As the name implies, the muscle spindle is a spindle shaped sensory organ, meaning that it is thicker in the middle and tapered at either end. It is a stretch receptor that is widely dispersed throughout most skeletal muscles. Our muscle spindles work as a mechanism in order to help protect a muscle from over stretching, causing a stretch reflex to happen as and when necessary.
Muscle spindles are specialised to sense changes in muscle length, particularly when the muscle changes length rapidly. Each muscle spindle is enclosed within a capsule and lies parallel to extrafusal fibers (ordinary skeletal muscle fibers). The muscle spindle contains specialised muscle fibers called intrafusal fibers. These intrafusal fibers have contractile proteins at each end ( actin and myosin) and a central region that is wrapped by sensory nerve endings. Because the intrafusal fibers of the muscle spindle lie parallel to the extrafusal muscle fibers, a stretching force applied to the muscle will stretch both the intrafusal and extrafusal muscle fibers. This will cause a sensory discharge from the muscle spindle that is carried towards the spinal cord. This then leads to a motor response, activation of the muscle that was initially stretched. This is known as a stretch reflex.
From a practical standpoint, static stretching exercises are typically done in such a way as to avoid activation of the muscle spindles. Moving slowly into a stretched position avoids activation of the muscle spindle. This is important because muscles are most easily stretched when they are relaxed. There are other times, however, when activation of the muscle spindle is desired during training. For example, Plyometric exercises are performed by rapidly stretching a muscle, and this is followed immediately by concentric action of the same muscle. This rapid stretch of the muscle will activate the stretch reflex, leading to a more powerful concentric action.