Final answer:
Eccentric contractions of the hamstrings control the descent onto a chair, preventing one from plopping down. These muscles also facilitate movements including pulling the lower leg towards the buttocks, moving and twisting the thigh, and maintaining posture.
Step-by-step explanation:
The action described involves eccentric contraction of the knee flexors, primarily the hamstrings, to control the lowering of the body onto a chair. Eccentric contractions occur when a muscle lengthens under tension, which in this case, helps to control the descent and prevent 'plopping' onto the seat. The hamstrings are responsible for several movements: they move the back of the lower legs up and back toward the buttocks as when kneeling; move the thigh down and back; and twist the thigh (and lower leg) inward. These muscles also assist in moving the thigh diagonally upward and outward, and are important for maintaining posture while walking and activities such as foot-tapping or jumping.