Aim: to define the input (muscle stimulation train) / output (torque at the bone segment) law when the stimuli change according to a frequency (torque / frequency relationship, TFR) or an amplitude (torque / amplitude relationship, TAR) triangle.

In ten young male subjects the tibialis anterior (TA) was stimulated at the main superficial motor point. TAR train: frequency 35 Hz; amplitude: varied linearly from Vmin (least EMG) to Vmax (largest EMG) to Vmin in 15 s; TFR train: amplitude Vmax; frequency: varied linearly from 2 to 35 to 2 Hz in 15 s. Tension during isometric ankle flexion for offline torque calculation and surface mechanomyogram (MMG) from laser distance sensor signal were recorded. UGL and DGL identified the up and down going limb of the TAR and TFR triangles.

During each limb TAR and TFR resulted almost linear and sigmoidal, respectively, and presented torque values during DGL > UGL for a given abscissa value. The same behaviour was reported for MMG / amplitude and frequency relationships but with different dynamics.

The not unique TA TAR and TFR behaviour can be partly explained considering that even in individual motor units (MU) an hysteretical response is present when stimulation pattern is aimed to increase/decrease the recruited MUs (due to MUs co-operation) or their firing rate (due to MUs catch like properties). The combined analysis of torque and MMG (which may mirror the degree of muscle shortening) may provide indirect data about active stiffness changes during linearly varying stimulation.