First, bones provide stability for our posture and enable dynamic movement through the attachments of muscles, tendons, ligaments, and other materials. Unlike other structures, muscles are the only musculoskeletal structures that can shorten and lengthen. Extrafusal fibers make our muscles relax, contract, and elongate and intrafusal muscle fibers are the main stretch receptors of the muscle. Tons of information is passed through this 'line' every second. There are three modes of muscle contraction eccentric, isometric, and concentric.
Eccentric contractions occur when the muscle needs to stop the body from moving.
For example, when a runner is moving forward and needs to stop or slow, muscles contract and control the slowing down motion. In mid- stride, the body kind of relaxes and becomes static and an isometric contraction occurs. In a concentric contraction, the muscle shortens which results in acceleration in the running motion. All these messages to and from the brain and muscles along with the contractions occurs quickly and with out us having to think about it. For example, let's take a look at what happens when a player is going up for a head ball.