The rate of force development - in cat and rat medial gastrocnemius motor units - An interspecies comparison

The rate of force development (RFD, calculated as dF/dt, is one of the most important parameters of force developed during motor unit (MU) or muscle contractions. However, it is not obvious to what extent it is determined by body mass, muscle size, and contraction force, and to what extent it reflects the properties of intramuscular contractile proteins and biomechanical relationships. Therefore, this study compared the RFD between cat and rat MUs, which have considerably different contraction time and force properties. It performed in vivo experiments on functionally isolated MUs, divided into slow (S), fast-resistant (FR), and fast-fatigable (FF) types. The twitch and tetanus RFD gradually increased from S to FF MUs in both species and correlated strongly with twitch and tetanus forces, respectively. The tetanus RFD of S, FR, and FF MUs did not differ significantly between cats and rats (1.2 vs. 0.89, 4.59 vs. 4.06, and 13.2 vs. 10.35 N/s, respectively; p > 0.05). Interspecies differences were observed only for the twitch RFD of S and FR MUs, being significantly lower in cats than in rats (p < 0.001). These findings indicate that the RFD is predominantly related to myosin properties, and interspecies differences in the twitch RFD of S and FR MUs have a biomechanical basis.