Effect of dynamic versus static stretching in the warm-up on hamstring flexibility

### Abstract

Recent studies have questioned the benefits of static stretching in the sports warm-up. The purpose of our research was to examine the acute effect of static and dynamic stretching in the warm-up, on hamstring flexibility using an intervention study design. Hamstring flexibility was measured using modifications of the Straight Leg Raise test to measure hip flexion range of motion in degrees. The reliability of the test setup was determined in a separate study (n=33), the results of which were also utilised to establish the relationship between static and dynamic SLR tests. There was a significant difference between flexibility measured by the Static-passive and the Dynamic-supine SLR test (p < .05); hence, these were utilised to assess static and dynamic flexibility, respectively, in the intervention study.

Twelve participants were randomly assigned to three interventions of 225 secs. stretch treatment on separate days: No stretching (Treatment 1), Static stretching (Treatment 2) and Dynamic stretching (Treatment 3) in a cross-over study design. When static stretching was included in the warm-up, there were statistically significant differences in pre and post static flexibility (t (11) = 4.19, p < .05). However, there was no significant difference in pre and post dynamic flexibility (t (11) = 0.72, p >.05). Following dynamic stretching there was a statistically significant improvement in both static (t (11) = 2.62, p <. 05) and dynamic (t (11) = 5.69, p < .05) flexibility. Non-parametric tests carried out on the data to corroborate the aforementioned findings.

Static stretching did not improve dynamic hamstring flexibility; however, dynamic stretching improved both dynamic and static flexibility. This has implications for the specificity of stretching in sport.

**Key words:* Range of Motion, hamstring, joint flexibility, Lower extremity, resting tension, stretching

### Abbreviations

ROM

range of motion

SPH

static passive hamstring flexibility test

DSUH

dynamic supine hamstring flexibility test

DSHWB

dynamic standing hamstring flexibility test with knee brace

DSHNB

dynamic standing hamstring flexibility test without knee brace (no brace)

SAID

Specific adaptation to imposed demands

### Introduction

Dynamic stretching consists of simulating movements that are representative of those frequently used in a particular sport (22). Examples of dynamic stretching include the toe walk, heel-walk, hand-toe hamstring stretch, military-walk, sumo groin stretch, and quadriceps kicks (31). In 1996, Alter (2) described a principle put forward by Wallis and Logan in 1964 for strength, endurance and flexibility training, called specific adaptation to imposed demands (SAID). “One should stretch at not less than 75 percent of maximum velocity through the exact plane of motion, through the exact range of motion, and at the precise joint angles used while performing skills in a specific activity” (2). The aforementioned principle lends support to the concept of dynamic flexibility training. There is a lack of studies that examine the effect of dynamic stretching on static as well as dynamic flexibility in the period preceding competition i.e. in the warm-up phase.

Numerous studies in recent literature examine the effects of static stretching on various performance variables (29, 37). In their 2006 study, Behm et al. (6) found decrements in knee extension, knee flexion, drop-jump contact time and counter movement jump height following an acute bout of static stretching. The analysis of the relationship between static stretching and performance focuses mainly on the variables of strength and power (30). Their study demonstrates that static stretching lowers the maximal strength of the knee flexors and extensors and may even hamper performance of activities involving maximal force output. If increased musculotendinous stiffness enables more efficient transmission of force, stretching just prior to activity might also decrease force output in skills such as jumping to attain maximum height and forceful throwing (12). Even a moderate duration of static stretching could result in quadriceps isometric force and activation decrements (33). Furthermore, it is theorised that this impairment of isometric force production could last for a period of up to 120 minutes.

The purpose of our research was to examine the acute effect of static and dynamic stretching in the warm-up, on hamstring flexibility using an intervention study design. The reliability of the experimental setup was established in a separate study (n=33) that was used to determine the relationship between the tests that measured static and dynamic hamstring flexibility. Analyses of variance and correlation analyses were computed on the collated data. An intervention design was used to determine how an acute bout of static or dynamic stretching affected hamstring flexibility as measured by a modified SLR test. Parametric (t-test) and non-parametric tests (Wilcoxon Matched-Pairs Ranks) were carried out to analyse the raw data.

### Method

#### Participants

Sixteen university students (n = 16) were recruited for the intervention study to examine the effects of dynamic and static stretching on hamstring flexibility. The final sample consisted of 12 students of which five females and seven males served as participants. Two potential participants did not complete all testing sessions and two participants’ data was excluded from the study due to measurement error. The average age of the participants was 24.8 ± 6.8 yrs. (mean ± SD). The average height and weight was 174.5 ± 4.5 cm. and 73.0 ± 15.7 kg. respectively (mean ± SD).

Participants were drawn from a variety of sporting backgrounds which predominantly involved the lower body (42). Most were actively training for a sport. All trained lightly a minimum of three times a week. A condition of entry to the study was that the subjects did not concurrently use any stretch or flexibility training in their regular training program (41). Screening questionnaires were provided to identify subjects with neurological or musculoskeletal abnormalities of the spine and lower limbs. Subjects were examined to determine hip, knee and ankle ROM and a brief examination of the lumbar spine was performed. The final participants were free of any bony or soft tissue injury to the spine and lower limbs. The participants were asked to carry out routine activities and not to exercise strenuously (10). They were also advised not to stretch the hamstrings and avoid initiating or changing any exercise program during the study (35).

All participants provided their written informed consent to participate in the study. Hamstring flexibility was measured in the dominant leg (19), identified by kicking a football towards a wall five times (11). This study received approval from the human ethics committee of the University of Canberra.

#### Materials and Procedure

Reflective markers attached to specific bony prominences utilised for biomechanical analysis (Figure 1). The functional orthopaedic knee brace, Knee Ranger II Universal (dj Orthopaedics, LLC, California, USA) helped to maintain 15º of knee flexion during pre and post-testing. Participants wore the knee brace only during testing and not whilst performing the intervention stretches. The Velcro strapping on the brace eased the removal and fastening process considerably. A warm-up consisting of five minutes of cycling on a stationary cycle ergometer (Exertech, Australia) at 60-70 W (6, 42) was employed. Testing was carried out at around the same time of the day for each participant involved in the intervention study (41). There was no stretching incorporated in the warm-up.

#### Modified SLR test for measuring hamstring flexibility

Previous studies examining stretch and contraction specific changes in ROM utilise the hamstring muscle group most frequently in humans and the SLR test is the most commonly used test (17). The contralateral or non-testing leg was partially flexed at the hip and knee, with a pillow rolled underneath the knee to stabilise the pelvis (11). A Velcro strap fastened around the pelvis and secured beneath the exercise bench to minimise pelvic rotation. In 1982, Bohannon (7) suggested that the pelvis and the contralateral thigh should be maintained in neutral position to decrease contribution to SLR-ROM. During testing, the participant was advised not to lift the upper body off the bench, and the arms were folded across the chest or placed beneath the head. This minimised the contribution from the trunk towards the effort of hip flexion.

The experimental setup included a camcorder placed perpendicular to the plane of motion. The camcorder was mounted on a tripod and placed at a distance of 10 metres from the test area (Figure 1). A PAL digital video camera (Canon MVX3i, Canon Inc., Japan) operating at 50Hz was used to video the participants performing the various flexibility tests. Dartfish ProSuite (Dartfish Connect 4.0, Dartfish Ltd., Fribourg, Switzerland) was used to capture the video data from the camera to a computer for two-dimensional analysis.

#### Measuring Flexibility

After the warm-up period, participants (n=12) undertook static passive (SPH) and dynamic supine hamstring flexibility (DSUH) tests to measure static and dynamic flexibility respectively. The reliability of this experimental setup and correlation between modifications of the SLR test was established in an earlier study involving 33 subjects.

##### Static Passive Hamstring Flexibility test

This test was performed in the supine position on an exercise bench. The functional knee brace was worn for testing. Passive stretching utilises an external agent to assist with the stretch. The participant used a Velcro strap around the ankle to assist with pulling the limb into hip flexion (Figure 1). The dominant leg was flexed to the terminal ROM or until a mild discomfort/tightness was felt in the back of thigh (5). This position was maintained for five seconds following which the limb was slowly lowered to the resting position.

##### Dynamic Supine Hamstring Flexibility test

The test was performed in the supine position on an exercise bench. Dynamic flexibility measures the ability to move a joint quickly through a non-restricted ROM. The participants were instructed to move the dominant limb into hip flexion using maximal effort and as quickly as possible or until a mild discomfort was felt in the back of the thigh. Dartfish analysis of the video frame that captured the terminal phase of movement was used to determine the angle of hip flexion.

Supine stretching is thought to better isolate the hamstrings, allowing for improved relaxation and is generally believed to be safer and more comfortable for people with a history of low back pain (15). Hence, the SPH test was used to measure static hamstring flexibility and the DSUH test was used to measure dynamic flexibility. Reliability testing demonstrated that there is a significant difference between flexibility measured by the SPH and DSUH hamstring flexibility tests (p<.001). There was also a significant difference between DSHWB (with knee brace) and DSHNB (without knee brace) tests (p = .003) and this result supported the use of the knee brace (dj Orthopaedics, LLC, California, USA) to maintain a fixed knee angle during flexibility testing.

An average hip flexion ROM was calculated for both and served as the final measure of hamstring flexibility (4). Post-testing was commenced immediately after the completion of the stretching intervention assigned for the day. In 2002, Klee et al. (26) suggested that participants should be retested as quickly as possible after the intervention stretches because resting tension started to increase after a three minute rest pause.

#### Stretching Program

##### Warm-up only/ No stretching: Treatment 1

No stretches were included in the warm-up, serving as a control. Participants cycled for 75 seconds on a stationary ergometer (Exertech, Australia) at 60-70 W with a 10 seconds rest pause between each of the five 75-second cycle periods. Total duration of cycling was 225 secs.

##### Static stretching: Treatment 2

Participants performed stretches for a total duration of 225 seconds (52). They performed three types of static stretches with a stretch time of 75 seconds for each (Table 1). This time equated to five stretches held for 15 seconds each (9, 29, 30, 34, 47,). A rest pause of ten seconds was allowed between stretches. Each static stretch was performed to the terminal range, defined as the point where the subject felt a mild discomfort or tightness in the back of the thigh (5). The static and dynamic stretching routines were appropriately timed so that the amount of time spent stretching was the same for each group, enabling comparison between the two groups (41).

##### Standing toe-touch

This stretch routine involved bending forward to touch toes whilst making sure that the knees remained fully extended. Participants held the stretched position for 15 seconds until a slight sense of discomfort or tightness felt in the back of the thigh. Ten seconds rest pauses were allowed after each stretch and when switching to a different stretch type.

##### Forward swing static stretch

The heel of the extremity to be stretched was supported on a treatment table to perform this particular stretch (35). The knee remained fully extended and the foot was positioned in relaxed plantar flexion. The pelvis was tilted anteriorly whilst bending forward at the waist avoiding flexion of the spine (15, 35), until the terminal range was reached or discomfort felt in the back of the thigh. This stretch position was held for 15 seconds and repeated five times on the dominant extremity.

##### Passive supine-sling stretch

This stretch was performed in the supine position whilst lying on an exercise treatment bench. A Velcro sling was passed around the ankle to flex the hip and consequently stretch the hamstring group of muscle. The stretch was held for 15 seconds to the terminal range of discomfort or tightness felt in the back of the thigh.

##### Dynamic stretching treatment

Five sets of seven to eight dynamic stretches equalled the amount of time spent (Table 1) on the aforementioned static stretching regimens. The aim was to allot the same amount of stretching time to the static and dynamic stretching interventions enabling comparison among the groups. The 15 seconds hold period for each static stretch equated to around seven to eight dynamic stretches. Five sets of dynamic stretches amounted to 225 seconds of total stretching time. There was a pause of 10 seconds between each set and another 10 seconds when changing over from one type of stretch to another.

Stretches were begun at low velocity and momentum was gradually built up to achieve at least 75% of maximum height and speed while performing the dynamic stretches. The SAID principle of specific adaptation to imposed demands formed the basis of the dynamic stretching routine. Participants stretched at 75% of the maximum velocity through a particular ROM whilst performing a sport-specific movement.

##### Dynamic leg swings

The dominant leg was flexed at the hip in a forward kicking action. The aforementioned SAID principle was applied during performance of all stretches (controlled stretching). Five sets of seven or eight forward leg swings or kicks (9) were carried out to a timed 225 seconds of stretching.

##### Crossed-body leg swings

Dominant leg swung across the midline of the body towards the opposite shoulder. This stretched the biceps femoris which is the lateral muscle of the hamstring group (40).

##### Standing bicycle-kicks

The dominant limb was put through a circumduction-like movement in a rhythmic cyclical manner incorporating the SAID principle (controlled stretching). Total time spent on this stretch was also 225 seconds.

#### Biomechanical analyses

The hip ROM in the dominant leg was used as an indirect measure of hamstring flexibility (44) and served as the only investigated parameter (Fully extended hip = 0°). Dartfish ProSuite (Dartfish Connect 4.0, Dartfish Ltd., Fribourg, Switzerland) is a complete video analysis software package, which includes all necessary functionality to analyse technical performance during and after training. Dartfish motion analysis software was used to quantify the degree of hip flexion. This system enables access to every video frame so that the terminal ROM of hip flexion can be accurately identified. Once the appropriate frame was identified, Dartfish was used to measure hip flexion accurately to the nearest degree. Intra-tester and operator reliability were tested by a repeat analysis of 15 participant performances.

#### Statistical Analysis

The principal dependent variable of interest was the change in hamstring flexibility measured by hip flexion ROM between pre and post-stretch measurements. The paired sample t-test compared the effect of the two treatments on static and dynamic hamstring flexibility. Non- parametric tests conducted on the collected data corroborate the aforementioned findings. Furthermore, Tukey’s Honestly Significant Difference (HSD) test explored the degree of change in static and dynamic flexibility. The data was analysed with the statistical package SPSS for Windows (version 12.1.0; SPSS Inc., Chicago, IL).

### Results & Disscussion

Various modifications of the SLR test were used to measure and compare hamstring flexibility in an earlier study that also tested for reliability (n=33). Static passive hamstring flexibility (SPH), dynamic supine hamstring flexibility (DSUH), dynamic standing hamstring flexibility with knee brace worn (DSHWB), and dynamic standing hamstring flexibility without knee brace (DSHNB). Subjects were tested on two separate occasions one week apart. Each subject had three trials for each tests for the two separate testing times resulting in a total of 30 scores. Test-retest was appropriate as subjects were tested at two points in time a week apart and a Cronbach alpha was used to test for internal consistency and reliability for the three trials of each week’s testing. The tests used in this study evidenced a very high degree of internal consistency for each trial by Occasion 1 and Occasion 2 as well as a high coefficient of reliability or stability as measured by the test-retest procedure (Table 3, Table 4).

Participants were randomly assigned to one of three interventions for each of three testing occasions:

1. No stretching (Treatment 1)
2. Static stretching (Treatment 2)
3. Dynamic stretching (Treatment 3)

A Paired-samples T-test was used to test for differences in static and dynamic flexibility from pre/post-test after each stretch intervention (Table 5).

Intervention Treatment 1, where the subjects did no stretching served as the control. Static and dynamic stretching (Treatment 2, Treatment 3) were the experimental treatments. Following Treatment 1 we expected measures of hamstring flexibility to remain unchanged from pre to post-test. However, our analysis revealed significant differences between pre and post score for static flexibility (t (11) = 2.76, p < .05). There was no significant difference between pre and post hip ROM measured by the dynamic flexibility test (t (11) = 0.315, p >.05). The mean value of difference between pre and post score for static flexibility (mean = 2.13, SD = 2.68) indicates that there is a substantial change.

When static stretching was included in the warm-up, there were statistically significant differences in pre and post static flexibility measurements (t (11) = 4.19, p < .05). However, there was no significant difference in pre and post dynamic flexibility measurements (t (11) = 0.72, p >.05). When dynamic stretches were included in the warm-up instead of static stretches, it was expected that there would be changes, at least, in dynamic flexibility of the hamstrings. The analysis shows that there were statistically significant differences in both static (t (11) = 2.62, p <. 05) and dynamic (t (11) = 5.69, p < .05) flexibility. This suggests that participants improved both their static and dynamic hamstring flexibility after dynamic stretching was included in the warm-up.

Non-parametric tests were carried out on the collected data to corroborate the aforementioned findings. Wilcoxon Matched-Pairs Ranks test was performed. The results were similar to those obtained following the Paired samples t-test. Following Treatment 1 (No stretching) there were resultant differences in the static hamstring flexibility (Wilcoxon, Z = -2.41, p < .05). Static stretching only influenced static flexibility (Wilcoxon, Z = -2.67, p < .05) of the hamstrings, while dynamic stretching produced changes in both static (Wilcoxon, Z = -2.39, p < .05) and dynamic flexibility (Wilcoxon, Z = -2.98, p < .05).

Furthermore, the differences in the degree of change in static and dynamic flexibility following dynamic stretching were explored using Tukey’s Honestly Significant Difference (HSD) test. The difference between the degree of improvement in static and dynamic hamstring flexibility following dynamic stretching were not statistically significant (Table 6). To corroborate these findings a Wilcoxon Matched-Pairs Ranks test was performed on pre-post differences of static and dynamic flexibility following dynamic stretching. The analysis failed to identify a significant difference in the changes demonstrated in both static and dynamic flexibility (Wilcoxon, Z = -0.178, p > .05).

The availability of state of the art software and improved video analysis techniques has changed the way flexibility is measured. The methods commonly being used have focussed on the measurement of static flexibility. With the growing trend towards using dynamic stretching and sport-specific drills in the warm-up, there is a need for measuring devices to adapt to these changes. We have provided a simple, reliable setup to measure flexibility. The inadequately defined relationship between flexibility and muscular performance or an athlete’s susceptibility to injury may be attributable to the lack of valid and reliable measures of flexibility (20). The drawback of flexibility assessment tools is the need for testing to be carried out within the confines of a laboratory. Although this study was carried out in a laboratory, the set-up could be used outdoors with the participant performing functional dynamic sporting movements.

Dynamic flexibility has been defined as a measure of the resistance throughout the ROM of a joint or a measure of joint stiffness (3). Dynamic flexibility is important in sport because it measures the ability of an extremity to move through a non-restricted ROM (36). The main findings suggest that static stretching improves static flexibility (p < .05) but may have no impact on dynamic flexibility (p > .05). Increasing ROM achieved through static stretching does not necessarily translate to improvements in dynamic flexibility. In 2004, Behm et al. (6) supported the concept that static stretching improved flexibility and ROM, however, it was believed that the relevance and specificity of the gains remained questionable.

In 1988, Alter (1) argued in support of the specificity of stretching: “ROM is a combination of active and passive ranges of motion and if passive stretching exercises are used to develop flexibility, then one should expect changes largely in passive flexibility” (p.179). Even a moderate duration of static stretching could result in quadriceps isometric force and activation decrements lasting for up to 120 minutes (33). The increase in static flexibility may not have translated into expected improvements in dynamic flexibility because of dampened hamstring activation following an acute bout of static stretching.

Static flexibility improved when no stretches were included in the warm-up as well as when the participants underwent a static stretching routine. Similar results were obtained in a other studies (44, 53). The 2003 study by Zakas et al. (53) indicates that flexibility improves significantly even when stretching is not included in the warm-up, however, any comparisons should be made with caution because of differences in methodology. The stationary cycling group in the study in 1997 by Wiemann and Knut (44) cycled for 15 minutes and demonstrated a significant improvement in hip ROM thereafter. They explain that this occurrence may be due to the decreased resting tension and a reduced stretch resistance following stationary cycling. However, other studies have shown that warming up before stretching does not complement the effectiveness of stretching (14, 45).

Following the inclusion of dynamic stretches in the warm-up, dynamic flexibility as well as static flexibility scores improved from pre-test to post-test. However, Tukey’s HSD test did not reveal significant differences between the degree of improvement of static and dynamic flexibility. Muscles have two types of receptors: the primary or annulospiral endings which measure changes in both muscle length and velocity, and the secondary or flower spray endings that measured changes in muscle length alone (2). Thus, Alter (2) reasons that dynamic stretching may be used to condition primary endings for a desired response, and sport-specific drills could be used in warm-up. Dynamic stretching may have caused activation of the primary annulospiral endings resulting in an increase in both static and dynamic flexibility. The dynamic stretching routine may have had a warming up effect, causing an increase in static flexibility.

There may be a need to consider the appropriate time for static stretching in the daily training schedule. There have been suggestions that static stretching may be useful in the cooling down period after a workout (18, 27, 31-32). Evidence remains in support of static stretching for long-term gains in flexibility (31, 39).

### Conclusion

The intervention study comparing the effects of static and dynamic stretching routines in the warm-up on hamstring flexibility demonstrated that dynamic stretching enhanced static as well as dynamic flexibility. Static stretching on the other hand did not have an impact on dynamic flexibility. This has implications for the use of static stretching in the warm-up for dynamic sport. The role of static stretching for injury prevention in dynamic sport is also being questioned.

### Application in Sport

The simplicity of the experimental set-up is the highlight of this research. Coaches can use our method of video analysis to monitor the effectiveness of stretching routines. A single person can carry out testing with ease and accuracy.

Dynamic stretching is synonymous with functional, sport-specific stretching and this research has demonstrated that dynamic stretching improves both static and dynamic hamstring flexibility. Static stretching has no impact on dynamic flexibility and should not be used in the warm-up; however, static stretches may be useful in the cooling down period of training for long term gains in flexibility.

Although our research has demonstrated the effectiveness of dynamic stretching in the warm-up, it is important to follow the training guidelines set aside in 2001 by Mann and Whedon (31) whilst implementing a stretching routine. Dynamic stretching may be most effective if performed according to the training principles discussed earlier, always making sure the needs and the capacities of the individual athlete receive precedence over general training goals.

### Acknowledgements

I would like to acknowledge my supervisors Dr. Mark Sayers and Dr. Gordon Waddington for their invaluable guidance. Their understanding and patience helped me overcome numerous hurdles en route to the completion of this thesis. I would also like to thank the sports studies staff for their help and advice.

I am thankful to the students of the University of Canberra (Sports Studies) for volunteering to participate in this research project. It was wonderful working with such cheerful and enthusiastic young people. Their willingness to participate and report at similar times for each testing session is much appreciated.

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### Tables

#### Table 1
Time spent on each stretch

(*) 10 seconds rest pause after each repetition and 10 seconds before switching over to the next type of stretch.
(a) 5 Stretches
(b) 5 Sets
(c) 15 seconds hold for each static stretch
(d) 7-8 swings/ kicks equivalent to around 15 seconds of stretching time for each set.

#### Table 2
Comparison of Dynamic and Static Hamstring flexibility measures in reliability study

Significant at p < .05
(a) All measurements are in degrees
(b) Number of participants performing each test = 33

#### Table 3
Cronbach alpha measure of reliability for each test repetition for two test sessions

(*) SEM – Standard Error of Measurement.

#### Table 4
Test – retest reliability

#### Table 5
Paired samples T test comparing the effect of the intervention treatments on dynamic and static hamstring flexibility

(*) Significant at p < .05
(a) Degrees of freedom = 11
(b) Number of participants undergoing each treatment = 12

#### Table 6
Tukey’s Honestly Significant Difference (HSD) test exploring differences in the degree of change in static and dynamic flexibility following dynamic stretching

### Corresponding Author
Gayle Silveira, MBBS
Modbury Hospital
Smart Road
Modbury, SA 5092
Australia
<[email protected]>
+6 (143) 172-1469