Purpose Time-Resolved Near Infrared Spectroscopy (TR-NIRS) was used to quantify tissue oxy- and deoxy-hemoglobin concentrations ([HbO2], [HbR]), and O2 saturation (stO2) in the oblique fibers of the vastus medialis muscle (VMO) and brain prefrontal cortex (PFC) during knee extension with and without blood flow restriction (BFR). Methods Six young healthy males performed three sets of knee extensions on a dynamometer (50% 1 RM), separated by 90 sec rest periods, in three conditions: 1) until fatigue without BFR (Fatigue); 2) until fatigue with BFR (100 mm Hg cuff constriction around thigh, BFR); 3) same number of repetitions from condition 2, without BFR (Matched). Each condition was performed on a separate visit. Results BFR was associated with higher VMO [HbR] (rest 1: 57.8 μM BFR vs. 35.0 μM Matched, p < 0.0001) and a significantly lower stO2 during recovery periods between sets (7.5 – 11.2 % lower than non-BFR conditions for rest 1 and 2, p < 0.0001). Using a piecewise linear spline method, a spike in [HbR] was observed before the onset of HbR clearance during recovery, causing HbR clearance to begin at a higher concentration (BFR: 81 μM vs. Matched: 62 μM, p = 0.029). [HbO2] kinetics during recovery were also affected by BFR, with longer duration (BFR: 51 s, Matched: 31 s, p = 0.047) but lower rate of increase (BFR: 58 μM/min, Matched: 89 μM/min, p = 0.004) during recovery. In the PFC, BFR was associated with increased [HbR], diminished increase in [HbO2], and higher subjective exertion. Conclusions These findings yield insight into possible physiological mechanisms of BFR, and suggest a role of TR-NIRS in monitoring and optimization of BFR exercise on an individual basis.

Background Changes in subcutaneous adipose tissue (AT) structure and metabolism have been shown to correlate with the development of obesity and related metabolic disorders. Measurements of AT physiology could provide new insight into metabolic disease progression and response to therapy. An emerging functional imaging technology, Diffuse Optical Spectroscopic Imaging (DOSI), was used to obtain quantitative measures of near infrared (NIR) AT optical and physiological properties. Methods 10 overweight or obese adults were assessed during three-months on calorie-restricted diets. DOSI-derived tissue concentrations of hemoglobin, water, and lipid and the wavelength-dependent scattering amplitude (A) and slope (b) obtained from 30 abdominal locations and three time points (T0, T6, T12) were calculated and analyzed using linear mixed effects models, and were also used to form 3D surface images. Results Subjects lost a mean of 11.7 ± 3.4% of starting weight, while significant changes in A (+0.23 ± 0.04 mm−1, adj. p < 0.001), b (−0.17 ± 0.04, adj. p < 0.001), tissue water fraction (+7.2 ± 1.1%, adj. p < 0.001) and deoxyhemoglobin [HbR] (1.1 ± 0.3 µM, adj. p < 0.001) were observed using mixed effect model analysis. Discussion Optical scattering signals reveal alterations in tissue structure which possibly correlate with reductions in adipose cell volume, while water and hemoglobin dynamics suggest improved AT perfusion and oxygen extraction. These results suggest that DOSI measurements of NIR optical and physiological properties could be used to enhance understanding of the role of AT in metabolic disorders and provide new strategies for diagnostic monitoring of obesity and weight loss.

Abstract. A quantitative and dynamic analysis of skeletal muscle structure and function can guide training protocols and optimize interventions for rehabilitation and disease. While technologies exist to measure body composition, techniques are still needed for quantitative, long-term functional imaging of muscle at the bedside. We evaluate whether diffuse optical spectroscopic imaging (DOSI) can be used for long-term assessment of resistance training (RT). DOSI measures of tissue composition were obtained from 12 adults before and after 5 weeks of training and compared to lean mass fraction (LMF) from dual-energy X-ray absorptiometry (DXA). Significant correlations were detected between DXA LMF and DOSI-measured oxy-hemo/myoglobin, deoxyhemo/myoglobin, total-hemo/myoglobin, water, and lipid. RT-induced increases of ∼6% in oxy-hemo/myoglobin (3.4 AE 1.0 μM, p ¼ 0.00314) and total-hemo/myoglobin (4.9 AE 1.1 μM, p ¼ 0.00024) from the medial gastrocnemius were detected with DOSI and accompanied by ∼2% increases in lean soft tissue mass (36.4 AE 12.4 g, p ¼ 0.01641) and ∼60% increases in 1 rep-max strength (41.5 AE 6.2 kg, p ¼ 1.9E-05). DOSI measures of vascular and/or muscle changes combined with correlations between DOSI and DXA suggest that quantitative diffuse optical methods can be used to evaluate body composition, provide feedback on long-term interventions, and generate new insight into training-induced muscle adaptations.

Near-infrared spectroscopy (NIRS) has long been used to measure tissue-specific O2 dynamics in exercise, but most published data have used continuous wave devices incapable of quantifying absolute Hemoglobin (Hb) concentrations. We used time-resolved NIRS (TR-NIRS) to study exercising muscle (Vastus Lateralis, VL) and prefrontal cortex (PFC) Hb oxygenation in 11 young males (15.3 ± 2.1 yrs) performing incremental cycling until exhaustion (peak VO2 = 42.7 ± 6.1 ml/min/kg, mean peak power = 181 ± 38 W). TR-NIRS measurements of reduced scattering (μs′) and absorption (μa) at three wavelengths (759, 796, and 833 nm) were used to calculate concentrations of oxyHb ([HbO2]), deoxy Hb ([HbR]), total Hb ([THb]), and O2 saturation (stO2). In PFC, significant increases were observed in both [HbO2] and [HbR] during intense exercise. PFC stO2% remained stable until 80% of total exercise time, then dropped (−2.95%, p = .0064). In VL, stO2% decreased until peak time (−6.8%, p = .01). Segmented linear regression identified thresholds for PFC [HbO2], [HbR], VL [THb]. There was a strong correlation between timing of second ventilatory threshold and decline in PFC [HbO2] (r = .84). These findings show that TR-NIRS can be used to study physiological threshold phenomena in children during maximal exercise, providing insight into tissue specific hemodynamics and metabolism.