Modeling of Lengthening Muscle: The Role of Inter-Sarcomere Dynamics

Every muscle fiber is made up of a large number of sarcomeres connected in series, so the tension must be the same in all of them. If the sarcomeres are identical, then velocity will also be the same in all, so any externally applied movements will be equally distributed among all the sarcomeres of the fiber. This leads to the common procedure of modeling a muscle fiber, or even a whole muscle, as a scaled sarcomere. Most cross-bridge modeling of muscle makes this assumption implicitly, bearing in mind that many experimenters have gone to great lengths, by use of segment length clamps or diffraction measurements, to make it at least approximately true in their experiments. Note that throughout this chapter the sarcomere is taken to be the fundamental unit of contraction. In many ways, the half sarcomere would be a more appropriate unit, and all the comments and calculations referring to sarcomeres could equally be taken to refer to half sarcomeres.