Whether you're a quarterback, gymnast, triathlete or everyday lifter, you use the stretch-shortening cycle (SSC). You not only use it, you rely on it to maximize your speed and power training with every rep, stride, throw and swing. This fundamental aspect of human movement is inherently ingrained in athletic development, but many athletes don't understand how it affects their training. So what is it, and how can we make use of it? Read on.
Defining the Stretch-Shortening Cycle
A stretched muscle builds tension in two primary ways: through the elastic properties of soft tissue and through activation of the muscle spindle, which causes involuntary contraction of muscle fibers that may otherwise be dormant. This, in a nutshell, is the SSC.
I explain this as a "supercharged rubber-band effect," enabling us to produce more force once the muscle has been stretched than from a standstill. On the flip side, the process may diminish our efforts in the gym by allowing us to rely on the SSC to assist with a given movement. Thus, not accounting for the process could increase risk of injury or simply render workouts less effective. Either way, understanding the SSC's impact on training is an integral part of advanced programming strategies like plyometrics or stop-and-go strength techniques.
3-Phase Approach to Rep Schemes
To further illustrate what happens during the SSC, we often break up training repetitions into three phases. For the sake of simplicity, imagine resistance training with a Bicep Curl. Muscle and connective tissue are elongated to an increasingly stretched position when you lower the weight. This is called the "eccentric phase" of the exercise, and it can be done quickly to maximize the stretch reflex (as in plyometric training) or slowly to minimize the stretch reflex and maintain eccentric mechanical load (important for increasing muscle size).
Throughout the bottom portion of the rep, when the muscle and tissue are stretched the farthest, signals travel through the nervous system, eliciting contraction of that muscle as a sort of protective mechanism to ensure no damage occurs from over-stretching. This portion of the rep—when movement stops to change direction—is called the "amortization phase." In this phase, a lot of tension is created by the stretch reflex, which can either be used to generate additional force (creating powerful concentric movement) or allowed to dissipate (contributing less to the upward phase of the movement.)
The upward movement of the exercise is called the "concentric phase," during which we normally instruct athletes to accelerate for maximum motor recruitment (number of muscle fibers engaged.)
After explaining the three phases of a rep, we can instruct our athletes specifically how to approach each phase of the rep to either minimize or maximize the stretch reflex associated with the SSC.
Maximizing the Stretch-Shortening Cycle
Power training is very much akin to skill acquisition. Although we do use overload techniques, we prioritize precision of movement when training for power production. For this reason, we focus on timing and body position throughout the movement. Timing of contraction is critical so that we contract voluntarily at the same time as our muscle spindle fires during the stretch reflex, minimizing time spent in the amortization phase.
We instruct our athletes to "find their tight spot"—i.e., the initial sensation of tightness while moving through the eccentric phase. They practice a few sets at low speeds and sub-maximal force to refine their timing. As they progress, we monitor their form to make sure other joints don't give too much (such as flexing too much at the spine), since this might absorb too much of the force they generate rather than transferring it to the ground. During a jump, for example, the athlete's shoulders, hips, knees and ankles should all hit their "tight spots" at the same time, while he keeps his spine rigid and his head high. This puts the athlete in a primed, mechanically advantageous position to get the most out of the stretch reflex. Once timing and form are solid and practiced to the point of redundancy, only then can we productively increase speed and force production.
Minimizing time spent in the amortization phase is key. Take too long at the bottom of the movement, and a lot of that would-be force generated from the stretch reflex simply dissipates. This is a learned skill, so focus on heavy refinement more than all-out fatigue. This is common practice in plyometric movements, but also a hallmark principle when training all dynamic movements, such as cutting or throwing. To elicit the highest contribution of power generated via the SSC, we need that highly refined timing and optimal skill set.
(Side note: All plyometric exercises are dynamic, but not all dynamic movements are considered "plyometric." For a solid overview of plyometric programming, check out my recent article on plyometrics.)
Changing direction is a great example of using the SSC for real-time power. Counter-movements perfectly illustrate athletes tapping energy from their SSC (almost subconsciously.) If you watch a tennis player during a volley, a pitcher during a wind-up, or an MMA fighter before he launches a powerful blow, all of them create movement in the opposite direction first to recruit more power from the SSC. As a matter of fact, many wrestlers and MMA fighters are taught to watch for counter-movements to predict an opponent's strike or move.
Athletic stance is also based on mechanical advantage–a primary component of employing the SSC. Stance plays a major role in how quickly and to what extent an athlete can create force. Joints slightly flexed, spine erect and shoulders square is a standard stance in many sports such as tennis, football and basketball. From this position, the athlete can more readily transmit force to a greater extent with a counter-movement than he or she could from a standstill.
Minimizing the Stretch-Shortening Cycle
Limiting the "bounce" effect during training sessions allows elastic energy to dissipate when the athlete changes direction, leaving him or her at a disadvantaged position to initiate contraction. This may sound like a bad thing, but consider: at least during strength training, we are trying to make exercises as difficult as possible. If the exercise is difficult, it will require more motor recruitment, force production and possibly be more effective. By limiting the elastic contribution during the rep, we enable a few things to take place that may be conducive to strength adaptations.
First, we lower the risk of injury from underlying issues of which we may be unaware. Micro-trauma, such as small tears, are common in athletic programs and may not present perceivable symptoms. The point that the athlete changes direction in a lift is the point where he or she must produce the most torque. If an adverse event is going to happen, it is more likely to do so in stretched positions, especially when engaging the bounce effect.
Second, we force the athlete to rely solely on voluntary contraction when initiating the concentric phase, since the contribution from the elastic properties is now minimized. It may still be warranted to elicit the stretch reflex in some cases, but not necessarily an aim of everyday practice. As an example, I use this "stop-and-go" approach when strength training with heavy weights during the off-season, but I might allow a small bounce effect in days leading up to competition. I probably err on the side of lighter lifts in the gym, using the stop-and-go method to avoid problems associated with lifting too much weight. I normally only maximize the stretch reflex on the field and during skill practice.
Forgetting the Stretch-Shortening Cycle
Still not sold on the importance of the SSC? Consider this: by neglecting the SSC, athletes might rely on the elasticity of their joints to assist them through a movement. Plyometric exercise is more left to chance, and power takes a back seat to circumstance if the athlete's timing and form have not been heavily refined. Any way you look at it, the athlete never realizes his or her full potential. Plateaus and injuries are right around the corner for those who don't account for the SSC's impact on their training and performance.
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