TWITCH ANALYSIS AS AN APPROACH TO MOTOR UNIT ACTIVATION DURING ELECTRICAL-STIMULATION

The mechanical twitch in response to increasing electrical stimulus in tensity, delivered both over the motor point and motor nerve, was reco rded in the first dorsal interosseous (FDI) and the adductor pollicis (AP), and only over the motor point in the soleus (Sol), lateral (LG), and medial (MG) gastrocnemius muscles of human subjects. The relation ship between intensity of electrical stimulation (ES) and twitch torqu e showed a positive linear regression in all muscles. In the FDI and A P the relationship was not significantly different when ES was applied at the motor point or over the motor nerve. At small intensities of a ctivation, ES induced larger twitch torques in the MG and LG, which co ntain a roughly equal proportion of stow and fast motor units (MUs) co mpared to the Sol, which is composed mainly of slow type fibres. Moreo ver, the relationship between ES intensity and twitch time-to-peak is best fitted in all muscles by a power curve that shows a greater twitc h time-to-peak range in its initial part for muscles containing a larg er proportion of fast MUs (LG, MG) than for muscles mainly composed of slow MUs (Sol). In conclusion, these results induced by ES at the mot or point and/or over the motor nerve confirm the concept of a reversed sequence of MU activation, as compared to voluntary contractions, and document this viewpoint in muscles of different function and composit ion. The reversed sequence of MU activation is more clearly evident du ring motor point ES.