The relationship between fusion index and stimulation frequency in tetani of motor units in rat medial gastrocnemius.

The relationship between tetanic fusion and frequency of stimulation was investigated in 81 motor units of the rat medial gastrocnemius. A fusion index was calculated for tetani evoked by trains with constant frequency stimuli. This index is expressed as the ratio of a/b, where a is the distance from a baseline to the maximal relaxation before the last component of tetanus and b is the peak amplitude of the last component. The fusion-frequency relation has a sigmoid shape. The steep portion of the relation corresponds to lower frequencies for slow motor units than for fast units. Among the fast units, FR motor units are fused at somewhat lower frequencies than FF ones. However, the increase in fusion index per 1 Hz change in frequency of stimulation for the three types of motor units is similar. This observation suggests a similar course of fusion within different motor units. The fusion index shows a correlation with the contraction time as well as with the half-relaxation time.