Poster 217: Changes in Dynamic Hip Muscle Moment Arms During Activities of Daily Living
Authors: Ruh Ethan , Frankston Naomi , Charles Shaquille , Mauro Craig , McClincy Michael , Anderst William , Johnson Camille 
Published: August 2023

Objectives: Hip muscle function is affected by surgical intervention and is of particular concern when considering post-operative rehabilitation of the joint1. However, little is known about the dynamic, in vivo function of the hip muscles during activities of daily living. Marker-based motion capture studies have associated dysplastic hip anatomy with shorter abductor moment arm lengths during walking2, but these measurements are complicated by soft tissue artifact3. Biplane radiography is a highly accurate technique for measuring in vivo bone motion of the hip4 and has recently been utilized to estimate dynamic muscle moment arms, which are important factors influencing force and torque capabilities. Previous research studies utilizing biplane radiography have identified shorter muscle lengths and moment arms in the implanted hip compared to the contralateral hip of individuals with total hip arthroplasty during the double support phase of gait2, but normative in vivo moment arms of the hip musculature in asymptomatic individuals have yet to be established. Further, it remains unknown how moment arms change between activities of daily living, particularly during deep flexion activities that challenge hip stability. Developing a normative dataset of dynamic in vivo moment arm lengths would provide a valuable reference for clinicians and physical therapists developing rehabilitation protocols to strengthen the hip musculature and increase stability. The objective of this study was to determine in vivo moment arms of the hip abductors, adductors, and external rotators during gait and squatting in a cohort of asymptomatic adults. We hypothesized that the maximum moment arm length during squatting would be longer than during walking and static standing. Methods: Young adults with no history of hip surgery, chronic hip pathology, or severe lower extremity musculoskeletal injury were recruited to participate in this IRB-approved study. Participants performed bodyweight squats and treadmill walking within a biplane radiography system while ground reaction forces were collected at 1000 Hz from a dual-belt instrumented treadmill. The support phase of gait was defined by vertical ground reaction forces over 50 N. Synchronized biplane radiographs were collected at 50 images per second to image three trials of each hip for each activity. Subject-specific bone models of the pelvis and proximal femur were created from computed tomography (CT) scans (average 0.37x0.3 mm in-plane resolution, 0.625mm slice thickness). Coordinate systems5 and muscle origin and insertion points for the gluteus medius (hip abductor), gluteus minimus (hip abductor), pectineus (hip adductor), and grouped external rotators1 were identified based upon bony landmarks on the 3D bone models (Figure 1). In-vivo bone motion during each activity was determined by matching digitally reconstructed radiographs, created from the CT-based bone models, to the biplane radiographs using a registration process with a validated accuracy4 of 0.3mm. The line of action for each muscle segment was determined for each frame of the biplane radiographs as the vector from the muscle origin to the insertion. Moment arm length (MAL) was calculated for each frame of the dynamic biplane radiographs as the perpendicular distance between the hip joint center (acetabulum center) and the line of action of each muscle1 and interpolated to gait cycle for walking or percent maximum hip flexion during squatting. Differences between the maximum MAL during static standing, walking, and squatting were assessed within each muscle group using ANOVA with post-hoc pairwise comparisons performed as necessary. Results: Data from 186 trials of 43 hips from 22 individuals were included in the analysis (9M, 13F; mean age 22.0±2.2years; BMI 21.3±5.2kg/m2). Average walking velocity was 1.0±0.2m/s and the average maximum hip flexion during squatting was 101.6±16.1°. Average static standing MAL was greatest in the gluteus medius and smallest in the pectineus (Table 1). Maximum MAL was significantly different between all activities for all muscle groups (all p<0.001) (Table 1, Figure 2). Contrary to our hypothesis, moment arms of the abductors were smaller during squatting compared to other motions. Conclusions: This study demonstrates the in vivo torque generating capabilities of muscles surrounding the hip (reflected by their moment arm) change considerably from the standing position to positions encountered during active functional motions. Muscles involved in hip abduction and stabilization have less mechanical advantage at greater hip flexion, highlighting the importance of strengthening these muscle groups during rehabilitation. Specifically, moment arms of the hip abductors and external rotators are shorter during walking and squatting compared to standing, while moment arms of the adductors are shorter during walking and greater during squatting and standing. Knowledge of these postural changes in mechanical advantage can be useful when designing targeted rehabilitation protocols to strengthen the hip musculature and improve stability. Further research into relationships between dynamic in vivo hip muscle moment arms and lengths will yield additional insight into force and torque generating capacity of muscles during activities of daily living. These results are limited to treadmill walking and bodyweight squatting in asymptomatic young adults. References: Hu, et al., J Ortho Surg and Res. 2020. 2) Song, et al., J Biomech. 2020. 3) Fiorentino, et al., Gait Posture. 2017. 4) Martin, et al., J Arthroplasty. 2011. 5) Wu, et al., J Biomech. 2002. [Table: see text]