# The Use of Free Weight Squats in Sports: A Narrative Review--Terminology and Biomechanics
## 6 Basic Squat Biomechanics
Although 2D approaches, often synchronized with a force plate, have been the norm, recent advances in 3D motion technology have enhanced the accuracy of inverse dynamic calculations. Using 3D video analysis combined with synchronized force plate(s) has resulted in lower extremity joint torques being a more commonly reported variable in the quantification of squatting movements. For example, [PERSON] and [PERSON] [99] reported hip and knee joint torque as a main contributor to concentric action during the squat. Depending on the bar position (e.g., high- or low-bar position) and different squat techniques, the contribution of the hip and knee also change as the body position changes. The magnitude of the forward lean of the trunk or the posterior displacement of the hip can all impact the knee and hip joint torques [98; 99]. For example, performing a high bar squat (21 kg) during which restricted forward knee movement caused a forward trunk tilt resulted in 302 N-m of hip torque and 117.3 N-m of knee torque [46]. Conversely, when the trunk angle was relatively vertical, and the knee was allowed to freely move forward, there was a hip torque of 28.2 N\(\cdot\)m and a knee torque of 15.1 N\(\cdot\)m. [PERSON] et al. [126] also reported that forward trunk lean is associated with a posterior hip shift during the descent of the squat. This pattern of movement is particularly evident during low bar squats, when restricting forward knee displacement. This technique can result in a larger moment arm for the weight relative to the hip, and greater hip torque at the initiation of the concentric action and results in substantial low back loading [144]. Investigating angular kinetics in different squat tasks and examining bilateral asymmetry between a healthy knee and a repaired or reconstructed knee can provide important insights into injury mechanisms as well as rehabilitation aspects. For example, [PERSON] et al. [94] investigated the bilateral differences in the high bar back squat performed by participants who had reconstructive knee surgery. Peak hip joint moment was markedly greater on the surgery side compared to the healthy side (1.67 Nm\(\cdot\)kg\({}^{-1}\) vs. 1.12 Nm\(\cdot\)kg\({}^{-1}\), \(p\) = 0.06). Additionally, there was a statistically significant difference in the peak knee joint moment for the surgery side compared to the non-injured side (surgery side: 1.02 Nm\(\cdot\)kg\({}^{-1}\) vs. non-injured side: 1.28 Nm\(\cdot\)kg\({}^{-1}\), \(p\) = 0.003). Furthermore, the ratio of average peak hip extensor and knee extensor torque was greater on the surgery side by 46.5% (\(p\) = 0.02) as compared to the non-injured side. Collectively [94; 145; 146; 147], the effort level at the knee and hip was different between the surgery side and healthy side. Importantly, this asymmetry may persist for several years [145; 146]. Conversely, for non-injured athletes, there is no supportive evidence that these types of asymmetries exist in the lower extremities. The general assumption is that non-injured athletes are fairly symmetrical, indicating that asymmetries occur as a result of injury, or post-surgery. Based on current data, from the rehabilitative standpoint, a goal would be to minimize the difference between the two sides during squatting and likely other tasks such as jumping. Electromyography (EMG): EMG is a method of evaluating the neural activation of muscles by measuring the muscles' electrical activity. Using EMG for the evaluation of squatting movements and other resistance training exercises has a relatively long history [148]. In previous EMG studies, investigators often targeted the superficial lower extremity muscles [68; 149; 150; 151; 152; 153; 154; 155; 156; 157; 158; 159; 160; 161; 162; 163; 164; 165; 166; 167; 168; 169; 170; 171; 172; 173; 174; 175; 176; 177; 178; 179; 18  
greater activation in the high bar squat [33; 44].