Mechanics
of the musculoskeletal system
The musculoskeletal system includes bones, joints, muscles, tendons and ligaments. These parts can be thought of as gear parts of a machine, in which there is a close link between the shape (anatomy) and function (physiology) of each element. The study of the musculoskeletal system as a machine allows us to understand how it works at a mechanical level. In this sense, in the musculoskeletal, or locomotor, system the bones are the levers and the joints are the couplings. In order to move, they both need a motor (or rather a transformer of chemical energy into mechanical energy) corresponding to the muscles. As structural cables, the tendons deliver the force developed by the muscles to the point where it should be, that is the point of intersection with the bones. Finally, ligaments regulate and constrain the movement of articular bone sectors, carrying out the function of machine’s fittings and safety stops.
To better understand how this system of levers, joints, pulleys, cables, connectors and fasteners works, we will take a closer look at the mechanics of movement. Skeletal muscles are attached to bones by tendons, and all together they form a lever system that the muscle sets in motion following its own contraction. Subsequently, with muscle relaxation, bone segments can return to their place.
This movement is made possible by the joints, which guarantee the mobility of the bones. In this lever system, the fulcrum «F» (the fixed point to which a lever is attached) is represented by the joint, the power «P» (one of the two driving forces to which a lever is subjected: the force required to do the work, or effort) by the muscle, the resistance «R» (the other driving force, or load) by the force opposing the force generated by muscles, which often coincides with the weight of the body portion which is shifted. Thus, the effort arm (the distance from P to F) it is represented by the distance between the articulation and the point where the muscle in action attaches. The load arm (the distance from R to F) is represented by the distance between the articulation and the centre of gravity of the shifted position.
In the musculoskeletal system, the three points F, R, P may combine in different ways labelled as first-class lever (when F is between R and P), second-class lever (when R is between P and F), or third-class lever (when P is between F and R). A first-class lever is designed basically to produce balanced movements when the fulcrum (F) is midway between the force (P) and the resistance (R), for example the joints between the first two cervical vertebrae and the cranium.
A second-class lever is designed to produce force movements, as a smaller effort can be used to advantage over a larger weight: an example is the plantar flexion of the foot, where the fulcrum (F) is represented by the metatarsal heads, the effort (P) is brought about by the contraction of the calf muscle attached to the heel and the resistance (R) is the weight of our body.
A third-class lever is designed to produce speed and range of motion movements. Most of the levers in the human body are of this type, but they are not favourable for force production, as they require a great deal of force to move even a small resistance: the use of this lever is in the gain in speed of movement of the weight. The biceps brachialis is a typical example in the body: the fulcrum (F) is the elbow, the effort (P) is applied by the biceps muscle, and the load (R) is in the forearm.
