3.2 The Stretch-Shortening Cycle

The short version

Here is a test you can do right now, in whatever room you are in. Jump once by dipping quickly and springing straight back up in one smooth motion. Then jump again, but this time lower into exactly the same squat depth, freeze there for a full three seconds, and only then jump. Almost everybody gets a few centimetres higher on the first one.

Nothing about your legs changed in between. Same depth, same effort, same intent. The only thing you altered was the pause — and the pause cost you height. That gap between the two jumps is the stretch-shortening cycle, and how big your gap is tells you how much your body gets out of a fast reversal.

This article is about where that free height comes from and how to stop wasting it. There is more than one thing going on, the effect goes stale surprisingly quickly, and the length of time your foot stays on the ground turns out to be the number that decides how you should train.

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Where the free height comes from

Your tendons do most of the springing, not your muscles. This surprises people. Picture the muscle and tendon as two things joined end to end: a motor and a rubber band. During a fast bounce the motor barely changes length at all — it just holds firm and acts like an anchor — while the rubber band stretches and snaps back. So when a jump feels springy, that is mostly tendon, with the muscle doing the unglamorous job of staying tight enough for the tendon to work against.

But it is not only a rubber band. At least four things pitch in at once, which is why nobody can point to a single tidy explanation. There is the energy stored in stretched tendon. There is a reflex that fires when the stretch happens fast, adding a little extra pull. There is the fact that the muscle was already switched on from the lowering part, so it does not have to start from cold. And there is a chemical head start inside the muscle fibres themselves. Think of four people shoving a stalled car at the same moment. Arguing about which one mattered most misses the point that the car moved.

The whole gain has a very short shelf life. Stretched tendon holds energy the way a stretched elastic band does, and if you hang around at the bottom, that energy quietly leaks away as heat. The reflex fades too. Half a second of stillness is enough to throw away most of the advantage, which is why a pause turns a bouncy movement into an ordinary one. There is no way to store it for later. Use it immediately or lose it.

Ground contact time splits the whole method into two halves. Roughly speaking, if the foot is down for less than about a quarter of a second, you are in the fast version: sprinting, hurdle hops, quick skips. Longer than that and you are in the slow version: a proper countermovement jump, a heavy change of direction, a takeoff with a big dip. Both are real, both are useful, and they do not train each other. Someone brilliant at deep, slow, powerful jumps can be surprisingly ordinary at the quick, twitchy stuff, and the other way round.

Being stiff is a trade-off, not a compliment. Stiffness here means how much your leg resists being squashed on landing. More of it means better energy return and quicker contacts. Less of it means you can absorb a heavy landing more safely. It is the same argument as car suspension: firm is fast on smooth tarmac and punishing on a rough road. You can train stiffness up or down, and which direction you want depends entirely on what your sport keeps asking of you.

There is a simple way to check whether you are using any of this. Measure a jump where you dip and go, then measure a jump starting from a dead stop in a squat. Compare the two. A decent gap means you are getting real value from the fast reversal. Almost no gap means the bounce is not happening, and that is a training problem worth fixing rather than a mystery.

The rest of the article goes into the detail: each of the four contributions on its own terms, how tendon and muscle share the work, the numbers behind the fast and slow split, how to train stiffness in either direction, and how to interpret that jump comparison properly. Those sections are more technical, so read them when you want the mechanism rather than the map.

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A spring, but not only a spring

The most common explanation is that the body acts like a spring: Stretch it and it snaps back. That is a reasonable first approximation, and elastic recoil in tendon is genuinely a large part of the answer. But it is incomplete, because the nervous system is also involved, and because the muscle is in a different state at the start of a countermovement jump than it is at the start of a paused jump.

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A useful way to picture it is a muscle-tendon unit made of two parts in series: The contractile fibres, which can generate force actively, and the tendon, which behaves passively but elastically. During a fast countermovement the fibres can hold a nearly constant length while producing high force, and the tendon does the stretching and recoiling. This arrangement is efficient, because the fibres operate in a favourable part of their force-length relationship while the tendon handles the energy storage.

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Four proposed contributors to enhanced stretch-shortening cycle outputA root node labelled enhanced concentric output branching into four contributors: Elastic energy return, reflex activation, prolonged active state and force potentiation, each with supporting detail.Four proposed contributors to enhanced stretch-shortening cycle outputEnhanced concentric output after a rapidstretchElastic energy returnTendon and aponeurosisstore energyReturned if shorteningfollows quicklyDissipates as heat duringany pauseReflex activationMuscle spindles detectstretch velocityShort-latency reflexraises motor driveContribution scales withstretch rateProlonged active stateMuscle is alreadyactivated at contactForce can rise beforeshortening beginsRemoves the delay ofbuilding tensionForce potentiationCross-bridges pre-loadedby the stretchHigher force at a givenlengthEffect decays withinmilliseconds

Figure 3.2.1 — No single mechanism explains the stretch-shortening cycle. The observed performance enhancement is the combined result of at least four contributions, whose relative importance changes with the speed and amplitude of the movement.

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The four contributions

• Elastic energy return. Tendon stretched under load stores energy and returns much of it during recoil. This is the largest contribution in fast, short-contact actions.
• Reflex activation. Muscle spindles respond to the speed of stretch and produce a reflex increase in motor drive. Because it depends on stretch velocity, this contribution is larger in faster movements.
• Prolonged active state. In a countermovement the muscle is already switched on and generating tension before shortening begins. In a paused jump, tension must be rebuilt from a lower level, which takes time the movement does not have.
• Force potentiation. Stretching an active muscle appears to leave cross-bridges in a state where they produce more force at a given length. The effect is real but short-lived, decaying within milliseconds.

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The relative weight of these four shifts with the movement. In a very fast, small-amplitude action such as a sprint ground contact, elastic return and reflex activation dominate (Komi, 2000). In a slower, larger-amplitude action such as a maximal countermovement jump, the prolonged active state and simple mechanical factors such as having more time to build force become relatively more important (Bobbert et al., 1996).

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The practical takeaway does not depend on resolving the mechanisms precisely: The faster the reversal from lengthening to shortening, the more of the benefit survives.

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The mechanics and physiology in detail

Muscle and tendon behave differently, and that is the point

Direct measurement of muscle fibre behaviour during stretch-shortening cycle movements, using ultrasound imaging of the working muscle, has repeatedly shown something that surprises people: The muscle fibres frequently do not lengthen much at all (Lichtwark & Wilson, 2006). During the eccentric phase of a hopping or running contact, the fibres may remain close to isometric or even shorten slightly, while the tendon and aponeurosis take up the lengthening (Fukashiro et al., 2006).

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This decoupling is functionally important. Muscle fibres produce force most economically when they are not changing length rapidly, and they are vulnerable to damage when forcibly lengthened under high load. By allowing the tendon to absorb the length change, the system protects the fibres and stores energy in tissue that returns it efficiently. Tendon returns a high proportion of the energy stored in it, whereas active muscle dissipates much more as heat (Roberts, 2002).

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The consequence for training is that stretch-shortening cycle performance depends on tendon properties as much as on muscle strength. Two athletes with identical squat strength can have very different reactive ability because their tendon stiffness differs.

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Stiffness: What it means and why more is not always better

Stiffness describes the relationship between the force applied to a structure and the deformation that results. A stiff leg deforms little under load; a compliant leg deforms more. In sport, leg stiffness and joint stiffness are regulated actively by the level of muscle activation around the joint, which means they are adjustable moment to moment as well as trainable over time.

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Higher stiffness suits short ground contacts. In sprinting and repeated hopping, a stiff ankle allows force to be transmitted quickly with minimal energy loss, and athletes with greater leg stiffness generally show shorter contact times and better reactive strength index values (Brughelli & Cronin, 2008).

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But stiffness has a cost. A very stiff system cannot absorb large amounts of energy through range, which is exactly what is required when landing from height or decelerating rapidly. An athlete who is stiff everywhere will be reactive but will struggle to attenuate force, and the tissue takes the difference. The trained athlete needs the ability to modulate stiffness according to task, being stiff when rebounding and compliant when absorbing.

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• When to bias stiffness. Short-contact activities: Sprint mechanics, repeated hopping, bounding, fast footwork. Trained with low-amplitude, high-frequency plyometrics and heavy isometric work.
• When to bias compliance. Landing from height, deceleration from speed, absorbing contact. Trained with eccentric work, landing drills and controlled deceleration.

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Fast and slow stretch-shortening cycles

The division at roughly 250 milliseconds of ground contact is a convention rather than a physiological boundary, but it is a useful one because it separates movements where elastic and reflex contributions dominate from those where muscular work and time-to-build-force dominate (Flanagan & Comyns, 2008).

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The distinction has direct programming consequences. Training that improves maximal countermovement jump height does not reliably improve sprint ground contact mechanics, and vice versa. An athlete whose sport demands both needs exercises from both categories, and it is worth being explicit about which category a given exercise belongs to rather than treating all jumping as interchangeable.

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Fast and slow stretch-shortening cycle comparedA table comparing fast and slow stretch-shortening cycle actions across ground contact time, joint excursion, dominant mechanism and representative exercises.Fast and slow stretch-shortening cycle comparedFast SSCSlow SSCGround contact timeUnder about 250 msAbove about 250 msJoint excursionSmall; ankle dominantLarge; hip and knee dominantDominant mechanismTendon elasticity and reflexMuscular work and active stateStiffness requirementHigh leg and ankle stiffnessModerate; range is usedTypical exercisesPogo hops, depth jumps, sprintingCountermovement and box jumpsSporting examplesSprint ground contact, repeated hopsMaximal vertical jump, heavy cut

Figure 3.2.3 — The two categories are trained by different exercises and transfer to different tasks. Assuming that improving one improves the other is a common programming error.

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Quantifying how well an athlete uses the cycle

The simplest field measure is the eccentric utilisation ratio: Countermovement jump height divided by squat jump height (McGuigan et al., 2006). A ratio close to 1.0 suggests the athlete gains little from the countermovement and may benefit from reactive and elastic work. A high ratio suggests they exploit the cycle well, in which case concentric strength may be the more useful target.

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The reactive strength index, calculated as jump height divided by ground contact time in a drop jump, addresses the fast stretch-shortening cycle directly (Flanagan & Comyns, 2008). Because it penalises long ground contacts, it captures the quality that matters in short-contact sport rather than height alone.

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Both measures are most useful as within-athlete tracking tools rather than for comparison between athletes, because absolute values depend heavily on testing protocol, drop height and instructions given. Consistency of protocol matters more than the specific numbers.

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Force against time: Countermovement jump compared with squat jumpTwo force-time curves. The countermovement jump curve shows an initial dip below bodyweight followed by a high peak, while the squat jump curve starts from a flat baseline and reaches a lower peak.Force against time: Countermovement jump compared with squat jumpCountermovement jumpSquat jump (from held position)StartMid-movementTake-off0BodyweightPeakTimeVertical forceUnweighting dipHigher peak

Figure 3.2.2 — The countermovement jump reaches a higher peak force and a greater impulse than a jump started from a static squat, despite identical intent and starting depth. The difference is the contribution of the stretch-shortening cycle.

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What happens when the cycle fails: Fatigue

Stretch-shortening cycle performance is unusually sensitive to fatigue. Sustained or repeated stretch-shortening cycle exercise produces a characteristic pattern of decline in which reactive measures fall before maximal strength does, and in which recovery can be biphasic: An initial drop, a partial recovery within hours, then a second decline over the following one to two days associated with inflammatory processes and muscle damage (Nicol et al., 2006).

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This has two practical implications. First, reactive measures are sensitive early indicators of accumulated fatigue, which makes a simple drop jump or countermovement jump test a useful daily monitoring tool. Second, plyometric work performed on fatigued tissue is not merely less effective, it is training a different and slower movement pattern while loading tissue that is less able to tolerate it.

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The observable signature of stretch-shortening cycle fatigue is lengthening ground contact time with maintained or falling jump height. When contact times start to drift upward within a set, the useful part of the session is over.

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Turning the mechanism into training decisions

Because the stretch-shortening cycle is a mechanism rather than an exercise, training it well is mostly about how exercises are performed and sequenced rather than about which exercises are chosen.

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Rules that follow directly from the mechanism

1. Minimise the transition. Cue speed off the ground rather than height achieved, because the transition duration determines how much benefit survives.
2. Match the exercise to the target quality. Use short-contact, low-amplitude work for fast stretch-shortening cycle qualities and larger-amplitude jumping for slow qualities.
3. Train tendon as well as muscle. Heavy slow resistance work and long isometric holds develop tendon stiffness on a timescale plyometrics alone does not address.
4. Keep sets short and rest long, because the mechanism depends on a fresh nervous system and intact tissue.
5. Monitor contact time, not just output. Rising contact time is the signal to stop.

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Why a pause destroys the benefitFour stacked rows showing the fate of stored elastic energy as the delay between the eccentric and concentric phases increases from milliseconds to a full second.Why a pause destroys the benefitImmediate reversal (tens of milliseconds)Most stored energy is recovered. Reflex contribution arrives during shortening. Output is highest.Short delay (around 100–250 ms)Some energy has dissipated. Reflex contribution reduced. Output still above a purely concentriceffort.Moderate delay (around 0.5 s)Most elastic contribution lost. Active state partly decayed. Output approaching concentric-onlyvalues.Long pause (1 s or more)Effectively a concentric-only movement. The stretch has provided no usable benefit.Benefit retainedBenefit lost

Figure 3.2.4 — The elastic and reflex contributions are perishable. As the transition lengthens, stored energy dissipates as heat and the reflex contribution decays, until the movement becomes equivalent to a purely concentric effort.

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A practical progression for developing the cycle

• Stage 1: Absorb. Altitude landings, snap-downs, drop landings held for two to three seconds. The athlete learns to accept force before trying to return it.
• Stage 2: Rhythm. Ankle hops, pogo hops, jump rope. Short contacts with small joint excursions, teaching the stiff-ankle behaviour that fast stretch-shortening cycle work depends on.
• Stage 3: Amplitude. Countermovement jumps, box jumps, broad jumps. Larger excursions with a fast reversal, developing slow stretch-shortening cycle qualities.
• Stage 4: Repeat. Hurdle hops, repeated broad jumps, bounding. Consecutive contacts where contact time must be held short across repetitions.
• Stage 5: Shock. Depth jumps and depth drops into rebounds. Highest eccentric load, lowest volume, reserved for athletes with an established base.

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Complementary work that is easy to neglect

Two training elements support stretch-shortening cycle development but are often left out. The first is heavy slow resistance training and sustained isometrics, which drive tendon adaptation (Bohm et al., 2015). The second is eccentric-specific work, which builds the capacity to accept the high forces that occur during the eccentric phase. An athlete who only performs fast plyometric work will develop the coordination but may lack the tissue capacity to tolerate progression.

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Foot and ankle strength deserves particular attention, because in fast stretch-shortening cycle actions the ankle is the primary site of elastic energy storage and return. Calf strength through full range, isometric calf holds and intrinsic foot work all contribute to reactive ability in ways that are not obvious from watching a jump.

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Sport applications

Sprinters live at the fast end of the spectrum. Ground contact at maximum velocity lasts around a tenth of a second, which is far too short for any meaningful voluntary adjustment. Performance depends almost entirely on pre-set stiffness and elastic return, which is why sprint-focused plyometrics emphasise very short contacts and minimal joint excursion.

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Basketball and volleyball athletes need both categories. An approach jump uses a longer, larger-amplitude countermovement, while repeated jumping in a rally demands short contacts and rapid recovery of elastic qualities. Testing both a countermovement jump and a drop jump gives a fuller picture than either alone.

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Distance runners benefit from stretch-shortening cycle work primarily through improved running economy. Better elastic energy return means less metabolic cost at a given pace, and low-level plyometric work has been shown to improve economy without necessarily changing maximal oxygen uptake.

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Combat athletes rely on the cycle in the trunk and upper body as well as the legs. A punch or a throw involves a rapid stretch of the trunk musculature followed by immediate shortening, which is why rotational medicine ball work is a direct application of the same principle.

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Common mistakes

• Assuming the tendon is the whole story. Elastic return is the largest contribution in fast actions but not the only one. Attributing everything to elasticity leads to neglecting the neural and active-state contributions that respond to intent and coordination.
• Pausing at the bottom of a jump. Even a brief hesitation discards a large part of the benefit. The pause is often unconscious and is best caught on video.
• Chasing stiffness everywhere. Stiffness helps rebounding and hinders absorbing. An athlete needs to modulate it, not maximise it.
• Training only slow stretch-shortening cycle work. Maximal jumping is easy to program and easy to measure, so it dominates many programmes. Sports with short ground contacts need the fast category explicitly.
• Ignoring tendon adaptation timescales. Muscle responds in weeks, tendon in months. Progressing plyometric intensity on a muscle timescale outpaces the tissue that carries the load.
• Testing inconsistently. Eccentric utilisation ratio and reactive strength index are only meaningful when drop height, instructions and equipment stay constant between tests.
• Continuing sets past the quality threshold. Once ground contact time lengthens, the athlete is practising a slower movement and loading fatigued tissue at the same time.

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Coaching cues

• Reverse it instantly
• Do not sink, snap
• Stiff ankle, quiet knee
• Off the floor before you think
• Same sound every contact
• Absorb when landing, spring when rebounding
• Feel the bounce, not the grind

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FAQs

What is the difference between the stretch-shortening cycle and plyometrics?

The stretch-shortening cycle is the physiological mechanism: A rapid muscle lengthening followed immediately by shortening. Plyometrics are the training exercises designed to exploit that mechanism. The cycle occurs constantly in normal walking and running whether or not you train it; plyometrics are the deliberate attempt to improve how well you use it.

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How much does the countermovement actually add?

In a vertical jump the countermovement typically adds a few centimetres of height compared with a jump from a held squat, which corresponds to a meaningful percentage improvement. The exact figure varies considerably between individuals, and the size of the difference is itself informative: A small difference suggests the athlete is not exploiting the cycle well.

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Does stretching before training reduce stretch-shortening cycle performance?

Prolonged static stretching immediately before explosive activity has been shown to reduce jump and sprint performance acutely, and one proposed reason is a temporary reduction in musculotendinous stiffness (Behm & Chaouachi, 2011). Short-duration static stretching within a broader dynamic warm-up appears to have little practical effect. For explosive sessions, a dynamic warm-up with some low-level hopping is the more sensible choice.

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Can I improve tendon stiffness, and how long does it take?

Yes. Heavy slow resistance training and sustained isometric contractions are the most consistently supported methods, and measurable changes in tendon properties are typically reported over periods of two to three months rather than weeks. This slower timescale is the main reason plyometric progression should be gradual.

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Why do my ground contacts get longer as a set goes on?

That is the signature of stretch-shortening cycle fatigue. The elastic and reflex contributions degrade before maximal strength does, so the athlete compensates by spending more time on the ground to build force. It is the most reliable practical signal that the set should end.

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Is the stretch reflex the main mechanism?

It is one contributor rather than the main one, and its relative importance rises with the speed of the stretch. In slower, larger-amplitude movements the prolonged active state and the simple availability of more time to develop force account for a larger share of the enhancement.

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Do upper-body movements use the stretch-shortening cycle?

Yes. A bench press with a controlled touch-and-go at the chest, a punch, an overhead throw and a medicine ball chest pass all involve a rapid stretch followed by shortening. The same principles about minimising the transition apply.

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Recommended videos

Each video below was chosen because it covers a specific part of this article in more depth than text alone allows.

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Related reading on FitXplor

• 3.1 Introduction to Plyometrics
• 3.3 Landing Mechanics and Force Absorption
• 3.9 Reactive Strength and Elastic Qualities
• 3.8 Jump Training and Vertical Power
• 3.4 Sprint Mechanics and Maximum Velocity

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References

Komi, P. V. (2000). Stretch-shortening cycle: a powerful model to study normal and fatigued muscle. Journal of Biomechanics, 33(10), 1197–1206.

Nicol, C., Avela, J., & Komi, P. V. (2006). The stretch-shortening cycle: a model to study naturally occurring neuromuscular fatigue. Sports Medicine, 36(11), 977–999.

Bobbert, M. F., Gerritsen, K. G., Litjens, M. C., & Van Soest, A. J. (1996). Why is countermovement jump height greater than squat jump height? Medicine & Science in Sports & Exercise, 28(11), 1402–1412.

Fukashiro, S., Hay, D. C., & Nagano, A. (2006). Biomechanical behavior of muscle-tendon complex during dynamic human movements. Journal of Applied Biomechanics, 22(2), 131–147.

Roberts, T. J. (2002). The integrated function of muscles and tendons during locomotion. Comparative Biochemistry and Physiology Part A, 133(4), 1087–1099.

Lichtwark, G. A., & Wilson, A. M. (2006). Interactions between the human gastrocnemius muscle and the Achilles tendon during incline, level and decline locomotion. Journal of Experimental Biology, 209(21), 4379–4388.

Brughelli, M., & Cronin, J. (2008). Influence of running velocity on vertical, leg and joint stiffness. Sports Medicine, 38(8), 647–657.

Flanagan, E. P., & Comyns, T. M. (2008). The use of contact time and the reactive strength index to optimize fast stretch-shortening cycle training. Strength and Conditioning Journal, 30(5), 32–38.

McGuigan, M. R., Doyle, T. L., Newton, M., et al. (2006). Eccentric utilization ratio: effect of sport and phase of training. Journal of Strength and Conditioning Research, 20(4), 992–995.

Bohm, S., Mersmann, F., & Arampatzis, A. (2015). Human tendon adaptation in response to mechanical loading: a systematic review and meta-analysis. Sports Medicine – Open, 1(1), 7.

Behm, D. G., & Chaouachi, A. (2011). A review of the acute effects of static and dynamic stretching on performance. European Journal of Applied Physiology, 111(11), 2633–2651.