It is well known that the stretch reflex is the key to plyometric or explosive training. However the exact muscle fiber recruitment and the interplay between the different muscle fibers are frequently misunderstood. Also the many different ways that the stretch reflex can be used is frequently unknown.
Learning how to use the stretch reflex when doing certain exercises can bring about great development of the fast twitch (explosive) muscle fibers with a quick, high intensity muscle contraction. Understand that maximal muscle power output and the potential for explosive movement is determined mainly by the proportion of fast twitch fibers involved in the movement.
For example, weightlifters and sprinters who must do more explosive movements have a considerably higher proportion of fast twitch fibers than bodybuilders, powerlifters and endurance athletes. High intensity is not necessarily dependent upon the use of near-maximal or maximal loads but more to the degree to which the relevant muscle fibers are recruited during the effort.
The terms “fast twitch” and “slow twitch” do not necessarily mean that fast movements recruit exclusively fast twitch fibers and slow movements slow twitch fibers. The fibers involved are determined by the force that is produced.
For example, the maximum force generated during rapid acceleration of a 220 pound bench press can easily exceed the maximum force produced during a slowly accelerated 330 pound bench press. Both a small load accelerated rapidly and a heavy load accelerated slowly, strongly involve the fast twitch fibers.
But the explosive movements rely heavily on the action of the fast twitch fibers. Also, accelerated movements recruit the muscle stretch (myotatic) reflex which can elicit a faster and more powerful contraction. The pre-stretch principle is well known in sports and in the plyometric (shock) method in explosive training.
Fast twitch fibers are the main contributors to force production in ballistic motion while the slow twitch fibers make their major contribution during very slow movements. Fast twitch fiber action can be impaired by the growth of slow twitch fibers since they appear to have a dampening effect on fast twitch contractions during quick or fast movements.
Thus, even though heavy resistance training serves as a powerful stimulus for the development and hypertrophy of both slow twitch and fast twitch muscle fibers, fast twitch fiber development requires special training.
Training the fast twitch fibers also includes the storage and release of elastic energy by the connective tissues in the muscle/tendon complex and should not be ignored by bodybuilders, powerlifters, power athletes and endurance athletes. This is done by involving the stretch reflex.
The stretch reflex entails accumulation of energy in the muscle-tendon complex prior to a quick explosive contraction, which accelerates the weight or object held. After this, the weight or object moves on its own momentum followed by some muscle involvement until movement stops.
When using the stretch reflex the muscles do not react in the same manner as during a slow movement. During slow movements the main muscles involved in performing the movement as well as their antagonists undergo contraction to control the movement via feedback over the full range of motion.
To use the stretch reflex to get a more explosive contraction you must program it in advance. This is needed so that the execution is safe and under control throughout the entire range of motion.
The bench press can be used to illustrate how the stretch reflex is used to elicit a more powerful muscle contraction. Begin the exercise with the arms straight, fully extended above the chest and holding a barbell. Lower the barbell at a normal rate of speed and as you approach the bottom position, before touching the chest, quickly change directions to accelerate the barbell upward.
In essence, you blast out of the bottom position and then allow the barbell to continue movement on its own inertia and later use the muscles to completely extend the arms. Pause after completion and then repeat for up to ten repetitions and no more.
During the exercise it is important that you inhale and hold the breath as you lower the barbell and execute the explosive change in direction and acceleration phase in the bottom position. Exhale after the barbell has passed the most difficult portion of the lift or when the arms are extended. The breath holding is needed during the explosive movements to stabilize the trunk for safety and to unsure effective execution.
When the bench press is executed using the stretch reflex, there is much greater force created in the initial pushing phase. The reason for this is that you use the energy accumulated on the down movement to execute the switch and upward acceleration of the barbell. Keep in mind that as you lower the barbell the triceps, pectorals and anterior deltoids undergo an eccentric contraction during which the muscles lengthen and more tension is created.
This muscle tension is then used to stop the downward movement and to switch to a brief isometric when the movement stops going down and then the concentric contraction with which you accelerate the barbell upward. The eccentric contraction is most important for eliciting the stretch reflex and for controlling the barbell on the down phase.
The stretch reflex allows you to not only use the strength of the muscles involved but also the energy accumulated on the down phase to enable you to generate even more power, using mainly the fast twitch fibers. If you pause in the bottom position, as often occurs in a slow movement, the energy gained in the eccentric contraction will be dissipated in the form of heat. As a result, you will have to generate additional concentric strength to push the barbell back up.
For more information on the stretch reflex read Explosive Plyometrics.