Power of the Stretch by strength coach Jamie Hale
Let’s start by performing a simple test. Crouch down into a squatting position and jump. Now, crouch down into a squatting position, pause two seconds and jump. The height of the second jump was probably significantly lower than the height of the first jump. The second jump disrupted the stretch – shortening cycle.
The stretch-shortening cycle occurs in a specific order. As the body performs an eccentric action the body stores potential kinetic energy. When the concentric action quickly follows the eccentric phase the kinetic energy is utilized. The stretch-shortening cycle is displayed in numerous sporting events. This sequence of movement is often referred to as reversible muscle action in sporting events. Examples of the stretch-shortening cycle would include the wind up by a pitcher in baseball. In powerlifting bench pressing and squatting are SSC movements. In boxing fighters usually perform a slight pre-stretch before throwing a punch. To appreciate this scenario viewing the punch in slow motion will probably be necessary. The success achieved in running and throwing events are to a large extent dependent on the efficiency of utilizing the SSC. An excellent point concerning the SSC is pointed out by Charles Staley in his book The Science of Martial Arts Training.
The deadlift and squat stimulate very similar movement patterns, but almost everyone can perform more weight in the squat than they can the deadlift. One reason being the squat begins with the eccentric phase. In comparison the deadlift begins with the concentric phase of the exercise; thus eliminating the SSC. The SSC preserves energy due to the storage of elasticity in the tendons and muscles. When trainees first begin training notice how fast their movements are. The reason this occurs is because it is the bodies natural tendency to exert the least effort possible. The SSC allows the body to perform the movement with less metabolic cost. When someone stands up from a sitting position notice they will always rock slightly backwards before standing (Staley : 1999), another example of the SSC.
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 (Siff : 2000). 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 (Zatsiorsky : 1995). 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 (Zatsiorsky : 1995).
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 (Zatsiorsky : 1995). 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).
A note on reversible muscle action training. In the U.S.A plyometrics is the term usually used to describe this type of training. Pliometrics (original spelling) means lengthening or eccentric phase of a movement. The term reversible muscle action or stretch-shortening cycle is more suitable for activities involving rapid stretching followed by explosive contraction. If hypertrophy is your main goal SSC should probably be performed in moderation. Remember SSC limits metabolic stimulation which is of paramount importance to bodybuilders. When the eccentric phase of an exercise is followed by a 1-2 second pause a great deal of the potential kinetic energy is lost; therefore having a higher metabolic cost to the body.
Siff, M.(2000) Supertraining Staley, C.(1999) The Science of Martial Arts Training Zatsiorsky, V.(1995) Science and Practice of Strength.