Rehabilitation, 27(2), 165-169.
11. Cohen, J. (1988). The effect size index: d. Statistical power analysis for the behavioral sciences. (pp. 284-288). Lawrence Erlbaum Associates, Publishers.
12. Couture, G., Karlik, D., Glass, S.C., & Hatzel, B.M. (2015). The effect of foam rolling duration on hamstring range of motion. Open Orthopaedics Journal, 2(9), 450-455.
13. Curran, P.F., Fiore, R.D., & Crisco, J.J. (2008). A comparison of the pressure exerted on soft tissue by 2 myofascial rollers. Journal of Sport Rehabilitation, 17, 432-442.
14. de Baranda, P. S., Rodríguez-Iniesta, M., Ayala, F., Santonja, F., & Cejudo, A. (2014). Determination of the criterion-related validity of hip joint angle test for estimating hamstring flexibility using a contemporary statistical approach. Clinical Journal of Sport Medicine, 24(4), 320-325.
15. Fairall, R.R., Cabell, L., Boergers, R.J., & Battaglia, F. (2017). Acute effects of self-myofascial release and stretching in overhead athletes with GIRD. Journal of Bodywork and Movement Therapies, 21(3), 648-652.
16. Fama, B.J., & Bueti, D.R. (2011). The acute effect of self-myofascial release on lower extremity plyometric performance. 2011. (Unpublished master‘s thesis Sacred Heart University, Fairfield, Connecticut, USA) Department of Physical Therapy and Human Movement Sciences,  Fairfield.
17. Favarini, R.A., Pereira, B.M., de Moraes, J. H., Monteiro, A.D., de Souza, A.M., Menzel, H.J., & Chagas, M.H. (2007). Cinematic analysis during a kick of taekwondo after passive static stretching exercise. In H.-J. Menzel & M. H. Chagas (Eds.). Conference Proceedings Archive of 25^th International Conference on Biomechanics in Sports, August 23-27, 2007, (pp. 598-601).  Ouro Preto: Brasil.
18. Fong, S.S., & Tsang, W.W. (2012). Relationship between the duration of taekwondo training and lower limb muscle strength in adolescents. Hong Kong Physiotherapy Journal, 30(1), 25-28.
19. Gajdosik R, Lusin G. (1983). Hamstring muscle tightness: reliability of an active-knee-extension test. Physical Therapy, 63(7):1085-90.  
20. Haddad, M., Dridi, A., Chtara, M., Chaouachi, A., Wong, D.P., Behm, D., & Chamari, K. (2014). Static stretching can impair explosive performance for at least 24 hours. Journal of Strength and Conditioning Research, 28(1), 140-146.
21. Halperin, I., Aboodarda, S.J., Button, D.C., Andersen, L.L., & Behm, D.G. (2014). Roller massager improves range of motion of plantar flexor muscles without subsequent decreases in force parameters. International Journal of Sports Physical Therapy, 9(1), 92-102.
22. Healey, K.C., Hatfield, D.L., Blanpied, P., Dorfman, L.R., & Riebe, D. (2014). The effects of myofascial release with foam rolling on performance. Journal of Strength and Conditioning Research, 2014, 28(1), 61-68.
23. Junker, D.H., & Stöggl, T.L. (2015). The foam roll as a tool to improve hamstring flexibility. Journal of Strength and Conditioning Research, 29(12), 3480-3485.
24. Junker, D.H, & Stöggl, T. (2019). The training effects of foam rolling on core strength endurance, balance, muscle performance and range of motion: A randomized controlled trial. Journal of Science and Medicine, 18(2), 229-238.
25. Kelly, S., & Beardsley, C. (2016). Specific and cross-over effects of foam rolling on ankle dorsiflexion range of motion. International Journal of Sports Physical Therapy, 11(4), 544-551.
26. Kendall, F.P., McCreary, E.K., Provance, P.G., Rodgers, M.M., & Romani, W.A. (2005). Muscles: testing and function with posture and pain. (p. 384-385). Baltimore: Williams & Wilkins.
27. Kilding, A.E., Tunstall, H., & Kuzmic, D. (2008). Suitability of FIFA’s “The 11” training programme for young football players–impact on physical performance. Journal of Sports Science and Medicine, 7(3), 320-326.
28. Konor, M.M., Morton, S., Eckerson, J.M., & Grindstaff, T.L. (2012). Reliability of three measures of ankle dorsiflexion range of motion. International Journal of Sports Physical Therapy, 7(3), 279-287.
29. MacDonald, G.Z., Penney, M.D., Mullaley, M.E., Cuconato, A.L., Drake, C.D., Behm, D.G., & Button, D.C. (2013). An acute bout of self-myofascial release increases range of motion without a subsequent decrease in muscle activation or force. Journal of Strength and Conditioning Research, 27(3), 812-821.
30. Madoni, S.N., Costa, P.B., Coburn, J.W., & Galpin, A.J. (2018). Effects of foam rolling on range of motion, peak torque, muscle activation, and the hamstrings-to-quadriceps strength ratios. Journal of Strength and Conditioning Research, 32(7), 1821-1830.
31. Miller, J.K., & Rockey, A.M. (2006). Foam rollers show no increase in the flexibility of   the hamstring muscle group. UW-L Journal of Undergraduate Research IX, 9, 1-4.
32. Okamoto, T., Masuhara, M., & Ikuta, K. (2014). Acute effects of self-myofascial release using a foam roller on arterial function Journal of Strength and Conditioning Research, 28(1), 69-73.
33. Pearcey, G.E., Bradbury-Squires, D.J., Kawamoto, J.E., Drinkwater, E.J., Behm, D.G.,  & Button, D.C. (2015). Foam rolling for delayed-onset muscle soreness and recovery of dynamic performance measures. Journal of Athletic Training. 50(1), 5-13.
34. Robertson, M. (2008). Self-myofascial release purpose, methods and techniques. Indianapolis, IN: Indianapolis Fitness and Sports Training.
35. Saiyed, M.Z., Pais, V., Shaikh, A., Shemjaz, A.M., & Pais, S. (2015). Relationship of limb girth, segmental limb length, hamstring flexibility with vertical jump in male sports players. International Journal of Current Research and Review, 7(4), 72-75.
36. Sefton, J. (2004). Myofascial release for athletic trainers, part 1: Theory and session guidelines. International Journal of Athletic Therapy and Training, 9(1), 48–49.
37. Su, H., Chang, N.J., Wu, W.L., Guo, L.Y., & Chu, I.H. (2017). Acute effects of foam rolling, static stretching, and dynamic stretching during warm-ups on muscular flexibility and strength in young adults. Journal of Sport Rehabilitation, 26(6), 469-477.
38. Sullivan, K.M., Silvey, D.B., & Button, D.C. (2013). Rollermassager application to the hamstrings increases sit-and-reach range of motion within five to ten seconds without performance impairments. International Journal of Sports Physical Therapy, 8(3), 228-236.
39. Vigotsky, A.D., Lehman, G.J., Contreras, B., Beardsley, C., Chung, B., & Feser, E.H. (2015). Acute effects of anterior thigh foam rolling on hip angle, knee angle, and rectus femoris length in the modified Thomas test. PeerJ 3:e1281.  Retrieved April 10, 2019 from: https://peerj.com/articles/1281/
40. Wiewelhove, T., Döweling, A., Schneider, C., Hottenrott, L., Meyer, T., Kellmann, M., Pfeiffer, M., & Ferrauti, A. (2019). A meta-analysis of the effects of foam rolling on performance and recovery. Frontiers in Physiology. 10(376), 1-15.