Biomedical Engineering Reference
In-Depth Information
The muscles and their ligaments, tough fibrous strands, together with the bones
maintain the structure constituted of the bones and the articulations at their
positions and are responsible for the movement of this structure, allowing it to
rotate or to twist within certain limits. Considerable damage occurs when the
ligaments are forced beyond their strength limit and rupture.
5.5.3 Muscle and Levers
There are three types of muscles in the body: the cardiac, the smooth, and the
skeletal muscle. The cardiac muscle is the muscle of the heart which has an
involuntary action. The smooth muscle, usually of involuntary action, is located
in the walls of hollow internal structures such as the digestive tract and the blood
vessels. The skeletal muscles are attached to bones and generate movements. These
muscles can be made to contract or relax under voluntary control.
Themuscle forcemust be treated as an action force on the bar (bone segment) of the
lever. The resistance force corresponds to the weight of the segment plus the weight
the external loads added to it (these forces are represented at their center of gravity).
A great number of muscles work in pairs to produce a given motion. Arm flexion
is an example. When the biceps muscle contracts, the arm flexes and the triceps, the
rear muscle, relaxes. To stretch an arm, the inverse of this occurs.
5.5.4
Identification of Levers in the Human Body
5.5.4.1 First Class Levers
In the body, this system of levers is often used to maintain posture or balance. One
example of a first class lever in the body is found with the skull pivoting on the first
vertebra, the atlanto-occipital articulation of the spine which acts as fulcrum. The
weight of the head is the resistance force which is balanced by the force of extensor
muscles (action force). The same principle is found in the intervertebral
articulations for both seated and standing postures in which the weight of the
trunk is equilibrated by the forces of the spinal erector muscle, acting on the
vertebral axis.
5.5.4.2 Second Class Levers
This system provides mechanical advantage and is rarely found in the human body.
The pushup is an example of a second class lever. In it, the fulcrum is at the toe-tip,
the weight, at the body's center of gravity, corresponds to the resistance force and
the action force is exerted by the arms during the pushup.
