Final answer:
Flexor muscles contract to decrease the angle between two bones, causing flexion. Extensor muscles work in opposition to straighten the joint. Muscles function in pairs to enable joint movement.
Step-by-step explanation:
The muscles that cause the angle between two bones to decrease are called flexor muscles. During a movement termed flexion, these muscles contract and bring bones closer together, effectively bending the joint and decreasing the angle between the bones involved.
This action can be observed in joints like the elbow where the biceps muscle contracts to bend the arm. Conversely, extensor muscles work to straighten the joint, increasing the angle between bones. Muscles work in pairs, with one muscle contracting and the flexor muscle elongating or relaxing in order to facilitate movement.