The relationship between hip extensor strength and contralateral and ipsilateral hip flexor muscle length in healthy men and women – United States Sports Academy Sports Journal

Authors: Ashley Calvillo1, Guillermo Escalante2, and Morey J. Kolber3
1Los Angeles Sunset Department of Physical Therapy, Kaiser Permanente, Los Angeles, CA, USA
2Department of Kinesiology, California State University- San Bernardino, San Bernardino, CA, USA
3Department of Physical Therapy, Nova Southeastern University, Fort Lauderdale, FL, USA
Corresponding Author:
Guillermo Escalante, DSc, MBA, ATC, CSCS*D, CISSN
California State University- San Bernardino
Department of Kinesiology
5500 University Parkway
San Bernardino, CA 92407
(909) 537-7236
(909) 537-7085 fax
[email protected]
Ashley Calvillo, PT, DPT, OCS is a physical therapist at Kaiser Permanente- Sunset in Los Angeles where she focuses on treatment of orthopedic injuries. Her research interests are in the areas of orthopedic physical therapy.
Guillermo Escalante, DSc, MBA, ATC, CSCS*D, CISSN is a Dean Fellow for the College of Natural Sciences and an Associate Professor of Kinesiology at California State University- San Bernardino in San Bernardino, CA. His research interests focus on body composition, improving muscle strength/hypertrophy/sports performance, sports injury prevention/rehabilitation, and sports nutrition.  
Morey J. Kolber, PT, PhD, OCS, CSCS*D is a professor of physical therapy at Nova Southeastern University in Fort Lauderdale, FL. His research interests are in orthopedics, diagnostic imaging, and regenerative medicine.
ABSTRACT
This study investigated the relationship between hip extensor (HE) strength to contralateral and ipsilateral hip flexor muscle length. Bilateral hip extension range of motion (ROM) was evaluated using the modified Thomas test using a hand-held goniometer in seventeen males (26 ± 7 yrs, 174.9 ± 6.72 cm, 79.4 ± 7.9 kg) and twenty-seven females (24 ± 2 yrs, 162.7 ± 6.40 cm, 67.2 ± 13.1 kg).  Participants were classified as: a) restricted hip flexors (hip extension ROM > 6° from horizontal), b) neither restricted nor normal hip flexors (hip extension ROM between 0° to 6° from horizontal), and c) normal hip flexors (hip extension ROM < 0° from horizontal). Peak isometric HE force was obtained via a Biodex dynamometer where maximum voluntary contraction (MVC) was determined. Correlations were used to determine the relationship between flexor length to contralateral and ipsilateral HE relative strength. A One-way ANOVA was used to examine HE relative strength in relation to hip flexor length classified as restricted vs neither vs normal. There were no correlations between right hip flexor length and contralateral HE strength (r = -0.228, p = 0.137), right hip flexor length and ipsilateral HE strength (r = -0.241, p = 0.115), left hip flexor length and contralateral HE strength (r = -0.193, p = 0.210), and left hip flexor length and ipsilateral HE strength (r = -0.111, p = 0.472). The One-way ANOVA revealed no significant differences between the groups for the most restricted hip flexor and contralateral HE relative strength (p = 0.179) nor for the most restricted hip flexor and ipsilateral HE relative strength (p = 0.670). No significant relationships were found between HE strength and contralateral or ipsilateral hip flexor length. Although it is commonly suggested that practitioners address hip flexor length to assist with improving gluteal muscle strength, the results of this study do not validate this clinical practice. Despite the results indicating no correlations, practitioners are encouraged to address these impairments from both a functional and performance based perspective.
Key Words: gluteus maximus, hip range of motion, functional limitations, performance limitations
INTRODUCTION
Gluteal muscle function has significant implications for lower extremity pathology due to its relevance in walking, running, and controlling lower extremity movements. The gluteus maximus is the largest muscle of the hip accounting for 16% of the total cross-sectional area (Winter, 2009) and weakness in the muscle has been associated with altered lower extremity kinematics that may contribute to injuries (Nadler et al., 2000; Powers, 2010; Steinberg et al., 2017; Tate et al., 2017). Although the gluteus maximus controls flexion of the trunk during upright posture and restrains femoral internal rotation, its major role is to extend the stance side hip and control flexion of the trunk during running (Ford et al., 2013; Teng & Powers, 2016). Decreased gluteus maximus strength is theorized to lead to increased reliance on the secondary hip extensor muscles, such as the hamstrings and hip adductors to produce hip extension torque (Macadam & Feser, 2019; Wagner et al., 2010). Dependency on secondary hip extensors may lend to greater tissue stress in the hamstring and hip adductor musculature, thus resulting in a higher risk of soft tissue injury (Franklyn-Miller et al., 2014; Kisner et al., 2017; Renström & Johnson, 1985) as well as decreased performance.
Restricted hip flexor muscle length is theorized to contribute to an acquired weakness of the hip extensor muscles, which may contribute to lower extremity injury (Mills et al., 2015; Moore & Hutton, 1980; Opar et al., 2012). Specifically, reciprocal inhibition of the gluteus maximus, secondary to dominant activity of the hip flexor muscles has been implicated as a contributing factor to acquired weakness of the hip extensor muscles (Sahrmann, 2013). Although it has been suggested that practitioners should address hip flexor length to potentially assist with improving gluteal muscle strength (Mills et al., 2015; Reiman et al., 2012), there is a paucity of research that validates the clinical theory of restricted hip flexor muscle length as an underlying factor contributing to hip extensor weakness. Mills et al. (Mills et al., 2015) investigated the relationship of hip extensor muscle activation, internal hip and knee extension moments during double‐leg squatting, and isometric hip extension/knee flexion strength in those with and without clinically restricted hip flexor muscle length. Although the authors reported that individuals with hip flexor tightness demonstrated less gluteus maximus activation (p = 0.008) with surface electromyography, participants with hip flexor muscle tightness produced similar net hip and knee extension moments to subjects without tightness.  Furthermore, in the aforementioned investigation, data was collected solely on the dominant limb with no comparison to the contralateral side.
The upper gluteus maximus reaches peak activity during mid‐stance to stabilize the pelvis and return it to a neutral position (Kokmeyer et al., 2014; Krebs et al., 1998; Perry J, 2010). Gluteal activity then diminishes prior to terminal stance as the posterior fibers of the tensor fascia latae remain active (Kokmeyer et al., 2014; Krebs et al., 1998; Perry J, 2010). Hip flexion contractures identified by muscle length testing (e.g. Thomas test) have been found to result in decreased peak hip extension range of motion (ROM) in terminal stance and contralateral step length (Kokmeyer et al., 2014; Lee et al., 1997). Given the gluteus maximus reaches peak activity during mid-stance, a decreased stride length as a result of restricted hip flexor length on the contralateral limb could theoretically lead to diminished gluteus maximus activation.
Thus, the purpose of this investigation was to compare hip extensor muscle strength from dynamometry to contralateral and ipsilateral hip flexor muscle length measured with a goniometer during the modified Thomas test. The investigators hypothesized that individuals with restricted hip flexor muscle length would exhibit decreased contralateral and ipsilateral hip extension strength.
 METHODS
Experimental Approach to the Problem
This study was conducted using a casual cross-sectional study design. The investigators assessed the participant’s hip extension ROM using the modified Thomas test position with a goniometer (Harvey, 1998). Isometric gluteus maximus strength was assessed using the Biodex System 4 (Biodex Medical Systems, Inc., Shirley, NY, USA) (Teng & Powers, 2016) .
Subjects
Subjects included 27 females (24 +/- 2 yrs., 162 +/- 6.4 cm, 67.2 +/- 13.1 kg) and 17 males (26 +/- 7 yrs., 174 +/- 6.7 cm, 79.4 +/- 7.9 kg).  Inclusion criteria were healthy men and women between 18-60 years of age with normal and clinically restricted hip flexor muscle length defined as hip extension ROM > 0° from horizontal. Participants who had a lower extremity injury to either limb, had any neurological deficits, and/or metabolic, cardiovascular, or systemic diseases were excluded from the study. Furthermore, participants who regularly used an assistive device for walking were excluded from the study. All subjects signed an informed consent, approved by the university’s institutional review board, and completed a physical activity readiness questionnaire prior to participation. After completing the documents, the participant’s height was measured with a stadiometer (Seca, Hamburg, Germany) and weight (mass) was measured with a standard scale (Detecto, Webb City, MO). Each participant was then familiarized with the testing procedures discussed in the next section prior to performing a self-selected intensity warm-up on a cycle ergometer for 5 minutes.
Procedures
Hip Flexor Length. Bilateral hip flexor muscle length (i.e., hip extension ROM) was evaluated using the Modified Thomas test (Harvey, 1998). The participant sat on the end of the plinth, rolled back on to the plinth, and held both knees to the chest. This ensured that the lumbar spine was flat on the plinth and the pelvis was in posterior rotation. The participant held the contralateral hip in maximal flexion with the arms, while the tested limb was lowered towards the floor. Hip extension ROM was measured using a hand-held goniometer aligned parallel to a line connecting the anterior superior iliac spine and the superior pole of the patella. Participants were classified into one of three groups based on their hip flexor muscle length: a) restricted hip flexor muscle length defined by hip extension ROM > 6° from horizontal, b) neither restricted nor normal (neutral) hip flexor muscle length defined as hip extension ROM between 0° to 6° from horizontal, and c) normal hip flexor muscle length defined as hip extension ROM < 0° from horizontal. The Intraclass Correlation Coefficient (ICC) for this methodology, which was determined by repeated measures of hip flexor length of the participants, was 0.86 with a Standard Error of Measurement (SEM) of 2.4 degrees by the investigator performing this assessment, who is a licensed physical therapist.
Hip Extension Strength. Peak isometric force was obtained via a Biodex System 4 dynamometer. The gluteus maximus maximum voluntary contraction (MVIC) was determined with the participant prone on a plinth. Participants were positioned with the knee extended and 30° of hip flexion by flexing their trunk forward onto the plinth. A nylon strap was secured over the posterior thigh, 5 cm proximal to the popliteal crease, to resist hip extension. Participants were instructed to push as hard as possible into the strap for 5 seconds for each of three trials. Participants were allowed to hold the plinth for stabilization and verbal encouragement was given throughout testing. Peak torque values (Nm) were identified and normalized to body mass (Nm/kg); the average of the three trials was used for statistical analysis.
Data Analysis
This study was conducted using a casual cross-sectional study design. The investigation compared peak hip extensor isometric strength to contralateral and ipsilateral hip flexor muscle length. A Pearson correlation coefficient was used to determine the relationships between hip flexor muscle length and contralateral hip extensor strength as well as hip flexor muscle length and ipsilateral hip extensor strength. Furthermore, two separate one way analysis of variances (ANOVAs) were analyzed to determine if there was a difference between the participant’s right and left hip extensor strength and their hip flexor muscle length; the participant’s hip with most restricted hip flexor muscle length (e.g., least hip extension ROM) was classified into the groups of restricted, neither restricted or normal, and normal hip flexor ROM as defined in the methodology section. Finally, dependent t-tests were run to determine if there was a difference (p ≤ 0.05) in contralateral and ipsilateral hip extensor relative strength and the participant’s most restricted hip flexor.  Restricted hip flexor ROM was defined as hip extension ROM > 6° from horizontal in the modified Thomas test and normal hip flexor ROM was defined as hip extension ROM < 0° from horizontal in the modified Thomas test. Individuals whose most restricted hip flexor length was between 0-6⁰ of hip extension were not used for the analysis for the dependent t-tests.
RESULTS
There were no significant relationships between hip extensor strength and contralateral or ipsilateral hip flexor ROM. The results of the correlations are displayed in Table 1 which indicate inverse, albeit weak correlations.
Table 1: Correlations between Contralateral and Ipsilateral Hip Flexor Length and Hip Extension Strength (n = 44)
HE, Hip Extensor; ROM, Range of motion
The one way ANOVAs revealed that there were no significant differences between the participant’s hip with the most limited hip flexor length classified as either restricted, neither restricted or normal (neutral), or normal (as defined in the methodology section) and their right hip extensor strength (F2,41 = 1.282, p = 0.288) and left hip extensor strength (F2,41 = 1.203, p = 0.311). Table 2 below shows the relative strength values (Nm/kg) of the right and left hip extensors for each of the 3 groups.
Table 2: Right and Left Relative HE Strength and Hip Flexor Length Classification (n = 44)
HE, Hip Extensor; Nm, Newton-meter; kg, kilogram; SD, standard deviation
Similar to the ANOVAs, the dependent t-tests also revealed that there was no difference between the contralateral (p = 0.181) and ipsilateral (p = 0.418) hip extensor strength and the participant’s most restricted hip flexor. In this analysis, the only subjects included were those that were defined as having either normal hip flexor mobility or restricted hip flexor mobility as defined in the methodology section; hence, only 22 participants were analyzed for this portion of the study.  Table 3 below shows the relative strength values of the subjects’ most restricted hip flexors and their contralateral and ipsilateral hip extensor strength (Nm/kg) for the 2 groups.
Table 3: Contralateral and Ipsilateral Relative HE Strength and Hip Flexor Length Classification (n = 22)
HE, Hip Extensor; Nm, Newton-meter; kg, kilogram; SD, standard deviation
CONCLUSIONS
The purpose of this study was to investigate the relationship between hip extensor muscle strength to contralateral and ipsilateral hip flexor muscle length. We hypothesized that individuals with restricted hip flexor muscle length would exhibit decreased contralateral and ipsilateral hip extension strength. The findings of this study revealed that a statistical significant correlation does not exist between hip extensor strength to contralateral or ipsilateral hip flexor length, thus rejecting our research hypothesis.
Restricted hip flexor muscle length has been proposed as a contributing factor to gluteus maximus weakness via reflexive inhibition (Sahrmann, 2013). Despite common acceptance of this clinical theory, little research has been conducted to substantiate this relationship. This is the first study to compare gluteal muscle strength to both contralateral and ipsilateral hip flexor muscle length. The findings of this investigation are consistent with Mills et al. (Mills et al., 2015) where hip extension strength did not differ between individuals with restricted hip flexor muscle length to those with normal muscle length. Although Mills et al. (Mills et al., 2015) did report greater gluteus maximus activation in the restricted hip flexor muscle length group as compared to those with normal hip flexor length, the increase in activation did not result in a difference in overall hip extensor strength between the groups. Taken together, these findings suggest that restricted hip flexor muscle length does not directly impact hip extensor muscle strength.
The findings of this study should be considered in the light of other study limitations. First, electromyography assessment of muscle activity was not utilized during MVIC testing of the hip extensors. Considering that the hamstrings work in conjunction with the gluteus maximus to decelerate the thigh in terminal swing, hamstring overuse is a potential neuromuscular compensation associated with gluteus maximus weakness (Wagner et al., 2010). Future research should include EMG assessment to quantify muscle activation during MVIC testing.  A second limitation of this study is that hip extensor strength was measured via MVIC; given that the hip extensor musculature works in both a concentric and eccentric manner for coordinated movement, it is unclear if the MVIC measurement accurately represents muscle performance with functional movement. Moreover, this investigation performed the MVIC hip extension strength test with the knee in full extension as opposed to having the knee placed in 90 degrees of knee flexion. Although this may increase bias to the hamstrings (as opposed to the gluteus maximus), it should be noted that Mills et al. (Mills et al., 2015) did place their participant’s knee in 90 degrees of flexion during their MVIC hip extension test and their results were similar to that reported in this investigation. Hence, it appears that placing the hamstrings in a position of active insufficiency during an isometric MVIC hip extension test may not alter the findings. Indeed, Sakamoto et al. (Sakamoto et al., 2009) reported that gluteus maximus activation was greater when participants extended the hip (lying prone) with the knee flexed to 90 degrees as compared to the knee fully extended; however, a systematic review of gluteus maximus activation during hip extension by Macadam & Feser (Macadam & Feser, 2019) classified the level of EMG activation for prone hip extension with the knee extended and flexed at 90 degrees to be in the same (<20% gluteus maximus MVIC activation) category as compared to other hip extension movements. Lastly, it should be noted that this study was performed on a sample of primarily college students; hence, the results of this investigation may not be generalizable to specific patient populations.
Although this investigation was not able to detect a significant relationship between hip flexor tightness and hip extension weakness, there remains a value in identifying these impairments. Muscle length of the hip flexors is necessary to achieve terminal stance during running or sprinting activities and hip extensor strength is needed for running, squatting, jumping, as well as lower extremity control.
APPLICATIONS TO SPORTS
Despite the results indicating no correlations, practitioners are encouraged to address these impairments from both a functional and performance based perspective. This study can only be generalized to the population that was investigated. Similar research of a different cohort may have identified different findings; thus, one cannot rule out with certainty the biophysiological concept of reciprocal inhibition of the hip extensors occurring from restricted hip flexors.
ACKNOWLEDGEMENTS
The authors would like to thank all of the participants for volunteering their time for this study and with the California State University kinesiology undergraduate student research volunteers that assisted with the data entry and scheduling of participants. This research was not funded.
REFERENCES

Abstract:
Women’s opportunities for competitive physical activity were limited in America until Federal Legislation, commonly referred to as Title IX, became law. It required American society to recognize a woman’s right to participate in sports on a plane equal to that of men. Prior to 1870, activities for women were recreational rather than sport-specific in nature. They were noncompetitive, informal, rule-less; they emphasized physical activity rather than competition. In the late 1800’s and early 1900’s, women began to form clubs that were athletic in nature. Efforts to limit women’s sport activity continued as they became more involved in competitive sports. This paper will present a history of women’s involvement in sport prior to the federal legislation enacted to eliminate sexual discrimination in education and sport.

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