EMG Amplitude, Onset Timing, and Functional Deceleration Ratio in Baseball Pitchers with Subacromial Pain Syndrome: A Secondary Analysis of a Randomized Controlled Trial

OBJECTIVES Subacromial pain syndrome (SAPS) in overhead athletes is characterized by altered neuromuscular control including impaired muscle activation magnitude, delayed onset timing, and reduced functional deceleration capacity. However, the relationship between activation magnitude, timing, and functional performance remains unclear. This study aimed to determine whether changes in electromyography (EMG) amplitude and onset timing are associated with changes in functional deceleration ratio (FDR).METHODS Forty male collegiate baseball pitchers with SAPS were included in a secondary analysis of a randomized controlled trial. The EMG amplitude and onset timing of eight trunk and shoulder muscles, along with isokinetic torque at 180°/s, were assessed before and after the intervention. Changes in EMG amplitude, onset timing, and FDR were calculated, and correlation and regression analyses were performed.RESULTS A weak positive correlation between changes in EMG amplitude and onset timing during eccentric ER was observed in the unadjusted analysis but was not statistically significant after correction for multiple comparisons. Positive associations with FDR were observed for the core and scapular stabilizers, whereas negative associations were observed for the prime movers. Primary participant-level regression analyses showed associations of intervention group and EMG amplitude with FDR, although some associations were attenuated in baseline-adjusted sensitivity analyses.CONCLUSIONS These findings indicate that changes in EMG activation magnitude and onset timing were associated with functional deceleration capacity, although the strength and independence of these relationships varied according to contraction condition and muscle group.