Ask the Professional: The Stretch-Shortening Cycle
What is the Stretch-Shortening Cycle and Why Should You Know About It?
Our bodies complete a variety of different movements every single day and some movements don't even require any thought at all. One of those naturally occurring things that we continually have happen without a second thought is the stretch-shortening cycle. This is an important occurrence in everyday life, but exceptionally important to be aware of when you are regularly exercising. Knowledge of the stretch-shortening cycle alone should allow for you to have a different approach or outlook on your own exercise.
The stretch-shortening cycle is the rapid cyclical muscle action where the muscle undergoes an eccentric contraction followed by a transitional period prior to the concentric contraction. Put simply, it is the “counter movement” your muscles make before changing direction allowing you to expend a greater amount of force. Your body utilizes the tension generated by the extra lengthening of the muscle in order to change your direction with greater force and power than moving the same load from a dead-stop position. More or less, it is essentially a spring-like action that causes a powerful rebound, the spring being the loaded muscle. This happens every time you prepare to jump! Just imagine trying to jump without the preparation, nearly impossible! The quicker that the muscle is contracted eccentrically, or the greater the force applied, the greater the reflex, or change of direction will be.
There are three phases to the stretch-shortening cycle: eccentric, amortization, and concentric. The eccentric phase includes the eccentric contraction, or loading of the muscle. The amortization phase is the transitional phase where the direction of the force is reversed, turning that eccentric load into a more powerful concentric phase. The concentric phase includes the concentric contraction, where you are applying the force to the external load in the opposite direction of the eccentric loading.
The stretch-shortening cycle can most commonly be observed in different types of jumping movements. However, it occurs ALL the time when any limb is going through any change of direction such as walking, running, twisting, raising your arms, touching your toes, etc. The speed of the stretch-shortening cycle is going to depend on the speed of the movement and/or force required. The stretch-shortening cycle in movements where the cycle is 250 milliseconds or less is classified as the “fast” stretch-shortening cycle. When movements are greater than 250 milliseconds, it is classified as the “slow” stretch-shortening cycle.
You can apply the knowledge of this concept directly to your exercise. Do you want to be able to generate maximum force? Limit the amount of time in the amortization phase so your concentric contraction takes place almost immediately after the completion of your eccentric contraction. However, you still need to pay attention to your form! Minimizing the amortization phase does not necessarily mean you should bounce the bar off of your chest during the bench press, or bounce out of the very bottom of the squat. While this may possibly be the quickest way to change the direction of the weight, it is at the expense of your form and in some cases your joint health as well. If you are passively bouncing through your movement, chances are you aren't maintaining tension in your muscles. Maintaining that muscle tension is key to getting the most out of the stretch-shortening cycle and producing the most force.
Beyond generating maximum force, knowledge of the stretch-shortening cycle is also a great tool to use to build strength. While the stretch-shortening cycle will help you generate the most force you're capable of producing, it's limited in increasing your maximum amount of force. In order to do that, you need to build strength both eccentrically and concentrically. You will also want to know how to limit the effect of the stretch-shortening cycle on your movement. This is where tempo reps come in to play, when you manipulate the variable of time within a given repetition. This typically occurs in the form of eccentric/negative reps or pause reps. For eccentric/negative reps, you are spending most of your time in the eccentric portion of the movement. For example, in the squat you might slow yourself down to the point where you need to count to 5 during your descent before you reach the bottom position of your squat. By spending more time in the eccentric portion of the movement, you are building more eccentric strength. Pause reps apply a similar concept. Instead of spending extra time in the eccentric portion of the movement, you spend that extra time at the bottom of the movement. So for the squat again, you may get to the bottom position and take a 3-second pause before pressing yourself back up. This almost completely eliminates the stretch reflex and allows for you to build strength and power in the concentric portion of your movement because you are unable to use that “spring effect” generated by the stretch-shortening cycle. Moving the weight then lies completely on the force you can generate with your concentric contraction alone. Manipulating the stretch-shortening cycle will help to build muscular strength.
Whether you are looking to maximize your force generation or increase your muscular strength, it pays to be familiar with the stretch-shortening cycle and how it works. It may be a variable that are already manipulating in your training, but it helps to know why. If you find yourself hitting a plateau in the amount of weight you're lifting, maybe it's time to start manipulating the stretch-shortening cycle and apply it to your training! You will very likely see some significant changes in your overall strength and maximum force generation.