An essential exercise for strength training of the lower limbs is the squat exercise. During this exercise, changes in lumbar lordosis are commonly used to indicate when the descent of the squat should cease, yet the behavior of the lumbar-scarum segments remains unclear. The purpose of this study was to quantify the lumbar-sacrum movements during the back squat, because the movement of the sacrum is influenced by the width of stance, this variable was also investigated. Thirty trained subjects, 18 men with 1 repetition maximum (1RM) squat of 123% (13.9%) of bodyweight and 12 women with 1RM squat of 93% (15.6%), performed a set of narrow and wide stance squats, each carrying an additional 50% of body weight as load. The timing and movement of the lumbar angle (T12/L1), sacrum angle (L5/S1), and lumbar flexion angle (lumbar lordosis) were measured in 3 dimensions for the ascent and decent phases. Men and women achieved similar lumbar angles for both width of stance and phase. Sacrum angles, lumbar flexion angles, and timing differed significantly (p < 0.05) between gender and width of stance. The lumbar flexion range during the descent phase for women in narrow and wide stance was 12.9° and 12.6°, respectively; for men, this range was significantly (p < 0.05) larger at 26.3° and 25.4°, respectively. Men and women developed different movement patterns for the squatting movement, and therefore, this needs to be considered in strength development and screening procedures. The lumbar spine became kyphotic as soon as a load was placed on the shoulders, and any teaching cues to maintain a curved lumbar spine when squatting must be questioned.