Age and sex related differences in scapular muscle excitation during loaded shoulder flexion

Context: The amplitude, time of peak excitation, and spatial distribution of excitation of the scapular muscles have been hypothesized to contribute to the increased risk of shoulder pain in older females. Objectives: To determine whether there are differences in the amplitude, % task time to peak, or spatial distribution of excitation in the four scapular muscles [upper (UT), middle (MT), lower (LT) trapezius and serratus anterior (SA)] during the raising phases of loaded shoulder flexion in the sagittal plane in four groups [older female (OF), older male (OM), younger female (YF), younger male (YM)]. Methods: 52 individuals were recruited to participate in the study. Participants completed five repetitions of loaded shoulder flexion holding a load equivalent to 30% maximum force while EMG of the SA, UT, MT, and LT. A two-way (age*sex) analysis of variance (ANOVA) was conducted for each muscle to determine the effect of age and sex on the dependent variables (average amplitude and percentage of task time to peak excitation). Statistical parametric mapping (SPM) t-tests were used to determine differences in the spatial distribution of the UT, MT, and LT between the ages and sexes. Results: No significant differences were found in the average amplitude of excitation between the groups for the UT, MT, LT, or SA. The % task time to peak was significantly later for the SA in the OM (p<0.01, d=1.44) and OF (p<0.01, d=1.33) groups compared to the YF group. Significant differences in the spatial distribution of the UT, MT, and LT were found between older and younger adults at 70-75% task time (p<0.05) and in the LT at 95-100% task time (p<0.05). Significant differences in the spatial distribution of the LT were found between the sexes at 70-75% (p<0.05) and 95-100% (p<0.05) task time. Conclusions: Differences in the % task time to peak excitation of the SA exist between OMs and OFs compared to YFs. Significant differences in the spatial distribution of the trapezius were found between the ages and ...