What Is Muscle?
Muscle is the body tissue which, because it has the power of contraction, enables the higher animals to move their bodies. Muscles are divided into two great groups, and muscle action can be classified as being either voluntary or involuntary, the former being controlled by the will, while the latter act independently. There are three types of muscle, skeletal or striated, cardiac, and smooth.
Most voluntary muscles are attached to the skeleton and range in size and shape to suit the particular jobs they perform. They can get into action within a few hundredths of a second, exert an enormous pull on the bone to which they are attached and, if necessary, support 1,000 times their own weight.
Involuntary muscles include heart muscle and the muscles of the digestive system, and the fibres of which they are composed are very much smaller than those of voluntary muscle. Because of its exceptionally rich blood supply, muscle is the most infection free of the body’s basic tissues. If it is over-taxed it tires and will stop contracting altogether, but if used normally will give little trouble.
Muscle tissues are derived from the mesodermal layer of embryonic germ cells in a process known as myogenesis. Muscles are predominantly powered by the oxidation of fats and carbohydrates, but anaerobic chemical reactions are also used, particularly by fast twitch fibers. These chemical reactions produce adenosine triphosphate (ATP) molecules that are used to power the movement of the myosin heads.
The term muscle is derived from the Latin musculus meaning “little mouse” perhaps because of the shape of certain muscles or because contracting muscles look like mice moving under the skin.
The action a muscle generates is determined by the origin and insertion locations. The cross-sectional area of a muscle (rather than volume or length) determines the amount of force it can generate by defining the number of “sarcomeres” which can operate in parallel. Each skeletal muscle contains long units called myofibrils, and each myofibril is a chain of sarcomeres.
Since contraction occurs at the same time for all connected sarcomeres in a muscles cell, these chains of sarcomeres shorten together, thus shortening the muscle fiber, resulting in overall length change.
The amount of force applied to the external environment is determined by lever mechanics, specifically the ratio of in-lever to out-lever. For example, moving the insertion point of the biceps more distally on the radius (farther from the joint of rotation) would increase the force generated during flexion (and, as a result, the maximum weight lifted in this movement), but decrease the maximum speed of flexion.
Moving the insertion point proximally (closer to the joint of rotation) would result in decreased force but increased velocity. This can be most easily seen by comparing the limb of a mole to a horse – in the former, the insertion point is positioned to maximize force (for digging), while in the latter, the insertion point is positioned to maximize speed (for running).