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The inputÂ¿output relationships for modulation of force by recruitment during intramuscular electrical stimulation were examined for cat sleus muscles and human finger and thumb muscles. Recruitment was modulated by varying either the pulsewidth or amplitude of a monophasic, rectangular, cathodal current pulse train. Force was a nonlinear function of either pulsewidth or amplitude, and the shape of the nonlinearity was the same regardless of which parameter was modulated. The charge per stimulus pulse was lowest if pulsewidth was modulated with a fixed, high amplitude stimulus. The shape of the nonlinear relationship between pulsewidth and force (recruitment characteristic) depended on stimulus amplitude, electrode location in the muscle and muscle length. In most applications the amplitude and location would be fixed, so force would be a two-dimensional nonlinear function of pulsewidth and muscle length. The results are discussed with respect to possible mechanisns of recruitment during intramuscular stimulation, and the implications of the nonlinearities on the proportional control of orthoses employing electrically stimulated muscles.