Put the barbell behind your neck, grab the barbell with both hands, keep your body straight, look straight ahead, and your feet are shoulder width apart. Bend your knees and squat slowly until your thighs are parallel to the ground or slightly below your knees. After holding, the quadriceps femoris will contract hard, kick and stretch the knee until it recovers. Action rhythm: squat for 2 -3 seconds, stay still 1-2 seconds, squat for 2 seconds.