The Stretch-Shortening Cycle (SSC)
The stretch reflex is utilized frequently during sport because most movements involve the two phases of muscular contraction. An eccentric phase, which is the muscle lengthening under tension, is followed by a concentric phase in which the muscle is shortened. Attaining a pre-stretch of the muscle causes it to be lengthened eccentrically so tension is developed in the muscle, similar to a rubber band. This stored energy created by the tension can be used to help increase the strength of the following concentric contraction. This concentric contraction must immediately follow being stretched or the tension created will dissipate as heat. An example is a quick countermovement before jumping which allows the quadriceps to be stretched eccentrically so that the following concentric contraction can be stronger. The amount of tension created by stretching the muscle is dependent on the degree and the speed of the muscle’s pre-stretch. Exploiting the elasticity of the muscle and the stretch reflex is referred to as using the stretch-shortening cycle. It has been shown that the faster the muscle is stretched eccentrically, the greater the force will be on the following concentric contraction.

In 1966, Yuri Verkhoshanski, a Soviet jumping coach, observed that jumpers with the shortest amount of ground contact time (amortization phase) displayed the greatest jumping performance. This led him to reason that maximal jumping performance requires muscles to be strong eccentrically so that they are able to withstand the high mechanical loading during the amortization phase. He believed that if the muscles are strong eccentrically, they will be able to quickly switch from overcoming the eccentric loading to immediately contracting concentrically to accelerate the body in the required direction. Again, this allows the athlete to take advantage of the tension created in the muscle during the eccentric stretch. Thus improvements can be made in jumping performance by increasing the amount of tension the athlete can generate during the eccentric contraction and by improving the reactive ability of muscles in switching from eccentric to concentric work. Yurkhoshanski’s findings are the basis of how plyometric training can exploit the stretch-shortening cycle to produce athletic improvements.

Utilizing Stored Elastic Energy
The key to utilizing stored elastic energy involved in the SSC is to minimize the conversion time between the eccentric and concentric phase of the movement. The attachment time between myosin and actin strands is very brief, usually 15 to 120 milliseconds. A long delay between the stretching and contracting phase of the movement results in increasing detachment of the myofilaments, which negates the potentioal to utilize the elastic energy stored in the muscle. There are two main factors involved with the SSC. The factors include muscle, and tendon elasticity as well as neural factors. When a tendon or muscle is stretched elastic energy is stored within it's structure.

The elastic energy is recoiled (similar to a recoiled spring) and used to increase the efficiency of the concentric phase of the movement. The level of stored energy is proportional to the applied force and the speed of the stretch. The magnitude of the stretch is a function of muscle and tendon stiffness. The stiffness of a muscle is variable and depends on the forces applied, while tendon stiffness is constant. The higher the tension in a muscle the harder it is to stretch. Studies show that elite athletes experience a higher level of stiffness in their muscles than in their tendons; thus elastic energy in elite athletes is primarily stored in the tendons.

The neural mechanisms most prominent in the SSC are the myotatic reflex, and the golgi tendon organ. The myotatic reflex receptors (muscle spindles) are specialized muscle fiber which detect length change within the muscle. The primary role of the muscle spindles is to set the muscle to a preset length. When the muscles are stretched the muscle spindles are also stretched. This causes muscle spindle discharge which results in alpha motoneuron release which results in reflex contraction of the stretched muscle.

This reflex enables the muscle to return to it's preset length. The Golgi tendon organ is located in the muscle-tendinous junction and senses change in the tension of the muscle. The primary role of the Golgi tendon organ is to prevent potentially injurious muscle tension. When this organ senses forces are excessive muscular contraction is inhibited.

One of the primary training goals for enhancing the SSC is to maximize the positive effects of the myotatic reflex while minimizing the negative effects of the Golgi tendon organ. This type of training could involve accelerative movements with or with out weights. Accelerative training in this case would refer to very rapid stretching followed by accelerative contraction.

Training with heavy weights at slow speeds can also be used to inhibit the Golgi tendon organ. Muscles and tendons are connected in a series. To visualize the stretch-shortening cycle picture two springs connected. The first spring (tendon) has properties that do not change under the influence of motion.

The second spring (muscle) has properties that vary depending on muscle stimulation. The level of muscle tension is not constant during movement. When an athlete is attempting to exert maximal muscle contraction, subconscious as well as conscious mechanisms are at work. The two neural mechanisms we spoke about earlier are displayed simultaneously. The goal is to maximize the benefits of the stretch reflex and minimize the actions of the Golgi tendon organ (discussed earlier).