MUSCLE CONTRACTION

The concept of shrinkage is different considering the isolated muscle fiber or the whole muscle belly. In the case of muscle fiber contraction is It is shortening its active and depends on the stimulation of the contractile machinery of the muscle fiber by the action the nerve stimulation. However, the contraction of a muscle belly in full does not necessarily mean that is shortened, and even if it is subjected to resistance forces, the muscle belly can be extended. This is because the muscles are divided into functional units, called motor units, which they are formed by all muscle fibers which they are innervated by the same motor neuron. Importantly, the distribution of fibers motor unit does not match the set fascicles by intramuscular connective tissue. When a motor neuron fires all fibers contract innervated by its axon, but not others. Thus, in the muscles contract synchronously different units motor, that is, that, at a given moment, in a muscle there is a variable number of motor units in contraction and relaxation other. The amount of contraction of a muscle it depends on the number of units motor are activated and frequency of stimuli motor neuron emitting fibers. The action of Muscle considered as a whole (shortened, lengthened or maintain their size) will depend on the number of units motor which are contracting at all times, regarding the resistance to which is subjected the whole muscle belly.