Gender differences between muscle activation and onset timing of the four subdivisions of trapezius during humerothoracic elevation

The aim of this study was to provide a description of gender differences of the normal muscle activation patterns of the four subdivisions of the trapezius (clavicular, descending, transverse, ascending) during arm elevation. Surface EMG was collected from these four subdivisions from twenty-two healthy subjects (12 females, 10 males) during arm elevation in the scapular plane. Percent activation (RMS) of each subdivision was compared between genders across arm angles and elevation phase and onset time of each subdivision was compared between genders. Females demonstrated significantly higher % activation levels for each subdivision (p < .05), except transverse trapezius (p = .36). A statistically significant difference for onset time was observed between gender for the descending trapezius, with a slower onset time for females (p < .05). Findings from this study support the theory that subdivisions within descending trapezius are preferentially activated and demonstrate that gender differences are present within the fourth subdivision of trapezius. As subjects were without shoulder pathology, the observed gender differences may be explained by normal motor control variations. However, as coordinated activation of trapezius is required for normal shoulder motion, it is recommended to include all four subdivisions of trapezius and address gender differences in future studies.