Regional flexion relaxation phenomenon in lumbar extensor muscles under delayed-onset muscle soreness: high-density surface electromyography insights.

Purpose: This study aimed to investigate whether lumbar delayed-onset muscle soreness (DOMS) impacts the magnitude of the flexion relaxation phenomenon regionally.

Methods: Eighteen adult participants (9 men and 9 women) performed flexion extension movement under two conditions (with and without DOMS). Lumbar muscle activation strategies were recorded using high-density surface electromyography (HDsEMG) on both sides of the trunk. To determine the spatial distribution of flexion relaxation phenomenon, flexion relaxation ratio of muscle activity was computed for all electrodes of the HDsEMG grid and the coordinates of the centroid (average position of flexion relaxation ratio across the HDsEMG grid) in the mediolateral and craniocaudal axis were calculated.

Results: The results revealed a cranial shift (~ 6 mm) of flexion relaxation phenomenon within the lumbar extensor muscles when DOMS was present (both sides: p < 0.05), possibly attributed to the increased recruitment of lumbar stabilizing muscles located caudally, which may serve as a guarding mechanism to pain.

Conclusion: These results highlight the importance of evaluating the entire lumbar region when assessing the flexion relaxation phenomenon.