The Deep Hip Muscles are Unlikely to Stabilize the Hip in the Sagittal Plane During Walking: A Joint Stiffness Approach

Objective: This study determined the contribution of the deep hip muscles to hip stability. Methods: Hip stability was defined as rotational hip stiffness in the sagittal plane, which was calculated for walking trials for 12 participants via an electromyography (EMG)–informed neuromusculoskeletal model which included all 22 hip spanning muscles. Three model configurations which differed in deep hip muscle excitations but had identical excitations for all other muscles were compared: (1) deep hip muscles informed by intramuscular EMG measurements (assisted activation); (2) deep hip muscles with simulated zero activation (no activation); (3) deep hip muscles with simulated maximal activation (maximal activation). Sagittal plane rotational hip stiffness across the gait cycle was compared between the three model configurations using a within-participant analysis of variance via statistical parametric mapping (p < 0.05). Results: Compared to the assisted activation configuration, hip stiffness (mean (95% confidence interval)) was 0.8% (0.7 to 0.9) lower in the no activation configuration, and 3.2% (3.0 to 3.4) higher in the maximal activation configuration. Conclusion: Regardless of activation level, deep hip muscles contributed little to sagittal plane rotational hip stiffness, which casts doubt on their assumed function as hip stabilizers. Significance: The merit of targeted deep hip muscle strengthening to improve hip stability in rehabilitation programs remains unclear.