A graphic analysis of the biomechanics of the massless bi-articular chain. application to the proximal bi-articular chain of the human finger.

In this paper a model is presented which visualises the biomechanical functioning of the loaded and unloaded theree-tendon bi-articular chain. This model allows to graphically determine in any position of the chain (i) the exact ranges of loads which can be sustained by the different motors; (ii) the motors forces; (iii) the feasibility of the (unloaded) equilibrium; and (iv) the conditions for the good controllability of the bi-articular chain. These results are applied to the proximal three-motor bi-articular chain of the human finger, when controlled by the superficial flexor, interosseus, and extensor only. It is shown that (i) the anatomic position of the superficial flexor and extensor is a prime determinant in the good functioning of this chain; (ii) the proximal bi-articular chain of the human finger can well sustain certain flexion loads but is structurally weak for extension loads; and (iii) the chain is not optimally controllable.