Force transmission and muscle mechanics

General principles

Muscle force is generated within sarcomeres within myofibers (i.e., muscle fibers). The sarcomere forces (summed according to the rules for serial and parallel arrangement) need to be exerted outside myofibers to be able to cause movement of body parts.

In short, the rules for summation of sarcomere effects indicate that forces are added for sarcomeres arranged in parallel, and shortening and shortening velocity are added for sarcomeres arranged in series.

There are two fundamental ways to look at force transmission.

1. The direct (mechanics) way: considers a force transmitted from the sarcomeres of a muscle onto, for example, the tendon and from there to bone to cause movement of a body segment.

2. The inverse mechanics way: deals with questions: which structures are the sources of so-called reaction forces that allow the muscle to exert force? An active or stretched passive muscle will shorten unless opposed by a load (an opposing force) that will prevent shortening to a length at which no force can be generated (slack length). This opposing force is a reaction force that is equal to the force exerted by the muscle (action = reaction), but has an opposite direction. It may be exerted by the tendon or by other structures arranged in series with the sarcomeres.

It is clear that the two approaches should yield the same answers. Even though the concept of inverse mechanics seems more complex initially, we will use this approach because it is easier to avoid mistakes.

As there are two types of structures arranged in series with sarcomeres, tendons and fascia, two types of force transmission are distinguished.

Myotendinous force transmission

Each myofiber (muscle fiber) is equipped (at least at one end) with a myotendinous junction (Fig. 3.1.1A). The thin filaments of the last sarcomere of myofibrils within myofibers are attached sideways through the sarcolemma to collagen fibers of the aponeurosis (tendon plate) that invade the invaginations of the myofiber but remain outside of the cell. The supramolecular structures involved in such connections are shown in Fig. 3.1.2A–C. The myotendinous loads exerted on the last sarcomere are transmitted to the next sarcomeres in series within each myofibril. If this is the only type of force transmission, forces in all sarcomeres in series within a myofibril need to be equal. If this is the exclusive reaction force available, and if all sarcomeres have identical properties, such sarcomeres will shorten until their identical force is in equilibrium with the reaction force. In static final conditions, this will yield identical sarcomere lengths within the myofiber.