Back muscle injury caused by retractor application during posterior spine surgery in rats was examined histologically and histochemically according to the postoperative time with reference to the retraction time-pressure relationship. The results were correlated to provide the risk factors for back muscle injury during posterior spine surgery. Back muscles were examined histologically and histochemically after application of quantitative pressure and retraction time. No previous study has assessed this relationship. Five groups were studied: Group 1, 1-hour low-pressure load group: Group 2, 1-hour high-pressure load group; Group 3, 3-hour low-pressure load group; Group 4, 3-hour high-pressure load group: and sham group. In each group, the multifidus muscle was evaluated 3 hours, 48 hours, 1 week, 3 weeks, and 6 weeks after surgery. In all groups except the sham group, degeneration of the muscle and neuromuscular junction was found at a very early postoperative time, but regeneration began at 1 week, 3 weeks, and 6 weeks after surgery. In all groups except the sham group, degeneration of the muscle and neuromuscular junction was found at a very early postoperative time, but regeneration began at 1 week, and recovery was attained by 6 weeks. The extent of muscle fiber necrosis and the severity of degeneration of the neuromuscular junctions showed a parallelism with the magnitude of the pressure load and retraction time. As the duration and pressure load increased, the time required for regeneration also increased. The fiber type grouping in group 3 and 4 was consistent with the severity of degeneration of neuromuscular junctions. The muscular degeneration and the regeneration was largely dependent on the retraction pressure-time product. These results suggest that denervation muscle injuries are likely secondary responses to muscle retraction injury in any case of posterior spine surgery.