The vertical jump is one of the most explosive physical movements executed in sport. In a number of sports, the higher the athlete is able to jump, the greater the prospects of success in that discipline. Basketball and volleyball are the two most prominent examples of sports where that correlation is plain. The jumping ability of an athlete is also an indicator of overall athletic ability, as there is a clear relationship between the ability to jump and the running speed that the athlete will develop over short distances. The National Football League, where prospective players are subjected to various physical tests, requires every player to be tested for both vertical leaps and 40-yd (37 m) sprints, irrespective of the position played.
The vertical jump is defined as the highest point that the athlete can touch from a standing jump, less the height that the athlete can touch from a standing position. The measurement of the jump is flawed if the athlete is permitted to take one or more steps before jumping, as the athlete will convert some of the energy developed in the step taken into the force of propulsion that generates upward lift. Basketball has numerous legends and other urban myths concerning the seemingly superhuman leaping ability attributed to certain players; one such player, former University of Louisville star Darrell "Dr. Dunkenstein" Griffith, was reputed to possess a 42 in (1 m) vertical leap. It is likely that the average National Basketball Association player 6 ft 6 in (1.97 m) or shorter has a vertical leap of between 25 and 30 in (0.63 and 0.75 m); taller and heavier players will usually not be able to jump as high.
Because jumping ability is a combination of leg strength and explosive power, jumping can be developed in the same fashion as any other muscular activity. The ultimate limit to how high any athlete can jump will be determined to a significant degree by the distribution of fast-twitch versus slow-twitch fibers present in the muscles of the legs. This distribution is a genetic determination. Fast-twitch fibers are those whose governing neurons, the component of the nervous system that receives the impulses generated by the brain to direct muscular movement, fires more rapidly, which in turn creates the more rapid muscle contractions required for speed. As a general proposition, an athlete with a greater distribution of fast-twitch fibers will be able jump higher than one with a preponderance of slow-twitch fibers.
Plyometrics is the best known of the jumping development exercise programs. Plyometrics training emphasizes speed and explosive movement, and a plyometrics program will typically consist of a series of bounding, hopping, and jumping drills. The object of a plyometrics program is to perform the exercises at maximum intensity. For this reason, plyometrics training must be approached with caution, and the athlete must progress slowly from one level to the next to reduce the risk of injury. Proper rest intervals must also be incorporated in to plyometrics training, as the exercises are intended to place significant stress on the target muscle groups.
A common, low-tech plyometrics method is performing box jumps, where the athlete jumps repeatedly from the floor to the top of the box and back again. By concentrating on the mechanics of the jump, directing propulsion from the balls of the feet and thrusting with an explosive extension of the legs, the ability of the athlete to land lightly and immediately return to the floor enhances motor control over the movement.
To build strength in the legs that will be compatible with the speed developed through successful plyometrics drills, squat and lunge exercises are important components. Squats are performed with free weights, where the athlete uses a weighted bar to carry out the exercise. The additional weight will be supported by the body through the abdominal, lumbar (low back), and gluteal muscles, in addition to the legs. This form of exercise permits the strengthening of the legs in conjunction with enhancing the core strength of the body, essential to the balance necessary to have the several muscle groups involved in leaping work in harmony.
Lunges, also performed with the athlete lifting free weights in each hand that are within the athlete's capabilities, will significantly strengthen the legs, without the risk of injury that may exist in the squat or plyometrics movements.
Muscular strength and explosiveness must be developed in conjunction with flexibility if the athlete is to maximize the jumping ability and reduce the risk of injury to structures such as the Achilles tendon and knee ligaments. Flexibility, when achieved through focused stretching programs, will serve to increase the range of motion in the joints essential to jumping: the ankles, knees, and hips. A common muscular deficiency that plagues athletes who require well-developed leaping ability is a lack of flexibility and resultant strength imbalance between the quadriceps (thigh) muscles and the hamstrings, the pair of muscles responsible for the flexion and the extension of the knee. Proper stretching will assist the athlete in the maintenance of an approximate 3:2 ratio in the relative strength of the quadriceps to the hamstring. When there is a significant deviation from that proportion, the knee and the muscles themselves are at greater risk of injury.
When the athlete proceeds with caution, with emphasis on the form of the training exercise, it is not uncommon to gain between 4 in and 9 in (0.1 and 0.2 m) in vertical jumps in periods as short as three months.