In strength training and fitness, the squat is a compound, full body exercise that trains primarily the muscles of thethighs, hips and buttocks, quadriceps (vastus lateralis, vastus medialis, vastus intermedius and rectus femoris),hamstrings, as well as strengthening the bones, ligaments and insertion of the tendons throughout the lower body. Squats are considered a vital exercise for increasing the strength and size of the legs and buttocks, as well as developing core strength. Isometrically, the lower back, the upper back, the abdominals, the trunk muscles, the costal muscles, and the shoulders and arms are all essential to the exercise and thus are trained when squatting with the proper form.The squat is one of the three lifts in the strength sport of powerlifting, together with deadlifts and bench press.The movement begins from a standing position. Weights are often used, either in the hand or as a bar braced across the trapezius muscle or reardeltoid muscle in the upper back. The movement is initiated by moving the hips back and bending the knees and hips to lower the torso and accompanying weight, then returning to the upright position.Squats can be performed to varying depths. The competition standard is for the crease of the hip (top surface of the leg at the hip joint) to fall below the top of the knee; this is colloquially known as "parallel" depth. Confusingly, many other definitions for "parallel" depth abound, none of which represent the standard in organized powerlifting. From shallowest to deepest, 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 ACLand 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 gradually descends, the hips and knees undergo flexion, the ankle 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. In a weight bearing squat, the heels should always maintain contact with the floor throughout the movement. Weight shifting forward on to the toes, and off the heels creates unnecessary stress on the knee joint. This added stress may lead to inflammation or other overuse injuries.Two common errors 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 aspinal disc herniation. Another error where health of the knee joint is concerned is when the knee is not aligned with the direction of the toes. If the knee is not tracking over the toes during the movement this results in twisting/shearing of the joint and unwanted torque affecting the ligaments which can soon result in injury. The knee should always follow the toe. Have your toes slightly pointed out in order to track the knee properly.