The motor units of cat medial gastrocnemius. Twitch potentiation and twitch-tetanus ratio.

Twitch potentiation and the associated changes in contraction time and 1/2 relaxation time have been studied in a sample of 78 medial gastrocnemius motor units from 8 cats. Potentiation was produced by repetitive stimulation of the motor units every 10 sec with a brief tetanus followed 2 sec later by a twitch. Fast twitch fatigue resistant units were found to potentiate more strongly than either slow twitch (contraction time greater than 45 msec) or fast twitch fatigable units. It is concluded that Type C muscle fibres are more susceptible to potentiation than either Type A or Type B fibres. In a sample of 88 motor units from the same experiments, values for twitch/tetanus ratio were compared amongst units sharing similar mechanical properties. Slow contracting units developed small tetanic tensions and had small twitch/tetanus ratios. Fast twitch non fatigable units had intermediate values for contraction strength and twitch/tetanus ratio overlapping the ranges found for both the slow twitch and the more powerful fast twitch fatigable units. It is concluded that differences in twitch/tetanus ratio for medial gastrocnemius motor units are primarily related to motor unit contraction strength rather than differences in muscle fibre type.