The other end, the distal attachment, is called the insertion . During contraction, the origin remains stationary and the insertion moves. Reversing the direction in which a joint bends is produced by contracting a different set of muscles. For example, when one group of muscles contracts, an antagonistic group stretches, exerting an opposing pull, ready to reverse the direction of movement. The contracting unit is the muscle fiber .
Muscle fibers consist of two main protein strands - actin and myosin .
Where the strands overlap, the fiber appears dark. Where they do not overlap, the fiber appears light. These alternating bands of light and dark give skeletal muscle its characterisitc striated appearance. The trigger which starts contraction comes from the motor nerve attached to each muscle fiber at the motor end plate . Acetylcholine is released at the motor end plate when the electrical impulse reaches the muscle fiber.