# The Use of Free Weight Squats in Sports: A Narrative Review--Terminology and Biomechanics
## 7 Safety and Injury Factors  
Concerns have arisen that squat exercises are injurious, particularly to the knee and lower back [21; 23; 160; 161]. For example, the knees and lower back appear to be common injury sites among lifters who regularly perform various squat exercises as a part of their training [162; 163]. These injuries are believed to be a result of excessive forces on the tissues or repetitive overload without sufficient recovery. Generally, tissues--particularly bone--are regularly exposed to three force types: compression, shear, and angular torque. Compressive forces deal with those directed axially, shear forces are directed transversely and typically cut across the axis of the tissue, and angular forces are those causing a twisting motion around the tissue axis. Even though it was not the primary aim of many past studies, it is logical to believe that compression force as well as shear force in the knee increase with additional external resistance for squatting exercises [82]. Excessive force projected through the joint is believed to be a factor related to greater injury potential. However, researchers question the degree to which squat exercises are actually contributing to acute/chronic pain in, and injuries to, the knee and lower back [22; 23]. Although some investigators and clinicians believe that injuries in the knee and lower back may result from performing squat exercises, there is little evidence that the exercise itself is the primary cause of the injury [23; 160; 164]. Moreover, these types of injuries may be due to overuse often associated with progressing training volume too fast as well as technique errors rather than an inherent problem with the exercise itself. In other words, this suggests that many, perhaps most, squat-related injuries can be prevented if lifters are properly instructed and perform and train the squat correctly. It should also be noted that among athletes, squats are rarely the only exercises being performed impacting the hip and knee joints, thus making it difficult to ascertain the impact of a single exercise on injury potential. In this section, studies and reviews are discussed in order to evaluate the speculation and concerns about pain and injuries in the spine (especially the lower back) and knee potentially resulting from squat exercises, particularly full squats. Spine: During axial compression of the spine, the vertebral body is typically the first structure to show damage and/or fracture [22; 165]. Squatting can place considerable compressive force on the vertebrae, particularly along the lumbar spine. When considering the lumbar spine (L-spine) region, it is logical to believe that as the external load increases during a squat, there will be higher axial compressive forces placed upon the body of the vertebrae, and if the weight is lifted faster, the magnitude of the compressive force increases due to higher acceleration of the CoM [144; 166]. In typical training scenarios, it is common to increase the squatting load during a session (or from session to session), which can increase spinal compression. For example, half and quarter squat exercises using loads of 0.8-1.6 \(\times\) body weight can result in vertebral compressive forces (L3-4) of 6-10 times body weight at the initiation of the concentric phase (76), so the compression force magnitude likely increases as external load increases [79; 167]. While compressive force magnitudes during squatting movements appear to be relatively high, it is important to note, based on cadavers, that the maximal compressive forces that the vertebral body can handle can be as high as 7800 N -10,000 N for typical untrained middle-aged males [23; 168]. Although it seems unrealistic to believe that the general population commonly experience compression forces this high, among well-trained male strength-power athletes, compression forces may reach as much as 37,000 N [57; 169]. Among females, vertebral CSA and compressive strength is lower than males [22].