# The Use of Free Weight Squats in Sports: A Narrative Review--Terminology and Biomechanics
## 7 Safety and Injury Factors
As noted, if a lifter squats more than twice body weight, tibio-femoral compressive forces can be quite large (i.e., up to 8000 N) [85]. With the same load, patellar ligament tension can increase up to 6000 N, but patellar ligament tensile strengths (untrained) have been measured at over 10,000 N [34]. The ultimate structural strength for the healthy knee appears to be capable of handling the forces encountered during heavy squatting. Therefore, those forces projecting through the joint are not necessarily a critical element for creating pain and injuries. As previously noted, injury could be due to ineffective lifting technique and improper training progression, such as adding loads too quickly. However, these same forces may be critical stimuli for adaptive mechanisms [27; 50]. Menisci and cartilage are also susceptible to mechanical influences, particularly compressive and shear forces. Concerns about degenerative changes of cartilage and the apparent increased risk for chondormalacia, osteoarthritis, and osteochondritis resulting from deep squats are apparently unfounded [27]. Although compressive and shear forces could induce cellular trauma, there are two protective mechanisms that must be considered. First, for articular cartilage (as in the tibio-femoral joint), load is transmitted not by elastic deformation (as might occur with severe impact), but rather through hydrostatic compression [193], which markedly reduces the potential for cellular trauma [194]. Secondly, in deep squats, such as the catch in weightlifting movements or during deep front squats, in which internal knee angles can be \(\geq\)40\({}^{\circ}\), posterior thigh and calf contact can diminish knee-joint forces markedly [179; 195]. Calculations of forces at deep squat knee angles have not considered soft tissue contact [27; 196] and must be examined critically [27]. Additionally, depending upon the CSA of hamstring and calf muscles, thigh-calf contact could begin at larger knee angles, from approximately 60\({}^{\circ}\)[23], reducing tibiofemoral [74; 113] and patellofemoral joint forces [24; 197]. Thus, the belief that deep squats increase the potential for injury and degenerative changes of the knee joint [18; 125] appears unsupported by objective evidence [22; 30]. As previously noted, knee forces are altered by a number of factors including knee angle. For example, partial squats (80\({}^{\circ}\)-100\({}^{\circ}\) internal knee angle) should be used with caution because, at the initiation of the concentric phase, these knee joint angles result in the highest patella-femoral [163; 198] and compressive stresses [168; 193] and occur with only a small tendon-femoral support surface [197; 198]. Thus, clinical recommendations for half or quarter squats in order to reduce the potential for degenerative changes in the knee joint may be counterproductive from an adaptive perspective [22; 97]. However, with a pre-existing injury or previous reconstruction of the PCL, it may be best to restrict knee flexion to \(\sim\)130\({}^{\circ}\), which can minimize posterior shear [17; 21]. Changes in forces with alternative squat exercises: It has been previously established that knee angles can influence the magnitude of knee compressive and shear forces in squatting. Studying squat variants, [PERSON] et al. [39] showed that single-leg squats (knees translated beyond toes at maximum knee flexion) generally produced smaller patello-femoral compressive/shear forces compared to two different types of dynamic wall squats, long (knees over ankles at maximum knee flexion) and short (knees translate beyond toes at maximum knee flexion). It is important to mention that the squatting tasks used in this study (one-leg squat and wall squat) are often used in clinical settings, while more traditional strength and conditioning exercises, such as the back squat, can produce higher patello-femoral compressive and shear forces, often due to greater loading.