How do the force-length properties of individual plantarflexors combine into the muscle group properties?

Misalignment between optimal lengths of individual muscles will affect the force-length (F-L) properties of a group of muscles acting synergistically at a joint. This misalignment can arise from differences in the properties of the muscles and of their tendons. To show how misalignment affects the group F-L properties, we measured the in situ F-L properties of kangaroo rat (k-rat) plantarflexors, both of the individual muscles and of the group. We performed sensitivity analyses to quantify how misalignment influences the maximal force and the width of the group F-L curve, two characteristics which vary inversely as misalignment changes. Misalignment between the k-rat plantaris and gastrocnemius was equal to 1.8 mm, which corresponded to nearly the greatest combined maximal force possible. The study outcomes can be applied more generally to a group that contains both uni- and bi-articular muscles, such as the human plantarflexors.