During the eccentric portion of plyometrics, what mechanism causes the muscle to lock in place and contribute to greater movement energy?

• A. Muscle spindles detect length change and trigger contraction
• B. Brain signals the muscle to fully relax
• C. Tendons passively lengthen without muscle involvement
• D. Golgi tendon organs initiate inhibition

Answer: (A)

The stretch reflex produced by muscle spindles is what drives this effect. When the muscle lengthens rapidly during the eccentric phase, the muscle spindle detects that sudden stretch and sends a reflex signal to contract. This reflexive contraction stiffens the muscle and effectively “locks” it in a ready state, while also storing elastic energy in the muscle–tendon system. When the movement switches to the concentric phase, that stored energy plus the reflex-ready state helps generate a more powerful push, contributing to greater movement energy.

Brain signaling to fully relax would reduce force output, so that’s not what enables the lock-in. Tendons can store energy, but passive lengthening without muscle involvement doesn’t explain the active, reflexive preparation. Golgi tendon organs tend to inhibit contraction to protect the tendon, not promote the rapid, forceful response seen here.