Theoretical considerations on cocontraction of sets of agonistic and antagonistic muscles

It is well known that static, non-linear minimization of the sum of the stress in muscles to a certain power cannot predict cocontraction of pairs of one-joint antagonistic muscles. In this report, we prove that for a single joint either all agonistic muscles cocontract or all are silent. For two-joint muscles, we show that lengthening and shortening of muscles corresponds closely to zero force and non-zero force states, respectively. This gives a new physiological interpretation of situations in which cocontraction of pairs of two-joint antagonistic muscles is predicted by these static non-linear optimization approaches.