Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
The musculoskeletal system (also known as the locomotor system) is an organ system that gives animals the ability to physically move using the muscles and skeletal system.
The human musculoskeletal system consists of the human skeleton, made by bones attached to other bones with joints, and skeletal muscle attached to the skeleton by tendons.
Hydrostatic musculoskeletal system
Among others, cnidarians and annelids, have a hydrostatic skeleton similar to a water-filled balloon, these animals can move by contracting the muscles surrounding the fluid-filled pouch, creating pressure within the pouch that causes movement. Animals such as earthworms use their hydrostatic skeletons to change their body shape as they move forward, from long and skinny to short and stumpy. Arthropoda have their muscles attached to an exoskeleton.
Muscle contraction initiation
In mammals, when a muscle contracts, a series of reactions occur. Muscle contraction is stimulated by the motor neuron sending a message to the muscles from the somatic nervous system. Depolarization of the motor neuron results in neurotransmitters being released from the nerve terminal. The space between the nerve teminal and the muscle cell is called the neuromuscular junction. These neurotransmitters diffuse across the synapse and bind to specific receptor sites on the sarcolemma (cell membrane of the muscle fiber). When enough receptors are stimulated, an action potential is generated and the permeability of the sarcolemma is altered. This process is known as initiation.