these other standards are: bottom of hamstring parallel to the ground; the hip joint itself below the top of the knee, or femur parallel to the floor; and the top of the upper thigh (i.e., top of the quadriceps) below the top of the knee. Squatting below parallel qualifies a squat as deep while squatting above it qualifies as shallow. Some authorities caution against deep squats; though the forces on the ACL and PCL decrease at high flexion, compressive forces on the menisci and articular cartilages in the knee peak at these same high angles. This makes the relative safety of deep versus shallow squats difficult to determine. As the body descends, the hips and knees undergo flexion, the ankle extends (dorsiflexes) and muscles around the joint contract eccentrically, reaching maximal contraction at the bottom of the movement while slowing and reversing descent. The muscles around the hips provide the power out of the bottom. If the knees slide forward or cave in then tension is taken from the hamstrings, hindering power on the ascent. Returning to vertical contracts the muscles concentrically, and the hips and knees undergo extension while the ankle plantarflexes. Common errors of squat form include descending too rapidly and flexing the torso too far forward. Rapid descent risks being unable to complete the lift or causing injury. This occurs when the descent causes the squatting muscles to relax and tightness at the bottom is lost as a result. Over-flexing the torso greatly increases the forces exerted on the lower back, risking a spinal disc herniation. Another error is when the knee is not aligned with the direction of the toes, entering a valgus position, which can adversely stress the knee joint. An additional common error is the raising of heels off the floor, which reduces the contribution of the gluteus muscles. Muscles used Agonist muscles Quadriceps femoris Gluteus maximus Adductor magnus Soleus Stabilizing muscles Erector spinae Rectus abdominis Internal and external obliques Hamstrings Gluteus medius and minimus Gastrocnemius Equipment Various types of equipment can be used to perform squats. A power cage can be used to reduce risk of injury and eliminate the need for a spotting partner. By putting the bar on a track, the Smith machine reduces the role of hip movement in the squat and in this sense resembles a leg press. The monolift rack allows an athlete to perform a squat without