# The Use of Free Weight Squats in Sports: A Narrative Review--Terminology and Biomechanics
## 4 Defining Squat Depth by Knee Angle  
When examining any of the various squatting techniques, one of the key areas of discussion revolves around the depth at which squats are performed [1]. This is often a difficult concept because the term depth suggests that a vertical displacement of the barbell is being quantified, when in reality the depth of the squat is a function of the knee and hip angle achieved during the squatting motion and the movement of the bar and body as a system [27; 57; 64]. Therefore, simply measuring or quantifying barbell displacement as a means of determining depth can be misleading [65] in that this type of measurement does not account for trunk lean or the actual knee and hip angles achieved during the squatting motion. Based upon this idea, a reasonable determination of the depth of any squatting movement should be quantified in relation to a specified knee angle achieved [43; 66] and not the vertical displacement of the bar (Table 1). The authors realize that tissue strain is a function of various knee, hip, and ankle angles and is covered elsewhere in this paper; however, it should be noted here that the knee angle and relation of the thigh to the floor offer easily discernable landmarks for standardization of depth. Although hip angle can affect tissue strain, it is quite difficult to measure/estimate during a squat performance. However, reasonable quantification of knee angle is possible both before and during a squatting movement using appropriate measurement tools [43; 66]. Furthermore, the authors' observations and that of other researchers indicate that when squatting to a specific knee angle, the change in hip angle during the movement remains relatively constant in comparison to the change in knee angle throughout the movement(s) [27; 57; 64]. Additionally, the terminal point of the squat movement can be more easily estimated through observation-for example, for a parallel squat in which the top of the thigh is parallel (or slightly below) with the floor. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & **Common Variant** & **Internal Angle of Knee** & **Description of Bottom Position** & **References** \\ \hline Back squat & Full squat & 40–45\({}^{\circ}\) & & Tops of the thighs are positioned so that they are below parallel when compared to the floor. & [45; 66; 67] \\  & & & & In this position the top of the thigh is slightly below parallel to the floor. Additionally, a straight horizontal line can be drawn from the top of the knee to the inguinal fold. & [45; 67; 68] \\  & & & & top of the knee to the inguinal fold. & \\ Half squat & 80–100\({}^{\circ}\) & & In this position the bottom of the thigh is approximately parallel to the floor. & [66; 67; 69] \\  & & & & Often categorized as halfway between a full squat and an upright standing position. Often used as part of strength–power–potentiating complexes. & [2; 66; 70] \\  & & & & During the box squat, the box height should be selected so that the bottom position of the squat is achieved when the top of the thigh is parallel to the floor. & [2; 66; 70] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Defining the squat variants and common depths. A point of confusion in the literature is how to clearly define joint angles. For example, some authors define squat depth in the context of the external knee angle [17; 22; 74], while others relate squatting depth to an internal knee angle [27; 43; 64; 66; 68; 70; 75]. In order to clearly present and define the various squatting depths, this review will reference the internal knee angle for the quantification of the various squatting depths that have been reported in the scientific literature or used in practical settings. Based upon these knee angles, four generalized squat depths can be established for bilateral movements, which include the full, parallel, half, and quarter squat [1; 2; 27].