Final answer:
A muscle contraction is the action of opening or lengthening muscle fibers at a joint. The flexor muscle contracts to cause a joint to bend, while the extensor muscle contracts to cause the joint to straighten.
Step-by-step explanation:
A muscle contraction is the action of opening or lengthening muscle fibers at a joint. When a muscle contracts, it generates tension or force, which can cause movement at the joint. The muscle that contracts to cause a joint to bend is called the flexor, while the muscle that contracts to cause the joint to straighten is called the extensor.