What are the three mechanisms for returning a contracted muscle to its resting length?

The three mechanisms for returning a contracted muscle to its resting length are:

1. Elastic recoil: When a muscle contracts, it stores elastic energy in its connective tissues. When the muscle relaxes, this elastic energy is released, causing the muscle to recoil to its resting length.

2. Viscous flow: When a muscle contracts, its myofilaments slide past each other. This sliding motion generates a viscous force that opposes the movement of the myofilaments. When the muscle relaxes, this viscous force disappears, allowing the myofilaments to slide back to their resting positions.

3. Active relaxation: In some cases, muscles can actively relax by using energy from ATP to pump calcium ions back into the sarcoplasmic reticulum. This decrease in calcium concentration causes the muscle fibers to relax.