WordNet
n. (physiology) a shortening or tensing of a part or organ (especially of a muscle or muscle fiber) [syn: contraction, muscular contraction]
Wikipedia
Muscle contraction is the activation of tension-generating sites within muscle fibers. In physiology, muscle contraction does not mean muscle shortening because muscle tension can be produced without changes in muscle length such as holding a heavy book or a dumbbell at the same position. The termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibers to their low tension-generating state.
Muscle contractions can be described based on two variables: length and tension. A muscle contraction is described as isometric if the muscle tension changes but the muscle length remains the same. In contrast, a muscle contraction is isotonic if muscle length changes but the muscle tension remains the same. If the muscle length shortens, the contraction is concentric; if the muscle length lengthens, the contraction is eccentric. In natural movements that underlie locomotor activity, muscle contractions are multifaceted as they are able to produce changes in length and tension in a time-varying manner. Therefore, neither length nor tension is likely to remain the same in muscles that contract during locomotor activity.
In vertebrates, skeletal muscle contractions are neurogenic as they require synaptic input from motor neurons to produce muscle contractions. A single motor neuron is able to innervate multiple muscle fibers, thereby causing the fibers to contract at the same time. Once innervated, the protein filaments within each skeletal muscle fiber slide past each other to produce a contraction, which is explained by the sliding filament theory. The contraction produced can be described as a twitch, summation, or tetanus, depending on the frequency of action potentials. In skeletal muscles, muscle tension is at its greatest when the muscle is stretched to an intermediate length as described by the length-tension relationship.
Smooth and cardiac muscle contractions are myogenic and can be modulated by the autonomic nervous system. The mechanisms of contraction in these muscle tissues are similar to those in skeletal muscle tissues.
Usage examples of "muscle contraction".
But once his trigger finger had become active Under its muscle contraction he was going to run into a phase of gross delay in the overall operation: the flesh of his finger alone would absorb several minor fractions of a second, and the spring mechanism of the gun would then begin using up the greatest proportion of the total time demanded from initiation to completion.
The charge ripping through the chauffeur's body broke his ribs with unrelieved muscle contraction, and the screaming stopped only when there was no more air to be forced through the lifeless throat.