During the descent phase of a squat, the quadriceps are primarily contracting:

During the descent phase of a squat, the quadriceps are primarily contracting in an eccentric manner. Eccentric contraction occurs when a muscle lengthens under tension, which is precisely what happens to the quadriceps as a person lowers their body toward the ground. As the knees flex and the body descends, the quadriceps work to control the downward movement and prevent the descent from occurring too rapidly. This control helps to stabilize the joints and maintain proper alignment, reducing the risk of injury.

In contrast to eccentric contractions, concentric contractions involve muscles shortening while generating force, which typically occurs when lifting or pushing against gravity. Isometric contractions would involve the quadriceps maintaining tension without changing length, commonly observed when holding a position stationary, such as at the bottom of the squat. Lastly, isokinetic contractions happen at a constant speed throughout the range of motion, which is not typical in standard squat performance. Therefore, acknowledging that during the descent of a squat the quadriceps are actively lengthening to control the movement highlights why eccentric contraction is the correct answer.