Reactive strength and the drop jump

RSI = jump height / ground contact time. A stiffness test, not a jump-height test. · 5 min read

Bounce versus absorb

A drop jump with a long, deep contact is a countermovement jump from a box. RSI then collapses because contact time sits in the denominator. Cue contact time. Typical bounce contacts are 150–250 ms. RSI of 1.5 is a field rule of thumb, not a law of nature — it moves with drop height, mass, and footwear.

The triceps surae and Achilles tendon store and return energy. That is why a thin calf envelope on the Z25 map and a poor RSI often arrive together. Girth is still not stiffness — pair the two measures.