Multifidi Muscle Characteristics and Physical Function Among Older Adults With and Without Chronic Low Back Pain

Abstract Objective To determine whether multifidi size, intramuscular fat, or both, are associated with self-reported and performance-based physical function in older adults with and without chronic low back pain (LBP). Design Case-control study. Setting Individuals participated in a standardized evaluation in a clinical laboratory and underwent magnetic resonance imaging (MRI) of the lumbar spine at a nearby facility. Participants A volunteer sample of community-dwelling older adults (N=106), aged 60 to 85 years, with (n=57) and without (n=49) chronic LBP were included in this secondary data analysis. Intervention Average right-left L5 multifidi relative (ie, total) cross-sectional area (CSA), muscle-fat infiltration index (MFI) (ie, a measure of intramuscular fat), and relative muscle CSA (rmCSA) (ie, total CSA minus intramuscular fat CSA) were determined from MRIs. Linear regression modeling was performed with physical function measures as the dependent variables. Age, sex, and body mass index were entered as covariates. The main effects of L5 multifidi MFI and rmCSA, as well as their interaction with group assignment, were compared as independent variables. Main Outcome Measures Medical Outcomes Study 36-Item Short-Form Health Survey physical functioning subscale, timed Up and Go, gait speed, and fast stair descent performance. Results Interaction terms between L5 multifidi MFI and group assignment were found to be significant contributors to the variance explained in all physical function measures ( P ≤.012). Neither the main effect nor the interaction with group assignment for L5 multifidi rmCSA significantly contributed to the variance explained in any of the physical function measures ( P >.012). Conclusions Among older adults with chronic LBP of at least moderate intensity, L5 multifidi muscle composition, but not size, may help to explain physical function.