The Effect of Static Stretch and Dynamic Range of Motion Training on the Flexibility of the Hamstring Muscles

By William D. Brady, PhD, PT, SCS, ATC; Jean M. Irion, Med, PT, SCS, ATC; Michelle Briggler, MS, PT

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According to Zachezewski (17), flexibility of muscle is "the ability of a muscle to lengthen allowing one joint (or more than one joint in a series) to move through a range of motion." Good muscle flexibility will allow muscle tissue to accommodate to imposed stress more easily and allow efficient and effective movement. More efficiency and effectiveness in movement as a result of enhanced muscle flexibility will assist in preventing or minimizing injuries and may enhance performance (1,2,5,6, 15-17). 

A variety of stretching activities has been presented in the literature in order to regain or maintain muscle flexibility and avoid a decrease in range of motion (ROM) that can impair functional activities in an individual. Some techniques used to increase flexibility in muscle include the ballistic stretch, the static stretch, and proprioceptive neuromuscular facilitation (2-5,7,8,12-14). 

Ballistic stretching is a bouncing, rhythmic motion and uses the momentum of a swinging body segment to vigorously lengthen the muscle. Although significant increases in ROM can be obtained from periodic ballistic stretching, many arguments exist that oppose this technique. The rapid production of high tension in a short period of time, which occurs during ballistic stretching, contradicts the use of low force over an extended time period, which has been shown to be maximally effective in permanent lengthening of soft tissue. In addition, the rapid increase in tension caused by the myostatic stretch reflex (which increases with magnitude and rate of stretch) can produce a strain or rupture of the tissue Because of these arguments, the use of ballistic stretching is infrequent (2,5,8,13). 

Introduced by Knott and Voss (7), proprioceptive neuromuscular facilitation involves techniques that use a brief isometric contraction of the muscle to be stretched prior to a static stretch. These techniques seek to facilitate the golgi tendon organ to inhibit the muscle in which it lies and to use the principle of reciprocal inhibition. The proprioceptive neuromuscular facilitation technique not only requires expertise to perform but also requires one-on-one intervention with another experienced individual (7,12). 

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Static stretching has been defined as elongating the muscle to tolerance and sustaining the position for a length of time (2,8). Limited research is available examining the optimal time a stretch should be sustained. According to Bandy and Irion (3) and Bandy et al (4), the optimal time a stretch should be held is 30 seconds one time per day. Benefits of this slower stretching technique include that the stretch prevents the tissue from having to absorb great amounts of energy per unit time, the slow stretch will not elicit a forceful reflex contraction, and this technique alleviates muscle soreness. According to Smith (14), static stretching has the least associated injury risk and is believed to be the safest and most frequent method of stretching. 

A relatively new method to lengthen muscles is called dynamic range of motion (DROM). The method is described by Murphy (9,10) as an alternative to static stretching, who suggests that DROM is a better stretch for lengthening muscle than static stretching. During DROM, a contraction by the antagonist muscle causes the joint crossed by the agonist (lengthening muscle) to move through the full ROM at a controlled, slow tempo. All movements are performed slowly and deliberately. If performed too quickly, a tendency to swing the extremity exits, causing the stretch reflex to be elicited at the endpoint of the movement in the lengthening muscle (10). Dynamic range of motion begins from a neutral position, followed by a slow movement (4-5 seconds) of the limb to end range, a brief hold at end range (4-5 seconds), and, finally, slowly (4-5 seconds) moving the limb back to the original neutral position using an eccentric contraction (9). Murphy (9,10) speculates that this contraction by the antagonist causes the lengthening muscle to relax due to the principle of reciprocal inhibition. Therefore, DROM is a more natural way to elongate the muscle and does so in a relaxed state, since the muscle is reflexively inhibited (9,10). Murphy (10) also suggests that strength is promoted because the movement is being performed by the muscles that actively move the involved joint. 

According to subjective testimony, DROM training may be a beneficial alternative to the more traditional stretching techniques. But, to date, no objective study has examined the effects of DROM on increasing flexibility of muscle. In addition, no research has compared DROM with the more frequently used stretching technique, static stretch, on the effectiveness of increasing muscle flexibility. Therefore, the purpose of the study was to compare the effects of DROM and static stretching of the hamstring muscles with a control group (performing no stretching activities) on increasing hamstring flexibility as measured by knee extension ROM. The null hypothesis to be tested is that if static stretch, DROM, and a control group are compared, no difference in knee extension ROM will occur following 6 weeks of training. 

METHODS 

Subjects 

Essentially, four criteria existed for participation in the study. First, each subject agreed to volunteer and complete the 6 weeks of training. Second, the subjects could not have any history of pathology to the hip, knee, thigh, or lower back. Third, each subject had to exhibit tight hamstring muscles, operationally de- fined as having greater than 30° loss of knee extension measured with the femur held at 90° of hip flexion (refer to "Procedures" section for details). Finally, subjects not involved in any exercise activity at the start of the study had to agree to avoid lower extremity exercise and activities other than those prescribed by the researcher, and those already involved in a regular exercise program agreed not to increase exercise intensity or frequency throughout the 6 weeks of training. 

Fifty-eight subjects (41 men and 17 women) met the established criteria and completed the study. Mean age for these subjects was 26.21 years (SD = 5.57, range = 22-46) .All subjects signed an institutionally approved informed consent statement prior to data collection. 

Equipment 

Measurements were performed using a double-armed, full circle protractor made of transparent plastic. The protractor measured degrees in 1° increments. Measurement and both arms of the goniometer were modified by adding extensions made up of two 12-inch wooden rulers, making each arm 43.18 cm (17 inches) in length. 

Procedures 

Prior to assignment to group, each subject who met the criteria for inclusion in the study was measured for flexibility of the right (arbitrarily chosen) hamstring muscle. Subjects were positioned supine with the right hip and knee flexed to 90°. In this position, the lateral malleolus, lateral epicondyle of the femur, and the greater trochanter of the femur were marked with a felt-tipped pen for goniometric measurement, During measurement, one researcher (MB) passively moved the leg to the terminal position of knee extension (defined as the point at which the researcher perceived resistance to stretch), while maintaining the 90° hip flexion position. Once the terminal position of knee extension was reached, the second examiner (JMI) measured the amount of knee extension with the goniometer using methods described by Norkin and White (11). Full knee extension was considered 0°. No warm-up period was allowed prior to data collection. 

The same examiners made all goniometric measurement for the entire study. Researchers involved in the measurements (JMI, MB) were not informed as to which group each subject was assigned. 

Prior to initiation of this research study, reliability of the measurement of the hamstring muscles using the procedures just described was evaluated in the researchers (JMI, MB) using a test-retest design. Ten subjects (seven males, three females), with a mean age (SD) of 25.76 years (4.34), who were not involved in this study agreed to participate in a pilot study to assess the measurement reliability. One week separated the first and second measurement, and the testers did not have information about the first measure when performing the second measurement. Mean values (SD) for pretest and posttest were 47.35° (8.17) and 47.19° (7.58), respectively. Analysis via intraclass correlation (ICC) (formula 1,1) revealed a reliability coefficient of .98. In addition, the same procedures using the same two re- searchers (JMI, MB) to measure hamstring flexibility were used in two additional studies (3,4), resulting in ICC reliability coefficients of greater than .90 in each study. 

Subjects were randomly assigned to one of three groups following the initial measurement of hamstring flexibility. Subjects assigned to Group I (N= 19; X age = 24.63, SD = 2.38, range = 22-31) performed a passive, static stretch for 30 seconds, and Group 2 (N = 19, x age = 25.53, SD = 4.86, range = 22-40) performed DROM. Group 3 (N = 20, x age = 28.35, SD = 7.58, range = 22-46) served as a control group and did not perform any stretching activities. 

Subjects in Groups 1 and 2 stretched five times a week for 6 weeks. Group 1 performed static hamstring stretches by standing erect with the left foot planted on the floor and pointing straight ahead (no hip internal or external rotation). The right hamstring muscles were stretched by placing the right calcaneal aspect on an elevated surface (high enough to cause a gentle stretching sensation in the posterior thigh) with the knee fully extended and toes pointed to the ceiling (again, no hip internal or external rotation). The subject then flexed forward from the hip, maintaining the spine in a neutral position, while reaching the arms forward until a gentle stretch was felt in the posterior thigh. Once this position was achieved, the stretch was sustained for 30 seconds. 

Group 2 performed DROM by lying supine and holding their hip in 90° of flexion. The subject then actively extended the leg (5 seconds), held the leg at the end of knee ex- tension for 5 seconds, and then slowly lowered the leg (5 seconds), which was considered one repetition. A research assistant timed each step to ensure the assigned time was used. The DROM movement was repeated for six repetitions. Performing DROM for six repetitions of 5 seconds each allowed 30 seconds of actual stretching time, which could then be later compared with the 30- second static stretches performed by the other group (Group 1). 

Performance of each stretching session by each subject was supervised and recorded on an attendance sheet to document compliance to the pro- gram. If a subject failed to attend a scheduled session, he/she stretched the following morning and the following afternoon. Any subject missing 4 days without stretching was eliminated from the study (one subject was dropped from both Group 1 and Group 2). 

After 6 weeks, all subjects were retested using the same procedures described in the initial testing. Two days separated the last day of stretching and the final measurement. 

Data Analysis 

An ICC (3,1) on the pretest and posttest scores of the control group 'was used to assess the reliability of the measurement. Means and standard deviations of the pretest and posttest measurements were calculated for each group. In addition, mean difference between the pretest and posttest scores (gain scores) was calculated for knee extension ROM. 

A 2 X 3 (test vs. group) two-way analysis of variance (ANOVA) with repeated measures on one variable (test) was performed to evaluate for significance. Following the significant interaction, three post hoc analyses were performed to interpret the test X group interaction. 

First, one dependent t test was calculated on the pretest to posttest change for each group (a total of three t tests was performed). The alpha level (0.05) was adjusted with the Bonferroni method by dividing 0.05 by the number of t tests performed (three) to prevent an inflation of the type I error rate. Therefore, in all analyses using the t test, the rejection region was p < .015. These dependent t tests were performed to assess which group(s) significantly increased hamstring flexibility after stretching for the assigned duration and frequency (including the control group). 

Second, to assess whether any significant differences existed in the pretest scores across the three groups, a one-way ANOVA was calculated. This analysis was performed to assess whether any significant difference existed between the three groups prior to the initiation of the study. 

Finally, a one-way ANOVA was calculated across the post test scores of the three groups to assess if any difference existed in the posttest scores. This analysis was performed to assess whether any difference existed between the three groups (including the control group). Significance for all statistical tests and all follow-up tests was accepted at the .05 level of probability, unless otherwise indicated. 

RESULTS 

The mean values for the pretest and post test measurements of the control group for degrees of knee extension were 40.95° (SD = 8.953) and 40.25° (SD = 9.33), respectively. The ICC (3,1) value calculated for pretest-posttest knee extension data of the control group was .97. 

The Table presents the means for pretest and post test measurements and gain scores for each 

group. Results of the two-way ANOVA indicated a significant difference for test (dl= 1,55; F= 104.13; p < .05) and interaction ( dl = 2,55; F = 36.17; p < .05), but no significant difference was found for group ( dl = 2,55; F= 1.32; p >.05) (Figure 1). 

In order to interpret the group x test significant interaction, three follow-up statistical analyses were performed. First, the three dependent t tests calculated (using Bonferroni correction to avoid inflation of the alpha level) on the pretest to post test change for each group indicated significant increases in hamstring flexibility in the groups that stretched (static: dl= 19, t= 8.00, p < .015; DROM: dl= 18, t = 6.98, p < .015), but no significant change in hamstring flexibility in the control group (dl= 19, t = 1.51, p > .015). 

Second, the one-way ANOVA calculated to assess whether any significant differences existed in the pretest scores across the three groups indicated no significant difference (dl = 2,55; F = 0.40; p > .05). Finally, the one-way ANOVA calculated to assess if any difference existed across the post test scores of the three groups indicated a significant difference (dl= 2,55; F = 6.81; p < .05). Tukey post hoc analyses indicated significant differences between the groups that stretched and the control group and between the two groups that stretched (i.e., the static stretching group appeared to increase ham- string flexibility to a significantly greater extent than the DROM group). 

Finally, in an attempt to summarize the data, an additional analysis using a one-way ANOVA on gain scores was calculated, revealing a significant difference between groups (dl= 2, F= 32.20, p < .05). Post hoc analysis using a Tukey test indicated a significant difference between the control group (gain = 0.70°) and both stretching groups, as well as a significant difference between the static stretch group (gain = 11.42°) and the DROM group (gain = 4.27°) (Figure 2). 

DISCUSSION 

Based on the results of the two- way ANOVA (Figure 1) and post hoc analyses, the null hypothesis that no difference would be obtained in knee extension ROM after 6 weeks if static stretch and DROM were compared with a control group (performing no stretching activities) must be rejected. The two groups that performed daily hamstring stretching for 6 weeks showed significantly greater gains in flexibility (as determined by increased knee extension ROM) than the control group. 

To our knowledge, this study is the first objective investigation as to the effects of DROM on changes in flexibility of muscle. Results support the claims by Murphy (9,10) that DROM will increase flexibility of muscle. But caution should be used in interpreting these results regarding DROM. Although the change in ham- string flexibility was statistically significant, the clinical significance of the change should be examined. The actual gains in knee extension ROM which occurred as a result of the in- creased flexibility of the hamstring muscles during 6 weeks of DROM were only about 4°, less than 1° per week, which brings into question the clinical relevance of using DROM. 

Based on the results of the post hoc analysis, a 30-second static stretch is more effective in increasing hamstring flexibility than DROM (Figure 2). Gains in knee extension ROM as a result of static stretching was nearly 12°, almost three times the improvement which occurred in the DROM group. 

The results obtained on the group stretching statically in this study are quite similar to the only two previous longitudinal studies investigating the effects of duration of static stretch (3,4) (performed in the same research lab). Bandy and Irion (3) compared the effects of three groups statically stretching the hamstring muscle (for 15, 30, and 60 seconds) with a control group which did not stretch. Fifty-seven subjects (40 men, 17 women) divided into the four groups stretched 5 days per week for 6 weeks. Results indicated that 30 seconds of static stretching of the hamstring muscle was as effective as the longer duration of I minute and significantly more effective at increasing hamstring flexibility than stretching for 15 seconds or not stretching at all. The group statically stretching for 30 seconds increased knee extension ROM 12.50° over the 6 weeks, similar to the increase in ROM which occurred in the present study. 

In a second study, Bandy et al (4) compared the effects of five daily frequencies and duration of static stretch on hamstring flexibility: 1) three 1-minute stretches; 2) three 30-second stretches; 3) one 1-minute stretch; 4) one 30-second stretch; and .5) a control, receiving no stretching activity. Ninety-four subjects (62 men, 32 women) divided into the five groups stretched 5 days per week for 6 weeks. Results indicated significant differences between the groups that stretched and the control group, but no significant difference between any of the stretching groups (i.e., all the stretching groups appeared to in- crease hamstring flexibility to the same extent). The results of the study suggested a 30-second duration was an effective amount of time to sustain a hamstring stretch in order to in- crease ROM. Over the 6 weeks of static stretching, the group stretching for 30 seconds increased knee extension ROM 11.50°, again similar to the present study. 

In designing this study, decisions were made concerning: 1) the duration of stretch to use in the investigation and 2) the number of repetitions of the stretching activities. The duration of 30 seconds used in the group that performed static stretch was chosen based on previous longitudinal studies performed in this research lab (3,4). This research, as was reviewed, indicated that the static stretch using a 30-second duration was more effective than a 15-second stretch and equally as effective as a 60-second stretch. 

In addition, an effort was made to compare stretching activities of equal duration. It did not appear appropriate to compare one repetition of DROM to one repetition of a 30- second static stretch, and a decision was made to control the actual time of stretch. Therefore, the DROM was performed six times, which allowed a total of 30 seconds in actual time of stretch. Given that in performing DROM, a 5-second knee extension and flexion was performed before and after the stretch, one may argue that the DROM group actually performed more stretching activities than the static stretching group. Despite this increase in activity in the DROM group, the group statically stretching for 30 seconds increased hamstring flexibility to a significantly greater amount than the DROM group. 

The present study was limited to the effects of stretching the hamstrings. Other studies are needed to evaluate the effects of stretching other muscle groups, such as the gastrocnemius, soleus, and iliotibial band. In addition, this study utilized a relatively young sample. Therefore, results and conclusions from this study are most applicable to a similar age group. Further research examining the effects of stretching on individuals in other age groups would be of interest. 

CONCLUSION 

Although the use of static stretch and DROM both resulted in an increase in hamstring flexibility (as determined by increased knee extension ROM), the results of the present study indicate that a 30-second static stretch was more effective than the DROM technique. Given that a 30- second static stretch performed one time per day over a 6-week period resulted in more than twice the gains in hamstring flexibility than performing DROM at the same frequency and duration, the use of DROM to effectively increase the flexibility of the hamstring muscle is in question. 

GROUP*

 

1 (n = 19)

2 (n = 19)

Control (n = 20)

x 

SD

x 

SD

x 

SD

Pretest

41.89

10.11

39.93

5.99

40.95

8.95

Posttest

30.47

9.05

35.68

5.95

40.25

9.33

Gain (difference between pretest and posttest

11.42

6.52

4.27

2.67

0.70

2.08

* Group 1 stretched statically for 30 seconds; Group 2 used dynamic range of motion and the control group did not stretch

TABLE: Mean standard deviation scores for pretest, posttest, and gain scores (in degrees) of knee flexion for each level of group.

JOSPT 

ACKNOWLEDGMENT 

The authors wish to thank Staci Rice-Smith and Jimmy Ishee for assistance with data collection.

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