• Y-K Kim
  • N. Dounskaia
  • R. Hinrichs
Keywords: arm swing, moment of inertia, extra mass, open kinetic chain principle


This study investigated the effect of adding extra mass on individual segments during the performance of an arm swing task in the horizontal plane. The amount of extra mass was 0, 25, 50, 75, and 100% of the mass of the segment on which the extra mass was placed (upper arm or forearm). The variables studied were arm swing speed (hand speed), positive muscle impulse, and system moment of inertia (MOI). The purpose was to see if adding extra mass sped up or slowed down the arm swing and why. Twenty subjects were instructed to produce their maximum hand swing speed over the target point during the horizontal non-dominant arm swing. It was found that the forearm extra mass elicited a significant decrease in the arm swing speed, while the upper arm added mass did not cause decreases in arm speed. Rather, moderate amounts of extra mass at the upper arm (25 and 50% extra mass) induced slight, although not significant, increases in arm swing speed (0.66% and 1.41% increase, respectively). These increases in speed were accompanied by small increases in both the positive muscle impulse and the system MOI with the upper arm extra mass. Significant increases in the system MOI accounted for the significant swing speed drop caused by the forearm extra mass. It was concluded that extra mass is not always detrimental to the arm swing speed. Extra mass added close to the axis of rotation either makes no difference or may actually help swing speed.