Jumping high requires strong muscles that can also contract rapidly. In addition to conventional resistance exercises that increase your muscles' capacity to develop force, you must also perform exercises that accelerate the rate at which force can be developed. Plyometric exercises are typically used for this purpose.
Vertical Jump
A jump is a movement that causes your center of mass to be projected upward with your feet leaving the ground, according to University of Colorado biomechanics professor R.M. Enoka. Jumps can be organized according to several goals including the maximal height that your hand can reach, the maximal horizontal distance that you can cover, the maximal height that your center of mass can be raised and the maximal time that you can spend off the ground. When your goal is to jump and reach as high as possible with your hands, it's called a vertical jump.
Countermovement
When you want to jump as high as you can, the natural tendency is to begin from an upright, erect position and then perform a short downward movement by bending at your hips, knees and ankles. This is called a countermovement because it occurs in a direction that is opposite from the direction you'll be traveling after jumping. The actual jump, which is performed rapidly right after this countermovement, involves forceful extension of the hips and knees along with forward and upward rotation of the arms at the shoulders.
Stretch-shortening Cycle
A countermovement involves performing an explosive, shortening muscle action directly after a rapid, lengthening one. As explained in "Strength and Power in Sport," this is a natural muscle function called a stretch-shortening cycle. A stretch-shortening cycle provides for an economical way to initiate movement and allows for enhanced performance. For example, when volleyball players were tested, they were able to jump 3 to 11 cm higher when performing a countermovement compared to jumping from a crouched position that was assumed without a rapid lengthening phase before. Performance improvement occurs in the stretch-shortening cycle when little delay is allowed between the lengthening and shortening phase.
Mechanisms
Several mechanisms contribute to the increased jump height that you can achieve by using the stretch-shortening cycle. Muscles have elastic tendencies, so stretching them rapidly before you jump results in elastic recoil within the muscle. The rapid stretch also activates the stretch reflex, which is an involuntary muscle activation strategy initiated by spindles that lie within muscle fibers. Muscle spindles respond to a rapid stretch by causing a reflex contraction in the muscle being lengthened. This is meant to protect the muscle, but in this case, also serves as the facilitator of a more forceful contraction because of the reflex activation.
Plyometric Training
Unlike conventional resistance training that is performed at slow speeds against heavy resistance, plyometric training involves explosive movements against lighter loads preceded by rapid countermovements. The objective is to train the stretch-shortening component of the movement, which is not possible during more controlled forms of lifting. In the "Journal of Strength and Conditioning Research," E.S. Villarreal and colleagues summarized 56 research studies and concluded that plyometric training seems to be an effective method for improving vertical-jump ability.
References
- "Neuromechanics of Human Movement"; R.M. Enoka; 2008.
- "Strength and Power in Sport"; P.V. Komi; 2003.
- "Journal of Strength and Conditioning Research;" E.S. Villarreal et al.; 2009.
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