Multifidus atrophy is localized and bilateral in active persons with chronic unilateral low back pain.

Objective: To compare the lumbar multifidi muscle volume devoid of fat local to the site of pain in persons with and without chronic unilateral lower lumbar pain.

Design: Prospective cross-sectional design.

Setting: University biokinesiology laboratory.

Participants: Active individuals (n=14) with chronic unilateral lower lumbar pain (>1 y) were matched for age, height, weight, and activity level with healthy individuals (n=14). Individuals with back pain had minimal disability (Oswestry Disability Index [mean ± SD], 14.9%±6.3%) at the time of testing.

Interventions: Not applicable.

Main Outcome Measures: Multifidus and erector spinae muscle volumes at the L5-S1 levels, multifidus muscle volumes at the L4 and S2-3 levels.

Results: Average multifidus volume was diminished by 18.1% between groups (P=.026) only at the L5-S1 levels. There was no difference between painful and pain-free sides. There were no volume differences between groups above L5, below S1, or in erector spinae at the L5-S1 levels.

Conclusions: The results of this study indicate that despite a low level of disability and an activity level similar to that of matched control subjects, considerable localized, bilateral multifidus atrophy is present. Such impaired size of the multifidus will likely reduce its capacity to control intersegmental motion, thus increasing the susceptibility to further injury. Unlike acute unilateral low back pain (LBP), muscle size is reduced bilaterally in persons with chronic unilateral LBP.