1.12.2009

Plyometrics

Introduction
Plyometrics is a type of exercise training designed to produce fast, powerful movements and improve the functions of the nervous system. Plyometric movements, in which a muscle is loaded and then contracted in rapid sequence, use the strength, elasticity and innervation of muscle and the surrounding tissues to jump higher, run faster, throw farther or hit harder, depending on the desired training goal. The goal is simple: increase the speed or force of muscular contractions.

Procedure
Plyometric training involves practicing plyometric movements to toughen tissues and train nerve cells to stimulate a specific pattern of muscle contraction so the muscle generates as strong a contraction as possible in the shortest amount of time. A plyometric contraction involves first a rapid muscle lengthening movement, followed by a short resting phase, then an explosive muscle shortening movement, which enables the muscles that work together in doing the particular motion. This type of training engages the myostatic-reflex, which is the automatic contraction of muscle when their stretch nerve receptors are stimulated.

Plyometric exercises use explosive movements to develop muscular power, the ability to generate a large amount of force quickly. Plyometric training acts on the nerves, muscles and tendons to increase an athlete's power output without necessarily increasing their maximum strength.

Physics of Plyometrics
Muscular power is determined by how long it takes for strength to be converted into speed. The ability to convert strength to speed in a very short time allows for athletic movements beyond what raw strength will allow. Thus, an athlete who has strong legs and can perform the free weight squat with extremely heavy weights over a long duration may get less distance on a standing long jump or height on a vertical leap than a weaker athlete who is able to generate a smaller amount of force but in a shorter amount of time. The plyometrically trained athlete may have a lower maximal force output, and thus may not squat as much, but his training allows him to shorten the amount of time required to reach his maximum force output, leading to more power from each contraction.

Muscle-Tendon Component
For a muscle to cause movement, it must shorten. This is known as concentric contraction. There is a maximum amount of force with which a certain muscle can concentrically contract. However, if the muscle is lengthened while loaded (eccentric contraction) just prior to the contraction, it will produce greater force through the storage of elastic energy. This effect requires that the transition time between eccentric contraction and concentric contraction (amortization phase) be very short. This energy dissipates rapidly, so the concentric contraction must rapidly follow the eccentric stretch. The process is frequently referred to as the "stretch shortening cycle," and is one of the underlying mechanisms of plyometric training.

Neurological Component
In addition to the elastic0-recoil of the musculotendous system, there is a neurological component. The stretch shortening cycle affects the sensory response of the muscle spindles and golgi tendon organs (GTO). It is believed that during plyometric exercise, the excitatory threshold of the GTO's is increased, making them less likely to send signals to limit force production when the muscle has increased tension. This facilitates greater contraction force than normal strength or power exercise, and thus greater training ability.

The muscle spindles are involved in the stretch reflex and are triggered by rapid lengthening of the muscle as well as absolute length. At the end of the rapid eccentric contraction, the muscle has reached a great length at a high velocity. This may cause the muscle spindle to enact a powerful stretch reflex, further enhancing the power of the following concentric contraction. The muscle spindle's sensitivity to velocity is another reason why the amortization phase must be brief for a plyometric effect.

A longer term neurological component involves training the muscles to contract more quickly and powerfully by altering the timing and firing rates of the motor units. Plyometric training conditions the neurons to contract with a single powerful surge rather than several disorganized contractions. The result is a stronger, faster contraction allowing a heavy load (such as the body) to be moved quickly and forcefully.

Plyometric Exercises
The following list is a sample of lower body plyometric exercises (low intensity):

Squat Jumps
  • Elapsed Time:
  • Purpose: Increase power/strength in the legs.
  • Exercise: Stand with feet shoulder width apart, trunk flexed forward slightly with back straight in a neutral position. Arms should be in the "ready" position with elbows flexed at approximately 90 degrees. Lower body where thighs are parallel to the ground and immediately explode upwards vertically and drive arms up. Do not hold a squat position before jumping up - keep the time between dipping down and jumping up to a minimum. Land on both feel. Rest for 1-2 seconds and repeat. Prior to takeoff, extend the ankles to their maximum range (full plantar flexion) to ensure proper mechanics.
Jump to Box
  • Elapsed Time:
  • Purpose: Increase power/strength in the legs.
  • Exercise: Stand facing box with feet slightly wider than hip-width apart. Lower body into a semi-squat position and immediately jump up onto box. Do not hold a squat position before jumping up - keep the time between dipping down and jumping up to a minimum. Feet should land softly on box. Step back down (not jump back down) and repeat.

Lateral Jump to Box
  • Elapsed Time:
  • Purpose: Increase power/strength in the legs.
  • Exercise: Stand side on to box with feet slightly wider than hip-width apart. Lower body into a semi-squat position and immediately jump up onto box. Do not hold a squat position before jumping up - keep the time between dipping down and jumping up to a minimum. Feet should land softly on box. Step back down (not jump back down) and repeat.
The following list is a sample of lower body plyometric exercises (moderate intensity):

Split Squat Jumps

  • Elapsed Time:
  • Purpose: Increase power/strength in the legs.
  • Exercise: Stand with feet hip-width apart. Take left leg and step back approximately 2 feet standing on the ball of back foot. Feet should be positioned at a staggered stance with head and back erect and straight in a neutral position. Lower body by bending at right hip and knee until thigh is parallel to floor then immediately explode vertically. Switch feet in the air so that the back foot lands forward and vice versa. Prior to takeoff, extend the ankles to their maximum range (full plantar flexion) to ensure proper mechanics.

Tuck Jumps
  • Elapsed Time:
  • Purpose: Increase power/strength in the legs.
  • Exercise: Stand with feet shoulder-width apart, knees slightly bent with arms at sides. Jump up bringing knees up to chest. Land on balls of feet and repeat immediately. Remember to reduce ground contact time by landing soft on fee and springing into air.



Lateral Box Push Offs
  • Elapsed Time:
  • Purpose: Increase power/strength in the legs.
  • Exercise: Stand to side of box and place the left foot on top of box. Push off the box using the left leg only and explode vertically as high as possible. Drive the arms forward and up for maximum height. Land with right foot on the box and left foot on the ground to the other side of the box. Repeat from this side.

Bounding
  • Elapsed Time:
  • Purpose: Exaggerated running motion focusing on foot push-off and air time.
  • Exercise: Jog into the start of the drill for forward momentum. After a few feet, forcefully push off with the left foot and bring the leg forward. At same time drive your right arm forward. Repeat with other leg and arm.

Bounding with Rings
  • Elapsed Time:
  • Purpose: Exaggerated running motion focusing on foot push-off and air time.
  • Exercise: Jog into the start of the drill for forward momentum. After a few feet, forcefully push off with the left foot and bring the right leg forward. At same time swing your left arm forward and land into the first ring, which is 3-4 feet out to the left, with the right foot. Continue and repeat with other leg and arm into second ring, which is now 3-4 feet up and to the right.

Box Drill with Rings
  • Elapsed Time:
  • Purpose: Increase power/strength in the legs.
  • Exercise: Stand with feet slightly wider than hip-width apart with your body facing the first ring. Hop forward using both feet and land in the first ring. Now hop to the left and land in the ring to the side. Now jump backwards to land in the ring behind you. Finish by jumping to your right to land in final ring. Rest and repeat. Keep ground contact between bounds to a minimum.


Lateral Hurdle Jumps
  • Elapsed Time:
  • Purpose: Increase power/strength in the legs.
  • Exercise: Stand beside object to be cleared. Bring knees up and jump vertically but also laterally off ground and over the barrier. Land on both feet and immediately jump to the other direction over barrier. Try not to pause between jumps or sink down into a squat position.

The following list is a sample of lower body plyometric exercises (high intensity):

Zig Zag Hops
  • Elapsed Time:
  • Purpose: Increase power/strength in the legs.
  • Exercise: Stand to the left of an agility ladder or similar object approximately 1-2 feet away. Forcefully push off both feet and land on the other side of the ladder. Repeat and land feet back on the other side, continue repeating and so on down the ladder. Do not "double hop" upon each landing and keep ground contact time to a minimum.

Single Leg Tuck Jump
  • Elapsed Time:
  • Purpose: Increase power/strength in the legs.
  • Exercise: This is the same as the tuck jump exercise above only one leg is used. Upon landing another jump is performed immediately with minimal ground contact time and with the same leg for the desired number of repetitions. This is repeated for the other leg after a rest period.
Single Leg Lateral Hops
  • Elapsed Time:
  • Purpose: Increase power/strength in the legs.
  • Exercise: Start by standing on one leg with your hands on your waist or at your sides. Proceed to hop to the side while maintaining your balance and hop back to the starting position. You can place a rope on the ground or any object on the ground. The object can be small in size and height or large to increase difficulty. Repeat continuously.

Depth Jumps
  • Elapsed Time:
  • Purpose: Increase power/strength in the legs.
  • Exercise: Stand on box with toes close to edge, feet shoulder width apart. Step off (do not jump off) box and land on both feet. Immediately jump up as high as possible and reach up with both hands towards. The jump should be vertical with no horizontal movement. Ground contact time should be short unlike the diagram. Landing should be soft. Start with a box height of 12 inches. Intensity can be increased by gradually increasing the box height to a maximum 42 inches but this is only for experienced athletes with a substantial strength training background.

The following list is a sample of knee injury prevention plyometric exercises:

Lateral Hops over Cone (20 reps)
  • Elapsed Time: 9.5-10 minutes
  • Purpose: Increase power/strength emphasizing neuromuscular control
  • Exercise: Stand with a 6" cone to your left. Hop to the left over the cone softly landing on the balls of your feet, and land bending at the knee. Repeat this exercise hopping to the right.

Forward/Backward Hops over Cone (20 reps)
  • Elapsed Time: 10-10.5 minutes
  • Purpose: Increase power/strength emphasizing neuromuscular control
  • Exercise: Hop over the cone softly landing on the balls of your feet and bending at the knee. Now hope backwards over the ball using the same landing technique. Do not snap your knee back to straighten it. Maintain a slight bend to the knee. Repeat for 20 reps.
Single Leg Hops over Cone (20 reps)
  • Elapsed Time: 10.5-11 minutes
  • Purpose: Increase power/strength emphasizing neuromuscular control
  • Exercise: Hop over the cone landing on the ball of your foot bending at the knee. Now hop backwards over the ball using the same landing technique. Do not snap your knee back to straighten it. Maintain a slight bend at the knee. Repeat for 20 reps. Now stand on the left leg and repeat the exercise. Increase the number of repetitions as needed.
Vertical Jumps with Headers (20 reps)
  • Elapsed Time: 11-11.5 minutes
  • Purpose: Increase height of vertical jump
  • Exercise: Stand forward with hands at your side. Slightly bend the knees and push off jumping straight up. Remember the proper landing technique. Accept the weight on the ball of your foot with a slight bend to the knee. Repeat 20 times and switch sides.
Scissors Jump (20 reps)
  • Elapsed Time: 11.5-12 minutes
  • Purpose: Increase power/strength of vertical jump.
  • Exercise: Lunge forward leading with your right leg. Keep your knee over your ankle. Now push off with your right foot and propel your left leg forward into a lunge position. Be sure your knee does not cave in or out. It should be stable and directly over the ankle. Remember the proper landing technique. Accept the weight on the ball of the your foot with a slight bend to the knee. Repeat 20 times.
Safety
Plyometric exercises involve an increased risk in injury due to the large forces generated during training and performance. Good levels of physical strength, flexibility and proprioception should be achieved before commencement of plyometric training.

Chu (1998) recommends that a participant be able to perform 5 repetitions of the squat exercise at 60% of their body weight before doing plyometrics. Core body (trunk) strength is also important. Flexibility is required for both injury prevention and to enhance the effect of the the stretch shortening cycle. Proprioception is an important component of balance, coordination and agility.

Always perform plyometrics on a somewhat giving surface, such as a gym mat or grass. Athletes over 240 pounds should be very careful and engage only in low-intensity plyometric exercises.
Be sure to consult a coach on proper technique for any plyometric exercise.

Sources
Plyometrics
http://en.wikipedia.org/wiki/Plyometrics
Phase 4 Plyometrics, Elizabeth Quinn
http://sportsmedicine.about.com/od/kneepainandinjuries/a/aa022202e.htm
Lower Body Plyometric Exercises
http://www.sport-fitness-advisor.com/plyometricexercises.html

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