Hip and Trunk Muscle Timing During Selected Movement Tasks: An Electromyographic Analysis

Purpose: The aim of this study was to study was to (1) identify the muscle firing pattern during the prone hip extension (PHE) and supine hip abduction (SHA) movement tasks and (2) examine the difference in intermuscular timing between the first muscle to fire and all remaining muscles, for each movement task. Methods: The intermuscular timing of muscle activation, based on an activation level of greater than 3 standard deviations beyond base activity, for specific trunk and lower extremity muscles were examined by surface Electromyography (EMG) in twenty-three healthy, active, pain free subjects (mean age ± SD, 27.2 ± 4.5 years). Data were collected from the dominant lower extremity of all subjects (12 males, 11 females). Computerized voice activation commands were used to record and synchronize the movement tasks and standardize the protocol. Mean timing and activation levels were identified for each muscle. A 1-sample t test was used to determine which muscles were activated later than the first muscle to fire. Results: Differences in intermuscular timing were identified for both movement tasks. PHE required trunk muscle activation prior to lower extremity muscle activation. The contralateral erector spinae was activated significantly earlier than the gluteus maximus (P=.004). A different pattern was identified for SHA. Trunk muscles, on the side of the moving limb, were activated before the muscles responsible for moving the limb. In addition, all of the trunk muscles were activated significantly later than the hip musculature (P≤.019). The hip muscles were the first to fire with the tensor fascia latae activated earliest. Conclusions: These movement tasks required different trunk and hip activation patterns. During PHE, a less stable pelvic position may have contributed to the earlier trunk muscle activation and a firing pattern with the gluteus maximus last in the sequence. The reverse occurred during the SHA movement task. A better understanding of muscle timing and activation patterns can assist clinicians in assessing patients and developing more effective rehabilitation programs.