A muscle becomes shorter when it contracts.
A muscle that contracts shortens whereas a muscle that relaxes lengthens.
When one muscle in a pair contracts the other expands.
Flexion
Flexion
The muscle that contracts when the leg is extended at the knee is call the gastrocnemius muscle
When the muscle contracts, the tendon pulls on the bone to create movement. The tendon is the connective tissue that attaches the muscle to the bone, so when the muscle contracts, it exerts force on the tendon, which in turn moves the bone.
When the bicep contracts the triceps relax
Relaxes
When the muscle contracts, it gets shorter. When it relaxes, it gets longer.
circular muscle
The site that undergoes most of the movement when a muscle contracts.