Matthew D. Spencer

Characterizing the profile of muscle deoxygenation during ramp incremental exercise in young men.

This study characterized the profile of near-infrared spectroscopy (NIRS)-derived muscle deoxygenation (Δ[HHb]) and the tissue oxygenation index (TOI) as a function of absolute (POABS) and normalized power output (%PO) or oxygen consumption (%VO2) during incremental cycling exercise. Eight men (24xa0±xa05xa0year) each performed two fatigue-limited ramp incremental cycling tests (20xa0Wxa0min−1), during which pulmonary VO2, Δ[HHb] and TOI were measured continuously. Responses from the two tests were averaged and the TOI (%) and normalized Δ[HHb] (%Δ[HHb]) were plotted against %VO2, %PO and POABS. The overall responses were modelled using a sigmoid regression (yxa0=xa0f0xa0+xa0A/(1xa0+xa0e−(−c+dx))) and piecewise ‘double-linear’ function of the predominant adjustment of %Δ[HHb] or TOI observed throughout the middle portion of exercise and the ‘plateau’ that followed. In ~85% of cases, the corrected Akaike Information Criterion (AICC) was smaller (suggesting one model favoured) for the ‘double-linear’ compared with the sigmoid regression for both %Δ[HHb] and TOI. Furthermore, the f0 and A estimates from the sigmoid regressions of %Δ[HHb] yielded unrealistically large projected peak (f0xa0+xa0A) values (%VO2p 114.3xa0±xa017.5; %PO 113.3xa0±xa09.5; POABS 113.5xa0±xa09.8), suggesting that the sigmoid model does not accurately describe the underlying physiological responses in all subjects and thus may not be appropriate for comparative purposes. Alternatively, the present study proposes that the profile of %Δ[HHb] and TOI during ramp incremental exercise may be more accurately described as consisting of three distinct phases in which there is little adjustment early in the ramp, the predominant increase in %Δ[HHb] (decrease in TOI) is approximately linear and an approximately linear ‘plateau’ follows.

Muscle deoxygenation to VO2 relationship differs in young subjects with varying τVO2

The relationship between the adjustment of muscle deoxygenation (∆[HHb]) and phase II VO2p was examined in subjects presenting with a range of slow to fast VO2p kinetics. Moderate intensity VO2p and ∆[HHb] kinetics were examined in 37 young males (24xa0±xa04xa0years). VO2p was measured breath-by-breath. Changes in ∆[HHb] of the vastus lateralis muscle were measured by near-infrared spectroscopy. VO2p and ∆[HHb] response profiles were fit using a mono-exponential model, and scaled to a relative % of the response (0–100%). The ∆[HHb]/∆VO2p ratio for each individual (reflecting the matching of O2 distribution to O2 utilization) was calculated as the average ∆[HHb]/∆VO2p response from 20 to 120xa0s during the exercise on-transient. Subjects were grouped based on individual phase II VO2p time-constant (τVO2p): <21xa0s [very fast (VF)]; 21–30xa0s [fast (F)]; 31–40xa0s [moderate (M)]; >41xa0s [slow (S)]. The corresponding ∆[HHb]/∆VO2p were 0.98 (VF), 1.05 (F), 1.09 (M), and 1.22 (S). The larger ∆[HHb]/∆VO2p in the groups with slower VO2p kinetics resulted in the ∆[HHb]/∆VO2p displaying a transient “overshoot” relative to the subsequent steady state level, which was progressively reduced as τVO2 became smaller (rxa0=xa00.91). When τVO2pxa0>xa0~20xa0s, the rate of adjustment of phase II VO2p appears to be mainly constrained by the matching of local O2 distribution to muscle VO2. These data suggest that in subjects with “slower” VO2 kinetics, the rate of adjustment of VO2 may be constrained by O2 availability within the active tissues related to the matching of microvascular O2 distribution to muscle O2 utilization.

Are the parameters of VO2, heart rate and muscle deoxygenation kinetics affected by serial moderate-intensity exercise transitions in a single day?

This study compared the parameter estimates of pulmonary oxygen uptake (VO2p), heart rate (HR) and muscle deoxygenation (Δ[HHb]) kinetics when several moderate-intensity exercise transitions (MODs) were performed during a single visit versus several MODs performed during separate visits. Nine subjects (24xa0±xa05xa0years, meanxa0±xa0SD) each completed two successive cycling MODs on six occasions (1-6A and 1-6B) from 20xa0W to a work rate corresponding to 80% estimated lactate threshold with 6xa0min recovery at 20xa0W. During one visit, subjects completed two series of three MODs (6A-F), separated by 20xa0min rest. VO2p time constants (τVO2p; 27xa0±xa010xa0s, 25xa0±xa012xa0s, 25xa0±xa011xa0s) were similar (pxa0>xa00.05) for MODs 1-6A, 1-6B and 6A-F, respectively. τVO2p had reproducibility 95% confidence intervals (CI95) of 8.3, 8.2, 4.7, 4.9 and 4.7xa0s when comparing single (1A vs. 2A), the average of two (1-2A vs. 3-4A), three (1-3A vs. 4-6A), four (1-2AB vs. 3-4AB) and six (1-3AB vs. 4-6AB) MODs, respectively. The effective Δ[HHb] response time (τ′Δ[HHb]) was unaffected across conditions (1-6A: 19xa0±xa02xa0s, 1-6B: 19xa0±xa03xa0s, 6A-F: 17xa0±xa04xa0s) with reproducibility CI95 of 5.3, 4.5, 3.1, 2.9 and 3.3xa0s when a single, two, three, four and six MODs were compared, respectively. τHR was reduced in MODs 6A-F compared to 1-6A and 1-6B (23xa0±xa05xa0s, 25xa0±xa05xa0s, 27xa0±xa06xa0s, respectively). This study showed that parameter estimates of VO2p, HR and Δ[HHb] kinetics are largely unaffected by data collection sequence, and the day-to-day reproducibility of τVO2p and τ′Δ[HHb] estimates, as determined by the CI95, was appreciably improved by averaging of at least three MODs.

Systemic and vastus lateralis muscle blood flow and O2 extraction during ramp incremental cycle exercise

During ramp incremental cycling exercise increases in pulmonary O2 uptake (Vo2p) are matched by a linear increase in systemic cardiac output (Q). However, it has been suggested that blood flow in the active muscle microvasculature does not display similar linearity in blood flow relative to metabolic demand. This study simultaneously examined both systemic and regional (microvascular) blood flow and O2 extraction during incremental cycling exercise. Ten young men (Vo2 peak = 4.2 ± 0.5 l/min) and 10 young women (Vo2 peak = 3.2 ± 0.5 l/min) were recruited to perform two maximal incremental cycling tests on separate days. The acetylene open-circuit technique and mass spectrometry and volume turbine were used to measure Q (every minute) and breath-by-breath Vo2p, respectively; systemic arterio-venous O2 difference (a-vO2diff) was calculated as Vo2p/Q on a minute-by-minute basis. Changes in near-infrared spectroscopy-derived muscle deoxygenation (Δ[HHb]) were used (in combination with Vo2p data) to estimate the profiles of peripheral O2 extraction and blood flow of the active muscle microvasculature. The systemic Q-to-Vo2p relationship was linear (~5.8 l/min increase in Q for a 1 l/min increase in Vo2p) with a-vO2diff displaying a hyperbolic response as exercise intensity increased toward Vo2 peak. The peripheral blood flow response profile was described by an inverted sigmoid curve, indicating nonlinear responses relative to metabolic demand. The Δ[HHb] profile increased linearly with absolute Vo2p until high-intensity exercise, thereafter displaying a near-plateau. Results indicate that systemic blood flow and thus O2 delivery does not reflect the profile of blood flow changes at the level of the microvasculature.

Pulmonary O2 uptake and muscle deoxygenation kinetics are slowed in the upper compared with lower region of the moderate-intensity exercise domain in older men

This study sought to determine the effect of the pre-transition work rate (WR) and WR transition magnitude on the adjustment of pulmonary oxygen uptake (VO2p kinetics) in older men. Seven men (69xa0±xa05xa0years; meanxa0±xa0SD) each performed 4–6 cycling transitions from 20xa0W to either a WR corresponding to 90% estimated lactate threshold (full step, FS) or 2 equal-step transitions (lower step, LS; upper step, US) to the same end-exercise WR as in FS. Gas exchange was analysed breath-by-breath and muscle deoxygenation (∆[HHb]) was measured with NIRS. The time constant (τ) for VO2p was greater in US (53xa0±xa017xa0s) and FS (44xa0±xa011xa0s) compared to LS (37xa0±xa09xa0s); τVO2p for US also trended (pxa0=xa00.05) towards being greater than FS. The VO2p gain in US (9.97xa0±xa00.41xa0mL/min/W) was greater than LS (9.06xa0±xa01.17; pxa0=xa00.06) and FS (9.13xa0±xa00.54; pxa0<xa00.05). The O2 deficit was greater in US (0.25xa0±xa00.08xa0L) than LS (0.19xa0±xa00.06xa0L); yet the ‘accumulated O2 deficit’ (0.44xa0±xa00.13xa0L; O2 deficit from LSxa0+xa0US) was similar to that of FS (0.42xa0±xa00.13 L; pxa0=xa00.38). The effective Δ[HHb] response time (τ′∆[HHb]) for US (36xa0±xa012xa0s) was greater than LS (27xa0±xa06xa0s; pxa0=xa00.07) and FS (26xa0±xa04xa0s; pxa0<xa00.05), suggesting that the slowed adjustment of muscle O2 extraction was associated with the slowed VO2 kinetics of the US. Despite already slowed VO2p kinetics, older men exhibit further slowing when small WR transitions are initiated from an elevated pre-transition WR, yet this results in no cumulative impact on O2 deficit. This slowing in US compared to LS does not appear to be related to local O2 availability.

Sex-related differences in muscle deoxygenation during ramp incremental exercise.

Sex-specific differences in the temporal profiles of fractional O2 extraction during incremental cycling were examined using changes in near-infrared spectroscopy (NIRS)-derived muscle deoxygenated hemoglobin concentration (Δ[HHb]) and breath-by-breath pulmonary O2 uptake ( .VO2p ) measurements. Subjects (men: n=10; women: n=10) Δ[HHb] data were normalized to 100% of the response, plotted as a function ( .VO2p, % .VO2p), power output (PO), and % PO, and fit with a piecewise double-linear regression model. The slope of the first segment of the double linear model was significantly greater in women compared to men when %Δ[HHb] was plotted as a function of .VO2p, % .VO2p and PO (p<0.05). Both sexes displayed a near-plateau in the %Δ[HHb] which occurred at an exercise intensity near the respiratory compensation point. Thus, young women display a poorer ability to deliver O2 to the exercising tissue compared to men and oxidative demands must be supplemented by a greater fractional O2 extraction.

Noninvasive estimation of microvascular O2 provision during exercise on-transients in healthy young males

Two methods for estimating changes in microvascular O2 delivery during the on-transient of exercise were evaluated. They were tested to assess the role of the adjustment of the estimated microvascular O2 delivery in the speeding of Vo2 kinetics during a Mod1-Hvy-Mod2 protocol (Mod, moderate-intensity exercise; Hvy, heavy-intensity priming exercise), in which Mod2 is preceded by a bout of Hvy. Mod pulmonary Vo2 (Vo(2p)) and deoxy-hemoglobin [HHb] data were collected in 12 males (23 ± 3 yr); response profiles were fit with a monoexponential. Signals were also 1) scaled to a relative % of the response (0-100%) to calculate the [HHb]/Vo2 ratio for each individual and 2) rearranged in the Fick equation for estimation of capillary blood flow (Q(cap)). A transient [HHb]/Vo2 overshoot observed in Mod1 (1.06 ± 0.05; P < 0.05) was absent during Mod2 (1.01 ± 0.06; P > 0.05); reductions in the [HHb]/Vo2 ratio (Mod1 - Mod2) were related to reductions in phase II τVo(2p) (r = 0.82; P < 0.05). For Q(cap), a near-exponential response was observed in 8/12 subjects in Mod1 and only in 4/12 subjects in Mod2. The Q(cap) profile was shown to be highly dependent on the [HHb] baseline-to-amplitude ratio. Thus, accurate and physiologically consistent estimations of Q(cap) were not possible in most cases. This study confirmed that priming exercise results in an improved O2 delivery as shown by the decreased [HHb]/Vo2) ratio that was related to the smaller τVo2 in Mod2. Additionally, this study suggested that Q(cap) analysis may not be valid and should be interpreted with caution when assessing microvascular delivery of O2.

Adjustments of pulmonary O2 uptake and muscle deoxygenation during ramp incremental exercise and constant-load moderate-intensity exercise in young and older adults

The matching of muscle O(2) delivery to O(2) utilization can be inferred from the adjustments in muscle deoxygenation (Δ[HHb]) and pulmonary O(2) uptake (Vo(2p)). This study examined the adjustments of Vo(2p) and Δ[HHb] during ramp incremental (RI) and constant-load (CL) exercise in adult males. Ten young adults (YA; age: 25 ± 5 yr) and nine older adults (OA; age: 70 ± 3 yr) completed two RI tests and six CL step transitions to a work rate (WR) corresponding to 1) 80% of the estimated lactate threshold (same relative WR) and 2) 50 W (same absolute WR). Vo(2p) was measured breath by breath, and Δ[HHb] of the vastus lateralis was measured using near-infrared spectroscopy. Δ[HHb]-WR profiles were normalized from baseline (0%) to peak Δ[HHb] (100%) and fit using a sigmoid function. The sigmoid slope (d) was greater (P < 0.05) in OA (0.027 ± 0.01%/W) compared with YA (0.017 ± 0.01%/W), and the c/d value (a value corresponding to 50% of the amplitude) was smaller (P < 0.05) for OA (133 ± 40 W) than for YA (195 ± 51 W). No age-related differences in the sigmoid parameters were reported when WR was expressed as a percentage of peak WR. Vo(2p) kinetics compared with Δ[HHb] kinetics for the 50-W transition were similar between YA and OA; however, Δ[HHb] kinetics during the transition to 80% of the lactate threshold were faster than Vo(2p) kinetics in both groups. The greater reliance on O(2) extraction displayed in OA during RI exercise suggests a lower O(2) delivery-to-O(2) utilization relationship at a given absolute WR compared with YA.

Muscle O2 extraction reserve during intense cycling is site-specific

The present study compared peak muscle deoxygenation ([HHb]peak) responses at three quadriceps sites during occlusion (OCC), ramp incremental (RI), severe- (SVR) and moderate-intensity (MOD) exercise. Seven healthy men (25 ± 4 yr) each completed a stationary cycling RI (20 W/min) test to determine [HHb]peak [at distal and proximal vastus lateralis (VLD and VLP) and rectus femoris (RF)], peak V̇O2 (V̇O(2peak)), gas exchange threshold (GET), and peak work rate (WR(peak)). Subjects also completed MOD (WR = 80% GET) and SVR exercise (WR corresponding to 120% V̇O(2peak)) with absolute [HHb] (quantified by multichannel, time-resolved near-infrared spectroscopy) and pulmonary VO2 (V̇O(2p)) monitored continuously. Additionally, [HHb] and total hemoglobin ([Hb]tot) were monitored at rest and during subsequent OCC (250 mmHg). Site-specific adipose tissue thickness was assessed (B-mode ultrasound), and its relationship with resting [Hb]tot was used to correct absolute [HHb]. For VLD and RF, [HHb]peak was higher (P < 0.05) during OCC (VLD = 111 ± 38, RF = 114 ± 26 μM) than RI (VLD 64 ± 14, RF = 85 ± 20) and SVR (VLD = 63 ± 13, RF = 81 ± 18). [HHb]peak was similar (P > 0.05) across these conditions at the VLP (OCC = 67 ± 17, RI = 69 ± 17, SVR = 63 ± 17 μM). [HHb] peaked and then decreased prior to exercise cessation during SVR at all three muscle sites. [HHb]peak during MOD was consistently lower than other conditions at all sites. A [HHb] reserve exists during intense cycling at the VLD and RF, likely implying either sufficient blood flow to meet oxidative demands or insufficient diffusion time for complete equilibration. In VLP this [HHb] reserve was absent, suggesting that a critical PO2 may be challenged during intense cycling.

Effect of moderate-intensity work rate increment on phase II τVO2, functional gain and Δ[HHb]

This study systematically examined the role of work rate (WR) increment on the kinetics of pulmonary oxygen uptake (VO2p) and near-infrared spectroscopy (NIRS)-derived muscle deoxygenation (Δ[HHb]) during moderate-intensity (Mod) cycling. Fourteen males (24xa0±xa05xa0years) each completed four to eight repetitions of Mod transitions from 20 to 50, 70, 90, 110 and 130xa0W. VO2p and Δ[HHb] responses were modelled as a mono-exponential; responses were then scaled to a relative % of the respective response (0–100xa0%). The Δ[HHb]/VO2 ratio was calculated as the average Δ[HHb]/VO2 during the 20–120xa0s period of the on-transient. When considered as a single group, neither the phase II VO2p time constant (τVO2p; 27xa0±xa09, 26xa0±xa011, 25xa0±xa010, 27xa0±xa014, 29xa0±xa013xa0s for 50–130xa0W transitions, respectively) nor the Δ[HHb]/VO2 ratio (1.04xa0±xa00.13, 1.10xa0±xa00.13, 1.08xa0±xa00.07, 1.09xa0±xa00.11, 1.09xa0±xa00.09, respectively) was affected by WR (pxa0>xa00.05); yet, the VO2 functional gain (G; ΔVO2/ΔWR) increased with increasing WR transitions (8.6xa0±xa01.3, 9.1xa0±xa01.2, 9.5xa0±xa01.0, 9.5xa0±xa01.0, 9.9xa0±xa01.0xa0mLxa0min−1xa0W−1; pxa0<xa00.05). When subjects were stratified into two groups [Fast (nxa0=xa06), τVO2p130Wxa0<xa025xa0sxa0<xa0τVO2p130W, Slower (nxa0=xa08)], a group by WR interaction was observed for τVO2p. The increasing functional G persisted (pxa0<xa00.05) and did not differ between groups (pxa0>xa00.05). The Δ[HHb]/VO2 ratio was smaller (pxa0<xa00.05) in the Fast than Slower group, but was unaffected by WR. In conclusion, the present study demonstrated (1) a non-uniform effect of Mod WR increment on τVO2p; (2) that τVO2p in the Slower group is likely determined by an O2 delivery limitation; and (3) that increasing Mod WR increments elicits an increased functional G, regardless of the τVO2p response.