# The Use of Free Weight Squats in Sports: A Narrative Review--Terminology and Biomechanics
## 7 Safety and Injury Factors
Thus, at equivalent external loads, female vertebrae are exposed to a relatively higher compressive force. However, during full squats, females demonstrated a lower range of lumbar flexion and a greater anterior tilt of the sacrum compared to males [22]. Females have also demonstrated a lower stiffness and greater RoM between lumbar spinal segments and may be more capable of developing muscular stabilization of the spine and thus maintaining a lordotic curve during deep squats [80]. Importantly, high compression forces observed among strength-power athletes, including females, especially those engaged in squatting for many years without spinal compression injury, suggests a positive training adaptation [169; 170; 171]. Axial compression force is not the only concern for lower back issues; loading and body position can create other forces, such as shear force (transverse) and angular torque, that could be associated with lower back issues [21]. From a mechanical perspective, changing the CoM position toward the anterior or posterior can increase or decrease the amount of shear and compressive forces within the lumbar spine. Early research indicated that with spinal flexion, there are greater trunk extensor and lumbar shear force demands, but a reduction in compressive forces [172]. It should be noted that back muscle activation brings about resultant shear forces on the joint that can be maintained at a relatively constant and safe level [90; 135]. This indicates that the musculature represents a safety mechanism that is particularly useful for maintaining a lordotic posture during lifting, when the muscles must provide the majority of the support moment [135]. These observations suggest that to minimize shear force in the spine, a relatively erect neutral to lordotic trunk position is recommended. Conversely, when a natural curvature of the lumbar area is maintained, the magnitude of the compressive force in the area can stay the same, but as the magnitude of lordotic curvature increases, the compression force also increases [165; 167]. Furthermore, [PERSON] et al., [103] employing real-time collection at 120 Hz using a 3D Magnetic Tracking Device, indicated that during lifting the spine becomes slightly flexed at 50% of body mass (even though the spinal muscles are contracting), particularly in men, partly due to structural differences between men and women. They indicated that the subtle alterations in spinal position would be quite difficult to see visually [103]. Thus, a more neutral spine may be the actual case during lifting, although the lifter should attempt to maintain a lordotic posture. Interestingly, [PERSON] et al. [173] used IMUs affixed at the spinous processes at T11, L2, and at the sacrum (S2), and these authors reported that when experienced powerlifters and weightlifters performed three repetitions of the squat at approximately 70% 1 RM, lumbar spinal alignments were adjusted in all three planes. The three-dimensional spinal alignment adjustments showed low variability and did not reach the outer ranges of lumbar spinal flexion or extension for the squat. It should also be mentioned that the capacity of the human movement system to adapt to tissue loading in order to maintain tissue homoeostasis and function is quite large [174]. Thus, to a point, the more the tissues are loaded, the stronger they become, and it is possible that subtle degrees of spinal flexion-extension may enhance tissue adaptation, making tissues stronger [175]. In any case, the development of good squat technique and the need for qualified instructors and coaches is paramount. A traditional method believed to help maintain a neutral or lordotic curvature in the spine when performing squat exercises is the use of a belt [161; 176]. However, some evidence indicates that wearing a belt has little or no effect [161]. For example, [PERSON] et al.