Plyometric training

What it is

Stretch, then shorten

A systematic review of older adults defined plyometric exercise as eccentric loading followed straight away by a concentric contraction¹. It named repeated jumping, hopping, bounding and skipping as typical forms¹. It did not count single jumps with a long rest after each landing¹. A review of distance runners added that the exercise uses body weight or loads up to 20% of the most a person can lift once⁵.

Another meta-analysis splits each movement into three phases²:

Pre-activation, an eccentric phase in which the muscle is stretched while working².
Amortisation, an isometric phase, in which the muscle works but the joint does not move².
Shortening, the concentric phase².

The quick switch from lengthening to shortening is called the stretch-shortening cycle². The ground contact of running, jumping and hopping is one example of it in the leg muscles, and similar phases occur in upper-body activities⁶. Researchers have classed these actions as fast when ground contact lasts under 250 milliseconds, and slow when it lasts longer⁷. Slow actions have been suggested to allow more force, and fast ones more speed⁷.

Why the stretch adds force

When a working muscle is stretched just before it shortens, the shortening produces more force, work and power than it would alone⁸. Proposed reasons include muscle activation before contact, reflexes set off by the stretch, and stored elastic energy springing back out of tendons⁸.

One ultrasound study watched the medial gastrocnemius, a calf muscle, during jumping⁸. The stretch and shortening happened in the tendon but not in the fascicles (bundles of muscle fibres)⁸. Studies of single muscle fibres suggest titin, a spring-like protein inside the fibre, may add to the effect⁸. Such fibre studies do not necessarily reflect muscle working in the body⁸.

What it loads

Tendons store and return energy

In one adult with a force sensor fitted around the Achilles tendon (heel cord), peak force differed by jump type⁹:

Peak Achilles tendon force (newtons). Bars start at zero; the table below gives each value with its source.

Jump (one adult)	Peak Achilles tendon force (newtons)
Squat jump (up from a held squat, with no dip)	2233⁹
Countermovement jump (a quick dip before going up)	1895⁹
Repeated hopping of about 7 cm	3786⁹

In the same person, elastic energy stored in the tendon was estimated at 34% of calf muscle work in hopping⁹. In ten men hopping on one leg, Achilles recoil returned about 16% of each hop’s total work¹⁰. Achilles tendon stiffness, the force needed to stretch it by a given amount, varied between people during one-legged hopping¹⁰.

The knee and the landing

In 9 active men stepping off platforms, ground reaction force (the push of the ground back up on the body) generally rose with height¹¹. It also rose with a stiffer landing style¹¹. In those men, the thigh muscles that straighten the knee absorbed the most landing energy of the three leg joints¹¹. In 16 experienced men doing drop jumps, force on the inner knee rose with drop height¹².

In 8 female athletes, soft landings used more knee bend and produced smaller ground forces than stiff landings¹³. More energy was taken up at the hip and knee in soft landings, and at the ankle in stiff ones¹³. A review of older adults argued that plyometric landings likely produce much less impact than deliberate hard landings used for bone¹.

In 6 NCAA Division I athletes doing eight plyometric exercises, knee forces and the speed of force rise on landing differed between exercises¹⁴. Peak ground reaction force may not have differed¹⁴. The patellar tendon is the band from the kneecap down to the shinbone. In 20 healthy adults, a single-leg forward hop and a single-leg countermovement jump fell in its top loading tier¹⁵. In 16 male athletes, a two-foot landing after a horizontal approach gave higher estimated patellar tendon force than landing after a vertical jump¹⁶.

Fatigue after a hard session

A review describes a two-step drop after exhausting hopping or jumping⁶. Performance falls at once, largely recovers within 1 to 2 hours, then falls again⁶. It is often lowest on the second day, when muscle soreness and damage peak⁶. Full recovery may take 4 to 8 days, and function can take longer to return than soreness⁶. A narrative review reported that short programmes on water or sand could give jump and sprint gains similar to usual plyometrics, with much less soreness¹⁷.

How the body adapts

Jumping, strength and speed

A meta-analysis of 32 studies in healthy adults found moderate effects on jumping and lower-body strength². In 26 controlled trials, countermovement jump height rose 8.7% and squat and drop jump height 4.7%¹⁸. In a meta-analysis of 56 studies, programmes of over 10 weeks, 20 sessions and 50 jumps a session seemed to give the largest jump gains¹⁹. Mixing jump types gave larger gains than one type, and added weight gave no extra benefit¹⁹. People with more sports experience gained more jump height, and men tended to gain more than women¹⁹.

One meta-analysis of 26 studies found programmes under 10 weeks, mixing jump types and including horizontal jumps, seemed to give the largest sprint gains²⁰. Another meta-analysis, in healthy adults, found no significant change in sprint performance². An updated review of 420 studies found links between dose and result were still unclear²¹.

Findings in runners also disagree^5,22. In 511 endurance runners, jump training improved 2 to 5 km time trials²². It gave small to moderate gains in running economy (energy used at a given speed)²². In 22 studies of long-distance runners, the effect on running economy was trivial, against a small effect of heavy resistance training⁵.

Muscle and tendon

In healthy adults, a meta-analysis found moderate gains in muscle thickness and fascicle length (the length of the bundles of fibres in a muscle)². It found a small effect on pennation angle, the angle at which fibres lie against the muscle’s line of pull². It found no significant change in muscle or tendon cross-sectional area (the size of a slice cut straight across it), or in muscle stiffness². One study in healthy younger adults found quick rises in muscle volume and fascicle length, measured at 2, 4 and 6 weeks⁸.

A narrative review concluded the gains likely come partly from more neural drive (nerve signal) to the working muscles¹⁷. It also named better coordination between muscles, and changes in the calf muscle-tendon unit, in muscle size or structure, and in single fibres¹⁷.

For tendon, a meta-analysis in healthy adults found a moderate effect on stiffness². Another meta-analysis in healthy adults found its five plyometric studies conflicted, with only one change reaching significance³. In two of those studies, Achilles stiffness rose after isometric or dynamic training but not after plyometric training³. A third meta-analysis found small stiffness gains from jump training and large ones from resistance training²³. Authors of one study suggested the short loading time in jumps may limit the signal reaching tendon cells³.

Bone

A narrative review reported that long-term training, 3 to 5 times a week for 5 to 12 months, raised bone mass in three groups¹⁷. These were children before or early in puberty, young women and premenopausal women¹⁷. Of 26 jumping studies in under-18s, 24 reported greater bone gains than controls, and 2 found no effect²⁴.

A meta-analysis pooled 18 jump-training trials with 666 adults²⁵. Bone density at the femoral neck, the angled part of the thigh bone below the hip, rose about 1.5% against non-jumping controls²⁵. Total hip density rose significantly only in younger adults, and the lower back showed no significant change²⁵. In a review of older adults, only 2 of 6 studies measuring bone showed positive results¹.

Across the lifespan

A review reports that a preceding dip raises jump height by 18 to 30% in adults, but only about 1 to 5% in children⁷. It adds that children activate their muscles less than adults before landing in fast hopping and drop jumps, and this rises with age into mid-adolescence⁷. A review of 7 low-quality studies in prepubertal children aged 5 to 14 found a large effect on running and jumping²⁶. That review concluded the research suggests it is safe when parents consent, children agree and safety guidelines are built in²⁶.

In 75 studies of people aged 6 to 18, plyometric training gave larger changes before the growth spurt, and resistance training around and after it²⁷. Around the growth spurt, boys appeared to gain more jump height and distance than girls²⁷. In 1,499 young male soccer players, jump training improved vertical jump and straight-line sprint performance²⁸.

In adults aged about 58 to 79, a review of 12 randomised studies found no study reporting more injuries related to the plyometric exercises¹. At most 1.4% of its 289 people left a study because of injury¹. It concluded the training likely raises strength, probably less than resistance training does¹.

What studies found about injury

A meta-analysis pooled 9 cluster-randomised trials, which assign whole groups, such as teams, by chance⁴. In its 14,394 sports participants, programmes with plyometric exercises were followed by fewer ACL injuries than usual warm-ups⁴. Subgroup estimates suggested a larger drop in males and for non-contact injuries, but were less precise⁴. In athletes under 19, programmes that included jumping showed a larger drop in injuries than programmes without it²⁹.

In 648 uninjured young females, various training programmes increased knee bend and reduced inward knee motion in jump landings³⁰. Other landing measures did not change significantly³⁰. In 976 female team-sport athletes, very low certainty evidence found programmes aimed at reducing injury raised knee bend by about 3 degrees³¹. Low to very low certainty evidence found no effect on ground forces³¹.

A review of jumping athletes found no landing factor that predicted the start of patellar tendinopathy (pain in the tendon just below the kneecap)³². No study had followed athletes forward in time³².

Connections

Plyometric training loads muscle, tendon and bone, with measured changes in each, though tendon findings are mixed^2,3,25. A review found muscle and tendon adapt at different speeds, and plyometric loading seems a stronger signal for muscle than for tendon³⁴. Each movement starts with an eccentric phase, the kind of muscle work covered on the eccentric training page². It has been compared with resistance training in runners, young people and older adults^1,5,27.

Warm-up programmes with jumping, covered on neuromuscular warm-ups, have been studied for ACL tears^4,29. The drop landing page covers landings and drop jumps, which several of these studies used^14,18. The Achilles tendon and patellar tendon pages cover structures loaded in these studies^9,15. The anterior cruciate ligament page covers the ligament whose injuries were counted⁴.