The influence of stimulation pulse frequency on the generation of joint moments in the upper limb.

Six muscles of the upper limb were stimulated transcutaneously. This communication reports the influence of the stimulation pulse frequency on the isometric joint moment generated by each muscle. A lower frequency of stimulation, the critical fusion frequency, was found for each muscle at which contractions ceased to be tetanic. This fusion frequency was correlated with muscle function. The magnitude of the joint moment was examined as a function of the stimulation pulse frequency for the six muscles tested. Parabolic curves were found to best fit this relationship; the magnitude of the moment reaching a maximum at typically 50 Hz, and often decreasing at higher frequencies. The slope of the linear portion of the relationship between the generated joint moment and the stimulation current intensity was shown to be a function of the stimulation pulse frequency. This function was found to be similar to the form of the joint moment versus pulse frequency curve. Fatigue curves were plotted at different stimulation frequencies; demonstrating reduced fatigue at lower stimulation frequencies. A model was presented of the fatigue curve as an exponential function of time. We conclude that a stimulation frequency of 15 Hz is optimal for the upper limb muscles with a working range of 15-50 Hz where stimulation frequency is one of the parameters used to modulate the muscle contraction force.