An evaluation of the muscle-bone unit theory among individuals with chronic spinal cord injury

Study design: Cross-sectional observation. Objectives: To explore the association between muscle size and function, and indices of bone strength among a sample of adults with chronic spinal cord injury (SCI). Setting: Ontario, Canada. Methods: Sixty-five participants (n=47 men) with chronic SCI (C1-T12 American Spinal Injury Association Impairment Scale (AIS) A-D) were recruited, mean +/- s.d. age 49.4 +/- 12.8 years and years post-injury 14.3 +/- 10.7. Muscle cross-sectional area (CSA) and indices of bone strength at the distal tibia and tibia shaft were measured by peripheral quantitative computed tomography. Muscle CSA was multiplied by tibia length to obtain muscle-bending moment (MBM), a surrogate of torque. Plantar flexor components of the lower-extremity motor scores (pf-LEMS) were used as clinical measures of muscle function. Pearson's correlations (r) were used to determine the strength of relationships. Results: Correlations were found between M BM and indices of bone strength at the distal tibia and tibia shaft (r=0.44-0.56), as well as between pf-LEMS and indices of bone strength at the distal tibia and tibia shaft (r=0.37-0.71). pf-LEMS had a stronger association with bone variables at the distal tibia compared with MBM (r=0.6 vs r=0.4). All relationships between muscle and bone remained significant when controlling for the duration of injury. Conclusion: It appears that lower limb muscle size and function are more strongly correlated with bone strength indices at the distal tibia than at the tibia shaft among individuals with SCI. The relationships between muscle and bone are clinically important, as muscle CSA and strength (motor scores) are potentially amenable to rehabilitation intervention(s). Spinal Cord (2012) 50, 147-152; doi:10.1038/sc.2011.99; published online 6 September 2011