Final answer:
During a countermovement jump, the knee joint is extended by the quadriceps muscle group, while the ankle joint is involved in plantarflexion powered by the calf muscles. The quadriceps and calf muscles play important roles in producing the necessary joint movements for a countermovement jump.
Step-by-step explanation:
During a countermovement jump, the knee and ankle joints undergo specific movements as a person pushes upward before takeoff. Let's start with the knee joint: as the person is preparing to jump, the quadriceps muscle group is activated to extend the knee joint. This muscle group includes the rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis. These muscles work together to straighten the knee joint. On the other hand, during ankle joint movement, the calf muscles are primarily involved. The main muscle involved is the gastrocnemius, which contracts to push off the ground and lift the body up.
In terms of muscle actions, the quadriceps muscles are the agonists for knee extension, meaning they are responsible for producing the movement. Conversely, the hamstrings are the antagonistic muscles that flex the knee joint. For ankle joint movement, the calf muscles are agonists for plantarflexion, which is the movement that lifts the body up. The tibialis anterior muscle, located on the front of the lower leg, acts as the antagonist to control the movement of dorsiflexion, which is lowering the heel to the ground.