Introduction

A sedentary lifestyle and prolonged periods of sitting can lead to the shortening and weakness of the hip flexors. Furthermore, a relationship between shortened hip flexors and less activation of the gluteus maximus was established by Mills et al. (2015). This may have an important influence on normal joint kinematics where a tight and weak hip flexor and a less activated hip extensor would probably cause a decrease in hip extension range of motion and an increase of anterior pelvic tilt. Similarly, Mills and colleagues found that the ratio of activation of gluteus maximus : biceps femoris was also reduced, despite similar net hip extension moments being created. In a sporting population, this may increase demand on the hamstring muscles, potentially predisposing them to strains or tears. In less active individuals, tight and weak hip flexors may be more important in increased lumbar lordosis and exaggerated pelvic anterior tilting, which may predispose someone to low back pain problems or gait alterations. Stretching of the hip flexors has been thoroughly studied in the past and showed reductions in the anterior tilting of the pelvis according to Preece et al. (2021). As stretching is a simple and effective method to use, the current authors wanted to investigate whether a difference exists between stretching with or without posterior pelvic tilt to improve hip extension mobility.

Methods

A crossover design was set up for this randomized controlled trial (RCT). Healthy and active individuals were included when they were between 18 and 35 years of age and trained for at least 2 days per week. To be eligible, the researchers set the requirement of having hip flexor tightness and thus a limitation of hip extension of less than -9.69°, measured with the modified Thomas test.

Two stretching techniques were compared.

1. Conventional hip extension stretch in a half-kneeling position
2. Hip extension stretch with an added posterior tilt

The conventional hip extension stretch required the participant to sit in a half-kneeling position. They were instructed to gently shift their pelvis forward while maintaining a neutral spine and upright posture. A stretch had to be felt in the anterior hip region of the leg positioned with the knee on the ground.

The hip extension stretch with posterior tilt was performed in a similar position, but a posterior tilt of the pelvis was added by instructing the patient to engage their core by tucking in the pelvis while advancing the pelvis forward. The aim was to engage the lower abdominal muscles and gluteus maximus muscle to minimize anterior tilting of the pelvis. The instructor ensured that no arching of the lower back took place.

Each participant performed both stretches; one technique with the left leg and one with the right leg. The order was randomly chosen. The stretch was held for 30 seconds and 2 repetitions per stretch were performed with a 30-second rest in between. The clinician gave verbal and visual instructions to ensure good performance. No pain was allowed during the stretch so the range of motion was adjusted in case discomfort was present.

The participants’ baseline sociodemographic data and their physical activity level using the International Physical Activity Questionnaire (IPAQ) were collected. The length of the femur was measured using palpation of bony landmarks.

The Reactive Hip Flexion Test was the primary outcome of interest. For this test, the participant was in a supine lying position with one hip flexed to 90° with the knee bent and the other leg in a neutral position with the knee and hip fully extended on the examination table. A dynamometer measured the reactive peak force generated by the hip flexors. The absolute force was normalized to the length of the femur and expressed as a torque in Newton/meters. The minimal clinically important difference is 4.83Nm.

A secondary outcome measure was the Active Knee extension in degrees for the contralateral lower limb at 90° of hip flexion. This was measured using a goniometer.

Results

Twenty-six participants were included in this study, totaling 52 hips. The participants had a mean age of 22.5 years and a mean BMI of 23.76 kg/m2. Their weekly physical activity equaled a mean of 120 minutes.

The primary outcome analysis reveals a significant group-by-time interaction with a medium effect size. The hip extension stretch using the posterior pelvic tilt led to a significant decrease in reactive hip flexor force of 4.85 Nm.

Active knee extension, as a secondary outcome, did not show any difference.

Questions and thoughts

In this study, the emphasis was mainly on stretching of the hip flexors. While it showed a meaningful reduction in the observed Reactive Hip Flexor Test, meaning that more extension and thus less hip flexion was generated, we must be aware that shortened muscles are often also weakened and this may lead to changes in the whole kinetic chain. In research, this is often labeled Lower Crossed Syndrome.

Rather than just focusing on stretching shortened muscles, we should strengthen these muscles more. By using eccentric exercises you can achieve both lengthening and strengthening in one exercise!

At first, I did not get why the authors used the Active Knee Extension test as a secondary outcome measure and the rationale for choosing this test was also not explained in the paper. I suppose that when greater knee extension at the contralateral leg is achieved during this test, this may be due to increased pelvic posterior tilting capacity and increased stretching tolerance of the hamstrings at the contralateral leg. This tension exerted on the pelvis by the contralateral hamstrings and position of posterior pelvic tilt will lead to an increase in posterior pelvic tilt on the ipsilateral side, causing a similar effect of hip flexion on the ipsilateral hip. So if the hip flexor length is improved, a similar posterior tilt during the Active Knee Extension on the contralateral side would give less hip flexion on the ipsilateral side. If then the Active Knee Extension increases at the contralateral leg without a reactive hip flexion on the ipsilateral hip, this can partly be attributed to improved hip flexor lengthening and thus a finding of improved hip extension mobility at the ipsilateral leg. If less hip flexion occurs on the ipsilateral side despite this posterior pelvic tilt during the active knee extension test, this may also indicate improved hip extension mobility on the ipsilateral side and more stretch tolerance of the hip flexors, also on the ipsilateral side while the stretch capacity of the hamstrings is challenged on the contralateral side. However, given there was no intervention aimed at improving hamstring length, it is quite logical that no improvement was seen in this test.

No measurement of hip extension range of motion was performed. Although a reduction in the Reactive Hip Flexor Test was observed, this could have been caused by an increased length tolerance of the hip flexor, without an effective improvement in hip extension range of motion. Furthermore, no baseline measurements of pelvic tilt occurred, which may have impacted the findings. Although the Reactive Hip Flexor Test is reliable by its objective measurement of hip flexion reaction, it also has a more subjective component in the execution by having to hold one hip in 90° of flexion. Slight deviations from this visually induced 90° flexion can lead to changes in the reactive peak force generated by the hip flexor muscles.

Talk nerdy to me

By using an objective measurement of hip flexor tightness with the Reactive Hip Flexor Test, the study tackled an important problem often present in research. This test was shown to have excellent reliability. The examiners also used a fixation belt to increase the measurement of the dynamometer. By calibrating the device before every measurement, and normalizing force to femur length, the authors did try to standardize their test and use valid measurements in their primary analysis, which we can only applaud.

Since this was a crossover randomized controlled trial, every subject experienced the intervention and control situation. The two different stretches were executed by every participant, one on each side. As such, every participant served as his or her control. There is a possibility that a participant has a weaker and “tighter” hip on one side compared to the other and that a difference between the left and right leg contributed to the effects found. However, in the absence of prior injuries, we can assume that both sides would show comparable ‘deficits’ in this non-athlete population. The baseline characteristics did not reveal any differences between the control and intervention groups (which were thus the right and left legs of each participant individually).

Take-home messages

The current study found that the hip stretch with the posterior pelvic tilt was more effective than the comparison. An objective test confirmed a reduced reactive hip flexor force, meaning the hip was positioned in less flexion and thus more extension of the joint took place. These findings can be used to improve hip extension mobility in healthy physically active individuals. Since the improvement exceeded the MCID we can be confident that the stretching is effective. I’d recommend evaluating hip flexor force since eccentric training might be worthwhile in case not only a range of motion deficit but also reduced strength is present.

Reference

González-de-la-Flor, Á., Cotteret, C., García-Pérez-de-Sevilla, G. et al. Comparison of two different stretching strategies to improve hip extension mobility in healthy and active adults: a crossover clinical trial. BMC Musculoskelet Disord 25, 853 (2024).