On the Action of Bi-Articular Muscles, a Review

Abstract

In various multi-joint movements where an animal is in contact with his environment, the interaction with the environment may constrain the control of joint moments and joint displacements. If the hand or foot has to exert an external force on the ground or on an object, the joint moments necessary to control the direction of this force are to be judged as independent from the joint displacements required in the same task. The moments required to control this force may even be opposite to the direction of the joint displacement at one of the joints. It is shown that bi-articular muscles can provide a unique solution to this type of conflict. These muscles are able to change the distribution of net moments and joint power over the joints that are crossed allowing the mono-articular muscles to contribute to positive power irrespectively of the required net moments in the joints crossed by these muscles. This results in a considerably more efficient utilisation of the work capacity of the mono-articular muscles than what would be possible with mono-articular muscles only. The mechanical reasoning where these concepts are based on is explained with help of a few simplified examples and their recent identification is placed in a context of ideas of the pioneers in this field of physiology and functional morphology.