Comparison of location, depth, quality, and intensity of experimentally induced pain in 6 low back muscles.

Introduction: The pattern of pain originating from experimentally induced low back pain appears diffuse. This may be because sensory information from low back muscles converges, sensory innervation extends over multiple vertebral levels, or people have difficulty accurately representing the painful location on standardized pain maps.

Objective: The aim of this study was to provide insight into the perception of pain from noxious stimulation of a range of low back muscles using novel depth and location measures.

Methods: Hypertonic saline (1 mL, 7% NaCl) was injected into bellies of longissimus (LO), quadratus lumborum (QL), superficial multifidus (SM), and deep multifidus (DM) at the level of the fourth lumbar vertebrae (L4) and in SM and DM at L5 using ultrasound guidance over 6 sessions. Fifteen participants reported depth, location, intensity, size, and descriptive quality of pain throughout the painful period (∼14 min).

Results: Pain was reported deeper (P<0.04) for DML4/L5 compared with SML4/L5, LO and QL; more cranial for LO compared with DML4 and QL (P<0.01); more lateral for LO compared with DML4 (P<0.02); and more lateral for QL compared with all other muscles at L4 (P<0.0001). Pain intensity was higher in DML4/L5 than all other muscles (P<0.04) for ∼3 minutes. Descriptive qualities varied slightly between muscles.

Discussion: Depth and lateral position may be the most critical descriptors to determine the source of acute lumbar muscular pain. Overlapping regions of pain may be explained by convergence of receptive fields, innervation of multifidus fascicles at multiple lumbar segments, and convergence of sensory input from different muscles to the same sensory cell bodies as demonstrated in the lumbar spine of animal preparations.