Comparison of two different stretching strategies to improve hip extension mobility in healthy and active adults: a crossover clinical trial

Abstract

Objective

To improve hip flexor flexibility, the commonly used hip extension stretch focuses on advancing the pelvis forward in a half-kneeling position, targeting only hip extension. This study aimed to compare the effectiveness of this traditional stretch with a new technique incorporating posterior pelvic tilt.

Methods

A randomized crossover clinical trial was conducted with in 26 healthy, physically active individuals (52 hips), aged 22.50 ± 4.10 years. Each participant performed both the hip extension stretch and the posterior pelvic tilt stretch (26 hips), with one technique applied to each limb: the posterior pelvic tilt stretch on one limb and the hip extension stretch on the other limb (26 hips). The reactive hip flexor test and active knee extension test were measured pre and post stretching techniques. A repeated measures ANOVA was performed to assess differences between groups.

Results

There was a significant group-by-time interaction with a medium effect size for the variable related to reactive hip flexor force (F = 4.775, p = 0.034, η2p = 0.087). The posterior pelvic tilt stretch had a statistically significant mean difference of 4.85 N·m (p = 0.003; 95%CI: 1.74; 7.96) compared with the hip extension stretch (mean difference = 0.06 N·m (p = 0.969; 95%CI-3.05; 3.17). No significant differences were found for active knee extension (p > 0.05).

Conclusions

The results showed that the posterior pelvic tilt stretching technique was effective in reducing the hip reactive flexor force when compared to conventional hip extension stretch.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-024-07988-9.

Introduction

Limitations in hip extension, often accompanied by increased anterior pelvic tilt, can alter kinematic patterns during several daily or sport-related activities [1–3]. In individuals with sedentary lifestyles, prolonged sitting periods contribute to a reduction in the range of motion (ROM), particularly in the hip flexors, which include the iliopsoas and rectus femoris muscles. This muscular shortening not only diminishes hip flexion strength and ROM but also leads to hyperlordosis of the lumbar spine and an exaggerated anterior pelvic tilt [1, 3, 4]. These biomechanical changes are closely associated with common pathologies in team sports, such as football, where hip flexor inflexibility can predispose athletes to injuries [5, 6]. Fortunately, these issues can often be mitigated through appropriate stretching techniques [1, 4].

There are various types of stretching methods, including static, dynamic, ballistic, and proprioceptive neuromuscular facilitation (PNF) [6]. Among these, static stretching is widely recognized for its simplicity and effectiveness in enhancing flexibility and ROM [7–9] with different dosages and protocols tested across numerous studies [3, 6, 7, 10, 11]. Some research has suggested that stretching the hip flexors may also positively impact balance and performance-related parameters, although the effects may vary depending on the specific technique used [3]. Moreover, stretching protocols have demonstrated significant improvements in hip extension ROM, particularly in individuals with limited flexibility. Thus, stretching the hip flexors is often recommended to enhance performance in sports requiring substantial hip extension ROM, such as dance, gymnastics, and football [3, 11]. A recent study highlighted that limited hip extension flexibility increases the risk of injuries in baseball pitchers due to restricted stride length during pitching [12].

To address the flexibility of the hip flexors, the most used stretch involves advancing the pelvis forward while in a half-kneeling position with the knee resting on the ground, effectively targeting hip extension. While this stretch is easy to execute, it primarily focuses on the extension component of the hip joint, neglecting the role of pelvic tilt. It is important to consider pelvic tilt because the hip flexors also contribute to anterior pelvic tilt when the femur is fixed. In practice, individuals often compensate for limited hip extension by increasing anterior pelvic tilt, which can exacerbate existing biomechanical issues [13].

Therefore, the objectives of this study were to investigate the effects of a posterior pelvic tilt stretching (PPTS) technique compared to the hip extension stretching (HES) technique on hip flexor and hamstring flexibility in healthy, physically active individuals. We hypothesized that incorporating a posterior pelvic tilt would result in greater improvements in hip flexor flexibility, without compromising hamstring flexibility, potentially offering better protection against injuries related to anterior pelvic tilt.

Materials & methods

Study design

This randomized crossover clinical trial was conducted among healthy and physically active individuals. The study adhered to the principles outlined in the Declaration of Helsinki and was approved by the Research Ethics Committee of the Universidad Europea de Madrid (CIPI: 22/160). All the participants signed a written consent before participating in the study. The trial was registered in the Australian New Zealand Clinical Trials Registry (ACTRN12624000202561) on February 29th, 2024. and conformed to the standards set by the Consolidated Standards of Reporting Trials (CONSORT) guidelines [14].

Sample size calculation

The sample size was determined using G*Power Software (v.3.1.9.2), considering an alpha error rate of 0.05 and a power of 0.8, assuming a medium effect size (f = 0.25 or Eta partial squared = 0.06) based on the primary outcome (the reactive hip flexor force) [15]. Therefore, at least 34 participants were included in the necessary sample size. For the sample size calculation, difference between two dependent means (matched pairs) statistical tests were applied in G*Power, considering that the measurements were taken from the same subjects but on different sides of the body. This approach ensured appropriate power to detect meaningful differences between the two stretching techniques.

Participants

The research was conducted with healthy, physically active participants from a university. Participant recruitment was achieved through the distribution of flyers, posting of posters, and placement of advertisements at the university. The inclusion criteria for this study were male and female individuals [16], aged between 18 and 35 years [17], and who maintained a training regimen of at least two days per week [18] and the modified Thomas Test was applied to identify participants with a specific degree of hip flexor tightness or limitation in hip extension. The cutoff point of -9.69º [19] means that participants who had less than this range of hip extension (indicating tight hip flexors) were eligible to participate in the study. This measurable limitation was essential to ensure that the stretching interventions were tested on individuals who actually needed flexibility improvement [20]. Exclusion criteria were individuals with a history of musculoskeletal lower limb or lumbopelvic conditions within the last five years, as well as those with neuromuscular, rheumatic, cardiovascular or neurological diseases, and those who had undergone previous surgical interventions or experienced fractures in the lower extremities or abdominal region.

Randomization and blinding

Participants were randomized into the two stretching protocols using simple randomization generated in Microsoft Excel, with a 1:1 allocation ratio. Blinding was maintained for the outcome assessors to minimize potential measurement bias. To reduce potential carryover effects, participants performed both stretching techniques on the same day with a 30-minute rest interval between sessions. While no washout period was included due to the acute nature of the study, the rest period was deemed sufficient based on the short-term effects of stretching.

Exercise protocol

The intervention protocol was aligned with the Consensus on Exercise Reporting Template (CERT) guidelines [21, 22].

Exercise type and dosage

Two stretching techniques were employed: (1) Conventional hip extension stretch in a half-kneeling position and (2) Hip extension stretch with an added posterior pelvic tilt. Each participant performed both techniques on both legs in a randomized sequence. Each stretching exercise was performed in two sets of 30 seconds (2 × 30”), with a 30-second rest interval between sets. This dosage was selected based on evidence suggesting that this duration is effective in inducing acute increases in flexibility without causing excessive muscle fatigue or discomfort [23, 24].

Participants were provided with detailed verbal and visual instructions on how to perform each stretch correctly. Demonstrations were conducted by a trained physiotherapist before the participants attempted the exercises. Emphasis was placed on maintaining the correct posture, particularly the alignment of the pelvis during the posterior pelvic tilt technique.

HES protocol

Participants were instructed to assume a half-kneeling position with one knee on the ground and the other foot placed forward. They were guided to gently shift their pelvis forward while maintaining a neutral spine and upright posture. The focus was on feeling a stretch in the anterior hip region of the leg positioned with the knee on the ground, specifically targeting the hip flexors, without allowing the lower back to arch or the pelvis to tilt excessively (Fig. 1A).

Fig. 1A.

Hip extension stretching technique

PPTS protocol

In a similar half-kneeling position, participants were instructed to engage their core by actively tucking the pelvis under into a posterior pelvic tilt while advancing the pelvis forward. This action was aimed at engaging the lower abdominal muscles and gluteus maximus to minimize anterior pelvic tilt. The stretch focused on deepening the engagement of the hip flexors while maintaining a stable lumbar spine and preventing compensatory movements, such as arching the lower back (Fig. 1B).

Fig. 1B.

Hip extension with pelvic tilt stretching technique

Participants were supervised during all stretching sessions to ensure correct technique and adherence to the protocol. Any deviations were immediately corrected by the supervising physiotherapist. Participants were also asked to rate their perceived exertion and discomfort on a scale of 1–10 after each set to monitor their response to the stretching interventions.

The intensity and execution of the stretches were adapted to each participant’s flexibility level and comfort. For those with limited initial flexibility or discomfort during the stretch, the range of motion was adjusted to ensure the exercise could be performed without causing pain.

Although the study focused on an acute intervention, guidelines were provided for future applications where progressive stretching might be necessary. Participants were informed that over time, as flexibility improves, the duration or intensity of the stretch could be gradually increased.

To maintain consistency across all participants, the same physiotherapist supervised all stretching sessions. The therapist used a standardized script to provide instructions and cues, reducing variability in how the exercises were delivered. Each participant’s performance was recorded in detail, including any adjustments made during the exercises and their responses to the intervention (e.g., perceived exertion, discomfort levels). This documentation was essential for analyzing the effectiveness of the interventions and identifying any patterns related to individual differences.

Although no adverse events were expected due to the low-risk nature of the stretching exercises, a protocol was in place to manage any potential issues. Participants were instructed to immediately stop the exercise if they experienced sharp pain or discomfort beyond the expected stretch sensation. The supervising physiotherapist was prepared to assess the situation and provide appropriate care or modifications to the exercise.

To motivate participants and enhance compliance, they were provided with information on the potential benefits of increased hip flexibility, particularly for improving athletic performance and reducing injury risk. Encouragement was given throughout the sessions, and participants were reminded of the importance of completing the exercises as prescribed. Feedback from participants was solicited at the end of each session to assess their understanding of the exercises and their comfort levels. This feedback was used to make any necessary adjustments to the instructions or execution of the protocol in subsequent sessions.

Descriptive variables

The sociodemographic data collected included information on the athlete’s sex (male or female), age (in years), height (in centimeters), weight (in kilograms), and body mass index (BMI) (calculated in kg/cm² using Quetelet’s index) [25]. In addition, the physical activity level measured by the International Physical Activity Questionnaire [26], the femur length [27] and the dominant limb (right or left) distribution was collected as frequency and percentage.

Primary outcome measure

The Reactive Hip Flexion (RHF) Test [15] was administered with the participant lying in a supine position on an examination table, their arms parallel to the body, and one lower extremity positioned with the hip flexed at 90 °and the knee bent, while the other lower extremity remained in a neutral position with the knee and hip fully extended.

The reactive peak force generated by the hip flexor muscles was quantified in Newtons using a handheld dynamometer (ActiveForce 2, Activbody, San Diego, California, USA) positioned 5 cm cranial to the superior edge of the patella. The handheld dynamometer was secured with a belt and calibrated before each measurement. The absolute force value was normalized to femur length and expressed as torque (N·m). Before the commencement of each trial, an initial tension reading was recorded by calibrating the handheld dynamometer to ensure a baseline reading of 0 N. The reliability of this test has been demonstrated to be excellent [15].

Secondary outcome measure

Active Knee extension (AKE) in degrees for the contralateral lower limb at 90 °of hip flexion was assessed using a standard universal goniometer (Orthopedic Equipment Co., Bourbon, USA). AKE measurement involves determining the degree of knee extension from the point of full knee flexion, as indicated in previous studies [28]. Before conducting the measurement, the examiner ensured the maintenance of the 90-degree hip flexion by placing one hand on the back of the thigh while positioning the universal goniometer on the lateral thigh to verify hip flexion alignment. Subsequently, the participant was instructed to actively extend the knee until experiencing an uncomfortable level of tension while maintaining hip flexion at 90 ° [15].

Systematic assessments were followed for each test to avoid bias or inconsistencies in the measurements. The participants were instructed to keep their lower back in contact with the table to minimize pelvic rocking during the RHF Test.

Statistical analysis

Statistical analysis was performed using IBM SPSS Statistics version 29.0 software for Windows (IBM, Armonk, NY, USA). To assess the data distribution, the Kolmogorov-Smirnov test and histogram examination were employed. For parametric variables (p > 0.05), descriptive statistics, including mean and standard deviation, were presented, while for non-parametric variables (p < 0.05), median and interquartile range were reported.

To compare the baseline characteristics of the two groups, an independent t-test was performed, considering the assumptions of homoscedasticity and sphericity. If these assumptions were met, two-way repeated measures analysis of variance (ANOVA) with a 2 × 2 design was conducted. The effect size was assessed using partial eta squared (η2p), with values of 0.01 interpreted as small, 0.06 as medium, and 0.14 as large. A 95% confidence interval was set for all analyses.

Results

Baseline demographic outcomes

Out of 33 volunteers who expressed interest in participating, 4 were excluded due to the presence of hip pathology, and 3 were excluded for exceeding the range of motion in the modified Thomas Test. A total of 26 participants (52 hips) were included in the study and divided into two groups: posterior pelvic tilt (n = 26) and hip extension (n = 26). The total sample included in the study had a mean age of 22.50 ± 4.10 years old, a mean weight of 75.73 ± 9.15 kg, a mean height of 1.78 ± 0.08 centimeters, a mean BMI of 23.76 ± 2.10 kg/m² and realized 120.86 ± 19.45 min per week of physical activity. Twenty-one participants had right-limb dominance (80.80%). Baseline outcome measures were not significantly different between the two groups (p > 0.05). The study reported no dropouts or losses to follow-up, ensuring that all participants who were randomized completed the study protocol. Compliance with the stretching protocols was monitored, and all participants adhered to the prescribed intervention.

Reactive hip flexor force

There was a significant group-by-time interaction with a medium effect size for the variable related to RHF force (F = 4.775, p = 0.034, η2p = 0.087). The posterior pelvic tilt group had 18.79 ± 8.40 N·m at baseline and 13.95 ± 7.28 post-stretching with a statistically significant mean difference of 4.85 N·m (p = 0.003; 95%CI: 1.74; 7.96) compared with the hip extension group (mean difference = 0.06 N·m (p = 0.969; 95%CI: -3.05; 3.17), which showed a RHF of 17.62 ± 8.97 N·m at baseline and 17.56 ± 9.74 N·m post-stretching (Fig. 2).

Fig. 2.

Differences between groups pre- and post-stretching for reactive hip flexor force

Active knee extension range of motion

No significant group-by-time interaction was observed for AKE (F = 2.925, p = 0.093, η2p = 0.055). The mean values of active knee extension at baseline and post-stretching were 28.46 ± 7.46 and 31.19 ± 7.26 degrees for the posterior pelvic tilt and hip extension groups, respectively. Mean active knee extension values at post-stretching were 30.19 ± 8.74 and 30.58 ± 8.00 degrees for posterior pelvic tilt group and hip extension group, respectively (Fig. 3).

Fig. 3.

Differences between pre- and post-stretching groups for active knee extension

Discussion

This study aimed to compare two self-administered stretching techniques designed to improve hip flexor flexibility and to evaluate whether a novel technique incorporating posterior pelvic tilt is more effective in reducing reactive hip flexor force in healthy, active individuals. The findings demonstrate that the PPTS was indeed more effective in reducing reactive hip flexor force than the HES. As such, this technique may be particularly beneficial for young, healthy individuals with limited hip flexor flexibility, potentially aiding in the prevention of musculoskeletal disorders, such as low back pain, which are often associated with restricted hip extension and lumbar hyperlordosis [4, 20].

The group-by-time analysis revealed a significant reduction in hip flexor force in the PPTS compared to the HES, with a medium effect size. This suggests that the posterior pelvic tilt technique effectively reduces the tendency for hip flexion during supine positioning with one limb at 90° of hip flexion and knee extension. Notably, the minimal detectable change for hip flexor reactive force was calculated to be 4.83 N·m [15], and the PPTS achieved a pre-post mean difference of 4.85 N·m, indicating a clinically relevant improvement. This reduction is important as it may enhance the biomechanics of walking and running, allowing for longer stride lengths without compensatory movements in the pelvis or lumbar spine. Additionally, the improved hip flexor flexibility observed in the PPTS could offer a protective effect against muscle strains in the hip flexors, such as the rectus femoris [29–31].

Regarding active knee extension, no significant differences were found between groups in the time × group interaction, indicating that neither stretch technique had a measurable impact on hamstring flexibility. Thus, the PPTS appears to be effective in reducing reactive hip flexor force without adversely affecting hamstring flexibility. This finding aligns with the study by Sandberg et al. [32], which investigated the effects of antagonist muscle stretching on agonist muscle performance, including jump height, torque, and EMG activity. The study concluded that stretching antagonistic muscles did not negatively impact the performance of agonist muscles, supporting the idea that stretching the hip flexors (antagonists) does not impair hamstring (agonist) flexibility. Therefore, the PPTS can be safely applied without compromising the flexibility of the hamstrings.

Given these findings, when implementing self-administered stretch techniques, individuals with reduced hip flexor flexibility, whether or not they have musculoskeletal disorders such as low back pain, should prioritize the posterior pelvic tilt stretch over the conventional hip extension stretch. This is particularly relevant as increased anterior pelvic tilt has been associated with a higher risk of injury in sports such as baseball or soccer [33, 34]. Moreover, reducing anterior pelvic tilt or lumbar hyperlordosis may decrease the eccentric load on the hamstring musculature during running, which could further contribute to injury prevention [33]. Although the PPTS involves movement of the proximal part of the joint, our study carefully monitored participants to ensure proper alignment, preventing compensatory spinal extension and overloading of posterior spinal structures. This method not only improved the stretch’s effectiveness but also minimized the risk of spinal compensation. Consequently, future studies should explore the inclusion of this technique into rehabilitation protocols for patients with conditions such as hip flexor strains, hip flexor groin pain, or iliopsoas tendinopathy to assess its long-term effectiveness.

This is especially relevant given the well-established interrelationship between hip flexors and hamstrings in the literature. Studies such as Ema et al. [35] have highlighted their coordinated roles in dynamic activities like sprinting. Both muscle groups contribute significantly to generating hip flexion and leg extension, emphasizing the importance of their balance in efficient movement. Similarly, Nagano et al. [36] underscored the biomechanics of muscles crossing the hip joint during sprinting, further supporting the functional link between these muscle groups. In the present study, we ensured that changes in hip flexor flexibility, achieved through the posterior pelvic tilt and hip extension stretches, did not negatively affect hamstring flexibilty by employing the AKE test.

While this study has several strengths, including its novel approach, practical application, and methodological design, it also has limitations. The study was conducted exclusively in young, healthy, physically active individuals, which limits the generalizability of the findings to populations with recent injuries or sedentary individuals. Additionally, this study did not analyze the biomechanics of the pelvis, such as baseline pelvic tilt during activities like walking or running, which could have provided further insights into the clinical implications of the intervention. Despite these limitations, the posterior pelvic tilt stretch technique holds promise as a preventive and therapeutic measure, particularly for managing rectus femoris strains, which are common in sports like soccer [37]. Another limitation of this study is the short 30-minute rest period between stretching sessions, which may not fully account for the prolonged effects of muscle and connective tissue responses to stretching. To mitigate this, we randomized the interventions between two different limbs, rather than applying both techniques on the same limb, reducing the likelihood of carryover effects.

Conclusions

The aim of this study was to investigate the impact of a novel posterior pelvic tilt stretching method on hip flexor flexibility among healthy, physically active subjects. Results indicated that this innovative technique successfully decreased the reactive force of the hip flexors without compromising hamstring function. Consequently, the posterior pelvic tilt stretch technique holds promise for both the prevention and treatment of rectus femoris strains, particularly prevalent in sports like soccer.

Electronic supplementary material

Below is the link to the electronic supplementary material.

Author contributions

AG: Formal Analysis, Investigation, Methodology, Writing–original draft. AS: Conceptualization, Supervision, Writing–review and editing. GGPS: Data curation, Formal Analysis, Investigation, Software, Writing–original draft. CC: Investigation, Methodology, Writing–original draft. DD: Investigation, Methodology, Writing–original draft. MP: Investigation, Methodology, Writing–review and editing. JADBM: Investigation, Methodology, Data curation, Validation, Writing–original draft. All authors reviewed the manuscript.

Funding

This study did not receive any funding.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Ethical approval

This study received approval from the Research Ethics Committee (CIPI: 22/160) of the Universidad Europea de Madrid (SPAIN).

Human Ethics and Consent to participate

The study adhered to the principles outlined in the Declaration of Helsinki. All the participants signed a written consent before participating in the study.