The effect of trunk position and pain location on lumbar extensor muscle recruitment strategies.

The aim of this study was to investigate the effect of trunk position and experimental lumbar pain location on lumbar extensor muscle recruitment strategies. Nineteen healthy participants (10 men and 9 women), aged 25.3 ± 4.7 yr, performed isometric back extension contractions in three positions (neutral, 45°, and 90° trunk flexion) and under three conditions (no pain, caudal pain, and cranial pain). Lumbar muscle activation strategies were recorded using high-density surface electromyography. The effect of position and pain condition on muscle activity amplitude and spatial redistributions was assessed. Muscle activity amplitude was 43% higher in 45° trunk flexion than in neutral position on both sides ( < 0.05). In the 90° trunk flexion, participants showed a more lateral spatial distribution than in the 45° trunk flexion on the left side ( < 0.01, 5.4 mm difference) and the neutral position on both sides ( < 0.05, 8.2 mm difference). In the 45° trunk flexion, participants exhibited a more lateral spatial distribution compared with the neutral position on the right side ( < 0.05, 3.7 mm difference). A lateral spatial redistribution of muscle activity was observed in the caudal pain condition compared with the no pain condition on the right side ( < 0.05, 3.0 mm difference). Individual responses to pain varied across all variables. Different trunk positions result in different distributions of activation within the lumbar extensor muscles, possibly based on regional mechanical advantage. No clear indication of location-specific pain adaptation and no effect of task-dependent pain adaptation were found, whereas individual-specific adaptations were observed. Changes in muscle activity amplitude and spatial redistribution of lumbar extensor muscles were observed in different trunk positions, potentially due to changes in their mechanical advantage. The results complement the current pain-adaptation theory by illustrating individual spatial redistributions of activation within lumbar extensor muscles during pain. The study found no clear indication of location-specific pain adaptation and no effect of task-dependent pain adaptation.