There is much controversy within the research community as to what types of stretching and which muscles should be stretched during a warm-up for optimal performance. Warm-ups before performance are believed to upregulate the muscular, cardiovascular, and neural systems to meet the demands of physical activity (28). Warm-ups generally incorporate a dynamic stretch (DS), movement rehearsal, and a cardiovascular component. The question regarding whether athletes should perform static stretching (SS) as part of their warm-up is still debated.
Static stretching was believed to enhance performance and reduce the risk of injury, but there is limited scientific literature to support this practice (29). Current research has shown that repeated bouts of pre-SS (static stretching before performance) can significantly attenuate lower body performance during athletic movements, such as the vertical jump (5,6,18,26,30,36,38,40) and sprinting (12,13,22). Hough et al. (18) found that SS of the lower body musculature for 1 set of 30-second holds (total stretch time of 7 ± 1 minutes) before performing a vertical jump decreased performance by 4.19 ± 4.47% compared with the nonstretch (NS) protocol. There are 2 proposed causes of this stretch-induced deficit: (a) Neurologically, SS may cause a reduction in muscle activity, decreasing muscular force output (5,17) and (b) Changes in mechanical factors associated with the viscoelastic properties of the muscular tendon unit (MTU), such as an increase in MTU passive compliance, which results in an increase in the time to maximal force production causing a decrease in total force (5,17).
The observation that SS may limit high strength or power output activities has resulted in athletes avoiding or being cautious when performing any SS before high power output activities (13,18,26,30,36). It is difficult to determine if this is fully warranted as some studies have found no significant difference between SS, NS, and even DS protocols on lower body performance measures (2,4,8,30,32). Dalrymple et al. (8) found that a SS protocol consisting of 3 sets of 15 second holds of 4 lower body muscles before performing a vertical jump produced similar performance outcomes to NS and DS protocols among female collegiate volleyball players. Samuel et al. (30) discussed the reason for the variation in the results and concluded that studies that found attenuation to performance after SS incorporated intense prolonged stretching interventions ranging from 8 to 30 minutes. Several of these studies used SS protocols targeting only a single muscle group (15,17), with stretching durations and volumes in excess of what is normally encountered in sport scenarios (3,15,18). Studies that found no SS-induced performance decrement incorporated a stretching protocol using multiple muscle groups and/or a volume of 3 sets of 15- to 30-second holds (2,4,8,32). These observations would suggest that there is some controversy that still exists related to the role of SS in preparation for athletic performance.
Although many studies have shown that SS may limit performance of explosive activities, it is possible that a stretch-induced deficit may actually enhance performance in an activity that requires high recruitment of an opposing muscle group. An example of this is when SS is used by clinicians and coaches for the treatment of limited hip extension range of motion (ROM) in their patients and athletes (19,20,31,37). Insufficient hip extension ROM is believed to be, in many cases, due to hip flexor muscle tightness. Yerys et al. (37) found that mobilization of the anterior hip capsule significantly increased gluteus maximus muscle strength during isokinetic testing. This study provides evidence that restrictions because of antagonistic muscular and passive tissues may limit the force production capabilities of the agonistic muscles. It was concluded that the limited strength of the gluteus maximus muscle before the mobilization treatment was due to a joint restriction causing an inhibitory effect on the gluteus maximus muscle. More recently, Sandberg et al. (31) performed SS of the hip flexors and dorsiflexor musculature for 3 sets of 30-second holds. They observed that SS increased vertical jump performance when compared with no stretching. This evidence would suggest that SS of hip flexors may result in greater hip extensor strength during explosive tasks, such as the vertical jump.
Stretching tight hip flexors has been suggested by some in the strength and conditioning industry to improve vertical jump performance (10). It is proposed that by decreasing resting tone of the hip flexor muscles using SS techniques, the hip extensor's torque during the ascending phase of the vertical jump will be increased, resulting in more net force directed to the ground propelling the individual higher (10). This may be true, in theory, if the stretch is able to target the uniarticular tissues of the hip flexor, or in other words, the antagonistic musculature of the hip extensors. It is possible that SS of the biarticular hip flexor muscle, rectus femoris, could cause a stretch-induced deficit and a loss in knee extension torque as it is a 2-joint muscle crossing not only the hip but also the knee joint (16). Considering that the knee extensor moment has been found to be one of the 2 main moments contributing to vertical jump performance (14,24), stretching of this muscle specifically could be contraindicated. It is therefore controversial as to whether SS of the hip flexor muscle group, which includes rectus femoris, will positively impact vertical jump performance as inhibition of this muscle may result in a net force loss in the desired direction.
The purpose of this study was to determine if SS of the hip flexor musculature will increase passive hip extension ROM and therefore increase vertical jump performance in individuals with variable hip flexor laxity. If hip flexor tightness does limit gluteal activation and hip extension torque, then the stretching of this muscle group should increase vertical jump performance, specifically as hip flexor tightness increases.
Experimental Approach to the Problem
A randomized control repeated measures with 3 within group factors and 2 between group factors design was used. The order in which the subjects completed the protocols was randomized and counterbalanced. This design was used to investigate how SS of the hip flexor muscles (rectus femoris, illiacus, and illiopsoas) affects vertical jump performance. Subjects were required to report to the laboratory on 4 separate occasions (D1, D2, D3, and D4). D1 was for subject familiarization of the stretching and testing protocols. Days 2–4 consisted of hip flexor stretching (HFS), hip extensor stretching (HES), and control (CON) conditions, administered in a randomized order. All sessions were separated by a minimum of 48 hours and a maximum of 7 days. Testing sessions were scheduled at similar times of the day for each subject to eliminate the possible effects of circadian rhythm on performance.
Fifteen recreationally active, healthy, college-age males were recruited. The mean (±SD) age, height, and weight of the subjects were 24.1 ± 2.4 years, 176.4 ± 5.4 cm, and 82.7 ± 8.3 kg, respectively. All testing was approved through the Internal Research Ethics Board. Subjects were provided with information regarding the purpose of the study, risks associated with participation, and informed consent. Subjects also answered the Physical Activity Readiness Questionnaire (PARQ) before participation. The subjects were asked to avoid lower body strenuous exercise within 24 hours of testing sessions. Subjects were screened and excluded from the study if they had any history of back, hip, or knee pathologies, lower body fractures within the last 3 years, and/or if they experienced pain in these areas that would inhibit physical activity.
During the familiarization visit, subjects were provided with a verbal description of the study and informed of the time commitment necessary to complete all testing days. The familiarization session was designed to allow the subjects to become comfortable with the testing measurements. Subjects completed the consent form, PARQ, and the injury questionnaire before randomly selecting a testing protocol order. In addition, each subject's height, weight, reach height, and age were recorded. Reach height was assessed by having the subject stand under the vanes of the Vertec with their dominant (reaching) hand closest to the Vertec. Then the subject, flat footed, reached as high as possible and touched the highest vane possible. Subjects were then familiarized with the modified Thomas test procedure and performed 3 maximal counter-movement vertical jump (CMVJ) attempts using the Vertec.
On all testing days, subjects reported to the laboratory and performed one of the 3 protocols in the order randomly chosen during the familiarization session. First, the subject's left and right hip ROM was measured. Next, the subject performed a general warm-up by pedaling on a cycle ergometer at 80 rpm with a workload of 50 W for 5 minutes. After warm-up, the subjects attempted 3 maximal CMVJs, and the highest jump was recorded. After the CMVJ testing, the corresponding protocol intervention (HFS, HES, and CON), randomly selected for that day, was administered on the subject. Finally, the subject's left and right hip ROM measurement was reassessed, and the subjects performed 3 more maximal effort CMVJs with the highest jump recorded.
For the HFS intervention, the investigator assisted each subject with a static stretch of the hip flexor muscles. Subjects were instructed to lie on a treatment table in the same position as the modified Thomas test. The investigator stretched the subject's hip flexors by pressing down on the base of the patella with his hand and stabilizing the nonstretched leg at 90° of both the knee and hip (Figure 1A). Stretches were held for 30 seconds and repeated 3 times for each leg with a 30-second rest between stretches during the time that the opposite leg was being stretched. The degree of ROM limitation for the stretch was set by the subject's perception of mild discomfort.
For HES intervention, the investigator assisted each subject with a static stretch of the hip extensor muscles. Subjects were instructed to lie on a treatment table in the same position as the modified Thomas test. The investigator stretched the subject's hip extensors by stabilizing the nonstretched leg in a neutral position with his hand and pushing the stretched leg with the other hand toward the subject's chest (Figure 1B). Stretches were held for 30 seconds and repeated 3 times for each leg with a 30-second rest between stretches during the time the opposite leg was stretched. The degree of ROM limitation for the stretch was set by the subject's perception of mild discomfort.
For the CON condition, the subject laid quietly on the treatment table for 3 minutes, a time equivalent to that of the intervention protocols.
“Passive Hip Range” of Motion Assessment
The modified Thomas test is widely used among the physical therapy community to assess the passive flexibility of the hip flexor muscles (25). This study used a modified procedure to standardize the modified Thomas test. The subject was asked to sit on the edge of a treatment table allowing their ischial tuberosity to clear the edge of the table, resting the subject's gluteal fold on the edge of the table, while keeping their feet flat on the ground. The subject then flexed the nontesting leg at the hip and knee bringing the knee to their chest. While grabbing the knee of the same leg with both hands, the subject slowly rolled back onto the treatment table with assistance from the investigator. The nontesting leg was externally supported at 90° of the hip and knee to allow the subject to completely relax during the test and standardize the pelvic tilt and lumbar lordotic curve of the subject's spine. The hip being measured was able to extend freely and unsupported off the treatment table to allow full extension without infringement from the treatment table. The investigator then verbally instructed the subject to take a deep breath in and out and relax. Once the subject had fully relaxed, the investigator took the measurements using a trigonometric method. The greater trochanter and the most posterior palpable aspect of the lateral epicondyle of the femur were landmarked with a marker. The distance between these 2 landmarks was measured with a standard tape measure, and the height of the 2 landmarks from the ground was measured with a stadiometer (Seca, United Kingdom) (Figure 2). Equation 1 was then used to calculate the hip angle. This procedure has been shown to have high intrameasurer test-retest reliability, with an intraclass correlation coefficient of 0.95 (33).
where Hypotenuse (H) = distance from greater trochanter (GT) to lateral epicondyle (LE); opposite (O) = height of GT (HGT) − Height LE (HLE).
Counter-Movement Vertical Jump Assessment
To assess the counter-movement vertical jump (CMVJ) of the subjects, the Vertec system was used. This system has been shown to have high reliability values (ICC = 0.88) (23). The subjects were instructed to stand under the vanes of the Vertec with their dominant hand closest to the Vertec. With their feet in a vertical jump position, the subjects were instructed, using maximal effort, to perform a CMVJ and reach as high as possible for the Vertec vanes. The subjects performed 3 attempts separated by a 1-minute rest interval. To calculate the exact vertical jump displacement for each subject, the investigator subtracted the subject's reach height from the highest vertical jump attained of the 3 attempts.
Maximal vertical jump performance on each testing day was normalized to the pretreatment vertical jump height, averaged within each treatment condition and compared between conditions using a single factorial repeated-measures ANOVA. Hip ROM post–warm-up was normalized to pre–warm-up values, averaged within groups and compared between groups using a repeated-measures ANOVA. A post hoc analysis using a paired sampled t-test was used to assess significance between treatment protocols. A Pearson's correlation coefficient was used to determine if there was a relationship between original hip tightness and increase in hip ROM after the HFS condition and to determine if there was a relationship between the increase in hip extension ROM and the change in vertical jump heights.
Stretching Effects on Vertical Jump Performance
A comparison of normalized CMVJ performance between HFS and HES conditions demonstrates a significant improvement in CMVJ for the HFS vs. the HES condition (HFS = 1.74% ± 0.73 vs. HES = −1.74% ± 0.65 and CON = −1.34% ± 0.96, N = 15, p ≤ 0.05, power = 0.97, Figure 3). There was no correlation between improvements in CMVJ and the increase in hip extension ROM after the HFS condition (r2 = 0.10, p > 0.05) indicating that changes in vertical jump could not be attributed to changes in hip flexor compliance (Figure 4).
Stretching Effects on Passive Hip Range of Motion
To determine whether the stretching intervention was related to hip flexor passive compliance, passive ROM was assessed before and after each of the intervention protocols. There was a significant increase in hip extension ROM after the HFS stretching protocol (6.54 ± 2.75%; p ≤ 0.05) when compared with the CON protocol (−1.73 ± 3.26%); however, no significant difference compared with the HES protocol (1.84 ± 2.79%) (power = 0.83, Figure 5). There was minimal correlation between initial hip extension ROM and the change in hip flexor ROM after the HFS protocol for both the left (r2 = 0.11) and right (r2 = 0.01) leg indicating that the initial compliance in the hip flexor muscle group did not predict response to stretch (Figure 6).
The purpose of this study was to investigate the effects of SS of the hip flexors on CMVJ height and passive hip extension ROM and to determine if the effects of SS will vary between subjects with tight or lax hip flexors. It was hypothesized that there would be an increase in CMVJ height and passive hip extension ROM after the stretching conditions. In support of the hypothesis, there was a significant mean increase of 1.74% ± 0.73 (1.02 cm) in CMVJ height after the HFS condition and a nonsignificant decrease in performance after the HES and CON conditions. The HFS protocol was effective in increasing passive hip flexor compliance; however, there was no correlation between the change in passive compliance of the hip flexors and the increase in vertical jump performance. In addition, those with tight hip flexors were not more likely to benefit from the HFS protocol. These results suggest that 3 sets of 30-second bouts of SS of the hip flexor muscle group can significantly increase vertical jump height independent of passive hip compliance changes.
There is limited evidence demonstrating the effects of stretching on the performance of opposing musculatures. Yerys et al. (37) investigated the effects of mobilization of the anterior hip capsule on hip extensor strength and found a 4% increase in isokinetic hip extensor force output. They concluded that the increase in hip extension strength was due to a decrease in mechanoreceptor-associated inhibition of the gluteus maximus muscle resulting in a subsequent increase in gluteus maximus activity; however, electromyography was not used to confirm this theory. Without assessing an increase in gluteus maximus muscular activity using EMG, it was not possible for the research team to quantifiably measure an actual increase in gluteus maximus activity because there are other prime movers of hip extension, such as the hamstring musculature that could be responsible for the increase in strength. It is possible that the hip extensor strength or activation did not change but rather less hip extensor force was used to counter anterior hip tissue resistance, causing an increase in expression of force output during hip extensor isokinetic strength testing.
Sandberg et al. (31) assessed activity changes in the quadriceps and hamstring musculature during isokinetic measures of the quadriceps after antagonistic (hamstring) SS. They observed a nonsignificant increase in quadriceps muscle EMG activity of 9.7% and a 16% nonsignificant decrease of hamstring activity after SS. In addition, there were significant increases in quadriceps torque output during isokinetic testing at 300°·s−1 after the antagonistic stretching protocol. In this same study, Sandberg at el. (31) also investigated the effects of SS of the hip flexors and dorsiflexor musculature on CMVJ performance. They found significant improvements in vertical jump performance after the SS protocol compared with the NS protocol. This study reported similar increases in vertical jump performance after a modest bout of SS of the hip flexor musculature, and therefore lends further evidence to the possibility that antagonistic stretching can improve the performance of agonistic musculature.
There are limited studies that found a significant increase in performance post-SS of the lower body when measuring power activities (31), while there is extensive research to suggest the contrary (5,6,18,26,30,36,38,40). The difference in these outcomes compared with this study and the study by Sandberg et al. (30) are based on the fact that they incorporated SS of all of the prime movers (agonistic), such as hamstrings, gluteal, shank, and quadriceps musculature. This study did find a nonsignificant decrease in vertical jump performance post-HES condition when compared with the control condition. This decrease was similar to the CMVJ performance decrement after the CON condition. Therefore, it is plausible that the decrease after the HES condition was not because of a stretch-induced deficit but rather a cool down effect associated with the passive nature of SS. Considering the combined results from the Sandberg et al. (31) and this study, it may be suggested that SS of antagonistic muscles, not agonistic muscles, should be incorporated into a warm-up.
There have been 2 main mechanisms proposed as the possible causal factor of a stretch-induced deficit in muscle performance after SS. Studies have investigated possible muscular activity changes resulting in altered reflex sensitivity (1,3,15), and musculotendinous passive compliance changes causing a reduction in muscular stiffness and ultimately force production (1,6,11,17,21,27,35,38). Variations observed in these studies may have been the result of the stretching protocol used as small volumes of SS have been shown to have limited impact on muscle compliance (2,4,8,32) In this study, there was no correlation between initial hip extension ROM and improvements in passive hip extension ROM after SS. Furthermore, there was no correlation between increases in hip extension ROM after the HFS condition and increases in CMVJ height. These results suggest that changes to the MTU may not be responsible for the stretch-induced deficits generally observed or the increase in agonistic performance outcomes but rather other factors may be at play. To better establish these relationships, a larger population should be used in combination with EMG analysis to confirm the relationship between hip laxity, vertical jump, muscle activation, and individual response to stretch.
The importance of the vertical jump for athletic performance during such sports as basketball, football, soccer, and volleyball is quite evident and is the reason it is used as an indicator for athletic ability in many testing protocols. This study has shown that SS of the hip flexors can increase vertical jump performance; however, whether there was a stretch-induced functional deficit to the hip flexor MTU after the application is not known because it was not measured in this study. This is a factor that strength and conditioning coaches must take into consideration when applying the results from this study to warm-up protocols, as a loss in hip flexor torque could limit sprint performance (9). Additional research is needed to assess whether SS of the hip flexors is warranted before activities that require high amounts of sprinting performance, and coaches should be conscientious regarding the application of such practices and the specific demands of their athlete's sport.
It is not clear whether all MTUs respond equally to a stretch-induced deficit. Evidence has suggested that males have less compliant musculature than females (21) and are therefore more susceptible to a stretch-induced deficit (8). If the hip flexor musculature in the subjects of this study were generally stiff because of prolonged shortened positions, this may cause a reduction of muscle length as seen in animal models (34). Drawing from this previous research, this would imply that individuals with tight hip flexors are at greater susceptibility to a stretch-induced deficit after SS (21). This may be the causal factor of the significant increase in vertical jump performance seen after such a low volume of SS. However, the poor correlation between hip extension ROM change and vertical jump improvements after the HFS condition suggest that the physiological mechanism associated with this increase in vertical jump height is unclear.
It is possible that trained athletes may respond differently to SS then untrained populations. The subjects for this investigation were recreationally active individuals and not athletes undergoing a vigorous training regimen. It is not clear at this time whether this would affect the performance outcome. In addition, there may be differences in outcomes associated with the testing of different genders (21) and perhaps even differences because of the history of use of SS as part of a warm-up routine (7,39).
In conclusion, this study found a significant increase in CMVJ height and passive hip flexor ROM after HFS; however, there was no correlation between the change in vertical jump height and passive hip flexor compliance change. These observations together suggest that antagonist muscle stretching induces agonist muscle activation, and that the performance changes observed are not solely due to the mechanical limitations imposed by antagonist muscle compliance. Future research should examine the effects of varying volumes of stretching and the effects of acute vs. chronic stretching protocols of the hip flexors on lower body performance. Studies should also explore the effects of SS of the hip flexors on vertical jump performance in varying populations, such as females and high level athletes. Investigating the effects of SS of the hip flexors on other lower body athletic markers, such as sprinting, would also be warranted to see if there is an actual attenuation of performances that require hip flexor muscular strength and power. Finally, future research should implement EMG assessment to determine whether there is an increase in gluteus maximus activation after stretching interventions of the hip flexor MTU.
When designing a warm-up routine for activities that favor high vertical jump performance in athletes, such as volleyball players, or before jump training, incorporating 3 sets of 30 seconds of SS of the hip flexor musculature is favorable. Because of the lack of research investigating the effects of HFS on sprinting performance, coaches should be cautious of stretching the hip flexors before athletic endeavors that require high amounts of hip flexor recruitment, such as those occurs during sprinting and kicking.
The authors thank all the subjects who volunteered for this study and mentorship from Dr Mardon Frazer.
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