The Influence of Adipose Tissue Thickness on Near-Infrared Spectrometry During Intra-contraction Knee Extension Exercise

Abstract

0169 Near-infrared spectroscopy (NIRS) has increasingly been used for non-invasive determination of human mVO2 and blood flow, by interpreting signal strength and scattering coefficients for changes in oxyhemoglobin, ([O2Hb]), deoxyhemoglobin, ([HHb]) and total hemoglobin, ([tHb]). The methodological constraints of NIRS due to the confounding influences of adipose tissue thickness (ATT) have yet to be fully elucidated. In addition, to our knowledge no studies have examined these relationships within muscle contraction. PURPOSE: To determine if changes in ATT affect changes in NIRS scattering properties during dynamic knee extension exercise METHODS: Three subjects (22.5 ± 1.7 years) performed two identical continuous, incremental, upright, knee extension exercise protocols at 40 cpm. 1. Frequencydomain NIRS spectrometry (OxiplexTS, ISS) measured [O2Hb], [HHb], and [tHb] in the rectus femoris. 2. Ultrasound (VIVIDTM 3 PRO in 2-D mode) measured minimum and maximum ATT (ATTU), within a sample field at different work rates during both peak muscle contraction and between contractions. ATT was also measured with a skinfold caliper (ATTC). influence of changes in scattering were observed in the signals by comparing the patterns of response during contraction and relaxation under two conditions: With the assumption that scattering was essentially constant, and with the assumption that scattering was dynamic. RESULTS: ATTC was always larger than ATTU, +5mm, +11mm, +20mm for the three subjects. ATTU between resting and exercise conditions surprisingly often increased at the highest work rate presumably as forceful contraction drove the muscle deeper into the sample field. Longitudinal variability of ATTU within an image field during peak work rates increased as a function of the resting ATT, with the greatest variability at contraction peak. Further, pronounced movement of the underlying muscle was observed in all subjects. For the subject with the greatest ATT, the assumption regarding the temporal variability of scattering led to a sustained offset in the responses. In the other two subjects there was no effect. Further, directional variability in the changes in [O2Hb], [HHb], and [tHb] signals with contractions was observed among subjects. CONCLUSIONS: Estimation with skinfold calipers may not accurately reflect ATT. The effect of ATT on scattering can influence the absolute estimates of [O2Hb], [HHb], and [tHb]. However, the directional differences in response patterns with contractions appear to be independent of ATT, and may reflect changes in muscle architecture within the field of view of the NIRS probe. Supported by AHA Grant-in-Aid 0151183Z