Loading conditions in the spine, hip and knee during different executions of back extension exercises

BACKGROUND: Back extension (BE) is a strength exercise for training the dorsal trunk and hip muscles. To optimise training recommendations that avoid overloading and possible injury, the aim of this study was to determine the loading conditions and the influence of different execution forms of BE on spine, hip and knee ranges of motion (RoMs), joint moments and muscle activity. METHODS: The kinematics, kinetics and muscle activity (EMG) of two execution types (BE(h): dynamic hip, BE(s): dynamic spine) and two versions (one-legged and two-legged) of BE were measured in 16 subjects. RoMs and external joint moments were calculated using an inverse dynamics approach and analysed with a linear mixed model. RESULTS: Although lumbar spine flexion was observed in both execution types, thoracic spine flexion predominantly occurred during BE(s), whereas thoracic spine extension was observed during BE(h). Larger maximal back and hip moments were observed for BE(h) than for BE(s). The activity of the dorsal back and hip muscles, as observed using EMG, was increased for one-legged executions. CONCLUSION: To strengthen the hips and lower back, BE(h) seem to be more efficient due to the higher moments, with higher or similar RoMs in the hip and lower back. One-legged BE(s) seem to provide an effective training for the hamstrings and hip regions without subjecting the spine to excessive loading, possibly promoting this as an effective exercise during training and rehabilitation.