3.9 Reactive Strength and Elastic Qualities

The short version

Reactive strength is the ability to turn a landing into a take-off before the landing has properly finished. It is the reason some people look strong in the gym and slow on the pitch, and it is very often the missing piece.

This article covers what it actually is, how to measure it, why it matters more than most of the numbers people chase, and how to train it without wrecking the tissue that makes it possible. Anyone who runs, cuts, jumps or strikes needs it. So does any coach staring at an athlete with brilliant gym numbers and disappointing field speed, and anyone bringing someone back from an Achilles, patellar or hamstring problem.

The main thing to hold onto is that this is a stiffness and timing quality rather than a strength quality. Short ground contacts build it and fatigue wipes it out. Get it right and your contacts get shorter, your top speed climbs, you turn faster, and you handle high-speed running far better. It assumes you already have landing mechanics (3.3), the stretch-shortening cycle (3.2) and jump basics (3.8) behind you.

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The reactive strength continuumChain of five jump types ordered from long to short ground contact time.The reactive strength continuumCountermovementjumpRoughly 500–600 mscontact; slowstretch-shorteningcycleBox jump and broadjumpRoughly 300–400 ms;mixed contributionDrop jump from lowheightRoughly 200–250 ms;fast stretch-shorteningcycle beginsDepth jumpRoughly 150–200 ms;elastic return dominantRepeat ankle hopsUnder 150 ms; tendonstiffness and reflexcontribution

Figure. The reactive strength continuum. Ground contact time is the organising variable, and each band trains a different balance of muscular work and tendon elasticity.

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Part 1 — Beginner Section: What Reactive Strength Is

The definition, in plain language

Reactive strength is how well you use the ground. When your foot lands, two things happen in the same instant: the impact stretches your tissue, and you are already trying to push off. Reactive strength is being able to do the second one before the first has finished.

In real life it looks like this. Someone with poor reactive strength lands, sinks, gathers themselves, and then pushes. Someone with good reactive strength lands and leaves. The whole exchange is over faster than a blink.

The bouncing ball picture

Drop a basketball and it comes most of the way back up. Drop a beanbag and it just sits there. Same height, same energy going in. What changed is what happened to that energy on contact: the basketball stored it in a stiff, springy wall and handed it straight back, while the beanbag swallowed it and turned it into heat and a dent.

People sit somewhere along that line. All of this training is really about dragging someone from the beanbag end toward the basketball end — while making sure the ball is strong enough not to burst.

Why this beats most of your gym numbers

There simply is not time to push. At full sprinting speed your foot is on the ground for roughly a tenth of a second (Taylor & Beneke, 2012). A deliberate muscle contraction takes longer than that just to reach full force. So at top speed you are not really pushing in any conscious sense at all. You are landing on a leg you stiffened in advance and letting the tendon get on with it.

Which is why a big squat does not automatically make anyone fast. A squat trains you to produce large force over a few hundred milliseconds. Sprinting needs useful force in under a hundred. Reactive strength is the bridge between those two worlds, and without it your strength turns up late and misses the moment completely.

Where a beginner should actually start

Not with depth jumps. Start with skipping, ankle hops and pogo hops done with a steady rhythm and quiet feet — plus the discipline to end the set the moment that rhythm falls apart. Ten minutes twice a week of genuinely well-executed low bouncing will do more over two months than any advanced plyometric programme you are not ready for.

And get strong alongside it. Reactive strength sits on top of a base of controlled lowering strength and stable joints (Haff & Triplett, 2016). If you cannot control a single-leg landing, there is no elastic system worth training yet, because your nervous system will deliberately keep that joint soft to protect you. It is not being awkward. It is refusing to hand you a spring you cannot hold onto.

Part 2 goes into the detail: leg stiffness and the spring-mass model, the reactive strength index and how to read it, testing protocols, and full training progressions. It is more technical, so read it when you want the mechanism rather than the map.

Part 2 — Advanced Section: The Science of Reactivity

Leg stiffness as the governing variable

Reactive tasks are usefully modelled with a spring-mass system: The body is a point mass bouncing on a single linear spring representing the whole limb. Vertical stiffness is peak ground reaction force divided by the vertical displacement of the centre of mass. Leg stiffness is peak force divided by the change in leg length (Butler et al., 2003).

Stiffness is not a fixed property. It is regulated moment to moment by the nervous system through pre-activation, co-contraction and joint angle selection. An athlete arriving at the ground with the ankle pre-set in slight dorsiflexion and the glutes and hamstrings already firing has a stiff, efficient limb. The same athlete arriving relaxed has a soft one. This is why reactive strength responds so quickly to training at first: Much of the early gain is a change in motor strategy, not tissue (Lloyd et al., 2012).

Pre-activation and the role of feed-forward control

Electromyography during hopping and running shows substantial muscle activity 50 to 100 milliseconds before ground contact (Komi, 2000). This pre-activation is anticipatory, not reflexive. The nervous system predicts the impact and sets limb stiffness in advance.

This has two coaching consequences. First, reactive training is heavily skill-based and improves with familiarity of the task and the surface. Second, unfamiliar or unpredictable surfaces temporarily reduce reactive performance, which is worth remembering when testing athletes in a new facility.

The short-latency stretch reflex

On top of pre-activation sits the short-latency stretch reflex, mediated by muscle spindles and arriving roughly 30 to 50 milliseconds after stretch onset (Komi, 2000). In fast stretch-shortening cycle actions this reflex contribution is real and adds to force during the propulsive phase. In slow actions there is time for higher-order control to intervene and the reflex contribution matters less.

Golgi tendon organs report tendon tension to the spinal cord through Ib afferents, and the familiar picture of a protective brake that chronic reactive training switches off does not hold up (Chalmers, 2002). Ib feedback is one input into task-dependent spinal and supraspinal control, and whether it reduces or supports motor output depends on the task, the phase of the movement and the joint position rather than on a fixed force threshold. Why a trained athlete absorbs landing forces that would fold an untrained one is better explained by pre-activation, timing and the tissue properties described below than by the withdrawal of an autogenic inhibition brake.

Tendon behaviour: The real engine

During a fast ground contact the muscle fascicles of the triceps surae operate close to isometrically while the tendon lengthens and shortens around them (Lichtwark & Wilson, 2006). The muscle acts as a strut; the tendon acts as the spring. This is the decoupling phenomenon, and it is the reason tendon stiffness rather than muscle strength dominates short-contact performance.

Two structural properties matter. Tendon stiffness determines how quickly stored energy is returned. Tendon cross-sectional area and material properties determine how much load the structure tolerates before damage. The first is trained by heavy slow resistance and long-duration high-load isometrics; the second by progressive, patient exposure over months (Arampatzis et al., 2007).

A dangerous mismatch arises when muscular strength and neural drive improve faster than tendon can adapt. The athlete can now generate forces the tendon has not been prepared for. This is the classic mechanism behind mid-season patellar and Achilles tendinopathy in athletes who made rapid strength gains in the off-season.

Fascial and connective tissue contribution

Force transmission is not confined to the tendon of the prime mover. Epimysial and intermuscular connective tissue transmit a meaningful proportion of force laterally between adjacent muscles, and the plantar fascia and long posterior chain structures behave as a continuous elastic system during bouncing gait. The windlass mechanism of the foot — where toe extension tensions the plantar fascia and raises the arch — is a genuine spring that contributes to push-off. Article 1.6 covers the connective tissue system in detail; the practical point here is that foot and ankle stiffness is trainable and materially affects reactive output.

Reactive strength index: Definition and interpretation

RSI equals jump height divided by ground contact time. A modified version, RSI-mod, divides jump height by time to take-off in a countermovement jump and is used where contact time cannot be measured directly (Flanagan & Comyns, 2008).

Interpretation depends on both terms. The most common testing error is to celebrate an increase in RSI that was produced entirely by jumping higher from a much longer contact — which is a different quality altogether. Fix a contact-time ceiling and enforce it.

Typical values

• Untrained or general population: RSI below 1.0. Contact times routinely above 300 ms.
• Trained team-sport athlete: RSI 1.5 to 2.2, contact times 180 to 240 ms.
• Well-trained sprinter or jumper: RSI 2.5 to 3.5 and above, contact times often under 170 ms.
• Meaningful change: Around 8 to 10 per cent in a healthy athlete over an eight-week block. Day-to-day noise is roughly 4 to 6 per cent.

Why reactive strength collapses first under fatigue

Reactive performance depends on high-frequency motor unit firing and on precise timing. Both are early casualties of central fatigue. In practice, RSI falls before maximal strength does and recovers more slowly. That makes it an excellent early-warning fatigue marker, and it also means reactive training placed at the end of a hard session trains the opposite of what was intended.

Part 3 — Practical Section: Training Reactive Strength

The three levers

Reactive strength improves through three distinct mechanisms, and a complete programme touches all of them.

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• Neural: Stiffness regulation and timing. Trained by short-contact bouncing at low to moderate amplitude, performed fresh, with strict quality control. Adaptation is fast — two to six weeks (Lloyd et al., 2012).
• Structural: Tendon stiffness and tolerance. Trained by heavy slow resistance and long isometrics at high load. Adaptation is slow — three to six months.
• Muscular: Eccentric strength and joint control. Trained by eccentric-emphasis lifting, single-leg work and landing progressions. Adaptation is intermediate — six to twelve weeks.

Programmes that use only the first lever produce athletes who improve for a month and then get injured. Programmes that use only the second produce athletes who are robust and slow.

Exercise selection by contact time

Programming the neural component

Volume. Count a contact as one ground strike the athlete is deliberately trying to be reactive on. Low-amplitude means pogo, ankle and line hops below roughly knee height; high-intensity means depth jumps, maximal bounds and anything from a drop. As conservative starting defaults rather than tested ceilings, 40 to 100 total contacts per session for low-amplitude work and 20 to 40 for true high-intensity work. Sprinting, jumping and change of direction in practice and matches are reactive contacts too, and belong in the same weekly count.

Set structure. Short sets. Five to ten contacts for bounding and hurdle hops; three to five for depth jumps; ten to twenty for pogo-style work. The set ends when contact time drifts, not when the rep count is reached.

Rest. Two to three minutes between high-intensity reactive sets. Sixty to ninety seconds for low-amplitude work.

Frequency. Two sessions per week for development, one for maintenance, with high-intensity work usually kept off consecutive days. That spacing is a conservative coaching default, not an evidence-based rule: no fixed recovery interval or universal weekly contact ceiling has been established for a general athletic population. Progress on symptoms, contact time, reactive output and training age instead, and be more conservative again where the athlete is in rehabilitation or working back towards sport (Chmielewski et al., 2006).

Position in the session. Immediately after the warm-up. Reactive work first, always.

Programming the structural component

Heavy slow resistance is the best-evidenced tendon intervention. The template that has held up across studies is three to four sets of six to eight repetitions, at loads producing genuine difficulty, with a three-second lowering and a three-second raising phase, three times per week, for at least twelve weeks (Kongsgaard et al., 2010). For the patellar tendon that means loaded squat and leg press variations; for the Achilles, seated and standing calf raises through full range.

Long-duration isometrics are the useful alternative when speed of contraction must be limited, for example in-season or in early rehabilitation. Five sets of 30 to 45 seconds at a high percentage of maximal voluntary contraction, performed in a mid-range joint position, is a defensible dose (Rio et al., 2015). Isometrics also have a short-term analgesic effect in reactive tendinopathy, which makes them useful before training (Rio et al., 2015).

Programming the muscular component

Eccentric-emphasis work builds the tissue tolerance that allows short contacts to be safe. Nordic hamstring curls, slow eccentric step-downs, and tempo split squats are the highest-value options. Two sessions per week of low volume — two to four sets of four to eight repetitions — is sufficient alongside everything else.

A twelve-week reactive strength block

• Weeks 1 to 3 — foundation. Pogo hops, ankle hops, line hops, drop-and-stick landings. 60 to 80 contacts, twice weekly. Heavy slow resistance introduced three times weekly. Establish baseline RSI at four drop heights.
• Weeks 4 to 6 — extension. Add bounding and low hurdle hops. 60 to 90 contacts, twice weekly. Continue heavy slow resistance. Introduce single-leg hops for distance.
• Weeks 7 to 9 — intensification. Depth jumps from the tested optimal height introduced at 20 to 30 contacts once weekly. Bounding volume maintained. Heavy slow resistance reduced to twice weekly to make room.
• Weeks 10 to 11 — realisation. Low volume, high quality. 20 contacts of depth jumps and maximal bounds, plus max velocity sprinting. Strength work reduced to maintenance.
• Week 12 — deload and retest. Half volume for the first four days, then retest the full RSI battery on day six.

Monitoring and autoregulation

• Contact time is the primary autoregulation variable. Set a ceiling before the session, measure with a jump mat or a 240-frames-per-second phone video, and end the set on breach.
• Sound is a free sensor. Loud contacts mean the athlete is now absorbing rather than reflecting.
• Weekly RSI trend. A sustained decline of more than 10 per cent over two weeks means the reactive dose is exceeding recovery.
• Tendon load monitoring. Morning stiffness lasting beyond a few minutes, or pain that increases across consecutive sessions rather than settling, means the structural component is behind the neural one.

Surfaces and footwear

Reactive work belongs on a firm but forgiving surface. Sprung timber, synthetic track and firm grass are ideal. Concrete increases impact peak without increasing useful stiffness training. Very soft surfaces such as deep sand or thick foam eliminate the elastic return entirely and turn a reactive drill into a strength-endurance drill — occasionally useful, but not the same stimulus.

Footwear should be relatively firm and low-drop for reactive work. Highly cushioned shoes lengthen ground contact and reduce the proprioceptive information the athlete needs to regulate stiffness.

Part 4 — Sport Applications

Sprinting. The purest expression. Reactive strength index correlates with maximum velocity more reliably than any strength test. Wicket runs, ankling, bounding and short-contact hopping form the backbone of a sprinter's plyometric programme, and depth jumps are used sparingly and only when fresh.

mixed martial arts (MMA). Reactive qualities show up in the ability to re-set stance quickly, to sprawl and immediately drive forward, and to recover balance after a missed strike. Train with lateral and multidirectional low-amplitude hops, sprawl-to-sprint drills and short-contact bounding. Volume must stay low because sparring already loads the system.

Boxing. Footwork is a continuous low-amplitude reactive task. Rope work, in-place pogo variations and lateral line hops transfer directly. The rear-foot drive of a power punch behaves like a short horizontal contact.

Wrestling and Brazilian jiu-jitsu (BJJ). Reactive strength is expressed against an opponent rather than the ground, and contact times are longer. Prioritise eccentric and isometric strength and low-amplitude hopping for tissue health rather than chasing high RSI.

Football (soccer). Repeated accelerations and cuts. Single-leg lateral and horizontal hops with short contacts, plus hamstring eccentric work, are the highest-value combination. Reactive volume must be counted alongside sprint and match load.

American football. Skill positions live in the short-contact world and benefit from full reactive development. Linemen work in a longer contact window; their reactive priority is the first two steps and the ability to reset after contact.

Basketball and volleyball. Extremely high native reactive volume from the sport itself. In-season additional reactive work should be minimal and targeted at tendon health rather than performance. Off-season is where RSI is built.

Hockey. The skating stride has a long push phase and no flight, so on-ice reactive demand is low. Off-ice reactive training is therefore valuable for general athleticism, injury resilience and the transitions between skating and stopping.

Rugby. High reactive demand in evasion and support running, combined with heavy collision loading. Progress reactive volume conservatively during contact-heavy phases.

Tennis. The split-step is a reactive drop-landing repeated hundreds of times per match. Train it explicitly: Split-step to lateral bound, split-step to sprint, with short contacts.

Baseball. Reactive qualities appear in the lead-leg block and in first-step quickness on the base paths. Lateral hops and short-contact linear work transfer best.

Olympic weightlifting and powerlifting. Limited direct application. Low-amplitude reactive work is best used as a warm-up potentiator and a nervous-system readiness check rather than as a training goal in itself.

Exercise Library

Pogo Hops (Low-Amplitude Bounce)

• Purpose. Develop ankle stiffness and the shortest tolerable ground contact.
• Primary muscles. Gastrocnemius, soleus. Secondary. Intrinsic foot muscles, tibialis anterior, quadriceps.
• Movement pattern. Fast stretch-shortening cycle (SSC), ankle-dominant. Difficulty. Beginner. Equipment. Bodyweight.
• Coaching cues. Stiff ankle set before contact; forefoot only; short, quiet, metronomic.
• Common mistakes. Heel contact; knee flexion; chasing height; letting rhythm decay.
• Progressions. Single-leg pogos, lateral pogos, pogos with a resistance band. Regressions. Hand-supported ankle hops.
• Sport applications. Sprinting, tennis, boxing footwork, return to running.
• When to use. Warm-ups and foundation blocks. When not to use. With acute Achilles or plantar pain.
• Programming. 3 to 5 sets of 10 to 20 contacts.

Alternate-Leg Bounding

• Purpose. Develop horizontal reactive strength and single-leg elastic capacity.
• Primary muscles. Gluteus maximus, hamstrings, triceps surae. Secondary. Quadriceps, trunk, hip flexors.
• Movement pattern. Alternating single-leg horizontal SSC. Difficulty. Advanced. Equipment. Bodyweight, forgiving surface.
• Coaching cues. Push the ground back and down, then float; drive the opposite knee through; stay tall.
• Common mistakes. Turning it into fast running; landing with the foot ahead of the hip; excessive volume on hard surfaces.
• Progressions. Bounds for distance, single-leg repeated bounds, resisted bounds. Regressions. Skipping for height, power skips.
• Sport applications. Sprint acceleration, football, rugby, long jump.
• When to use. Off-season and pre-season, fresh. When not to use. With hamstring sensitivity or within 72 hours of a match.
• Programming. 4 to 6 sets of 6 to 10 contacts, 2 to 3 minutes rest.

Hurdle Hops (Continuous)

• Purpose. Train repeated short contacts under a defined amplitude.
• Primary muscles. Triceps surae, quadriceps, gluteus maximus. Secondary. Hamstrings, trunk.
• Movement pattern. Fast SSC, bilateral vertical. Difficulty. Intermediate. Equipment. Hurdles.
• Coaching cues. Stiff ankles; minimal contact between hurdles; cycle the knees, do not tuck.
• Common mistakes. Hurdles too high; tucking to clear; heavy flat landings; continuing once rhythm decays.
• Progressions. Higher hurdles, single-leg hurdle hops, hurdle hop to sprint. Regressions. Line hops, low mini-hurdles.
• Sport applications. Basketball, volleyball, sprinting, court sports.
• When to use. Development blocks. When not to use. As a conditioning circuit.
• Programming. 4 to 6 sets of 5 to 8 hurdles.

Depth Jump for RSI

• Purpose. Maximal reactive strength development and drop-height testing.
• Primary muscles. Quadriceps, gluteus maximus, triceps surae. Secondary. Hamstrings, trunk, tibialis anterior.
• Movement pattern. Fast SSC, bilateral. Difficulty. Advanced. Equipment. Plyo box, jump mat if available.
• Coaching cues. Step off; pre-tension before contact; touch and go; stop on contact-time breach.
• Common mistakes. Excessive drop height; deep squat on landing; performing when fatigued.
• Progressions. Drop height increased only when RSI is maintained; depth jump to hurdle; depth jump to sprint. Regressions. Drop landings, hurdle hops.
• Sport applications. Sprinting, jumping events, American football skill positions.
• When to use. Intensification blocks, fresh, low volume. When not to use. In-season for high-jump-volume sports, or with tendon symptoms.
• Programming. 4 to 6 sets of 3 to 5 repetitions, 2 to 3 minutes rest, once weekly.

Heavy Slow Resistance Calf Raise

• Purpose. Build Achilles tendon stiffness and load tolerance.
• Primary muscles. Gastrocnemius, soleus. Secondary. Tibialis posterior, intrinsic foot muscles.
• Movement pattern. Slow isolated plantarflexion. Difficulty. Beginner. Equipment. Machine, barbell or dumbbells, step.
• Coaching cues. Three seconds down, three seconds up; full range at both ends; knee straight for gastrocnemius, bent for soleus.
• Common mistakes. Bouncing; partial range; too light to drive tendon adaptation.
• Progressions. Load increases; single-leg; deficit range. Regressions. Bodyweight double-leg, seated variation.
• Sport applications. Every running and jumping sport; central to Achilles rehabilitation.
• When to use. Three times weekly during structural blocks. When not to use. As a replacement for reactive work.
• Programming. 3 to 4 sets of 6 to 8 repetitions with a 3-second eccentric and 3-second concentric.

Recommended Viewing

Individual video URLs change over time, so each entry names the channel and the exact search topic within it.

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• ALTIS — search for "wicket runs" and "plyometric progression". The clearest available demonstration of short-contact mechanics in a sprint context.
• Cal Dietz — search for "triphasic isometric" and "reactive strength". Explains the eccentric-isometric-concentric framing that underpins reactive development blocks.
• E3 Rehab — search for "Achilles tendinopathy" and "heavy slow resistance". The best free summary of tendon loading evidence.
• Squat University — search for "plantar fascia" and "foot stiffness". Useful anatomy behind the windlass mechanism.
• PJF Performance — search for "elastic strength" and "ankle stiffness". Practical coaching progressions for court-sport athletes.
• Institute of Human Anatomy — search for "Achilles tendon" and "plantar fascia". Cadaveric footage that makes the spring model concrete.

Frequently Asked Questions

Can I improve reactive strength without plyometrics? Partially. Heavy slow resistance and isometrics will build tendon stiffness, and that raises the ceiling. But the timing and stiffness-regulation component only improves through short-contact tasks.

Is a higher RSI always better? Up to the point where the athlete can tolerate the associated loads. An athlete with an RSI of 3.0 and a chronically irritated patellar tendon is worse off than one at 2.4 who trains every week.

How much reactive work is too much? When contact times lengthen within sessions, when RSI trends down for two weeks, or when tendon stiffness persists into the following morning. Those three signals catch almost every case.

Should reactive training be done barefoot? Low-amplitude work, sometimes. High-amplitude and depth work, no. The impact peaks are too high to remove footwear protection while also increasing training load.

Does jumping rope build reactive strength? It builds ankle stiffness and rhythm at very low amplitude, which is a genuine but limited part of the picture. It will not develop the high-force reactive qualities that bounding and depth jumps target.

My RSI dropped after a strength block. Is that bad? Usually not. Accumulated fatigue suppresses reactive output first. Reduce volume for one to two weeks and retest before drawing conclusions.

Research Summary

The literature supports several consistent conclusions. Reactive strength index relates strongly to sprint performance, particularly maximum velocity, and to change-of-direction ability. Leg and vertical stiffness are modifiable by training and are regulated largely through pre-activation rather than reflex activity alone. Muscle fascicles operate near-isometrically during fast ground contacts while the tendon performs most of the length change, which places tendon mechanical properties at the centre of reactive performance. Heavy slow resistance and long-duration high-load isometrics produce measurable increases in tendon stiffness and cross-sectional area over 12 weeks or more, whereas plyometric training alone produces smaller structural change (Arampatzis et al., 2007; Kongsgaard et al., 2010). Optimal drop height for depth jumping is individual and should be selected by reactive strength index rather than by a fixed prescription (Flanagan & Comyns, 2008). Reactive performance is among the earliest measures to decline under accumulated neuromuscular fatigue (Komi, 2000).

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References

Arampatzis, A., Karamanidis, K., & Albracht, K. (2007). Adaptational responses of the human Achilles tendon by modulation of the applied cyclic strain magnitude. Journal of Experimental Biology, 210(15), 2743-2753. https://doi.org/10.1242/jeb.003814

Beattie, K., Carson, B. P., Lyons, M., & Kenny, I. C. (2017). The effect of maximal- and explosive-strength training on performance indicators in cyclists and runners. Journal of Strength and Conditioning Research, 31(4), 999-1015. https://doi.org/10.1519/JSC.0000000000001584

Butler, R. J., Crowell, H. P., & Davis, I. M. (2003). Lower extremity stiffness: Implications for performance and injury. Clinical Biomechanics, 18(6), 511-517. https://doi.org/10.1016/S0268-0033(03)00071-8

Chalmers, G. (2002). Strength training: Do Golgi tendon organs really inhibit muscle activity at high force levels to save muscles from injury, and adapt with strength training? Sports Biomechanics, 1(2), 239–249. Read on Europe PMC

Chmielewski, T. L., Myer, G. D., Kauffman, D., & Tillman, S. M. (2006). Plyometric exercise in the rehabilitation of athletes: Physiological responses and clinical application. Journal of Orthopaedic & Sports Physical Therapy, 36(5), 308–319. Read on PubMed

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. https://doi.org/10.1519/SSC.0b013e318187e25b

Kongsgaard, M., Qvortrup, K., Larsen, J., Aagaard, P., Doessing, S., Hansen, P., Kjaer, M., & Magnusson, S. P. (2010). Fibril morphology and tendon mechanical properties in patellar tendinopathy: Effects of heavy slow resistance training. American Journal of Sports Medicine, 38(4), 749-756. https://doi.org/10.1177/0363546509350915

Komi, P. V. (2000). Stretch-shortening cycle: A powerful model to study normal and fatigued muscle. Journal of Biomechanics, 33(10), 1197-1206. https://doi.org/10.1016/S0021-9290(00)00064-6

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. https://doi.org/10.1242/jeb.02434

Lloyd, R. S., Oliver, J. L., Hughes, M. G., & Williams, C. A. (2012). The effects of 4 weeks of plyometric training on reactive strength index and leg stiffness in male youths. Journal of Strength and Conditioning Research, 26(10), 2812-2819. https://doi.org/10.1519/JSC.0b013e318242d2ec

Rio, E., Kidgell, D., Purdam, C., Gaida, J., Moseley, G. L., Pearce, A. J., & Cook, J. (2015). Isometric exercise induces analgesia and reduces inhibition in patellar tendinopathy. British Journal of Sports Medicine, 49(19), 1277-1283. https://doi.org/10.1136/bjsports-2014-094386

Taylor, M. J. D., & Beneke, R. (2012). Spring mass characteristics of the fastest men on Earth. International Journal of Sports Medicine, 33(8), 667-670. https://doi.org/10.1055/s-0032-1306283

Haff, G. G., & Triplett, N. T. (Eds.). (2016). Essentials of strength training and conditioning (4th ed.). Human Kinetics.