Squat (exercise)
A squat is a strength exercise in which the trainee lowers their hips from a standing position and then stands back up. During the descent of a squat, the hip and knee joints flex while the ankle joint dorsiflexes; conversely the hip and knee joints extend and the ankle joint plantarflexes when standing up. Squats are considered a vital exercise for increasing the strength and size of the lower body muscles as well as developing core strength. The primary agonist muscles used during the squat are the quadriceps femoris, the adductor magnus, and the gluteus maximus. The squat  also isometrically uses the erector spinae and the abdominal muscles, among others. The squat is one of the three lifts in the strength sport of powerlifting, together with the deadlift and the bench press. It is also considered a staple exercise in many popular recreational exercise programs. Form. The movement begins from a standing position. Weight is often added; typically in the form of a loaded barbell, but dumbbells and kettlebells may also be used. When a barbell is used, it may be braced across the upper trapezius muscle, which is termed a "high bar squat", or held lower across  the rear deltoids, termed a "low bar squat". 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. Although it may be confusing, many other definitions for "parallel" depth abound, none of which represents 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 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 Stabilizing muscles 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 having to take a couple of steps back with weight on as opposed to conventional racks. Not many  powerlifting federations allow monolift in competitions (WPO, GPC, IPO). Other equipment used can include a weight lifting belt to support the torso and boards to wedge beneath the ankles to improve stability and allow a deeper squat (weightlifting shoes also have wooden wedges built into the sole to achieve the same effect). Wrist straps are another piece of recommended equipment; they support the wrist and help to keep it in a straightened position. They should be wrapped around the wrist, above and below the joint, thus limiting movement of the joint. Heel wedges and related equipment are discouraged by some  as they are thought to worsen form over the long term. The barbell can also be cushioned with a special padded sleeve. Chains and thick elastic bands can be attached to either end of the barbell in order to vary resistance at different phases of the movement. This may be done to increase resistance in the stronger upper phase of the movement in order to better meet a person's 1RM for that phase. Bands can also be used to reduce resistance in the lower weaker phase by being hung from a power rack and the barbell being increasingly supported  by them as it is lowered. This can help someone to overcome a 'sticking' point. A squat performed using these techniques is called a variable resistance squat. Variants. The squat has a number of variants, some of which can be combined: Injury considerations. Although the squat has long been a basic element of weight training, it has not been without controversy over its safety. Some trainers claim that squats are associated with injuries to the lumbar spine and knees. Others, however, continue to advocate the squat as one of the best exercises for building muscle and strength. Some coaches maintain  that incomplete squats (those terminating above parallel) are both less effective and more likely to cause injury than a full squat (terminating with hips at or below knee level). A 2013 review concluded that deep squats performed with proper technique do not lead to increased rates of degenerative knee injuries and are an effective exercise. The same review also concluded that shallower squats may lead to degeneration in the lumbar spine and knees in the long-term. Squats used in physical therapy. Squats can be used for some rehabilitative activities because they hone stability without excessive compression on the tibiofemoral  joint and anterior cruciate ligament. Deeper squats are associated with higher compressive loads on patellofemoral joint and it is possible that people who suffer from pain in this joint cannot squat at increased depths. For some knee rehabilitation activities, patients might feel more comfortable with knee flexion between 0 and 50 degrees because it places less force compared to deeper depths. Another study shows that decline squats at angles higher than 16 degrees may not be beneficial for the knee and fails to decrease calf tension. Other studies have indicated that the best squat to hone quadriceps, without inflaming  the patellofemoral joint, occurs between 0 and 50 degrees. Combining single-limb squats and decline angles have been used to rehabilitate knee extensors. Conducting squats at a declined angle allows the knee to flex despite possible pain or lack of mobilization in the ankle. If therapists are looking to focus on the knee during squats, one study shows that doing single-limb squats at a 16-degree decline angle has the greatest activation of the knee extensors without placing excessive pressure on the ankles. This same study also found that a 24-degree decline angle can be used to strengthen  ankles and knee extensors. Different Sets For Squats Forced repetitions are used when training until failure. They are completed by completing an additional 2-4 reps (assisted) at the end of the set. Partial repetitions are also used in order to maintain a constant period of tension in order to promote hypertrophy. Lastly, drop-sets are an intense workout done in at the end of a set which runs until failure and continues with a lower weight without rest.