Research in Review: Does a Shortened Hip Flexor Limit the Hip Extensor and Lower Extremity Biomechanics?

Kyle Stull
Kyle Stull
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Using the overhead squat assessment as one of the series of assessments, this study examined the affect of hip flexor length on lower extremity strength, activation and biomechanics.

Journal Article:

Mills, M., Frank, B., Goto, S., Blackburn, T., Cates, S., …, Padua, D. (2015). Effect of restricted hip flexor muscle length on hip extensor muscle activity and lower extremity biomechanics in college-aged female soccer players. International Journal of Sports Physical Therapy, 10(7), 946-955.

Purpose of the Study:

The purpose of this study was to compare lower extremity strength, muscle activation, and biomechanics between individuals with and without limited hip flexor muscle length. The authors hypothesized that individuals with restricted hip flexor length would exhibit less hip extension strength, greater internal knee extension moment, and lesser internal hip extension moment compared to individuals with normal hip flexor length. The authors additionally hypothesized that individuals with restricted hip flexor length would also display less gluteus maximus activation and elevated biceps femoris activation compared to those with normal hip flexor length.

Study Participants:

Forty female, college-aged soccer players were recruited for the study. 20 displayed normal hip flexor length, and 20 displayed shortened hip flexor length. All participants were involved in competitive collegiate soccer for one hour or more at least twice per week, had no history of lower extremity, spine, abdominal, vestibular, or mild traumatic brain injury in the last 3 months.

Procedure or Methods:

A Certified Athletic Trainer measured hip extension ROM of the dominant leg using the modified Thomas Test and a digital inclinometer. Those that demonstrated more than 15° hip extension were included in the normal category, and those that displayed 0° of extension were included in the restricted group.

Surface EMG was used to measure the maximum velocity isometric contraction (MVIC) of the gluteus maximus and biceps femoris of the dominant limb.

Motion analysis was used to collect hip and knee kinematic data.

Muscle activation, kinematic, and kinetic data were collected during the descent phase of the double-leg squat to mimic hip and knee flexion deceleration neuromuscular control during physical activities. The squat was performed with participants barefoot, feet shoulder width apart, toes pointed straight ahead, and arms extended overhead. Participants were instructed to achieve at least 60° of knee flexion during the squat.


There was a significant difference in hip extension ROM between the restricted and normal groups. The shortened hip flexor group demonstrated significantly less gluteus maximus activation compared to the normal group during the squat. The shortened hip flexor group also demonstrated more than 2.5 times more biceps femoris activation relative to gluteus maximus activation compared to the normal group. There were no differences between hip extension strength or internal hip and knee extension moments between groups.


This study is the first to compare glute strength and hip extensor muscle activation patterns in individuals with restricted versus normal hip flexor muscle length during a squatting task. The results suggest that gluteus maximus activation was significantly less in the restricted group compared to the normal group. However, activation of the biceps femoris was significantly higher in the restricted group compared to the normal group. These findings support the author’s hypothesis that gluteus maximus activation is affected by hip flexor muscle length and that those with shortened hip flexors may increase activation of the hamstrings.

Take away for NASM-CPTs:

This study is important for the NASM-CPT to consider for two primary reasons. First, the study supports the use of the overhead squat assessment as a functional tool to identify movement compensations. While the authors of this study did not investigate findings such as an anterior pelvic tilt, they did identify the squat as being related to hip extension range of motion testing.

Second, the study confirms that shortened hip flexors inhibit optimal contraction of the gluteus maximus. However, to maintain force production during hip extension, the biceps femoris increased activation relative to the gluteus maximus.

Therefore, the CPT can use these findings to support the use of foam rolling to inhibit the hip flexor and biceps femoris muscles followed by static stretching of the muscles and then followed by gluteal activation and strength training to restore optimal movement.

The Author

Kyle Stull

Kyle Stull

Kyle Stull, DHSc, MS, LMT, NASM-CPT, CES, PES, NASM Master Instructor, is a faculty instructor for NASM. Kyle is also an Adjunct Professor for Concordia University Chicago.


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