Muscles move when the motor nerve attached to it get an impulse which makes the synapse (nerve ending on the muscle) to release a certain chemical on to the muscle cell - this (specific)chemical reacts with specific proteins in the muscle cell which would trigger a movement in the muscle by contracting or releasing the muscle (by fluid motion).
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The muscle cells move by contraction and relaxation by utilizing the energy in the form of ATP. Muscle contraction is initiated bythe signal sent by the CNS to the muscle fibres. During contraction, the muscles fibres shorten, the muscle pulls on the ligamentsthat connect the bones and the movable body parts. During relaxation the muscle fibres come to their original position. Thismovement of muscles cells or the muscle fibres results in the movement of the body parts.