# The Use of Free Weight Squats in Sports: A Narrative Review--Terminology and Biomechanics
## 6 Basic Squat Biomechanics
Typically, during the eccentric phase, it is important to produce relatively slow and constant negative bar velocity (negative because the bar moves downward) in order to keep the body position and movement in control, rather than creating a rapid and potentially deleterious descent. This is important especially when instructing novice level lifters that are learning appropriate techniques. However, it should be noted that in some cases, lifters may use a somewhat faster descent or partial descent (near the bottom of the descent) which may augment the eccentric maximum force levels and produce a subsequent higher concentric force and power output [115]. Higher descent velocities may also be used in \"speed squats\", in which lighter weight squats are to be completed as fast as possible with good technique. Higher eccentric velocities may offer a task-specific or greater stretch shortening cycle/plyometric training adaptation [115; 116]. From both a performance and clinical perspective, frontal/transverse planes of hip and knee movements during squatting have been studied [117; 118; 119]. Typically, total medial-lateral knee displacement (frontal plane) up to \(\approx\)3 cm can be expected in a back squat [117]. Excessive hip adduction and knee valgus characteristics are not considered an optimal movement pattern in the squat and may increase long-term injury potential, particularly of the knee joint [120; 121]. Additionally, it is unclear as to how hip adduction alters the typical muscle activation patterns. Although hip adductors show considerable activation, some studies have indicated substantially greater activation of the vastus medialis and gluteus media [28; 122], while others have not [79; 123]. However, the degree to which knee valgus motions are related to the bi-lateral squat maximum strength as a result of hip adduction is unclear [77; 124]. Valgus motion could be a result of multiple factors such as (1) sub-optimal maximum hip muscle strength, (2) technique preferences, (3) habits from years of training, and (4) abnormal anatomical structure (e.g., excessive Q-angle) [73; 77]. There is conflicting information available regarding the relationship between hip muscle strength and lower extremity valgus. A meta-analysis has indicated that lower extremity dynamic valgus was positively, but weakly, associated with hip strength (external rotation) in single leg ballistic tasks among women [124]. While it is reasonable and important to understand the mechanisms producing undesirable mechanics, additional investigation is necessary to understand the impact of these mechanisms on the hip and knee joints, particularly as it effects long-term injury potential. Focusing specifically on bar kinematics of the squat, an excessive amount of multi-planar bar displacements such as tilting, uneven extension, rotation, and excessive anterior displacement are considered undesirable actions, as this creates an inefficient squat movement pattern. Among trained powerlifters, total anterior displacement of the bar (shoulder), knee, and system CoM has been shown to be somewhat greater during a high bar squat compared to a low bar back squat, while posterior displacement of the hip was larger for the low bar squat, particularly during the eccentric phase [41]. Trunk forward tilt was clearly greater in the low bar squat [41]. Additionally, the peak joint movements during extension of the knee and ankle tended to be higher for the high bar squat, while the peak hip movement is larger during the low bar squat [41]. From a coaching perspective, minimizing the forward lean of the trunk is recommended, particularly during the initial phase of learning how to squat [105; 125]. If there is no excessive posterior hip displacement, the forward lean of the trunk leads to anterior displacement of the bar [126]. Aberrant mechanics can limit performance of most \"vertical\" resistance training exercises.