Methodological considerations for near-infrared spectroscopy to assess mitochondrial capacity after spinal cord...

Background: Skeletal muscle mitochondrial activity is reduced by ∼ 50-60% after SCI, resulting in impaired energy expenditure, glucose utilization and insulin sensitivity. Near infra-red spectroscopy (NIRS) is a non-invasive tool that can be used to assess mitochondrial capacity. Objectives: (1) Highlight methodological limitations impacting data acquisition and analysis such as subcutaneous adipose tissue (SAT) thickness, movement artifacts, inadequate muscle stimulation, light interference, and ischemic discomfort. (2) Provide technical considerations to improve data acquisition and analysis. This may serve as guidance to other researchers and clinicians using NIRS. Study Design: cross-sectional observational design. Settings: Clinical research medical center. Participants: Sixteen men with 1 > year post motor complete SCI. Methods: NIRS signals were obtained from right vastus lateralis muscle utilizing a portable system. Signals were fit to a mono-exponential curve. Outcome Measures: Rate constant and r² values for the fit curve, indirectly measures mitochondrial capacity. Results: Only four participants produced data with accepted rate constants of 0.002-0.013 s^-1 and r² of 0.71-0.87. Applications of studentized residuals ≥2.5 resulted in sparing data from another four participants with rate constants of 0.010-0.018 s^-1and r² values ranging from 0.86-0.99. Conclusions: Several limitations may challenge the use of NIRS to assess mitochondrial capacity after SCI. Acknowledging these limitations and applying additional data processing techniques may overcome the discussed limitations and facilitate data sparing.