Muscle stretching is a common form of exercise. Athletes, recreational enthusiasts, and clinicians are among those who use stretching exercises. The advantage of stretching is primarily associated with increased joint range of motion (ROM). Current evidence supports the fact that stretching increases the motion in a healthy joint and restores movement to an injured or restricted body segment (5,6,21,33). Although the acute effect of stretching on sport performance has been called into question (26,29,31,35), sufficient evidence supports the long-term use of stretching to improve flexibility and athletic performance, contribute to injury prevention, and reduce postactivity muscle soreness (3,6,12,20,28,32,33,42,45,47,48,50). Improvements in joint ROM have been reported after a single stretching bout and after long-term stretching interventions (5,12,17,19–21,28,38,42,43,46,47,50). The potential physiological adaptations that allow for ROM changes include modifications in musculotendinous length, increased stretch tolerance, and viscoelastic stress relaxation (28,32,33).
Various stretching protocols have been studied in prior research, with the emphasis on optimal stretch duration and frequency per day (5,6,12,24,42). There appears to be consensus that the total stretching time per day is more important than the duration of a stretch hold position. Cipriani et al. (12) found that the total daily stretching time was the key factor related to ROM gains, more so than the number of repetitions or the duration of a single stretching bout. Specifically, 6 repetitions of 10 seconds performed twice daily produced a similar increase in the ROM when compared with 2 repetitions of 30 seconds, performed twice daily. Thus, the main factor for either group was 1 minute of stretching, repeated 2 times each day. These findings support the work of others who also found that total time, regardless of the daily dosing, was the main factor influencing the effectiveness of stretching. Roberts and Wilson (37) found that 9 repetitions of 5 seconds a day produced similar gains in ROM when compared with 3 repetitions of 15 seconds a day. Doebler et al. (21) found that a static stretch time of 2 minutes per day yielded greater gains in the ROM at the hip joint than did durations <2 minutes.
Despite this, there remain many aspects of stretching that deserve attention. One area in particular is the effect of ROM when a stretching regimen is discontinued. This is a practical topic in both rehabilitation and athletics. For instance, it has been shown that adherence to exercise programs decreases after discharge from rehabilitation or physical therapy (40). Research studies suggest that the cessation of a stretching program will result in an eventual loss of joint ROM (28,36,38,43,47,50). If ROM is not sufficiently retained after a stretching program, then patients and athletes may be more susceptible to reinjury and may never achieve optimal flexibility for performance. Willy et al (47) found that within 4 weeks of cessation of a regular stretching program, joint ROM returned to baseline. These findings, however, are in conflict with the finding of Guissard et al. (28) who found that a stretching protocol consisting of 10 minutes of passive static stretching per day, performed 5 times a week, for 6 weeks resulted in approximately 74% retention in motion up to 4 weeks after the cessation of the program. The stretching protocol was much more involved than that of Willy. Hence, the more frequent stretching may have contributed to the better retention. In addition, Rancour et al. (36) demonstrated that intermittent stretching can attenuate the decline in hip ROM, after the cessation of a daily stretching program. The subjects in this study were able to attenuate or even maintain hip ROM gains, while stretching only 2 d·wk−1.
All 3 of these studies examined the effects of cessation of stretching with some evidence that the gains made during stretching can be retained for up to 4 weeks. The Rancour et al. (36) study demonstrated that stretching as infrequently as 2 d·wk−1 could maintain some level of flexibility.
Most studies that examine the influence of regular stretching only look at intermittent time points, such as, the beginning, middle, and end of the study. Therefore, it is not possible to determine if the gain and decline in ROM occurred immediately before the start or cessation of stretching or if the changes occurred incrementally over the course of total time period. Further, it is unknown as to how much the frequency (i.e., days per week and repetitions per day) influences the rate of gain or the rate of decline in ROM. Finally, the influence of the individual's sex, which has been demonstrated to influence flexibility, has not been studied in terms of varying stretching protocols. Men have been shown to adapt quicker to fitness protocols than women, whereas women have been shown to demonstrate greater flexibility than men (1,2,7–9,11,14,15,22,23,25,27,39,43,49). It is not clear whether men or women respond differently to stretching frequency.
The purpose of this study was to document the rate of gain and rate of decline in hamstring flexibility over the course of an 8-week stretching and cessation program and to examine whether men and women respond differently to stretching. We intended to determine if less frequent stretching each week (i.e., 3 d·wk−1) would yield similar changes in flexibility as high frequent stretching each week (i.e., daily stretching). Because Rancour et al. (36) found that as little as 2 d·wk−1 could maintain or possibly improve flexibility, we did expect that differences in weekly stretch frequency (i.e., daily or 3 times each week) would result in similar gains over time. We chose 4 different stretching prescriptions that would provide a variety of stretch frequency over the course of each week, ranging from highly frequent (daily, 2 times per day) to least frequent (3 times per week, once per day). From practical experience, we are aware of athletes who stretch highly frequently, whereas others struggle to find the time each week. Because athletes and individuals involved in fitness must find the most efficient means to address their fitness, flexibility, and performance training, we wished to determine if less frequent stretching could yield similar gains as more frequent stretching; this could be helpful to the individual in terms of time management. We also did not expect that sex would influence the rate of change. Research is inconclusive as to whether men or women gain at a different rate in response to stretching. Finally, we intended to document the actual rate of change over time, on a weekly basis, to provide information that is currently missing from the literature. Current research has examined the changes over intermediate time points but not on a weekly basis. Particularly from an injury and recovery standpoint, it is important to know the rate of change in flexibility that might be expected on a weekly basis.
This study examined 4 different stretching frequency programs ranging from 14 times each week (daily, twice each day) to 3 times each week (3 times each week, once each day). In addition, this study followed the rate of gain and rate of loss over the course of 4 weeks of stretching and 4 weeks of cessation from stretching. It was hypothesized that hamstring ROM would increase at a similar rate for all the 4 stretching programs, during the 4-week stretching phase. It was further hypothesized that the rate of decline would be comparable for all the groups, regardless of the stretching program. Sex as a factor was not expected to contribute to differences between groups over time. Men and women differ in flexibility and in response to fitness and conditioning programs; however, we did not expect that these 2 groups would respond differently to stretching programs.
Experimental Approach to the Problem
This study used a randomized, single-blind, longitudinal, quasiexperimental design with repeated measures. The dependent variable was hip ROM as measured with a goniometer for hip flexion. Hip ROM was measured as an indication of changes in flexibility of the hamstring muscle during the course of the study. The 3 independent variables were time (9 time points over 8 weeks), group assignment (4 different stretching protocol groups and 1 control group), and sex (male or female). The subjects were randomly assigned to 1 of the 5 groups. Hence, this was a multifactorial design which included time, stretching protocol, and sex as the 3 factors.
This study was approved by the Institutional Review Board (IRB) for Research Ethics at San Diego State University, protocol approval number 23009. Furthermore, all the subjects signed an informed consent form approved by the IRB at San Diego State University, before being enrolled in this study. Seventy-three healthy adults between the ages of 18 and 50 years were initially screened for the study. All the subjects came from the University campus population (students, faculty, and staff); the subjects were not part of any formal athletic program (i.e., all the subjects were nonathletes). After initial screening, 11 subjects were excluded based on the following exclusion criteria: all the subjects must be free of lower back pain, hip, or knee pain at the time of the study (n = 2); the subjects could not be currently involved in a hamstring-stretching program (n = 6); the subjects could not be pregnant (by self-report) (n = 0). In addition, the subjects were excluded if they presented with excessive hamstring ROM (i.e., >100° of hip flexion in the straight-leg-raise test) (n = 5). Based on these criteria, a total of 62 subjects (30 male, 33 female) mean age = 24.91 years, SD = 6.40, range = 18–46 years were initially enrolled in the study. The subjects enrolled in the study refrained from any hamstring stretching, during the course of 8 weeks, except for the stretching protocol imposed by the study itself. There were no other restrictions placed on the subjects' activities of daily living or any other recreational activities. The data from each of the variables were evaluated and met the criteria for skewness and the assumption of a normal distribution. Descriptive statistics for the 62 initial subjects are presented in Table 1.
The total duration of this study was 8 weeks. During the first 4 weeks of this period, the subjects participated in a static hamstring-stretching program or were assigned to a control group of no intervention. The 4 stretching programs varied in terms of weekly frequency. This study chose to use static stretching because it has been shown to provide similar gains in motion, over an extended period of time with dynamic stretching (50). Static stretching is defined as a sustained tension applied to a muscle group in a slow and controlled manner (3,6,20,47). In addition, static stretching is commonly used by the general public, based on its simplicity (17). In this study, the subjects assigned to a stretching protocol performed static stretching for the initial 4 weeks of the study. Stretching was then discontinued for the final 4 weeks (cessation period). All stretching was performed independent of any active exercise or possible recreational programs. The subjects were instructed to avoid stretching 2 hours before an active exercise regime or performance, and within 1 hour after the exercise program.
Stretching Protocol and Control
This study tested 4 different stretching protocols (S14, S7, S6, and S3) and a control group (C). The subjects were randomly assigned to 1 of these 5 groups. The protocols differed in terms of frequency and total weekly stretching time: (a) daily stretching, twice each day (14 min·wk−1): S14; (b) daily stretching, once each day (7 min·wk−1): S7; (c) 3–4 d·wk−1 stretching, twice each day (6–8 min·wk−1): S6; (d) 3–4 d·wk−1 stretching, once each day (3–44 min·wk−1): S3; and (e) a control group refrained from any hamstring stretching (0 m·wk−1): C.
After the prescreening process to ensure eligibility, the subjects were randomly assigned to 1 of the 5 groups (i.e., S14, S7, S6, S3, and C), as illustrated in Figure 1. Only the subject and one investigator knew the subject's group assignment. All the remaining investigators involved in the measuring process remained blind, including the data analyzer.
A standing one-legged hamstring stretch was used for the purpose of this study, as described by Cipriani et al. (12). The leg to be stretched was placed on an elevated surface, approximately the height of the midthigh, with the knee near full extension (i.e., not locked). While maintaining a neutral spine (i.e., slight lordosis), the subject flexed at the hip, drawing the anterior abdominal area toward the anterior thigh, until an uncomfortable stretching sensation was experienced in the posterior thigh of the elevated leg. The subjects maintained this position of discomfort throughout the duration of the stretch, increasing forward flexion as needed to sustain the sensation of discomfort for the duration of the stretch time.
The stretch position was held for a total of 30 seconds, using a standard countdown stop watch. All the subjects were issued a standard countdown stop watch. After a 10-second rest, the stretch was repeated for an additional 30 seconds. The total stretch time for a session was 1 minute. The same procedure was repeated for the contralateral leg. Depending on the group assignment, stretching was performed either 1 or 2 times each day and the designated number of days per week.
The subjects refrained from all hip ROM stretching for the final 4 weeks of the study. During this time, the subjects could continue with their normal activities (i.e., those activities that they were presently engaged in).
A standard 12-in. double arm plastic goniometer was used to take weekly ROM measurements of hip joint flexion. Goniometric assessments have been shown to yield reliable data in measuring lower extremity joint ROM and are considered appropriate when measurements are taken by the same tester (10,13). Using the procedures described by Cipriani et al (12) and Norkin and White (34), all the subjects were measured on a weekly basis for the entire duration of this study (8 weeks). Including the preintervention measure, all the subjects were measured 9 times over the course of the study. The day of the week for each measurement period (preintervention, week 1, week 2, etc.) was the same for all the subjects, as was the time of the day. Three investigators measured the subjects. One investigator placed the subject in the straight-leg-raise position, securing the leg to be measured. The second investigator measured the hip flexion angle with the goniometer and recorded this value on a card. The card was then given to a third investigator who entered the data in to a data spreadsheet. All 3 individuals involved in the acquisition of data were blind to the group assignment of the subjects. Subject data were recorded using a coding system that did not reveal group assignment. Finally, all the subjects were asked to self-report their average stretching time for each week. The self-reported times are provided in Table 2.
The independent variables (factors) in this study were time (9 time points), the stretching protocol (5 levels), and sex (male and female). The dependent variable was hip flexion ROM measured at the angle formed by the hip and the pelvis, with the knee extended. Intrarater reliability of the data, with a single measurer was evaluated before the study with an ICC1,3 of 0.92 (0.88–0.96). The initial power analysis of having at least 11 subjects in each group was met (effect size d = 1.5, alpha = 0.05, 1 − beta = 0.95). A factorial (3-way) analysis of variance was used to analyze the interaction between group × sex × time. Two-way analyses of variance (ANOVAs) were performed in the absence of a 3-way interaction and investigated the time ×sex and group × time interactions, during the stretching time period and also during the cessation time period. These analyses were followed by a simple repeated-measures ANOVA for time, in the absence of a 2-way interaction. Univariate approaches for main effects and t-tests for simple main effects were also used as required depending on the presence or absence of any interaction effects. A p value ≤ 0.05 was established as the criteria for statistical significance.
Fifty-three subjects completed the entire 9 weeks of the study. Nine subjects were lost to follow-up at the midpoint measure of 4 weeks. Table 1 includes the descriptive data of those lost to follow-up along with those who remained in the study. There were no differences in terms of initial hip ROM or age between those who completed the study and those who dropped from the study. The subjects lost to follow-up did not differ from the remaining subjects in terms of age, gender, or initial hip ROM values. Loss to follow-up was attributed to lack of interest (n = 2), school obligations (n = 5), and too much time commitment (n = 2). At baseline, the average age and hip ROM were not different across the 5 groups (p > 0.05). The average self-reported stretching time for each week can be found in Table 2. All group self-report times are significantly different (F = 58.2, p < 0.05) from each other except the S7 and S6 groups, which were not different.
The 3-way ANOVA was significant for interaction, when including the control group (F = 2.89, p < 0.05). However, once the control group was removed from the analyses, the 3-way ANOVA was no longer significant for the interaction effects (F = 0.46, p > 0.05). The subsequent 2-way ANOVA for time × sex was not significant for interaction (F = 2.53, p > 0.05). The 2-way ANOVA for group × time was also not significant for interaction (F = 1.59, p > 0.05). Hence, it was determined that neither sex nor stretching group influenced the changes over time. The main effect for time was significant (F = 266.64, p < 0.001). All the 4 stretching groups demonstrated significant gains in hip ROM from the premeasure to week 4 (mean gain = 18.1 ± 6.3°, p < 0.05, eta2 = 0.88). In addition, all the 4 groups demonstrated significant gains in ROM when compared with the control group (p < 0.05); the control group did not demonstrate any change in hip motion at any point in the first 4 weeks (p > 0.05). When comparing the 4 stretching groups to each other, the change in ROM did not differ statistically between any of the 4 groups (p > 0.05) at any time point during these 4 weeks. Finally, the change from week to week was not significant. However, when comparing the initial hip ROM with hip ROM at weeks 2–4, these time points were all significantly greater (p < 0.05) from the initial. Thus, although week-to-week changes were not significant, the subjects had achieved significant gains from pretesting by week 2. Finally, when collapsing groups based on stretching frequency, a significant interaction was detected. The individuals who stretched at least 6 times per week (S14, S7, and S6) gained greater hip ROM than did those who stretched 3 times per week (S3), a relative difference of 24.9 vs. 16.8% (F = 5.20, p < 0.05, eta2 = 0.11). Table 3 provides the average range of hip motion values for the 5 groups, during the first 4 weeks of the study. The table contains weekly ROM values for each group and the relative changes between the initial measure and week 4.
Table 4 provides the mean ROM values across the 5 groups during the cessation period. The 3-way ANOVA was significant for an interaction, when including the control group (F = 2.91, p < 0.05). However, once the control group was removed from the analyses, the 3-way ANOVA was no longer significant for the interaction effects (F = 1.76, p > 0.05). As during the stretching period, the control group did not change over time during the cessation period. The subsequent 2-way ANOVA for time × sex was not significant for an interaction (F = 3.63, p > 0.05). The 2-way ANOVA for group × time was also not significant for an interaction (F = 1.72, p > 0.05). Hence, it was determined that neither sex nor stretching group influenced the changes over time during the last 4 weeks. The main effect for time was significant (F = 115.58, p < 0.001, eta2 = 0.76). The 4 stretching groups all demonstrated a decrease in hip ROM over time, from week 4 to week 8 (mean loss = 9.2 ± 3.1°, p < 0.05). There were no differences in the change in motion between any of the 4 groups (p > 0.05) at any time point. Just as with the stretching period, when looking only at the factor of time, week-to-week changes were not significant. The decline in motion did not approach a significant level until week 8, when compared with week 4. Finally, the final hip ROM values at week 8 were significantly greater than the hip ROM values at the onset of the study. All the stretching group subjects retained approximately 11.2% of the gain in ROM after the 4-week cessation period (F = 82.78, p < 0.05, eta2 = 0.70). Table 4 provides the average ROM values for the 5 groups, during the final 4 weeks of the study (the cessation period). The Table contains weekly ROM values for each group and the relative changes between week 4 and week 8.
A 2-factor ANOVA showed no interaction between sex and time (p > 0.05). A significant main effect for sex existed throughout the study, with women demonstrating greater hip ROMs than men did at each time point (p < 0.05). The rate of change over time however was not significant (Table 5). Figure 4 illustrates the rate of gain and loss in ROM between male and female subjects over the 8-week period.
The intent of this study was to determine if weekly stretching frequency would influence the rate of gain and the rate of loss in hamstring flexibility. As expected, the control group did not demonstrate any change in hip ROM over the course of 8 weeks. However, each of the stretching groups did demonstrate a change in hip ROM; each group gained hip ROM during the stretching period of the study (Figure 2), and each group lost hip ROM during the cessation period (Figure 3). The gains in our study (18.1°) are similar to those reported in previous studies and perhaps slightly greater than in some others (4,12,18,36,39). For example, Sainz de Baranda and Ayala (39) in their investigation of different stretching routines on hamstring flexibility reported a mean gain of 15.14° on the straight-leg test. However, Bandy et al. (4), reported less gain for each of their stretching groups (i.e., mean gain of 10.50, 10.05, 10.45, and 11.50°), respectively, in their study of different stretching frequencies on hamstring flexibility. The intensity of the stretching in our study might have contributed to these gains. The subjects in our study were encouraged to continue leaning into the stretch to maintain a constant feeling of discomfort.
In terms of weekly frequency, which was our primary interest, we found that all the 4 groups gained at similar rates, whether stretching twice each day, every day, or only 3 times each week. The group that only stretched once each day, 3 times each week demonstrated the smallest change between the 4 stretching groups. Thus, even those who stretched only 3 d·wk−1, provided they stretched twice each day, gained at a similar rate as those who stretched every day and those who stretched every day, twice each day (Figure 4). It appears that stretching 3 times each week can yield ROM gains similar to a daily stretching program, provided the stretching is performed at least twice a day. These results are somewhat in support of those obtained by Rancour et al. (36), in that even 2 d·wk−1 of stretching, twice per day, was able to maintain the flexibility gained after a typical stretching program. It is likely that even the occasional attention to ROM on the part of the subjects was able to provide sufficient physical stress to promote ROM gains. We cannot determine if these gains are a result of reduced sensitivity to being stretched or if the gains are a direct result of increased connective tissue length.
Finally, we observed that the gains in ROM, during the stretching period, occurred incrementally over time (Figure 2). There was no single week in which gains were greater than in any adjacent week. Individuals should expect gains in flexibility to occur gradually over time. We cannot determine however at what point gains would eventually plateau because we only followed stretching for 4 weeks. Previous research suggests that ROM gains may plateau at around 6 weeks of stretching (12).
In terms of loss of ROM when a stretching program is discontinued, all the subjects lost ROM at the same relative rate, once stretching ceased, regardless of their prior stretching protocol. Hence, independent of the stretching frequency (i.e., whether a subject stretched every day, twice a day, or only once a day, 3 d·wk−1), once a stretching program was discontinued, the rate of decline was similar for all the subjects. The findings from this study provide evidence that the decline in ROM does occur incrementally over the course of 4 weeks (Figure 3). Even with this gradual loss, our subjects retained a significant improvement in hip ROM at the end of 4 weeks. We are not able to determine how long before subjects would return to their baseline values, but we can speculate that individuals can expect to retain some flexibility even after 4 weeks of cessation of a stretching program.
In regard to our findings between the sexes, as expected, the female subjects in our study demonstrated higher hamstring ROMs at pre, week 4, and week 8. These findings support those of previous studies, which show that women are more flexible than men are for most body regions (1,2,7,25). However, the relative rate of change over time did not differ between men and women (Figure 5). These findings contradict those of Starring et al. (43) who observed that male subjects are not expected to gain or maintain as much ROM as female subjects. We attribute the inconsistencies to the difference in the methodologies of these 2 investigations. Starring et al. examined individuals with tight hamstring muscles and used a mechanical stretching devise; however, our study recruited subjects with normal hamstring flexibility and had subjects perform a 1-legged hamstring stretch without the use of additional equipment. It is possible that individuals with tight muscles may respond differently to stretching compared with individuals with less tight muscles. The effect of a mechanical stretching device can also potentially influence stretching gains because subjects are no longer in control of the stretching force. Also, although there is evidence suggesting that men tend to respond faster to exercise programs compared with women, this difference was not exhibited in our stretching study (16,30,41,44).
Our inability to find a significant difference in the stretching protocols may be attributed to a small sample size in each of the 4 groups. We had sufficient sizes in each group, based on our a priori power analysis; dropouts from each group adversely affected our group sizes. Larger groups might have provided sufficient power to detect significant differences, particularly in relation to the cessation period. However, the actual differences between groups, at any time point, were so small (i.e., <3°) that we do not believe these differences are practically significant.
An additional limitation of this study was that outside behaviors of the subjects were not controlled. The subjects could continue with their normal activities, regardless of their impact on hip motion. We are hopeful that our randomization and screening process controlled for this outside factor; however, we are uncertain (i.e., the subject's behavior was not closely monitored). Finally, this study used a sample of convenience. In this sample, all of our subjects were between the ages of 18 and 50, and the subjects were healthy and free from injury. Future studies should attempt to recruit a more diverse sample including those with musculoskeletal injuries; this will allow the findings to be generalized to other populations. Moreover, different muscles should be studied to see if similar findings are present in different muscle groups.
When prescribing a stretching program for patients, clients, or athletes, it is important to identify the optimal frequency and hold duration of a stretch. Thirty-second hold time appears to be effective, based on this study and previous studies. We now have evidence that stretching as infrequently as 3 d·wk−1 can be as equally beneficial as stretching every day, in terms of hip ROM. Our data support at least 6 times per week of stretching, in any 1-week period, which can be accomplished either once each day, on a daily basis, or twice each day and only 3 d·wk−1. Less frequent stretching (i.e., 3 d·wk−1 rather than daily) might appeal to individuals who struggle to fit stretching into their already busy schedule of training and conditioning. Athletes can consider devoting only a few days each week to stretching, while continuing with other forms of conditioning and training. Fewer days each week might also appeal to individuals who do not enjoy the process of stretching, yet require the benefits of this activity. Hence, a reduced frequency protocol might provide greater compliance for these patients, clients, or athletes. Coaches, trainers, and therapists should also expect to see fairly consistent changes over time, with a stretching program. We did not find that the weekly gains were significantly different from 1 week to the next. Based on our data, we recommend that individuals strive to stretch 2 times per day, to optimize the ability of stretching to create ROM gains. Further, as observed by others, it is possible to maintain gains in ROM by continuing with stretching only 2 or 3 d·wk−1. Individuals should be aware though that the gains achieved with a stretching program will decline gradually over time if all stretching ceases.
This study was funded by the American Council on Exercise (total award $15,383.00). The authors thank the ACE for their financial support. The funds for this study provided timers and gift cards for the subjects and funding for the research assistants. Furthermore, the authors wish to express their special gratitude to the following individuals for their assistance with subject management: Anthony Montijo, Elizabeth Holt, Lindsay Shelp, Rochelle Johnson, Jaci Lee, Michael Murray, and Tiffany Michelin.
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