# The Use of Free Weight Squats in Sports: A Narrative Review--Terminology and Biomechanics
## 7 Safety and Injury Factors
MASF values were found to be up to 500 N at 160\({}^{\circ}\) of internal knee flexion with a heavy external load (250 kg) [189]. While shear forces are influenced by squat speed, none of the previous studies reported extensively regarding the relationship between angular knee flexion velocity and the magnitude of shear forces. Specifically for MPSF, knee joint angular velocity seems to be an important factor for predicting load force as previous studies showed different magnitudes of MPSF with a range of external loads (147-250 kg) and internal knee flexion anglesfrom 117\({}^{\circ}\) up to 50\({}^{\circ}\), representing approximately 45% of limit tensile strength [81; 151; 190]. Calculated shear forces on the ACL were found to be considerably lower, typically less than 1000 N and less than 30% limit tensile strength [27; 187]. Thus, shear forces developed in the cruciate ligaments are well within tensile strength tolerance. Note that based on evidence from recreational trained subjects, anterior-posterior shear forces may be less in males compared to females, resulting from greater co-activation among males (hamstrings and quadriceps) [104]. Additionally, some evidence suggests that squat training increases co-activation, thus reducing shear forces [76]. Compressive force: Knee compressive forces occur where the femur and tibia (i.e., tibio-femoral joint), and patella and femur (i.e., patello-femoral joint), are in contact. Compressive forces for both joints generally increase as the knee flexes during a low bar squat motion and reach a maximum at the peak knee flexion angle [39]. [PERSON] et al. [39], describing the low bar squat, also reported differences in patello-femoral forces at different internal knee angles (120\({}^{\circ}\)\(p\) = 0.001; 110\({}^{\circ}\)\(p\) = 0.001; 100\({}^{\circ}\)\(p\) = 0.002; and 90\({}^{\circ}\)\(p\) = 0.002), indicating that as the knee flexes, greater forces at the patello-femoral joint are created. However, it is important to note that increases in force at this joint are largely due to alterations in mechanical advantage at the knee joint. As documented by [PERSON] et al. [39], underpinning mechanisms of knee pain associated with patello-femoral pain syndrome are poorly understood. Thus, the greater force application at the patello-femoral joint cannot be clearly defined as a primary reason for the pain. Past studies have indicated that knee compression forces of a powerlifter squatting can be greater than twice body mass. The tibio-femoral compressive force at the internal knee flexion of 50\({}^{\circ}\) was approximately 8000 N, at 120\({}^{\circ}\) was 5500 N, and at 150\({}^{\circ}\) was 3500 N [151; 161]. Although these values seem very high, especially at an internal knee flexion of 50\({}^{\circ}\) (i.e., equivalent to near a full squat position), it appears that these values are well below the threshold for patellar ligament or tendon failure, indicating a substantial reserve before acute failure occurs. Indeed, the patellar ligament (untrained) can handle up to 15,000 N before failure. Although information is lacking, the quadriceps tendon has a much larger CSA than the patellar ligament and potentially could withstand even greater forces [189; 191; 192]. Interestingly, an internal knee flexion of approximately 130\({}^{\circ}\) seems to be a threshold for compressive and shear forces in the tibio-femoral and patello-femoral joints. After 130\({}^{\circ}\), forces markedly rise with continued flexion, as occurs in deeper squats (or leg presses) [82]. The recommendation was to limit the internal knee flexion angle to 130\({}^{\circ}\) for selected cases in rehabilitation [82]. However, this does not mean that internal knee flexion \(\geq\)130\({}^{\circ}\) is injurious. Considering the data from these studies [189; 191; 192], proper technique and a reasonable training process still seem to be key factors for minimizing the risk of pain and injury.