keeps our joints from moving too far and dislocating? We could say that the ligaments and joint capsule fill
that role, and we would be partially correct. Joint capsules and ligaments do check movement in certain directions, but should be the last line of defense against moving a joint too far. They provide static stability for a joint and help to guide the arthrokinematics of that joint, but shouldn’t be relied on as the
primary structures to stop a joint from moving too far – especially when movement forces are fast, explosive or repetitive. For example, when preparing to throw a baseball I will wind up by abducting and externally rotating my shoulder joint. This is a direction of movement that is commonly associated with glenohumeral dislocation; do I really want to trampoline my bones off my capsule and ligaments every time I throw?
Instead, I want to be able to decelerate and stop that movement into abduction/external rotation before banging into delicate connective tissue. I want my pectoralis major and subscapularis to catch the bone before it lands on my collagen. This is where the muscle spindle comes in. The role of the muscle spindle is to recognize the length of the muscle in which it is embedded and the physiological limit of the joint it crosses. It is a specialized type of muscle fiber that responds to stretch and then stimulates a contraction of its associated muscle group. The classic example is the knee-jerk reflex. When the patellar tendon is tapped with a reflex hammer, a quick stretch is applied to the quadriceps muscle. Some of the various muscle stretched, stimulating a reflex arc that results in a contraction of the quadriceps as a whole – the knee straightens. The CNS interprets the knee tap as a movement of the knee into flexion and as a lengthening of the quadriceps. This then stimulates the contraction of the muscle to prevent too much movement into knee flexion or too much stretch of the quadriceps.