By: John Evans MS
It is extremely common for me to read content that makes assumptions either biased in experience, or sometimes even science, without looking at a variety of confounding factors. A good example of this is when individuals discuss the stretch-shortening cycle in the context of jump training. Many times scientists or coaches assume that they understand it perfectly because they have read a published article on the internet or in a published journal. Understand this: the stretch-shortening cycle is complex. It isn’t as easy as, “Here is a rubber band and this is what happens in the muscles and why you need to move fast.” This sort of misinformation is not helpful, and does not serve to build up the coaching community because it is a gross oversimplification of what is ACTUALLY happening during any athletic movement.
The stretch-shortening cycle is a physiological process that occurs when an individual goes through a lengthening and subsequent shortening of a musculotendinous unit(10). In other words, the tendon and muscle are stretched while lowering, and shortened on the way up, typically resulting in more force than would have been generated without the former(11). For example, an athlete performing a standing jump will jump higher if they perform a quick downward action. Commonly coaches will use this as an assessment of an athlete’s ability to utilize the stretch-shortening cycle, or to assess whether they have some sort of strength deficit that needs to be addressed. That said, this is not limited to jumping. The stretch-shortening cycle happens in most any athletic movement, and can explain some of the variation between sub-elite athletes and the truly elite(7).
Summary: The stretch-shortening cycle is when a muscle and tendon are stretched and shortened quickly, resulting in an increase in performance through the stretch reflex, muscle contraction, and tendons.
THE STRETCH REFLEX
The stretch reflex is a reflex that occurs in muscles when tiny sensory organs called Myotatic spindles are stretched very quickly resulting in an increase in muscle activation(15). That said, it is extremely important to also consider that an aggressive stretch stimulates another sensory organ, the Golgi tendon organ(21). This sensory organ functions to inhibit the muscle from contracting when it senses high tension. These two reflexes can combat each other, but in highly trained individuals the Golgi tendon organ can actually function to inhibit the antagonist through reciprocal inhibition(21). In other words, instead of relaxing the muscles you want to be active, it relaxes the muscles that need to be passive, which improves your force output(1,18–20).
Summary: The stretch reflex is one component of the stretch-shortening cycle and increases the force a muscle produces following a quick stretch. The muscle may have a lower output due to the influence of the Golgi tendon organ in untrained individuals.
Although stretching a muscle quickly can result in a greater muscular contraction, volitionally trying to contract the muscle is important to maximize the force output(1,4,8,14). If the athlete doesn’t attempt to contract their muscles forcefully, the movement will not be optimized, as the neurons controlling muscular contraction will not be stimulated. Connecting the dots in a movement takes time, but being able to contract muscles at the time is major contributor to athletic success(6). As it pertains to the stretch-shortening cycle, the athlete must practice the skill they want to improve. There must be an active and volitional choice to apply effort at the correct time, ultimately yielding increased output during any stretch-shortening cycle(7). This is explained by the increased the release of excitatory neurotransmitters, the release of calcium from the sarcoplasmic reticulum, and the rate of cross-bridging between myosin and actin(14). Ultimately this process is mediated by the neurons, which are controlled by the peripheral and central nervous system(15).
Summary: Optimizing the stretch-shortening cycle involves the voluntary choice to contract muscles at the correct time.
ENERGY STORAGE AND RELEASE
Lastly, the stretch-shortening cycle is characterized by the storage and release of energy from the tendon(3,9,12,16). This process can yield a great deal of kinetic energy that serves to increase the net impulse the athlete is able to generate. Tendons are viscoelastic, meaning it can change stiffness depending on the tension within the muscle(13). They are a stiffer spring when there is more tension within the muscle, and a softer spring at rest.
Because of this property, the tendon’s elastic return is highly variable(13,17). It is the result of distance the tendon is stretched at a given stiffness(17). The more the tendon is stretched at a greater stiffness, the more energy will be returned(13,17). This is relatively complex, but as the muscle contracts, it stiffens the tendon. When an athlete’s foot is contacting the ground, this contact results in the muscle producing force(2). Because the muscle is attached the tendon, the tendon stiffens in conjunction with the amount of force produced in the muscle. In sprinting, as the athlete reaches the amortization period during contact, the forces are typically very high inside of the muscle(5). During this portion of the leg cycle, it is possible for the muscle to stay the same length, and the tendon to lengthen and consequently shorten(16). In other words, the muscle is staying the same length while the tendon is able to release energy in proportion to the stiffness and length of the tendon. This process likely is not isolated to the amortization phase, but does help illustrate the importance of the tendons in athletic movements.
Summary: The tendon is able to store and release energy in proportion to the stiffness and length of stretch.
MISCONCEPTION NUMBER 1
“The stretch-shortening cycle only occurs in the legs during a counter movement jump or depth drop.”
The stretch-shortening cycle occurs all throughout the body. Any time an athlete stretches a muscle very quickly, followed by an aggressive shortening of the muscle, a stretch-shortening cycle occurs.
Here are a few examples:
1. The oblique, abdomen, and shoulder when striking a golf ball
2. The calf when contacting the ground in a sprint
3. The rotator cuff while throwing a baseball
4. The abdomen during a tuck in a backflip
5. The lateral hip when cutting in football
Specifically to jumping, it occurs sequentially as stated below:
1. In the hip flexor and shoulder flexors during the push from the penultimate step into plant
2. In the plant leg hamstring as it is repositioned in front of the body
3. In the knee extensors, plantar flexors, hip extensors, and truck extensors as the athlete begins to jump upwards
This is not an exhaustive list, but does help to illustrate the importance of musculature that oftentimes ignored. Because the stretch-shortening cycle is occurring to some degree at almost every phase of the jump, a strong argument can be made for incorporating specificity into the training plan, as well as developing general coordination throughout the entire body.
Summary: The stretch-shortening cycle occurs throughout the entire body during an athletic movement.
MISCONCEPTION NUMBER 2:
“The stretch reflex is the stretch-shortening cycle.”
The stretch reflex is a piece of the stretch-shortening cycle, but it is not explain the entire cycle. The stretch reflex is a reflex that can assist in the improvement of performance. It is highly sensitive to fatigue, and not limited to a given time frame. There are different latencies periods for a stretch-shortening cycle. In other words, the reflex may be stimulated by a muscular stretch, but there is a delay between the stretch and subsequent increase in contractile force. This is likely related to the electromechanical delay. There are short, medium, and long latency stretch reflexes. An athlete will improve their stretch-shortening cycle if they are able to utilize the stretch reflex that occurs within the time domain of that movement. Jumping involves around between 500-150ms, a massive time range, meaning there is potentially a lot of time to illicit a strong stretch reflex.
Summary: The stretch reflex is one component of the stretch-shortening cycle.
MISCONCEPTION NUMBER 3:
“The stretch-shortening cycle is a muscular process.”
The stretch-shortening cycle is not isolated to just the muscles. It involves the nervous system because of the stretch reflex, the tendon storing and releasing energy, and the muscles volitionally contracting to maximize force. The stretch-shortening cycle explains the culmination of a series of events that occurring throughout the body during an athletic movement. Great leapers have a perfect storm: high motor unit recruitment, low neural inhibition and stiff tendons.
Summary: The stretch-shortening cycle is multi-factorial.
MISCONCEPTION NUMBER 4:
“The stretch-shortening cycle only occurs when tissue is passively stretched aggressive or vice versa.”
The stretch-shortening cycle isn’t simple. Because there are a variety of factors that influence the stretch reflex, tendon stiffness, and contractile velocity, the stretch-shortening cycle can occur across a continuum. For faster, larger, aggressive stretches where muscle tissue is not fatigued, there appears to be a greater increase in the resulting force of that tissue. Stretch-shortening cycles can occur during passive and active stretches to a greater or lesser degree. Many times jumpers are able to utilize the stretch-shortening cycle on every step leading up to a jump even though they may not be applying maximum effort.
Summary: The stretch-shortening cycle occurs across a continuum.
MISCONCEPTION NUMBER 5:“A shorter, faster stretch is always better at eliciting a greater impulse.”
While it is tempting to assume that a shorter, faster stretch is better, and in isolated situations this may be the case, one must consider the impact that impulse has on an athlete’s performance. If an athlete such as a pitcher or golfer were to shorten their movement into the shortest most aggressive movement, it would look like throwing a dart or an aggressive putt. Obviously these athletes are benefiting from a large range of motion, as it allows them to generate force in line with the force-velocity curve. That said, these athletes do perform a stretch-shortening cycle very fast, but within the context of the movement they are performing. Specific to the examples above, a golfer achieves the greatest amount of stretch in the torso at the end of the swing, and a pitcher when the shoulder is in the greatest amount of external rotation. Jumpers benefit from a deeper squat and a long push into the plant for this exact reason. The triple extensors are able to generate for over a long time if the athlete is able to get into and out of a deep knee position quickly.
Summary: A long and fast stretch may be most beneficial for performance when the event is not constrained by time.
MISCONCEPTION NUMBER 6:“The stretch-shortening cycle happens all at once”
Similar to the idea that the stretch-shortening cycle is only relevant at the knee and ankle, one must be careful not to assume that the stretch-shortening cycle occurs at one instant. Movement is dynamic. It isn’t as a bow firing an arrow. The body is constantly changing position as different segments move into position at different times. For example, as the athlete plants the foot, the jumper experiences a stretch-shortening cycle in the hip extensors, then the knee extensors, followed by the plantar flexors. Some stretch-shortening cycles are longer and others are shorter, but the result is typically a well-timed summation of forces. As a rule of thumb, many stretch-shortening cycles begin at the most proximal joint, and finish with the most distal; however there is a lot of synchronization that occurs between the limbs.
Summary: The stretch-shortening cycle in athletic movements typically involves segmentation and synchronization of proximal to distal limbs.
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