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

Craig JC, Broxterman RM, Wilcox SL, Chen C, Barstow TJ. Effect of adipose tissue thickness, muscle site, and sex on near-infrared spectroscopy derived total-[hemoglobin + myoglobin]. J Appl Physiol 123: 1571-1578, 2017. First published September 21, 2017; doi: 10.1152/japplphysiol.00207.2017 .-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 that 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), RF (-26 µM/mm), GS (-54 µM/mm), and FDS (-33 µM/mm). 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. NEW & NOTEWORTHY Near-infrared spectroscopy (NIRS) is an important tool for physiologists and clinicians. However, adipose tissue greatly attenuates the signals from these devices. Correcting for this attenuation has been suggested based on the strength of the relationship between NIRS-derived measurements and the adipose tissue thickness. We show that this relationship is unique to the muscle site of interest but may not be sex specific. Accurate quantification of underlying tissue mandates researchers correct for adipose tissue thickness.

W' expenditure and reconstitution during severe intensity constant power exercise: mechanistic insight into the determinants of W'.

The sustainable duration of severe intensity exercise is well-predicted by critical power (CP) and the curvature constant (W'). The development of the W'BAL model allows for the pattern of W' expenditure and reconstitution to be characterized and this model has been applied to intermittent exercise protocols. The purpose of this investigation was to assess the influence of relaxation phase duration and exercise intensity on W' reconstitution during dynamic constant power severe intensity exercise. Six men (24.6 ± 0.9 years, height: 173.5 ± 1.9 cm, body mass: 78.9 ± 5.6 kg) performed severe intensity dynamic handgrip exercise to task failure using 50% and 20% duty cycles. The W'BAL model was fit to each exercise test and the time constant for W' reconstitution (τW') was determined. The τW' was significantly longer for the 50% duty cycle (1640 ± 262 sec) than the 20% duty cycle (863 ± 84 sec, P = 0.02). Additionally, the relationship between τW' and CP was well described as an exponential decay (r(2) = 0.90, P < 0.0001). In conclusion, the W'BAL model is able to characterize the expenditure and reconstitution of W' across the contraction-relaxation cycles comprising severe intensity constant power handgrip exercise. Moreover, the reconstitution of W' during constant power severe intensity exercise is influenced by the relative exercise intensity, the duration of relaxation between contractions, and CP.

Prediction of Lunar- and Martian-Based Intra- and Site-to-Site Task Performance.

BACKGROUND: This study aimed to investigate the feasibility of determining the physiological parameters associated with the ability to complete simulated exploration type tasks at metabolic rates which might be expected for lunar and Martian ambulation. METHODS: Running V̇O2max and gas exchange threshold (GET) were measured in 21 volunteers. Two simulated extravehicular activity field tests were completed in 1 G in regular athletic apparel at two intensities designed to elicit metabolic rates of ∼20.0 and ∼30.0 ml · kg(-1) · min(-1), which are similar to those previously reported for ambulation in simulated lunar- and Martian-based environments, respectively. RESULTS: All subjects were able to complete the field test at the lunar intensity, but 28% were unable to complete the field test at the Martian intensity (non-Finishers). During the Martian field test there were no differences in V̇O2 between Finishers and non-Finishers, but the non-Finishers achieved a greater %V̇O2max compared to Finishers (78.4 ± 4.6% vs. 64.9 ± 9.6%). Logistic regression analysis revealed fitness thresholds for a predicted probability of 0.5, at which Finishing and non-Finishing are equally likely, and 0.75, at which an individual has a 75% chance of Finishing, to be a V̇O2max of 38.4 ml · kg(-1) · min(-1) and 40.0 ml · kg(-1) · min(-1) or a GET of 20.1 ml · kg(-1) · min(-1) and 25.1 ml · kg(-1) · min(-1), respectively (χ(2) = 10.2). Logistic regression analysis also revealed that the expected %V̇O2max required to complete a field test could be used to successfully predict performance (χ(2) = 19.3). DISCUSSION: The results of the present investigation highlight the potential utility of V̇O2max, particularly as it relates to the metabolic demands of a surface ambulation, in defining successful completion of planetary-based exploration field tests.

Constructing quasi-linear V̇O2 responses from nonlinear parameters.

Oxygen uptake (V̇O2) kinetics have been shown to be governed by a nonlinear control system across a range of work rates. However, the linearity of the V̇O2 response to ramp incremental exercise would appear to be the result of a linear control system. This apparent contradiction could represent a balancing of changing V̇O2 kinetics parameter values across a range of work rates. To test this, six healthy men completed bouts of ramp incremental exercise at 15, 30, and 60 W/min (15R, 30R, 60R, respectively) and four bouts of an extended-step incremental exercise. V̇O2 parameter values were derived from the step exercise using two monoexponential models: one starting at time zero and encompassing the entire stage (MONO), and the other truncated to the first 5 min and allowing a time delay (5TD). The resulting parameter values were applied to an integrative model to estimate the ramp responses. As work rate increased, gain values increased (P < 0.001 for MONO and 5TD), as did mean response time (or time constant) values (MONO: P < 0.001; 5TD: P = 0.003). Up to maximal V̇O2 (V̇O(2 max)), the gains of the estimated ramp responses from both models were not different from the gains of the actual observed V̇O2 responses for 15R and 30R (15R: 11.3 ± 1.2, 11.7 ± 0.7, 10.9 ± 0.3; 30R: 10.5 ± 0.8, 11.0 ± 0.5, 10.7 ± 0.3 ml O2·min(-1)·W(-1), for actual, MONO, 5TD, respectively) but were significantly greater for 60R (8.7 ± 1.0, 9.9 ± 0.4, 10.3 ± 0.3 ml O2·min(-1)·W(-1) for actual, MONO, 5TD, respectively). Up to 80%V̇O(2 max) gain values were not significantly different for any ramp rate (P > 0.05 for all). We conclude that the apparent linearity of the V̇O2 response to ramp incremental exercise is consequent to a balancing of increasing time constant and gain parameter values.

Standardized Exercise Tests and Simulated Terrestrial Mission Task Performance.

BACKGROUND: Missions to terrestrial destinations (i.e., asteroids, the Moon, and Mars) will consist of physically challenging mission-critical tasks. These tasks, coupled with the negative physiological effects of prolonged microgravity exposure, create a plausible situation in which physical requirements may exceed an astronaut's physical capacity. Therefore, the objective of the current study was to evaluate the association of aerobic fitness and muscular strength parameters with performance during two field tests designed to simulate upper-body mission-critical activities. METHODS: There were 70 subjects who completed a material transport field test requiring the loading, transport, and unloading of geological samples and a device operations field test consisting of tasks associated with equipment set-up and the operations of controls and valves. The relationships between test duration and the following measurements were determined: running Vo(2max), gas exchange threshold (GET), speed at Vo(2max) (s-Vo(2max)), highest sustainable rate of aerobic metabolism [critical speed (CS)], and the finite distance that could be covered above CS (D'); and arm cranking Vo(2peak), GET, critical power (CP), and the finite work that can be performed above CP (W'). RESULTS: CP (r = -0.66), CS (r = -0.56), and arm cranking Vo(2peak) (r = -0.54) were most strongly correlated with the material transport field test and decision tree analysis revealed CP as the best predictor of performance. For the device operations field test, CP (r = -0.70), CS (r = -0.62), and arm cranking peak power output (r = -0.56) were significant predictors. DISCUSSION: Arm cranking tests are strongly associated with upper-body dependent tasks, highlighting that the nature of mission tasks needs to be considered when evaluating astronaut physical capacity.

The effect of resting blood flow occlusion on exercise tolerance and W'.

It has previously been postulated that the anaerobic work capacity (W') may be utilized during resting blood flow occlusion in the absence of mechanical work. We tested the hypothesis that W' would not be utilized during an initial range of time following the onset of resting blood flow occlusion, after which W' would be utilized progressively more. Seven men completed blood flow occlusion constant power severe intensity handgrip exercise to task failure following 0, 300, 600, 900, and 1,200 s of resting blood flow occlusion. The work performed above critical power (CP) was not significantly different between the 0-, 300-, and 600-s conditions and was not significantly different from the total W' available. Significantly less work was performed above CP during the 1,200-s condition than the 900-s condition (P < 0.05), while both conditions were significantly less than the 0-, 300-, and 600-s conditions (P < 0.05). The work performed above CP during these conditions was significantly less than the total W' available (P < 0.05). The utilization of W' during resting blood flow occlusion did not begin until 751 ± 118 s, after which time W' was progressively utilized. The current findings demonstrate that W' is not utilized during the initial ∼751 s of resting blood flow occlusion, but is progressively utilized thereafter, despite no mechanical work being performed. Thus, the utilization of W' is not exclusive to exercise, and a constant amount of work that can be performed above CP is not the determining mechanism of W'.

Upper Body Aerobic Exercise as a Possible Predictor of Lower Body Performance.

BACKGROUND: Aerobic exercise capacity provides information regarding cardiorespiratory health and physical capacity. However, in many populations the ability to measure whole-body or leg aerobic exercise capacity is limited due to physical disability or lack of appropriate equipment. Clinically there is a need to evaluate aerobic capacity in individuals who cannot use their legs for locomotion. In astronauts the habitable space for exercise testing in the next generation of space exploration systems may be restricted and may not support the traditional lower body testing. Therefore, the purpose was to determine if upper body physical performance could estimate lower body aerobic capacity. METHODS: Maximal O2uptake (Vo(2max)), gas exchange threshold (GET), and the highest sustainable rate of aerobic metabolism [arm cranking critical power ((A)CP) and lower body critical speed ((L)CS)] were determined in 55 conditioned men and women during arm-cranking and treadmill running. RESULTS: Vo(2max) and GET (48.6 ± 7.6 and 29.0 ± 4.8 ml · kg⁻¹ · min⁻¹, respectively) were significantly lower during arm-cranking exercise compared to running (27.1 ± 7.6 and 13.5 ± 2.6 ml · kg⁻¹ · min⁻¹, respectively). The Vo2at ACP was significantly lower than the Vo2at the (L)CS (18.4 ± 5.01 vs. 39.5 ± 8.1 ml · kg⁻¹ · min⁻¹, respectively). There was a significant correlation between arm-cranking and lower body Vo2max, GET, and the Vo2at (L)CS and ACP. Backward stepwise regression analyses revealed that arm-cranking physical fitness could explain 67%, 40%, and 49% of the variance in lower body Vo(2max), GET, and (L)CS, respectively. DISCUSSION: Results suggest arm-cranking exercise can be used to obtain an approximation of lower body aerobic capacity.