BACKGROUND: Out-of-sequence pelvis and trunk rotation are associated with increased forces at the shoulder during pitching. Assessing electromyographic (EMG) amplitudes of the involved trunk and shoulder musculature across sequence patterns throughout the pitch may reveal different levels of muscle activity. The purpose of this investigation was to examine differences in throwing-side external oblique and infraspinatus activity between chronological (CS) and discordant (DS) sequencing patterns. METHODS: Surface EMG data were recorded at a sampling interval of 1/1500 on the throwing-side external oblique and infraspinatus of thirty-three youth baseball pitchers (11.9 ± 1.7y [range 10-15y], 1.61 ± 0.14m, 49.4 ± 14.0kg). Kinematic data were collected using an electromagnetic tracking system (150 Hz). Maximal voluntary contractions (MVC) of each muscle were performed for three seconds. Participants pitched five fastballs at regulation distance. Electromyographic data were processed with a root mean square calculation and normalized using the average from two peak MVCs. Sequencing patterns were determined by the order in which the pelvis and trunk segments reached their peak angular velocity. In a DS pattern, the trunk angular velocity peaked before the pelvis. Of the 33 pitchers, 16 pitchers displayed both CS and DS patterns. Muscle activation for each pitcher were time-normalized from peak knee height to follow through (0 to 100 percent) and each sequence was compared with paired T-tests using 1-Dimensional statistical parametric mapping. RESULTS: Pitch speeds were not different (t =-.11, p = .91) between CS (24.1±4.1m/s) and DS (24.1±4.2m/s) sequences. Across the pitching movement, mean muscle activity between CS and DS differed the most at 76% of the pitch for external oblique (15 vs 11 %MVC) and 97% of the pitch for infraspinatus (56 vs 48 %MVC), respectively. Yet, no significant differences were observed for external oblique (t<3.8,p>.05 0-100%) and infraspinatus (t<3.9,p>.05 0-100%) between sequences across 100 percent of the pitch. CONCLUSIONS: Approximately half of the pitchers in this sample displayed both sequencing patterns highlighting intra-pitcher variability in pelvis and trunk kinematics. While DS can increase shoulder kinetics, additional work assessing muscle activation patterns and pitching sequences is needed to further understand the control and execution of the pitching movement.

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