The Effects of Alterations to Upper Limb Inertial Properties on Vertical Point-to-Point Movement
Purpose: Several studies have examined intralimb coordination between the shoulder and elbow joints during target-oriented movements. These studies have observed consistent patterns in coordination despite changes in movement variables such as speed, direction, and inertia. Researchers used intersegmental dynamics to quantitatively analyze these patterns between shoulder and elbow joints while systematically changing values of these movement variables. Some studies have examined central nervous system adaptations to inertial changes at the elbow and entire arm during a movement, but none have examined inertial changes to the upper limb. Methods: Five male and five female participants aged 27 to 39 years (mean age = 33 ± 4.3 standard deviation) performed a maximal speed, point-to-point, reversal hand movement in the sagittal plane with and without a 2.2 kg. weight attached to their dominant, right upper arm. To determine the effects of the added mass, dependent t-tests were performed on elbow and shoulder peak muscular torques generated during the reversal region of the movement. Results: A significant increase in shoulder joint torque (p < 0.05), a significant increase in movement time (p < 0.05) and a non-significant decrease in elbow muscular torque (p = 0.1074) was shown to achieve the movement objective with the added weight. Conclusions: While future studies may result in more conclusive findings, this study showed a pattern of increased shoulder torque and decreased elbow torque due to the added inertia. Larger shoulder torque was needed to overcome the added inertia and move at high speed. As a result of the higher shoulder force, interaction torque at the elbow increased, and a reduction in elbow torque was needed to control the hand path and accurately hit the targets. This pattern supports Bernstein's proposal that passively arising phenomena (i.e., interaction torque) is exploited during multi-segment movement.