Knee extension describes the movement where the leg straightens by increasing the angle at the joint. This action happens when the quadriceps muscles contract to push the weight upward from the floor. Success in the clean and jerk depends heavily on the power generated during this specific movement. An athlete drives the barbell upward by fully straightening their legs during the final pull phase. Strong extension allows the lifter to transfer force from the ground into the heavy metal bar. Lifters practice squats to improve the strength and speed of this critical leg straightening process.