The aims of the study were to evaluate the relative and absolute variability of upper (T11-L2) and lower (L2-S2) lumbar spinal alignment in power- and weightlifters during the deadlift and back squat exercises, and to compare this alignment between the two lifting groups. Twenty-four competitive powerlifters (n = 14) and weightlifters (n = 10) performed three repetitions of the deadlift and the back squat exercises using a load equivalent to 70% of their respective one-repetition maximum. The main outcome measures were the three-dimensional lumbar spinal alignment for start position, minimum and maximum angle of their spinal alignment, and range of motion measured using inertial measurement units. Relative intra-trial reliability was calculated using the two-way random model intraclass correlation coefficient (ICC) and absolute reliability with minimal detectable change (MDC). The ICC ranged between 0.69 and 0.99 and the MDC between 1 degrees-8 degrees for the deadlift. Corresponding figures for the squat were 0.78-0.99 and 1 degrees-6 degrees. In all participants during both exercises, spinal adjustments were made in both thoracolumbar and lumbopelvic areas in all three dimensions. In conclusion, when performing three repetitions of the deadlift and the squat, lumbar spinal alignment of the lifters did not change much between repetitions and did not differ significantly between power- and weightlifters.