# The Use of Free Weight Squats in Sports: A Narrative Review--Terminology and Biomechanics
## 7 Safety and Injury Factors
Knee: In the past, the squat was believed to be quite injurious to the knee [184]. Unfortunately, many, particularly in the medical profession, still believe that squatting is injurious to the knee (and other structures). Indeed, excessive tibio-femoral shear and compressive forces are a concern for many clinical professionals when considering the use of squat exercises as a part of a rehabilitative process with their patients. Because of this concern, investigators are often interested in analyzing forces transferred through the knee (i.e., tibio-femoral joint), as well as patella-femoral joints [22; 101]. Compression and shear force: Compression forces are those axially directed forces from the \"top and bottom\" of a structure. Knee compressive forces can be generally divided into tibio-femoral and patello-femoral [22; 101]. Shear forces are perpendicular to the axial direction. Several factors can affect knee compressive and shear forces. Increased velocity of movement, squat depth, and loading are related to increases in forces applied to the tissues [75]. Indeed, increased acceleration can increase compression and shear forces to a greater extent than loading alone, particularly in the transition from the descent to the ascent phase of the squat [22; 27]. Additionally, lack of proper control of the descent phase can result in ballistic contact between the hamstrings and calf muscles, potentially increasing the stress on knee ligaments [17; 34; 179]. However, it should be noted that weightlifters routinely use semi-ballistic descents in the catch phase of the snatch and clean and jerk but have not shown excessive knee ligament problems [53; 160; 163]. Nevertheless, there is a potential for injury with rapid descents or bouncing out at the bottom; therefore, it is logical to perform squat exercises with a controlled speed, particularly in the eccentric phase. For advanced speed-strength training, the squat can be performed explosively with high velocity to maximize the training adaptation, provided bouncing is avoided. However, for novice level and very young lifters, certified/licensed strength and conditioning coaches and personal trainers should teach those lifters to perform a descent that is controlled to a degree that allows maintenance of body control and appropriate positions (e.g., neutral or lordotic spine). As long as reasonable control is maintained, especially in the descent phase of the squat, concerns for damage such as osteoarthritis, ligament tears, and meniscus tears in the knee joint can be minimized. Following is a summary of literature addressing the shear and compressive force issues at the knee joint. Shear force: Shear force is the result of forces created in opposite directions to one another; during squatting, the greatest forces are projected posteriorly [35; 185]. Shear forces can be particularly injurious to knee cartilage and cruciate ligaments, particularly the ACL. Tibio-femoral compression forces stabilize the knee joint and help to reduce shear forces [27; 186]. For the knee joint during squatting, shear force is generally calculated in a plane with respect to the alignment of tibia and femur. Shear force reduction depends upon appropriate technique application and proper alignment of body segments during squatting [27; 170]. Based on cadavers, tensile strength (load at failure) of the PCL for young men is about 4000 N. The tensile strength is about 2000 N for the ACL and decreases with age [187; 188]. Mean posterior shear force (MPSF) creates a tensile load on the PCL when performing a squat. Load values for MPSF range widely depending on external loads and specific joint angles. With similar external loads and similar populations tested, studies have reported similar MPSF load values of 1783 to 1868 N at an internal knee angle of 117\({}^{\circ}\) (at a quarter squat depth) and 90\({}^{\circ}\) (at a half squat depth) [81; 118]. Mean anterior shear force (MASF) creates a tensile load in the ACL when performing a squat.