Age and Gender Effects on the Proximal Propagation of an Impulsive Force Along the Adult Human Upper Extremity

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 degrees 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 (F-1) 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.