Good article I just read in this month's "Journal of Strength and Conditioning" from the NSCA
It has previously been stated that the squat is one of the most popular and important exercises in developing strength and power and is commonly incorporated into strength and conditioning and rehabilitation programs (2, 11, 29). Individuals performing the squat use a variety of techniques that are of personal preference and believed to reduce the risk of injury or be more beneficial for development of specific muscles (13). It is therefore the purpose of this article to identify an optimal technique for the squat based on the selected literature.
Forces Exerted on the Knee
Contrary to popular belief, research has demonstrated that the squat does not place excessive strain on the anterior cruciate ligament (ACL) (6, 11, 12, 17, 22, 24), even though compressive and shear forces to the knee have been shown to increase as knee flexion increases, reaching peak values near maximum knee flexion (11). Research has shown that the squat exercise produces significantly less anterior displacement when compared to the open kinetic chain (OKC) exercise of the leg extension, thus reducing the amount of strain placed on the ACL (6, 12, 17). Similar results regarding peak posterior cruciate ligament (PCL) forces have also been found, with forces >4.5 times body mass during isokinetic and isometric leg extension, compared with 3.5 times body mass during the squat (12, 29), indicating that the squat may result in a lower risk of injury than the leg extension. It is worth noting that PCL forces increase with increased knee flexion (28, 33) and therefore with the depth of squat. Other research has found that during the squat the ACL was subject to small forces when the knee was less than 50? flexion and that at greater angles the PCL rather than the ACL receive greater loads (13, 29). ACL forces were also much lower when the squat was performed with heals on the ground compared with when the squat was performed with heels elevated during both the descent (26 ? 31 Nm, 95 ? 40 Nm) and the ascent (49 ? 57 Nm) (29).
?Off-season? strengthening programs of professional soccer players demonstrated no significant increases in anterior-posterior tibiofemoral translation in athletes using the squat exercise as part of their off-season training program (24). Therefore, it would appear that not only is the squat a safer leg strengthening exercise than OKC exercises (6, 12, 17), but used consistently in training, it does not increase ACL strain (24) and therefore may reduce the risk of injury comparatively speaking. Other research into OKC and CKC exercises demonstrated performance benefits of CKC exercise, which resulted in a greater increase in strength and power compared with OKC exercise (5).
A deeper squat tends to lead to anterior displacement of the knees and may increase strain on the ACL and meniscus (11). However, research has demonstrated that restricting anterior movement so that the knees do not pass beyond the toes results in a decreased knee torque from 150.1 ? 50.8 Nm (unrestricted) to 117.3 ? 34.2 Nm (restricted). Unfortunately, restricting anterior movement of the knees resulted in an increase in hip torque from 28.2 ? 65.0 Nm (unrestricted) to 302.7 ? 71.2 Nm (restricted) (14). The restricted squat also increased forward lean, which was shown to increase lumbar shear forces (14). Therefore, restricting anterior movement of the knee in an attempt to reduce the forces exerted on the knee may disproportionately increase forces exerted on the hips and lumbar spine. A study investigating head positioning found that in a downward gaze, results in hip and trunk flexion are comparable to a forward or upward gaze (9); therefore, it is recommended that the athlete maintain a forward or upward gaze to prevent an increase in lumbar shear forces.
There is continued debate among strength and conditioning experts regarding the most appropriate foot placement and squat depth, not only in terms of stresses on the knee but also in terms of recruitment of muscles.
Squat Depth and Muscle Activation
Electromyographic studies have found that increased squat depth (half squat 45?, parallel squat 90?, full-depth squat 125?) resulted in a greater percentage contribution of the gluteus maximus during the full-depth squat (7, 23). Vastus medialus oblique data suggest a trend in which the contribution of the vastus medialus oblique increases, in terms of electrical activity, in the partial squat depth but less so during a full squat (7). During the eccentric phase of the weighted-back squat, the relative contributions of 4 muscle groups (vastus medialis, vastus lateralis, biceps femoris, gluteus maximus) at the 3 depths tested were not statistically different for both average and 80-peak integrated electromyographic data (7). Therefore, muscle activation did not differ based on squat depth. However, it is worth noting that the loads used were submaximal (25% body weight and 100?125% body weight) and that recruitment patterns may alter at near-maximal loads.