Principal component analysis of upper-extremity joint moment variability during hammering

The goal of this study was to examine effects of hammer symmetry of inertia on upper-extremity joint moment variability. Moment variability is important in quantifying tissue exposure to stress variations. Nineteen male participants (age: 20-40 year) swung four hammers with known inertial properties while movement was recorded. Joint moment components were calculated with inverse dynamics. Moment component variability was explained with principal component analysis. Higher inertia symmetry hammers resulted in greater explained variability (up to 18%) for wrist internal-external, elbow abduction-adduction and elbow flexion moment components. For the most inertia symmetric hammer a different structure of moment variability between principal components was found for some moment components. These results suggest that more inertia symmetric hammers result in decreased moment diversity. Decreased moment diversity represents a more constant dose of loading exposure applied to some upper-extremity tissues and may indicate less variable stresses and increased potential for musculoskeletal disorder.