There is a lack of research on stretching and flexibility. Many clinicians recommend stretching, but few have attempted to prove its effectiveness. Suggested benefits include improved athletic performance and functional gains (6,14,33). In addition, stretching has maintained a time-honored role in health and fitness (1,5,27,23). Many factors influence an individual's flexibility. In previous studies, age, race, gender, circadian rhythms, tissue temperature, strength training, stiffness, and warm-up have influenced flexibility (3,13,16,17,21,25-27,29,36).
A proper stretching program is key to improving flexibility. Some research suggests that stretches be held for 30 seconds, with at least 3-4 sets (12,20,32). For maximum improvement in flexibility, it has been recommended that stretching be done 5 or more times per week (34). Some studies have shown that proprioceptive neuromuscular facilitation (PNF) and contract-relax (CR) stretches may be most effective (7,9,10,18,22,24,28,34). Ballistic stretching seems to be less effective and, anecdotally, may cause injury (32). Static (or passive) stretches have some benefit but may not work as well as PNF stretches (8,11,16). Active stretching such as ballistic and PNF requires individuals to volitionally contract muscles. The PNF stretches use contraction of antagonist and then relaxation (CR). Alternatively, they can also employ contraction of the agonist of the lengthened muscle then relaxation (contract-relax, antagonist-contract [CRAC]). The CRAC stretches are reported to be more painful and cause more muscle trauma than other types of stretches (16). Additionally, nerve glide stretches, termed neuromobilization, are active stretches in which the nervous system is made taut and then slack. The PNF stretches may be done in combination with neuromobilization maneuvers. An example of a neuromobilization maneuver would be the slump test. The slump test is a seated straight leg raise (SLR) in which a patient's neural structures have progressive stretch applied to elicit painful symptoms. Neural traction is experienced in the intervertebral foramen by actively dorsiflexing the ankle while flexing the cervical spine. Kornberg and Lew (15), in a small, uncontrolled study, have suggested that adding the slump maneuver to a treatment regime facilitated an athlete's return to full function after a hamstring strain. If abnormal neural retraction predisposes the hamstrings to strain, then theoretically stretching these neural structures should improve flexibility and protect against injury.
Our study attempts to determine whether active stretches are more effective than passive stretches and whether adding a neuromobilization maneuver to active stretches enhances the stretch. To our knowledge, no other study has looked at these combined active stretches.
Experimental Approach to the Problem
As clinicians and trainers, we often wonder what stretches are most effective and how long it will take to see change. To determine whether active stretches are more effective than passive stretches, some common stretches used in therapy, training rooms, and health clubs were chosen to represent the independent variable (control group, 90/90 active, 90/90 passive, SLR active with mobilization, and SLR passive). To reveal change, hamstring flexibility (dependent variable) was measured initially, at 4 weeks, and at 8 weeks.
Institutional review board approval was obtained before recruitment of subjects. One hundred subjects were recruited by word of mouth with a target age range of 18-80 years. Actual participants ranged in age from 21 to 57; there were 45 women and 55 men. The mean age of participants was 33, with a mode of 30. Exclusion criteria included hypermobility (defined as initial hamstring length greater than 90°), history of hamstring tear, upper motor neuron disease, lower motor neuron disease, and past participation in formalized stretching programs. Informed consent was obtained from all subjects.
Reliability of Hamstring Length Measurement
Goniometric hamstring length via the knee angle was measured in the supine position. All subjects had their lumbar lordosis supported with a lumbar roll. The distal tip of the lateral malleolus was marked. Therapist 1 placed the hip into flexion until the femur was perpendicular to the exam table. Therapist 2 then placed a goniometric lever over the femur and the other lever toward the marked lateral malleolus. Therapist 1 then extended the knee until firm end feel was achieved; the knee angle was then recorded (see Figure 1). The ankle was relaxed during the final knee angle measurements. All measurements were taken using the right leg. Inter- and intrarater reliability testing was done before proceeding with measurements on the study subjects.
Comparison of Hamstring Stretches
Preintervention measurements of hamstring length were taken, using the method described above. The supine knee angle was used as the major measurement of hamstring length. These subjects were then randomly assigned to 1 of 5 groups: group A-control; group B-90/90 passive stretch; group C-90/90 active stretch (antagonist contraction); group D-SLR active-assisted stretch (added neuromobilization component); and group E-SLR passive stretch. Groups B, C, D, and E performed 3 sets of the assigned stretch. Each stretch was held for 30 seconds, with sets to be done 5 d·wk−1. Subjects in each group were asked to keep logs of their stretching activity. The control group (group A) was asked to continue their normal activities and not to change their activity routines for the next 12 weeks. The 90/90 passive stretch (group B) was performed supine with a strap. Flexing the hip until the femur was perpendicular to the floor standardized the hip angle. By placing a strap around the ankle, each subject applied force to achieve passive knee extension (see Figure 2). The 90/90 active stretch (group C) was performed supine, without a strap. Flexing the hip until the femur was perpendicular to the floor standardized the hip angle. Subjects applied active tension by actively extending their knees via quadriceps contractions. Subjects clasped their hands across their thighs for balance and to keep their hip angles steady, with the femur perpendicular to the floor (see Figure 3). The SLR active-assisted stretch (group D) was performed supine against a wall, with the knee extended to 180°. The hip angle varied across subjects. Each subject was asked to bring the femur as close to perpendicular to the floor as possible. Each subject placed a heel against a corner of a wall, and passive tension was applied to the posterior hamstrings. Additionally, subjects “pumped the foot” by actively dorsiflexing and plantarflexing the foot. A strap was used to aid in pumping the foot (see Figures 4A and 4B). This added ankle motion is generally considered to be a neuromobilization maneuver (4).
The SLR passive stretch (group E) was performed supine against a wall, with the knee extended to 180°. The hip angle varied across subjects. Each subject was asked to bring his or her femur as close to perpendicular to the floor as possible. Each subject placed a heel against a corner of a wall, and passive tension was applied by gradually increasing the hip flexion angle (see Figure 5).
Each subject received a picture book illustrating his or her selected hamstring stretch. Each subject (in groups B through E) was given 1-on-1 instruction, on the same day as random assignment, on how to do the selected stretch. Instructions were to begin the stretching the next day. Subjects were given a daily compliance log to complete. Each subject also answered a questionnaire regarding demographics and perceived hamstring stiffness. Subjects in groups B-D reported prestudy activity levels for cardiovascular, strengthening, and stretching activities. Cardiovascular activities consisted of running, swimming, aerobic classes, and walking. Strengthening activities included weight training and participation in resistance training classes. Stretching activities consisted of stretching exercises and yoga. The percentages of subjects in each group who reported prestudy cardiovascular, strengthening, and stretching activities are summarized in Table 1.
Initial hamstring measurements were taken on all subjects. The right leg was used for all measurements. At 4 and 8 weeks, hamstring length measurements were taken again. Results were recorded as degrees of knee flexion angle. A larger knee flexion angle reflects tighter hamstrings, whereas a smaller knee flexion angle reflects looser hamstrings. Perceived level of hamstring tightness score was recorded initially, at 4 weeks, and again at 8 weeks, using the following scale: 0 = no perceived level of tightness, 1 = occasionally feel hamstrings are tight, 2 = frequently feel hamstrings are tight, and 3 = constantly feel hamstrings are tight.
Data were processed by checking for item nonresponse, distributional forms (e.g., normality of continuous data elements), and creating derived variables. SAS version 9.1 (SAS Institute Inc., Cary, NC) statistical software was used for all statistical analyses. Frequencies and percentages were calculated for categorical data, and mean and SD were calculated for numeric data. Associations among numeric measures were tested using Spearman correlations. Chi-square and Fisher exact tests were used to test relationships between bivariate categorical data. Groups A, B, C, D, and E were compared using analysis of variance (ANOVA) on the 4-week follow-up measure of range of motion after adjustment for initial measure, gender, age, and frequency of exercise. The Tukey-Kramer multiple comparisons test was used to detect significant pairwise differences between groups.
Finally, the repeated-measures of range of motion over time (initial, 4-week, and 8-week measures) were modeled using the MIXED procedure in SAS. The MIXED procedure models the dependence of within-person data and incorporates observations from each subject regardless of missing data. The longitudinal model simultaneously examined the following explanatory variables: activity protocol group, gender, age, and frequency of exercise. For all statistical analyses, significance was considered at p ≤ 0.05. Intraclass correlation coefficients (ICCs) were calculated to measure intra- and interrater agreement (the reliability between raters). The ICCs give a measure of agreement between raters on numeric data that is more informative than a Pearson correlation coefficient. For example, the pairs (1, 10), (2, 20), and (3, 30) have a Pearson correlation coefficient of 1.00. The ICC of these pairs, however, is close to zero, because this statistic incorporates information about the magnitude of variation between raters.
Of the 100 subjects initially recruited, 13 subjects failed to participate in further data collection at the 4-week or 8-week time points (82 subjects had complete data; 5 subjects had 8-week data but no 4-week data). There was no evidence of loss to follow-up based on type of stretching technique (Fisher exact test, p = 0.5012). Demographic pooling of data revealed no significant differences in age, exercise activity of individuals, or initial hamstring measure. However, groups differed significantly by gender (chi-square test, p = 0.0006). Group B (90/90 passive stretching group) had the highest percentage of men, and group C (90/90 active stretching group) had the lowest (see Table 2). Compliance log results were similar in all treatment groups.
The ICC values to measure intrarater reliabilities for hamstring measure were 0.75 and 0.85. The ICC value to measure interrater reliability for hamstring measure was 0.69. Values of ICC in this range are deemed acceptable.
Using ANOVA modeling, the 4-week range-of-motion measure was modeled using the explanatory variables of initial measure, activity protocol group, gender, age, and frequency of exercise. The ANOVA model explained 75% of the variation in response (R 2 = 0.7496). After adjustment for other explanatory variables in the model, initial measure (p < 0.0001), activity protocol group (p = 0.0237), and gender (p = 0.0103) were found to be significantly related to the 4-week range-of-motion measure. Age and frequency of exercise were not found to be significant. Women had significant improvements in range of motion compared with men. Using the Tukey-Kramer multiple comparison procedure to test for pairwise differences, group D (SLR active-assisted stretch) was found to differ significantly from group B (90/90 passive stretch; p = 0.0318), with group D exhibiting significant improvements in range of motion compared with group B. Results of ANOVA testing are given in Table 3. Negative regression coefficients indicate improvements in range of motion.
A longitudinal analysis of the initial, 4-week, and 8-week repeated range-of-motion data was conducted. In a longitudinal analysis, the trajectory of repeated measures is considered the “outcome,” and its shape (a linear or curvilinear evolution) is explained by a set of predictor variables. For the range-of-motion repeated-measures data, the trajectory for each individual was modeled as a linear evolution with the following predictor variables: type of stretch performed, gender, age, and frequency of exercise. Mean values of each group are plotted in Figure 6. Three factors significantly predicted the linear trajectory of the range-of-motion data: type of stretch performed, gender, and frequency of exercise. Group E (SLR passive stretch) demonstrated the largest improvement of flexibility at 8 weeks. The women's flexibility improved more than that of their men counterparts. The more frequently the exercise was performed, the more range of motion improved. Further testing showed significant improvement in flexibility for group E (SLR passive stretch) compared with group B (90/90 passive stretch; p = 0.0066). There was a trend toward significance between group B (90/90 passive stretch) and group D (SLR active-assisted stretch; p = 0.0652), with group D showing more improvement of flexibility than group B. Results of longitudinal analysis are given in Table 4.
The objective of this study was to compare 4 different stretching techniques to determine which one was most efficacious at improving hamstring flexibility. The 2 types of stretches studied were active and passive. The results of this study demonstrate that both active and passive stretches were more efficacious compared with the control group. At 4 weeks, improvements in hamstring flexibility were seen in both groups C and D. Group C used a 90/90 active stretch, which is a modified contract/relax PNF stretch, and group D performed an SLR with a neuromobilization component. The passive stretch group (group E) using a supine SLR against a wall also achieved an improvement. Both the 90/90 passive stretch group and the control group showed no improvement with hamstring flexibility. After 8 weeks of stretching, the SLR passive stretch group (group E) achieved the most improvement in hamstring length.
The range-of-motion improvements in the SLR passive stretch group (group E) may be attributed to an increase in stretch tolerance found with static stretching (19). These improvements also may be attributable to the viscoelastic property changes that occur with “creep,” whereby the tension in the muscle-tendon unit diminishes over time (32).
As with previous research, PNF stretches seem to be as beneficial as passive stretches. Furthermore, the addition of a neuromobilization component may be beneficial to hip flexion and knee extension range of motion. The improvement seen in the neuromobilization group emphasizes the fact that flexibility is influenced not only by muscle elasticity but also by connective tissue/nervous tissue extensibility. One previous study showed a quicker return to play for injured athletes who used neuromobilization techniques (31). Unlike previous studies, our results show no correlation with hamstring flexibility and age.
Compliance was not directly measured in this study. While participating in the study, all subjects kept stretching diaries. Individuals reported that PNF stretches were more engaging and less boring than static stretches. A recent study showed no retention of knee range of motion at 4 weeks after a 6-week stretching program had been implemented (35). This conveys the importance of maintaining a stretching program. The improvements seen with the active and passive stretches suggested in this study will likely only continue if the stretching prescription is maintained indefinitely.
Despite the randomized controlled design, this study has limitations. Not all items could be controlled, such as the activity levels of the participants. Also, the subjects were performing the majority of these stretches on their own, without the supervision of a therapist. Although the therapist watched them doing the stretches correctly in follow-up visits, there was no way to determine whether they were really keeping their hips at 90/90 when stretching independently. Researchers attempted to obtain measurements at the same approximate time of day for each measuring session to eliminate flexibility variations with circadian rhythms. Additionally, a high variability of data was observed in each group. Generalization of this study to clinical practice should be confined to the demographics of the individuals we studied.
Future studies on flexibility or stretching could be considered using functional, weight-bearing movements. Questions on the safety of neuromobilization maneuvers warrant investigation. Moreover, a clinical outcomes study on the utility of these types of stretches in subjects with specific injury diagnoses would be helpful.
Clinically, these types of hamstring stretching may be a useful first-line agent in treating hamstring strains, remodeling chronic hamstring and surrounding tissue dysfunction, recovering elasticity of tissue end feel, and improving hamstring flexibility for performance. However, the effect of stretching on performance is controversial. Winchester et al. (37) found an adverse effect on sprint performance after 10 minutes of presprint static stretching in collegiate runners. In addition, a study by Bazett-Jones et al. (2) found that 6 weeks of a hamstring-stretching protocol neither negatively nor positively impacted athletic performance or sprint and vertical jump outcomes.
Functional progression from these active hamstring stretches to triplanar, weight-bearing stretches should be incorporated to return individuals to specific types of activity or play. The entire kinetic chain should be assessed, and the hamstring stretches should not be used in isolation. Although we suggest that both active and passive stretches are beneficial, they are not necessarily functional. Recent research has shown that performing exercises that improve neuromuscular control are more effective for rehabilitating pelvic muscle injuries than stretching exercises alone (30). Active and passive stretches should be viewed as a means to the end, not the end.
Our research affirms that changes in flexibility take time, achieving end ranges intermittently and with diligent training. The more frequently someone stretched and the further they pushed into the range, the more range they achieved. This supports the need to educate patients that remodeling tissue is time dependent and should be undertaken consistently and frequently and, most importantly, that they should achieve end range by over pressure.
In summary, our study indicates that improvement in hamstring flexibility was greatest for the SLR passive stretch. Our results also show that using PNF in the 90/90 active stretch provided better knee range-of-motion improvements than the 90/90 passive methods did. Future studies on more functional weight-bearing stretches should be performed to define their role.
The authors thank the staff and patients of the Rehabilitation Institute of Chicago, Center for Spine and Sports Rehabilitation.
The authors declare no conflicts of interest related to this study. The results of the present study do not constitute endorsement by the authors or the NSCA.
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Keywords:© 2009 National Strength and Conditioning Association
hamstring flexibility; viscoelasticity; flexibility