Age and gender effects on the proximal propagation of an impulsive force along the adult human upper extremity.

Abstract

We tested the null hypotheses that neither age, gender nor muscle pre-cocontraction state affect the latencies of changes in upper extremity kinematics or elbow muscle activity following an impulsive force to the hand. Thirty-eight healthy young and older adult volunteers lay prone on an apparatus with shoulders flexed 75° and arms slightly flexed. The non-dominant hand was subjected to three trials of impulsive loading with arm muscles precontracted to 25, 50, or 75% of maximum pre-cocontraction levels. Limb kinematic data and upper extremity electromyographic (EMG) activity were acquired. The results showed that pre-cocontraction muscle level (p < 0.001) and gender (p < 0.05 for wrist and shoulder) affected joint displacement onset times and age affected EMG onset times (p < 0.05). The peak applied force (F1) occurred a mean (± SD) 27 (± 2) ms after impact. The latencies for the wrist, elbow, and shoulder displacements were 21 ± 3, 29 ± 5, and 34 ± 7 ms, respectively. Because the latencies for elbow flexion and lateral triceps EMG were 23 ± 5 and 84 ± 8 ms, respectively, muscle pre-activation rather than stretch reflexes prevent arm buckling under impulsive end loads.