Effects of instability direction on early neuromuscular responses of trunk and hip muscles during rapid squatting.

INTRODUCTION: Evidence suggests that unstable support-surface conditions alter trunk muscle activity during exercise. However, previous studies predominantly focused on the overall magnitude of muscle activity. Consequently, the effect of instability direction on the temporal characteristics of early neuromuscular responses during dynamic tasks remains poorly understood. This study investigated the effects of anteroposterior (AP) and mediolateral (ML) instabilities on early neuromuscular responses in the trunk and hip muscles during rapid squatting. METHODS: Twenty healthy adult men performed rapid squats in a within-subject repeated-measures design under AP, ML, and stable floor (ST) conditions. The AP and ML conditions were created by orienting a uniaxial rocker board to permit rocking in the respective directions. The auditory cue was presented at a random time between 4.0 and 6.0 s after trial initiation. Surface electromyography (EMG) was used to record muscle activity in the internal oblique/transversus abdominis (IO/TrA), external oblique (EO), rectus abdominis (RA), erector spinae (ES), gluteus medius (GMed), and gluteus maximus (GMax). Primary outcomes were EMG onset latency and early muscle activity (%MVC) during the first 0-75 ms after EMG onset, representing the initial phase of neuromuscular activation with relatively limited influence of sensory feedback. Data were analyzed using two-way repeated-measures analysis of variance (ANOVA) (condition × muscle). RESULTS: For EMG onset latency, significant main effects of condition and muscle were observed; however, no significant interaction was observed. Post hoc comparisons indicated that the onset latency was significantly shorter in the AP condition than in the ST condition. Early muscle activity showed significant main effects of condition and muscle and a significant condition × muscle interaction. RA activity was significantly lower in the AP and ML conditions than in the ST condition. No significant differences were found in the other muscles. DISCUSSION: These findings provide limited evidence for direction-specific responses because no clear AP-ML differences were observed. Muscle-specific differences in early activity were mainly limited to lower RA activity under both unstable conditions relative to the ST condition.