Effects of a Hip Flexor Stretching Program on Running... : The Journal of Strength & Conditioning Research (2024)

Introduction

The popularity of running is undeniable, with over 50 million people participating each year in the United States alone (¹⁷). Unfortunately, participation in running is often accompanied by injury with annual incidence rates ranging from between 19.4 and 92.4% (²³). Generally, it is accepted that approximately 50% of runners experience an injury each year (²). Although the cause of these injuries is complex and multivariate, flexibility has been anecdotally linked with increased risk of injury.

An increased risk of low back pain (LBP) may be caused by decreased flexibility of the hip flexor musculature, structural limitations of the anterior hip capsule, or a combination of both. Research has demonstrated that individuals with LBP exhibit a reduction in passive hip extension when measured in a static position, compared with asymptomatic individuals (^16,22). Restrictions in passive flexibility have the potential to affect the active, or dynamic, range of motion during functional activities. Thus, reduced passive hip extension may translate into reduced active hip extension during dynamic movements such as walking or running. Reduced active hip extension flexibility has been correlated with greater anterior pelvic tilt during running (^5,18). In turn, greater anterior pelvic tilt has been associated with greater extension of the lumbar spine during running (¹⁹). Because it is postulated that excessive lumbar spine extension may contribute to LBP, the kinematic relationship between hip, pelvis, and lumbar spine, also known as the lumbo-pelvic-hip (LPH) complex, provides a theoretical injury mechanism to help explain LBP during running. Limited passive hip extension flexibility has been associated with LBP in runners (¹), which further supports the LPH complex as an injury mechanism. However, further investigation is needed to understand the effect that limited passive hip extension flexibility may have on the kinematics of the hip, pelvis, and lumbar spine during running.

Although static stretching has been shown to improve the range of passive extension at the hip (^12,27), its effect on active hip extension is less clear. For instance, Moreside and McGill (¹³) found that improvements in passive hip extension flexibility did not necessarily result in improvements in active hip extension flexibility in a group of young, healthy participants during several functional dynamic movement tasks. Watt et al. (²⁴) reported similar results in a group of healthy elderly participants. However, when they analyzed a subset of elderly participants who displayed limited active hip extension during walking, the authors measured an increase in both passive and active hip extension during walking after a 10-week static stretching program. Despite the widespread practice of stretching in runners, no study has yet examined the effects of static stretching on active hip extension during running.

In summary, reduced passive hip extension flexibility may put an individual at risk of LBP by decreasing active hip extension while increasing both anterior pelvic tilt and lumbar spine extension during running. Therefore, the purpose of this study was to analyze the effect of a 3-week stretching program on passive hip extension as well as on the sagittal plane kinematics of the LPH complex during running. It was hypothesized that greater passive and active hip extension would be measured after the 3-week stretching program, in addition to reductions in both anterior pelvic tilt and lumbar spine extension.

Methods

Experimental Approach to the Problem

The study design used a one-group pretest-posttest design to determine whether significant improvements in passive hip extension would result in changes in hip, pelvis, and lumbar spine motion during running. Thirty-six participants were screened to find individuals with limited passive hip extension to complete the stretching intervention. The 20 participants who qualified for the study completed a 3-week static hip extension stretching to determine the effects of increased flexibility on sagittal plane movements of the hip, pelvis, and lumbar spine during running. Data were collected, and joint angles of the hip, pelvis, and lumbar spine were calculated before and after the 3-week stretching intervention. The independent variable was the stretching intervention, and the dependent variables were the joint/segment angles of interest.

Subjects

Participants were between the ages of 18–40 years and regularly engaged in physical activity involving running for at least 30 minutes, 3 times per week. Subjects were excluded if they were: pregnant; currently experiencing any pain in the lower extremities or spine; had suffered an injury to the lower extremity or spine that limited activity in the past 3 months; had undergone major surgery to the lower extremity or spine at any time; or if they were not comfortable running on a treadmill without the use of handrails. Subjects were also required to exhibit limited passive hip extension flexibility based on screening measurements. Before the screening session, all participants were informed of the purpose of this study and provided written informed consent using a form approved by the institutional review board of the University of Kentucky.

A 4.1° (SD = 7.5°) increase in active hip extension flexibility during walking after a stretching program has been reported previously (²⁴). Therefore, we estimated that there would be a 5° increase during running (due to greater joint range of motion during running). An a priori sample size of 19 was calculated using an alpha level of 0.05, a power of 0.8, and an estimate of the change in hip extension of 5°. Participants were recruited until the required sample size had completed the protocol. A total of 36 participants were screened for the study. A total of 4 subjects were excluded from the study because they failed to complete the flexibility protocol or follow-up testing: of those screened, 24 met the inclusion criteria of limited passive hip extension flexibility. A total of 4 subjects were excluded from the study because they failed to complete the flexibility protocol and follow-up testing session: 2 subjects because of busy schedules, 1 subject due to a sprained ankle, and 1 subject failed to adhere to the stretching program. Therefore, 20 participants (mean ± SD: 27 ± 5 years, range: 21–38 years, 1.71 ± 0.06 m, 67.62 ± 11.44 kg) consisting of 11 females (mean ± SD: 28 ± 6 years, 1.69 ± 0.06 m, 62.17 ± 8.69 kg) and 9 males (mean ± SD: 28 ± 5 years, 1.74 ± 0.06 m, 74.27 ± 11.22 kg) completed the entire study.

Procedures

Screening

Participants were required to complete one screening visit to establish whether they demonstrated limited passive hip extension. During the screening session, participants underwent a passive hip extension flexibility assessment using a modified Thomas test (MTT) (²⁷). Measurements were considered negative if the thigh dropped below the horizontal and positive if the thigh remained above the horizontal. Participants were considered to have limited hip extension flexibility if they produced a flexibility value of −1.5° or lower, which represents the 50th percentile of a young adult male population (¹¹).

Data Collection

Participants qualifying for the study completed 2 data collection sessions, separated by 3 weeks of static stretching. Passive hip extension measurements and a gait analysis were conducted during both the baseline (PRE) and follow-up (POST) sessions by a single tester. After the baseline session, participants were taught to perform the stretches.

Motion Capture

A standardized neutral running shoe (New Balance, R662WSB, Boston, MA, USA) was provided to each participant. Retroreflective markers were placed on the trunk, pelvis, and lower extremity (see Supplemental Digital Content for a full description of the kinematic model, https://links.lww.com/JSCR/A91). The gait analysis was completed using 10 motion capture cameras (Motion Analysis Corp., Santa Rosa, CA, USA) and an instrumented treadmill (Bertec, Columbus, OH, USA) recording at 200 and 1000 Hz, respectively. A standing calibration trial was captured, and then participants ran on the treadmill at a self-selected speed equivalent to an easy 30-minute run. After a 5-minute acclimation period, marker trajectory data were collected and tracked using Cortex software (Motion Analysis Corp.). The average running speed for the 20 participants was 2.85 ± 0.38 m·s⁻¹. Participants were constrained to the same self-selected speed during the PRE and POST sessions to account for speed as a confounding factor.

Kinematics

Further data processing, including filtering and calculating joint/segment angles, was conducted using Visual 3D software (C-Motion, Inc., Germantown, MD, USA). Raw marker trajectory data were filtered using a fourth-order low-pass Butterworth filter with a cut-off frequency of 8 Hz. A sagittal-frontal-transverse cardan sequence was used to quantify joint and segment angles for the hip (thigh relative to pelvis) and pelvis (relative to the laboratory coordinate system), respectively. To assess lumbar spine extension, a 2D projection angle adapted from the study by Junquiera et al. (⁸) was used, where the midpoint of the posterior superior iliac spine (PSIS) markers was used instead of a marker at the lower edge of the sacrum. Kinematic data of the least flexible limb based on the results of the MTT screening were selected for further analysis. Initial contact was identified using vertical ground reaction force data. The peak values for the kinematic variables of interest, including hip extension, anterior pelvic tilt, and lumbar spine extension, were defined as the greatest values observed during the gait cycle. The ensemble mean value for 5 consecutive gait cycles of each kinematic variable was calculated for each subject, which were then averaged to find the overall mean for all subjects.

Stretching Protocol

All participants were assigned the same 3-week home-based stretching protocol. The duration of the protocol was based on previous research demonstrating that 3 weeks of static hip flexor stretching resulted in significant improvements in passive hip extension (²⁷). The stretches consisted of a standing hip flexor stretch and a kneeling modified lunge stretch (Figure 1) to be performed daily. Ten repetitions were completed for each stretch, with a 10-second hold and a 10-second rest for each repetition. After explaining the procedures, the stretch was demonstrated by the researcher, then performed by the participant in the presence of the researcher to ensure proper technique. Participants were encouraged to maintain their daily activities, with the addition of the stretching protocol as the only exception. A daily calendar with written instructions describing the program details was provided, allowing participants to log compliance with the program. Compliance was quantified as a percentage of the total number of available stretching sessions. Although participants were encouraged to stretch every day, they were required to stretch at least 5 days each week (71.4%) for compliance to the program to be met. For the 20 participants, compliance was found to be 86.7 ± 9.0%.

Statistical Analyses

All kinematic and flexibility-dependent variables of interest were compared before and after the 3-week stretching program using dependent t-tests. Cohen's d effect sizes were also calculated to help interpret the effect of the stretching program, with designations of small (d = 0.2), medium (d = 0.5), and large (d = 0.8). Correlations were performed between the change in passive hip extension and: (a) the change in active hip extension; (b) the change in anterior pelvic tilt; and (c) the change in lumbar spine extension. In addition, correlation analyses were conducted for stretching compliance vs. the change in passive hip extension as well as stretching compliance vs. the change in active hip extension. Interpretation of Pearson correlation coefficient (r) followed guidelines set out by Hinkle et al. (⁷): very high (r > 0.90), high (r = 0.70–0.90), moderate (r = 0.50–0.69), low (r = 0.30–0.49), and negligible (r = 0.00–0.29). All statistical analyses were performed using SPSS statistical software (version 22; SPSS, Inc., Chicago, IL, USA) with a significance level of p ≤ 0.05.

Because the reliability of the MTT has been questioned in the past (¹⁵), a reliability analysis of the single tester performing the MTT during this study was performed. Modified Thomas test data from the initial screening was compared with the MTT data from the baseline data collection for 17 subjects. The analysis yielded an intratester standard error of measurement of 3.5°.

Results

Mean values for passive hip extension, as well as sagittal plane kinematics of the LPH complex, are presented in Table 1. The results of the study showed a significant increase in passive hip extension after the 3-week static stretching program (Table 1). In addition, individual changes in passive hip extension that each participant made after the 3-week stretching program are presented in Figure 2. Nineteen of 20 participants demonstrated an increase in passive hip extension (mean = 10.7 ± 6.9°, range = −8.3 to 24.0°).

Despite the significant improvement in passive hip extension after the 3-week stretching program, there were no differences in peak active hip extension, anterior pelvic tilt, or lumbar spine extension during running between PRE and POST (Table 1). The sagittal plane kinematics of hip extension (Figure 3A), anterior pelvic tilt (Figure 3B), and lumbar spine extension (Figure 3C) remained nearly identical through the entire gait cycle after the 3-week stretching program. In addition, no relationship was found between the change (between PRE and POST) in passive hip extension with either the change in peak active hip extension, peak anterior pelvic tilt, or peak lumbar spine extension during running (Table 2).

Finally, there were no relationships between stretching compliance and change in passive hip extension (r = −0.176, p = 0.457) or between stretching compliance and change in peak active hip extension angle (r = 0.019, p = 0.937).

Discussion

The results of the study supported the hypothesis that an increase in passive hip extension would be observed after a 3-week stretching program. However, contrary to the second hypothesis, no increase in active hip extension or reductions in anterior pelvic tilt or lumbar spine extension were observed during running after the static stretching.

The magnitude of the improvement found for passive hip extension (10.6°) was similar to values reported in previous studies (^12,27). Winters et al. (²⁷) reported improvements in passive hip extension of 13° after 3 weeks of static stretching. Similarly, a 13.6° increase in passive hip flexibility was reported after 6 weeks of static stretching in healthy young men with limited hip mobility (¹²). The significant increase in passive hip extension found in this study shows that the stretching protocol was indeed successful in terms of improving passive flexibility at the hip. The fact that passive hip extension improved in 19 of the 20 participants further highlights the effectiveness of the stretching program. Moreover, the improvement in passive hip extension was relatively large compared with the SEM calculated for the reliability analysis, thus indicating that the improvements were not a consequence of measurement error.

Although an increase in passive flexibility was found after the stretching program, we can only speculate regarding the underlying mechanism. Increases in muscle extensibility can be achieved either by structural changes of the tissue (e.g., passive and series elastic components) (²⁵) or a person's tolerance to an uncomfortable stretch sensation (^4,10). Magnusson et al. (¹⁰) concluded that the improvements in range of motion after stretching for 20 days were not accompanied by changes in tissue properties, but were rather the consequence of an augmented stretch tolerance. Similarly, Folpp et al. (⁴) reported that stretching 5 days per week for 4 weeks resulted primarily in improvements in a subjects' tolerance to the stretch. Therefore, given the short duration of our stretching program, it seems likely that the changes in passive hip extension may be due to greater stretch tolerance.

Despite the increase in passive hip extension, no increase in active hip extension during running was observed after stretching. Furthermore, there were no reductions in anterior pelvic tilt or lumbar spine extension. These results are consistent with those of Moreside and McGill (¹³), who reported no significant changes in active hip extension during several functional movements, despite observing an increase in passive hip extension after 6 weeks of static stretching in a healthy young population. Watt et al. (²⁴) also found that an increase in passive hip extension after a 10-week supervised stretching program did not correspond to an increase in active hip extension during walking in a healthy elderly population. Thus, our findings in conjunction with other literature indicate that improvements in passive hip extension are not accompanied by changes in active hip extension in a healthy population, regardless of the activity performed (functional movement, walking, and running), the age of the sample, or the duration of the stretching program.

Although an increase in the mean active hip extension was not observed for the sample in the current study, individual variation in the response to the stretching program warranted a correlation analysis to further explore the relationship between passive and active hip extension. However, no relationship was found between the changes in passive and active hip extension. This provided further evidence that improvements in passive flexibility do not necessarily translate to a greater range of motion during dynamic movements.

The poor association found between passive and active hip extension suggests that flexibility is not the only variable influencing the dynamic range of motion at the hip. Some alternative factors that may help explain the lack of relationship between passive and active hip extension include the geometry of the joint, the speed at which the dynamic task is performed, and considerations related to motor learning. Wilson et al. (²⁶) showed that joints have a preferred path of least resistance based on both the geometric parameters of the articular surfaces and the surrounding tissues. The tissues and ligaments surrounding a joint do play a role in motion, but they must work together with the articular surface geometry to actually produce motion at the joint. Alternatively, it is possible that static (passive) flexibility may not be a limiting factor for a healthy population when running at submaximal speeds (¹⁴). Specifically, running at submaximal speeds may not have required our participants to approach the end range of motion at the hip. Our findings are in contrast to a study conducted on walking, which reported that an elderly population who displayed limited active hip extension underwent significant improvements in both passive and active hip extension during walking after static stretching (²⁴). However, the increases they reported in active hip extension were also accompanied by an increase in stride length, suggesting that the participants may have reached their end range of motion at the hip during the walking task. Considering this evidence in conjunction with our findings, it is suggested that passive flexibility may play a role in dynamic movements, albeit only when those dynamic movements are pushing individuals to use their full range of motion. Finally, given the experience level of the study population, it is likely that the task has become essentially automatic due to years spent running with the same movement pattern (³). Therefore, even if improved passive hip extension provided the individual with the physical capability to run with greater hip extension, it is possible that they would not automatically alter their gait pattern unless instructed specifically to do so.

Several limitations should be noted in this study. First, error due to marker placement is always present, although a single trained investigator performed all marker placement to avoid intertester variability. Also, the current study analyzed hip, pelvis, and lumbar spine kinematics during treadmill running. It has been demonstrated that treadmill running, although overall displaying very similar kinematic patterns to overground running, does produce subtle differences in the kinematics of the LPH complex (²⁰). Therefore, caution is needed when applying the results of this study to overground running settings. In addition, the generalizability of the results of this study may be limited. The study population was composed of young, experienced runners; so, the results may not apply to older individuals or novice runners. Also, considering that all subjects were healthy, caution should be taken when relating these results to a population with LBP. It has been reported that individuals with LBP display different trunk, pelvis, and lower-extremity kinematics compared with healthy individuals (^6,9,14,21,28). Although the participants of our study displayed limited passive hip extension similar to individuals who suffer from LBP, they were free from pain.

Practical Applications

Static stretching is frequently used in many rehabilitation and training programs. The results of this study confirmed that a 3-week static hip extension stretching intervention is an effective way to increase passive flexibility. However, the improvements in passive hip extension did not translate to a change in the sagittal plane kinematics of the hips, pelvis, and spine during running. The absence of any changes in the kinematics of the LPH complex as well as the absence of a correlation between passive and active hip extension suggest that passive flexibility of the joint is not the only factor that determines range of motion at the hip during running at submaximal speeds. Although static stretching alone is not sufficient to alter kinematics of the LPH complex in young and healthy individuals running at submaximal speeds, it is possible that an increase in range of motion at the hip may be beneficial when running at or near maximal speeds.

Acknowledgments

There are no conflicts of interest for any of the authors. The authors thank Dr. Brian Noehren for his contributions to the study, as well as Ethan Stewart and Jessica Quinn for their assistance with the data collections.

References

Supplemental Digital Content

• JSCR_2018_04_13_METTLER_1_SDC1.docx; [Word] (13 KB)