Instrumentation

Muscle-activity data were obtained using a surface electromyography (EMG) system (Bagnoli; Delsys Inc, Natick, MA) with a common mode rejection ratio greater than 100 dB and an input impedance greater than 10¹⁵ Ω at a sampling rate of 1000 Hz. The surface EMG electrodes were single differential sensors (model DE-2.1; Delsys Inc) with 2 parallel bars 1 cm apart that were each 1-cm long and 1-mm wide. The skin was prepared by scrubbing the area using a cotton ball soaked with rubbing alcohol. Electrodes were placed over the muscle bellies and in line with the muscle-fiber orientation of the gluteus maximus, posterior portion of the gluteus medius, and TFL bilaterally according to guidelines for surface electrode placement.²⁷ A disposable ground electrode was placed on the posterior elbow over the olecranon process. After electrode placement, muscle activity was visually inspected during volitional gluteal contractions, standing hip internal and external rotation, and hip abduction and during maximal voluntary isometric contraction (MVIC) trials to ensure proper electrode placement.

A 10-camera motion-capture system (Vicon Motion Systems Ltd, Centennial, CO) was used to collect 3-dimensional trunk and lower extremity kinematic data at a sampling rate of 100 Hz. The motion data were synchronized with the EMG data in Vicon Nexus (version 1.8.5; Vicon Motion Systems Ltd). Retroreflective markers were placed bilaterally and secured with tape on the trunk, pelvis, and lower extremities as previously described.²⁸ Briefly, markers were secured bilaterally with tape over the first and fifth metatarsal heads, calcanei, medial and lateral malleoli, medial and lateral femoral epicondyles, greater trochanter, anterior-superior iliac spines, iliac crests, and acromioclavicular joint. Markers were also placed over the sacrum, spinous process of C7, and sternum. Rigid clusters of 4 markers were placed laterally on each shank and thigh and secured with hook-and-loop fasteners.