FROM TWITCH TO TETANUS - PERFORMANCE OF EXCITATION DYNAMICS OPTIMIZEDFOR A TWITCH IN PREDICTING TETANIC MUSCLE FORCES

In models of the excitation of muscles it is often assumed that excita tion during a tetanic contraction can be obtained by the linear summat ion of responses to individual stimuli from which the active state of the muscle is calculated The purpose of this study was to investigate whether such a model adequately describes the process of excitation of muscle. Parameters describing the contraction dynamics of the muscle model used were derived from physiological and morphological measureme nts made on the gastrocnemius medialis muscle of three adult Wistar ra ts. Parameters pertaining to the excitation dynamics were optimized su ch that the muscle model correctly predicted force histories recorded during an isometric twitch. When a relationship between intracellular calcium and active state from literature on rat muscle was used, the m uscle model was capable of generating force histories at stimulation f requencies of 20, 40, 60 and 80 Hz and other muscle-tendon complex len gths which closely matched those measured experimentally - albeit forc es were underestimated slightly in all cases. Differences in responses to higher stimulation frequencies between animals could be traced bac k to differences in twitch dynamics between the animals and adequate p redictions of muscle forces were obtained for all animals. These resul ts suggest that the linear summation of responses to individual stimul i indeed gives an adequate description of the excitation of muscle.