Plyometrics is the science examining the explosive movement generated by muscle power, with particular application to sport training and performance. As power is the product of speed plus strength, the ability of an athlete to generate force in a short period of time is at the root of developing greater explosive effect.
It is an anatomical fact that muscles in the human body contract in order to generate force, a process known as concentric contraction. Further, if the muscle is lengthened in the period immediately prior to contraction, the muscle will develop greater power. The lengthening process is known as an eccentric contraction. The shorter the time period between eccentric and concentric muscle contractions, the greater the power capable of being generated by the muscle.
Plyometrics training is designed to develop rapid alterations of eccentric and concentric contractions, while constant resistance is applied to the target muscle or muscle group.
Plyometrics was first popularized by the former East Germany state sport trainers in the 1970s, as a method of building the myotactic reflex, the term used to describe this stretch reflex in the muscle that enhances the power of contraction. As an example, when a person performs repeated forward jumps, there will be stretch of the quadriceps (located on the front of the thigh) muscles in the first jump. The subsequent contraction of this muscle will make the jump immediately following more powerful.
Sports in which explosive speed and power are at a premium are those in which plyometrics training techniques are often employed—the delivery of an effective punch in boxing, various positions in American football, basketball, and volleyball are examples.
A typical plyometrics workout for a sport in which explosive lower body power is key might include ground-level jumping on soft surfaces such as padded mats or grass, with a progression to jumping over cones or foam barriers. As a general rule, lower body plyometrics are performed on semi-resilient surfaces. Advanced plyometrics jumping often involves bounding exercises, both in straight lines and in patterns. The exercise routines emphasize speed, not endurance, with a number of repetitions typically performed quickly. When accompanied by both an active warm up, designed to elevate the athlete's core body temperature, as well as a stretching program and other weight training to build an overall strength base, there is considerable evidence that plyometrics will improve the explosive movement capabilities of an athlete.
However, plyometrics training has generated debate regarding athlete safety in the sport science community. A number of experts have criticized plyometrics as being compared to high-impact aerobics, a fitness activity that fell into disfavor due to the risk of injury to participants' lower body joints from excess stress on the musculoskeletal system. There is little question that some athletes, because of their physical structure, fitness level, and related factors, are poor candidates for a plyometrics training program.
Any training method that builds strength through an explosive movement, such as a plyometrics jump program, carries with it an greater inherent risk of injury. This danger can be minimized if the athlete incorporates the plyometrics drills into a broad-based weight training and stretching program. The strength of the muscles of the lumbar (lower) back, the gluteal muscles (the buttocks and pelvic muscles), and the abdominals is important in providing stability to the body both at the moment of moving explosively into a jump, as well as upon the body landing.
These long-term injury concerns underscore a number of critical safety factors to be applied to all plyometrics training programs. Given the intensity of the training techniques, extreme caution should be employed before a young athlete participates in plyometrics. The stresses of the exercises create a potential for injury to a young athlete's skeletal growth plate (the epiphysis).
Further, an introduction to and maintenance of a plyometrics regime must be progressive in nature. Even where there is little by way of equipment, and the exercises appear simple, the stresses on the body must be introduced over time. In this regard, an athlete must also assess the effect of any previous injury or physical condition on the ability to benefit from plyometrics.
Other critical safety factors that should form a part of any plyometrics program are the use of foam or other soft barriers, boxes or jumping surfaces that will not twist on impact, and the provision for built-in rest periods of at least 48 hours between training sessions.
The ultimate measure of the success of any athletic training program is directly related to an athlete's success in competition. However, to directly assess the benefits of a plyometrics program, as well as planning further training, the athlete or the coach must determine an appropriate baseline through periodic field testing.