The Navy SEAL Physical Fitness Guide 161
lengthens. The take-off occurs via concentric contraction of the muscles. During this phase,
the muscle shortens as it contracts, and actual work (i.e., movement of the body through
space) is performed.
The amortization phase is the period of time from the beginning of the lengthening
phase to the beginning of the take-off phase. This is the most important phase when it
comes to plyometric training. During this phase, the muscle must convert the muscular
tension generated during the lengthening phase to acceleration in a selected direction
during the takeoff phase. The elastic properties inherent within the muscles and
neuromuscular reflexes (the stretch reflex) are responsible for this conversion. Plyometric
training may increase the efficiency of this conversion. The goal of plyometric training is
to decrease the amount of time in the amortization phase and thereby increase speed.
Preparation for Plyometric Training
Plyometric exercises should be undertaken only once an adequate strength base has
been developed. Most sources define an adequate strength base for lower body plyometrics
as the ability to squat or leg press 1.5 to 2.0 times your body weight for one maximum
repetition. For upper body plyometrics, larger athletes (weight greater than 115 kg or 250
lbs.) should be able to bench press their body weight and athletes weighing less than 115
kg (250 lbs) should be able to bench press 1.5 times their body weight.
Plyometric training should never be undertaken if you
have any leg, hip, arm, or shoulder injury.Safety in Plyometric Training
Several steps can be taken to ensure that plyometrics training is safe. These
measures include using an appropriate surface, footwear, and equipment, and proper
technique.
Surface
Plyometrics should not be performed on hard surfaces such as concrete or steel, nor
should they be performed on soft surfaces such as sand. The best surface is a grass
field, followed by artificial turf or wrestling mats. Wrestling mats should not be
too thick (> 15 cm) since they will increase the time in the amortization phase. The stored
energy gained during the lengthening phase will be lost, and this will defeat the purpose of
plyometric training.
Footwear