Figure 5.

(A) Schematic of muscle-tendon unit showing muscle fiber in series with the tendon. Note the pennation angle, φ, of the muscle fiber relative to the tendon and that the total tendon length, ℓt, is twice that of the tendon on either end of the muscle fiber, ℓt/2. (B) Schematic of muscle fiber with the contractile element and parallel elastic component. The nonlinear tendon stiffness is given by k. The force produced by the contractile element, Fm, is a function of ℓm and νm, while the tendon force, Ft, is a function of ℓt. The total muscle fiber force, Fm, is the sum of FAm and Fpm.