Author Topic: Physics of training for explosiveness: shock training in particular  (Read 1437 times)

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I found this read on shock training to be pretty interesting, talking about how the muscle-tendon unit is overloaded through shock training, and how the energy restitution comes mainly from the tendons, and thus only shock training, and depth jumps can provide a specific overload to those units. The author of this document also did not favor weighted depth jumps, however I did not understand why and that is what I'd like to question in this article.

Also at question is the validity of these methods. Shock training may just be a myth afterall:

Before share the equations let me share anecdotally a few experiences that contradict the notion that building horsepower through traditional strength training improves the same qualities that plyometric are purported to improve.

1. Improving strength in my squats and lunges did not improve my depth jumps. They increased my ground contact time and I could not absorb forces from even a drop of 30'' without caving considerably at the knees.
2. Training depth drops from 30'' and subsequently depth jumps however did reduce my ground contact time and decrease the amount of knee and hip flexion upon landing, thus making my legs feel "stiffer."
3. Heavy squat jumps also made my legs feel stiffer and reduced the amount of flexion in the hips and knees from high drops.


Based on anecdotes 1 and 3 above I questioned how the forces involved in weighted depth jumps or jump squats would differ from those involved in depth jumps. Also at hand are the following questions:
What percentage of forces produced during a depth jump come from contractile elements of muscles, visco-elastic properties of tendons, and from a reconstitution of energy? How does this differ in a jump squat, a weighted depth jump, and maximal squats?
In this first post I will simply demonstrate calculations regarding differences in ground reaction force produced from a depth jump and weighted depth jump.

Upon landing from the initial drop in a depth jump, a ground reaction force (GRFs) is produced from the ground into the point of contact on our feet, while we decelerate from an initial velocity to a velocity of 0m/s. Afterwards we produce forces and reconstitute energy into the jump.

To calculate GRF, we use these equations:
GRF = mass (velocity)/time + weight (mass *gravity)
velocity = sqrt(2gd)

100kg athlete depth jump from 1m and 0.5m with weight. How much load does the athlete need to add to himself in a 0.5m depth jump in order to produce the same GRFs produced from an unloaded 1m depth jump? The athlete takes 0.15s to decelerate.

Velocity = sqrt(2*9.81m/s*1) = 4.4m/s
GRF = 100kg*4.4m/s/0.15s + 100kg*9.81m/s = 3914N

to calculate mass needed to produce same GRF from 0.5m drop as from 1m drop:
velocity = 3.1m/s
3941N = 3.1x/0.15m/s + 9.81x

rearrange and total mass = 127.5kg - 100kg = 27.5kg needed.

Question: Despite the same GRF, will the time to decelerate be the same? I would hypothesize yes. Would ground contact time be the same? Would forces produced during the jump be the same? For the last two I don't know. I would like some help in discussing that. Thanks.

"Performance during stretch-shortening cycle exercise is influenced by the visco-elastic properties of the muscle-tendon units. During stretching of an activated muscle, mechanical energy is absorbed in the tendon structures (tendon and aponeurosis) and this energy can subsequently be re-utilized if shortening of the muscle immediately follows the stretching. According to Biscotti (2000), 72% of the elastic energy restitution action comes from tendons, 28% - from contractile elements of muscles.