When Oxygen readers see the word “plyometrics,” they likely think of box jumps or “jump training” as a broad-based approach to improving power. But when plyometrics was developed about 50 years ago, it was described as a “shock method” of training. Improvement in performance came following a drop from height and subsequent immediate jump. The focus was on keeping the time between landing and takeoff short. So let’s look at the science of “plyo” to show how the original concept can include core training for a tummy you will be proud to display.
Forces and Phases
Increased muscle force in a plyometric exercise stems from three physiological components: (1) Elastic energy in muscle tissue and tendons is stored during a rapid stretch and can be released if a contraction immediately follows that rapid stretch; (2) when a muscle is stretched, the microfilaments that cause muscle contraction may be at the best length to bind and produce force during a subsequent contraction; and (3) there is resistance from muscle tissue itself (a kind of “rubbery” quality of muscle) during an unstimulated stretch, and this “passive” resistance adds to the subsequent force generated.
There also are neurophysiological factors in plyometrics. Muscle spindles (located near the muscle belly/tendon tie-in) protect the muscle from damage caused by a too-strenuous stretch. When a stretch occurs rapidly, the muscle spindle senses potential danger and sends a signal to the central nervous system. The nervous system responds reflexively, immediately contracting the muscle in order to protect it. Plyometrics uses this reflex by adding the force of the spindle-initiated contraction to the muscle’s subsequent contraction during an explosive movement.
Now that you have a grasp of the forces involved in plyometrics, here’s how it all comes together in training. In the first phase, the muscle is stretched when a force is exerted on it. That force could occur upon landing or when the muscle encounters a resistance (such as a medicine ball). The muscle lengthens in this phase, called the eccentric phase, and the muscles involved in resisting that lengthening movement are being “pre-loaded.” In the simple case of running, when your foot strikes the ground, your calf muscle lengthens under the load. So in the eccentric phase of a plyometric movement, a muscle (like the calf) has stored energy.
Let’s skip to the third phase, known as the concentric phase. Here, stored energy is used to increase the force of the subsequent contraction. Because power is the product of force and velocity, the more force you provide to any contraction, the more power the muscles involved develop. In the case of running, more force is available upon a rapid stretch, and therefore the ensuing explosion coming off the toes on the next stride is greater.
But the phase we skipped is critical. The second one is called the amortization phase. When something is amortized, its cost is distributed over time. In plyometrics, the stored energy is distributed as heat over the time frame of the amortization phase — the period after the muscle’s lengthening ends and before the shortening (concentric contraction) begins. The longer that phase, the more energy is distributed and lost, leaving less to be added to the subsequent contraction, so the shorter the amortization period, the better. In running, the longer your foot stays on the ground (“foot-strike time”), the less explosive you are.
When performing plyometrics, keep the idea of amortization foremost in your mind. Always concentrate your efforts on minimizing the time you engage the resistance. You’ll see that as part of your fat-loss plan, plyometrics can be a figure-flattering bonus.
Plyo Exercise: 45-Degree Sit-Up
With a medicine ball weighing 4 pounds (and no more than 10 pounds), grab a partner who will spot you for this exercise. Sit with your trunk at a 45-degree angle. Your partner should be in front of you with the medicine ball.
While you hold your hands outstretched, your partner throws you the medicine ball. Catch the ball with both hands, immediately propelling the ball back to your partner with your trunk remaining at 45 degrees. The key here is threefold. First, try to minimize any trunk extension upon receiving the ball. Second, your return throw should be as immediate and explosive as possible. Third, the force used to absorb the impact of the ball and return it to your partner should come from your abdominal muscles. You can increase the intensity of this exercise by simply using heavier medicine balls.
In upper-body plyometrics, we count throws or catches. In general, when starting out, keep total plyometric volume low. Protect your back — don’t do multiple sets or perform these more than once per week until you are properly conditioned.