Proposed actions of bi - articular muscles and the design of hindlimbs of bi - and quadrupeds Gerrit Jan

In contemporary textbooks on functional anatomy, the actions of muscles which cross more than one joint are often described according to the degrees of freedom which they might influence about the joints crossed. Such actions, however, might in principle also be performed by sets of mono-articular muscles crossing the same joints. In the past centuries many authors have provided convincing evidence that muscles which cross more than one joint are able to perform unique actions that could by no means be realized by sets of mono-articular muscles. This paper provides an overview of the literature from these previous centuries. The major action of bi-articular muscles appears to be their capacity to couple movements in adjacent joints. The consequences of this action (which was already identified more than three centuries ago) can be explained in various different contexts. Examples are presented which may help to understand the principles which underly the general design of the hindlimb of man, hoofed animals and predators. “Nature, just in everything, has made the size of the muscles to accord with the usefulness of the actions they must perform”. De usu partium, I 171 Galen (131-201 AD) (transl. by May, 1968) 0167-9457/94/$07.00