Get those muscles moving by integrating movement into your workstyle. Move your printer across the room to force you to get up out of your chair. The new wider keyboards, such as those which are ergonomically designed many unintentionally increase muscle tension during use with a mouse. Be aware that a higher levels of muscular tension significantly increases your risk of computer-related disorders. Muscle Contraction by Reed Ferber, Graduate Student - Sports Medicine Contraction of a muscle is definately a process that takes more than a page to describe but here's a shot.
Contraction occurs via individual contractile units called sarcomeres.
Sarcomeres are composed of actin and myosin filaments which slide past one another upon stimulation from a nerve. As a sarcomere is stimulated the actin and myosin filaments slide past one another and essestially shorten. Millions of sarcomeres make up a muscle fiber and several muscle fibers are innervated by a single motorneuron or nerve. This is the motorunit. A stimulus from a nerve causes the motor unit or group of muscle fibers to contract as all the sarcomeres shorten.