A stretch-shorten cycle (SSC) is an active stretch of a muscle followed by an immediate shortening of that same muscle.
In biomechanical terms, this is an eccentric contraction followed by a concentric contraction. In an eccentric contraction (lengthening contraction), the muscle elongates while under tension due to an opposing force greater than the muscle can generate. Rather than working to pull a joint in the direction of the muscle contraction, the muscle acts to decelerate the joint during the movement. A concentric contraction (shortening contraction) is a type of muscle contraction in which the muscles shorten while generating force.
There is a significant performance benefit associated with muscle contractions that take place during SSCs.
This stretch-shorten cycle occurs naturally in many sport activities, including running, jumping, and all other activities in which muscles are suddenly stretched by impact or other external forces, for example when a wind-up movement is performed before a throw.
SSC plays an important role in the generation of power in an efficient golf swing, especially during the transition phase, where the body segments are changing direction sequentially and stretching the muscles and connective tissue around the joints. In the transition phase, each body segment — the hips (pelvis), then the chest (thorax), then the lead arm, and finally the hands and club — change direction from backswing to downswing in a precisely timed sequence. Each segment changes direction while the next segment in the kinetic chain is still moving back, thus stretching the muscles in an eccentric contraction and augmenting the subsequent muscle (concentric) contraction forces in the downswing.
Most exercise scientists and coaches believe that greater performance in SSC movements is due to recovery of elastic energy and increased muscle activation due to the stretch reflex — energy from an external quick stretching force is temporarily stored in the muscles and tendons, and then released through elastic recoil.
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