PT Tip of the Month Archive

What is plyometric exercise?

The word “plyometric” comes from the Greek word pliometric (“plio” meaning increase and “metric” meaning measure), meaning “to increase the measurement.”Plyometric exercise is defined as a lengthening of the muscle-tendon unit followed directly by a shortening of the muscle-tendon unit. To put it simply, you are quickly putting a stretch on a muscle before quickly contracting or shortening the same muscle. This stretch-shortening cycle enhances the ability of the muscle-tendon unit to produce maximum force in the shortest time. Typically in physical therapy, plyometric exercises are initiated at a low intensity and progressed to a high intensity as the athlete or patient becomes stronger. Progression to higher intensity exercises are thought to prepare the musculoskeletal system for rapid movements and high forces, which are similar to the demands the athlete will face with return to sport. Examples of typical plyometric exercises include jumping drills, landing drills, and lateral cutting maneuvers.

Phases of plyometric exercise

Loading

The first phase of plyometric exercise is called the loading phase. Some researchers refer to this phase as the eccentric, deceleration, yielding, or cocking phase. Regardless of the name, during this phase is when the muscle-tendon unit of the muscle is stretched as load or kinetic energy is applied to the joint. This may occur as an athlete is in flight from a previous jump or if an athlete is preparing to catch a heavy load. Stretching of the muscle-tendon unit elicits a stretch in certain muscle receptors which will create movement, and also store elastic potential energy.

Coupling

The transition between loading and unloading phases is called the coupling phase. The coupling phase is, for all intents and purposes, when there is an isometric contraction of the muscle. This means that for the most part, the muscle does not change in length despite changes in the joint angle, vertical ground reaction force, or the athlete’s center of mass.

Unloading

The phase immediately after the coupling phase is called the unloading phase. This phase is also known as the rebound, shortening, or push-off phase. During this phase, the muscle-tendon unit begins to shorten.

Momentum

Plyometric activities terminate in this phase. This is when the body segment continues to move as a result of the forces generated during the unloading phase (i.e. continued upward motion during a jump).

Plyometric exercise and rehabilitation

There is scientific literature supporting the use of plyometric exercise to enhance an athlete’s performance. Many athletic conditioning programs involve plyometric training at a sub-maximum level and focuses on achieving proper technique during the exercise. There are numerous studies indicating that this type of training is effective in decreasing lower extremity injuries and improving athletic performance. Performance is optimized when the exercise imparts higher forces and faster speeds of movement. Contact time is also an important factor in plyometric training. Contact time is the amount of time your feet or hands are touching the ground. It is important that during plyometric training, the contact time be minimal. This can be adjusted by altering the intensity of the exercise or making sure that the transition between loading and unloading phases is as continuous as possible.

Various plyometric training programs indicate that 6 to 15 weeks of training improves athletic performance. The combination of plyometric and weight training for the lower extremities may result in up to 90% increase in leg strength after 6 weeks of training, according to one study. Other benefits include a faster rate of force production and a delayed onset of muscle fatigue. Several research studies indicate that plyometric training decreases risk for lower extremity injury in female athletes. One study by Hewett et al indicated that female athletes who participated in plyometric exercise had a decreased incidence of knee injury by 3.6 times compared to female athletes not participating in plyometric exercise. There is less research on the effects of plyometric training for rehabilitation of upper extremity injuries. There is some preliminary data indicating that plyometrics may improve joint position sense and postural control in the upper extremities, however more research needs to be conducted in order to substantiate these claims.

Plyometric training is indicated for patients who have a desire to return to activities which involve more explosive movements. Typically, plyometric training is initiated in single planes of motions(such as forward and backwards), at slower speeds, and with low to moderate forces. As a patient progresses, exercises are prescribed in varying and combined planes of motion, with increased speed, and with higher forces, simulating the type of sport the patient wants to return to.

Plyometric training is contraindicated in patients with acute pain, in patients who are immediately post-op, or in patients who have joint instability. Although your physical therapist will be able to appropriately prescribe the frequency of plyometric training you should undergo, high intensity plyometric exercises are often incorporated 2x/wk for healthy, non-injured patients in order to allow for proper rest and muscle recovery. During physical therapy, because plyometric exercises are initiated at lower intensities, some patients may be able to tolerate more frequent bouts of exercise without adverse effects. Research indicates that 48 to 72 hours of rest provides optimal time for recovery between plyometric training sessions.

Please remember that plyometric training can be harmful if done too frequently, with too many repetitions, or with too high of an intensity. Therefore it is important to have proper guidance when starting plyometric training. Your physical therapist will be able to prescribe exercises at an appropriate intensity depending on the ability of the muscle tissues to handle load with proper technique.

If you are a patient or athlete who is injured and wants to return to higher level sporting activities, please contact 617-232-PAIN (7246) for our Brookline office and 617-325-PAIN (7246) for our West Roxbury office in order to schedule a physical therapy evaluation.

References

  1. Chmielewski TL, Myer GD, Kauffman D, Tillman SM. Plyometric exercise in the rehabilitation of athletes: physiological responses and clinical application. JOSPT. 2006;36:308-319.

  2. Heiderscheit BC, McLean KP, Davies GJ. The effects of isokinetic vs. plyometric training on the shoulder internal rotators. JOSPT. 1996;23:125-133.

  3. Hewett TE, Ford KR, Hoogenboom BJ, Myer GD. Understanding and preventing ACL injuries: current biomechanical and epidemiologic considerations – update 2010. N Am J Sports Phys Ther. 2010;5:234–251.

  4. Hewett TE, Ford KR, Myer GD. Anterior cruciate ligament injuries in female athletes: Part 2, a meta-analysis of neuromuscular interventions aimed at injury prevention. Am J Sports Med. 2006;34:490-498.

  5. Hewett TE, Stroupe AL, Nance TA, Noyes FR. Plyometric training in female athletes. Decreased impact forces and increased hamstring torques. Am J Sports Med. 1996;24:765-773

  6. Risberg MA, Holm I, Myklebust G, Engebretsen L. Neuromuscular training versus strength training during first 6 months after anterior cruciate ligament reconstruction: a randomized clinical trial. Phys Ther. 2007;87:737-750.

  7. Souissi S, Wong DP, Dellal A, Croisier J, Ellouze Z, Chamari K. Improving functional performance and muscle power 4-to-6 months after anterior cruciate ligament reconstruction. JSSM. 2011;10:655-664.

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