Engineering approach (Myology)

The engineering approach treats the body as if it were a mechanical system. It applies mechanical analogies to understand the musculoskeletal system. There are for example mechanical analogies for the joint and for the muscles. The behaviour of a muscle can be described in a muscle-model. There are different types of muscle-models, for example the Hill-type muscle model.

Mechanical analogies for the joint
Where the degrees of freedom are the independent movement possibilities, the joints can be seen as:
 * A hinge joint, for example the fingers joints and the elbow joint. These joints have 1 degree of freedom (flexion, extension).
 * A saddle joint, for example the thumb joint. This joint has 2 degrees of freedom (flexion, extension, adduction, abduction).
 * A ball and socket joint, for example the shoulder joint and the hip joint. These joints have 3 degrees of freedom (flexion, extension, adduction, abduction, axial rotation).

Mechanical analogies for the muscles
Muscles are treated as separate entities. They can shorten against resistance and resist lengthening. Muscles can be seen as independent actuators. Actuators are objects that convert energy into motion. An example is the pneumatic artificial muscle, or the air muscle. It’s a device made of a flexible and inflatable membrane. The air muscle contracts when it’s activated.

The Hill-type model
The Hill-type model consists of a contractile element (CE), and two non-linear (= the output is not directly proportional to its input) spring elements (= an elastic object used to store energy). One non-linear spring element is in series (SE), and the other one is in parallel (PE).
 * The CE represents the actine and myosine cross-bridges, and it generates active force. It can shorten when it is activated and it is freely extensible when it is not activated.
 * The SE represents the tendon and the intrinsic elasticity of the myofilaments and crossbridges. It allows a rapid change of the state of the muscle from inactive to active. It is also an energy storing mechanism.
 * The PE represents the connective tissues (epimysium, perimysium, endomysium). It is responsible for the passive behaviour of the muscle when it is stretched, even when the CE is not activated.

The combination of the force-length characteristics of CE, SE and PE is the net force-length characteristics of the muscle.