One motion, different strategies: Intra-individual spinal movement variability during a repeated flexion task

Spinal movement variability is a normal feature of repetitive motions and has been hypothesized to differ between people with and without low back pain. However, normative values for intra-individual variability are currently lacking, making it difficult to judge when the variability becomes abnormal. This study used a combination of principal components analysis, single component reconstruction, and coefficient of variation to assess intra-individual variability of 3 blocks of 10 repeated spinal flexion movements in a group of 15 healthy individuals. Spinal flexion movements were assessed using motion capture cameras and a 19 × 3 matrix of retroreflective stickers on the spinous processes and bilateral paraspinal muscle bellies of S1-C7 spinal levels. All participants showed lower range of motion coefficients of variation in the lumbar spine (1.9-25.3 %) compared to the thoracic spine (7.9-30.1 %). To explain ≥80 % of the total variance within movements, 2-5 principal components were needed for each participant. Single component reconstruction revealed magnitude changes, waveform differences, and phase shifts as common sources of variability. These changes were usually observed when the coefficient of variation exceeded 10 % for that region of the spine. In conclusion, healthy individuals display varying levels of intra-individual spinal movement variability. The sources of variability can be interpreted using a combination of principal components analysis and single component reconstruction.