Relationships between magnitude of handedness and bilateral asymmetry of shoulder muscle activation during a unilateral overhead fatiguing task in right-handed individuals.

Previous literature has shown a relationship between side dominance and asymmetry in some features of motor control. However, the magnitude of handedness (i.e. how side-dominant someone is) is unclearly related to bilateral differences in shoulder muscle activity and how it changes with fatigue. We expected that people with the highest scores on the handedness laterality quotient would display a greater side dominance (difference between their right and left sides) in their shoulder activation patterns, both in the absence and presence of fatigue. Thirty right-handed adults completed two sessions of an overhead fatiguing task: one with their dominant and the other with their non-dominant arms. Electromyographic (EMG) data was collected from six shoulder muscles (deltoids, rotator cuff and biceps) using either surface and in-dwelling electrodes, and activation amplitude was calculated using root-mean-square (RMS). Asymmetry was calculated using a Limb Symmetry Index equation. Two statistically significant correlations were found, only in females, between the handedness laterality quotient (degree of right-handedness) with no-fatigue anterior deltoid EMG RMS asymmetry (p = 0.004), and with fatigued middle deltoid EMG RMS asymmetry (p = 0.006). Handedness magnitude should be considered when recommending shoulder injury prevention approaches for overhead work, but only in females. During apprenticeships, trainers should consider using EMG and Limb Symmetry Index equations (which determines the laterality quotient) to assess shoulder muscle asymmetry, especially in shoulder injury prevention for women who perform overhead tasks.