Contractile properties and fatiguability of the human adductor pollicis and first dorsal interosseus: a comparison of the effects of two chronic stimulation patterns.

We have developed a method for simultaneously measuring the isometric force of the adductor pollicis (AP) and first dorsal interosseus (FDI) muscles of the hand. Both muscles were found to have very similar force-frequency relationships, relaxation rates and fatigue indices. The effect of 2 patterns of chronic electrical stimulation on the force production, contractile properties and fatiguability of the AP and FDI were compared in 2 groups of 5 subjects. One pattern consisted of uniform 10 Hz stimulation and the other a random pattern containing a few high frequency bursts against an essentially low frequency background. Both patterns were found to increase the fatigue resistance of both muscles by approximately 30% and caused a smaller and less consistent shift in the force-frequency relationship such that more force was generated at low frequencies of stimulation. There was a decrease in both the maximum voluntary and tetanic force after stimulation with the uniform 10 Hz but not the non-uniform patterns. The maintenance of force with the latter pattern may be due to the presence of the high frequency bursts.