Effect of adipose tissue thickness, muscle site, and sex on near-infrared spectroscopy derived total-[hemoglobin + myoglobin]

Adipose tissue thickness (ATT) attenuates signals from near-infrared spectroscopy (NIRS) and diminishes the absolute quantification of underlying tissues by contemporary NIRS devices. Based on the relationship between NIRS-derived total-[Hemoglobin+Myoglobin] (total-[Hb+Mb]) and ATT, we tested the hypotheses that the correction factor for ATT: 1) is muscle site specific; 2) does not differ between men and women; and 3) exclusion of the shortest source-detector distance from data analysis increases total-[Hb+Mb]. Fourteen healthy subjects (7 men) rested in a neutral body position (supine or prone) while measurements of total-[Hb+Mb] and ATT were taken at four muscles common to resting and exercise studies: vastus lateralis (VL), rectus femoris (RF), gastrocnemius (GS), and flexor digitorum superficialis (FDS). ATT averaged 6.0±0.4 mm across all muscles. Every muscle showed a negative slope (r²: 0.6-0.94; p<0.01) for total-[Hb+Mb] as a function of ATT: VL (-34 μM·mm^-1), RF (-26 μM·mm^-1), GS (-54 μM·mm^-1), and FDS (-33 μM·mm^-1). The projected total-[Hb+Mb] at 0 mm ATT (y-intercept) was 452, 372, 620, and 456 μM for VL, RF, GS, and FDS, respectively. No differences were found between the sexes within VL, RF, or FDS, but men had a greater projected total-[Hb+Mb] at 0 mm for GS (688±44 vs 552±40 μM; p<0.05). Exclusion of the shortest source-detector distance increased total-[Hb+Mb] by 12±1 μM (p<0.05). The present findings demonstrate that total-[Hb+Mb] should be corrected for ATT using muscle site specific factors which are not sex specific, except in the case of GS.

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