Previous studies addressing the relationship between blood flow (Q˙) and oxygen uptake (V˙O2) have primarily focused on the whole body (8,23,37) or on the larger vessels in the exercising limbs (19,35). This can predominantly be attributed to methodological constraints. Furthermore, it has been predicted (22) and shown (15) that the adjustment of muscular blood flow (Q˙m) in the microcirculation of active skeletal muscle might differ from that measured in the larger conduit arteries. In this context, near-infrared spectroscopy (NIRS) can provide a noninvasive alternative, allowing measurements of muscle oxygenation (or O2 extraction) at the level of the microcirculation. On the basis of the different absorption spectra of the near-infrared light at different wavelengths, NIRS can differentiate between the two major forms of hemoglobin (Hb) and myoglobin (Mb), i.e., oxy[Hb + Mb] and deoxy[Hb + Mb]. Deoxy[Hb + Mb] can be considered as a surrogate of microvascular O2 extraction (7,14), and according to the Fick principle (arteriovenous O2 difference (C(a-v)O2 = muscular oxygen uptake (V˙O2m)/blood flow (Q˙m))), it can be assumed that the pattern of deoxy[Hb + Mb] during exercise can provide insight into the relationship between the muscular blood flow (Q˙m) and oxygen uptake (V˙O2m), especially because deoxy[Hb + Mb] seems to be less affected by changes in blood volume under the NIRS probe during exercise compared with oxy[Hb + Mb] (6,9,14,26). Therefore, NIRS can provide unique, noninvasive insight into the control mechanisms governing the Q˙m/V˙O2m relationship that is an important determinant of the aerobic exercise capacity in athletes and some patients.
Generally, it is accepted that the steady-state relationship between blood flow and V˙O2 is linear, with a positive intercept on the blood flow axis. Consequently, C(a-v)O2 or fractional O2 extraction displays a hyperbolic profile when plotted as a function of work rate (8,10,23,35). If deoxy[Hb + Mb] reflects fractional O2 extraction, a similar hyperbolic pattern could be expected for deoxy[Hb + Mb] as a function of work rate. Recently, however, it has been demonstrated that the pattern of deoxy[Hb + Mb] during a continuous incremental exercise (ramp exercise) follows a sigmoid rather than a hyperbolic model, suggesting a dynamic nonlinear relationship between Q˙m and V˙O2m, i.e., an initial rapid increase in Q˙m at the onset of the ramp followed by a less pronounced increase in Q˙m (Fig. 1) (5,11). This less pronounced increase in Q˙m could be related to central and/or local hemodynamic factors. It is well known that as work rate increases, the cardiovascular responses are regulated by a withdrawal of parasympathetic activity and an increase in sympathetic activity, with the contribution of the sympathetic activation growing with increasing work rate. It has been reported that the increase in sympathetic activity is accompanied by a slowing of the cardiovascular responses compared with the withdrawal in parasympathetic activity (40). Therefore, it is possible that a slowing effect on the cardiovascular responses occurs during ramp exercise as work rate increases, especially on the increase in cardiac output (CO). In this context, Stringer et al. (39) reported a nonlinear increase in CO during ramp exercise, more specifically a less pronounced increase at higher work rates, which is in contrast with the widely demonstrated linear relationship between CO and V˙O2 during constant work rate transitions (36). This nonlinear increase in CO could be associated with a nonlinear Q˙m/V˙O2m relationship at the microvascular level. Local factors might also contribute to the less pronounced increase of Q˙m at higher work rates during ramp exercise. It has been reported in rat muscle that "slow-twitch" muscles are characterized by faster Q˙m kinetics relative to the V˙O2m kinetics compared with "fast-twitch" muscles (3,27). A different relationship between the Q˙m and V˙O2m kinetics inherent to muscle fiber types could induce a slowing effect on Q˙m because ramp exercise is presumably characterized by a shift from a predominantly slow-twitch to a mixed slow-twitch and fast-twitch muscle fiber recruitment. If the nonlinear Q˙m/V˙O2m relationship during ramp exercise is related to changes in Q˙m kinetics originated by central and/or local hemodynamic mechanisms, then it could be expected that this relationship will be specific to ramp exercise. The incremental ramp exercise protocol is characterized by a continuous increase in work rate and thus by non-steady-state conditions, which is in contrast to the steady-state conditions of the incremental step exercise.
It is also possible that regional differences in muscle blood flow (e.g., [17,33]), motor unit recruitment (12), and fiber-type distribution (24), especially between the proximal and distal portions of a single muscle, might influence the Q˙m/V˙O2m relationship and, as a consequence, the pattern of deoxy[Hb + Mb] during incremental exercise. Mizuno et al. (31) reported significant differences in the oxygenation status during incremental static knee extension exercise between the distal and proximal sections of the musculus vastus lateralis. Also, Kime et al. (18) and Koga et al. (20) observed substantial heterogeneity in the NIRS response between different measurement sites on the same muscle during cycle exercise, especially at low to moderate intensities.
Thus, the aim of the present study was to compare the pattern of deoxy[Hb + Mb] during both steady-state (step) and non-steady-state (ramp) incremental cycle exercise in a group of trained cyclists. Because the central and/or local mechanisms that could be at the origin of the nonlinear Q˙m/V˙O2m relationship could be related to changes in Q˙m kinetics and, therefore, are inherent to the non-steady-state conditions of the ramp protocol, it would predict that the sigmoid pattern of deoxy[Hb + Mb] will be specific to the ramp protocol. Therefore, we hypothesized that the pattern of deoxy[Hb + Mb] follows a sigmoid profile in response to the non-steady-state (ramp) exercise and that this profile will not be present in response to the incremental steady-state (step) exercise. In addition, to evaluate the possible regional heterogeneity of muscle deoxygenation, we measured the pattern of deoxy[Hb + Mb] on the proximal portion as well as on the distal portion of the musculus vastus lateralis.
A group of 10 male cyclists, mean ± SD age 19.4 ± 1.6 yr, volunteered to take part in this study. The subjects had a mean body mass of 67.6 ± 4.6 kg and a mean height of 1.77 ± 0.03 m, and they had at least 3 yr of competitive cycling experience. All subjects were informed about the protocol and the aim of the study, and they signed an informed consent approved by the ethical committee of the university hospital. A medical history questionnaire and an examination including rest and exercise ECG were performed before the start of the study. All subjects were declared to be in good health, and none of them presented medical contraindications for participation in the study.
The experimental protocol of the present study consisted of two incremental cycle exercises on an electromagnetically braked cycle ergometer (Excalibur Sport; Lode, Groningen, The Netherlands) performed in a random order, i.e., a ramp exercise (non-steady-state) and a step exercise (steady-state), both performed until exhaustion. In the ramp protocol, the work rate increased linearly and continuously with a rate of 35 W·min−1 (i.e., ∼0.58 W·s−1). A previous study in our laboratory revealed that this ramp slope would lead to exhaustion in approximately 12 min in a group of well-trained subjects (5). The actual ramp increase in work rate was preceded by 3 min of rest on the cycle ergometer and by 3 min of baseline cycling at 40 W. In the step protocol, the work rate increased by 40 W·3 min−1, starting at 40 W. A baseline value of 40 W was used to ensure that possible changes in mechanical efficiency at very low work rates would not interfere with the linear increase in V˙O2 (4). In both protocols, the subjects were instructed to cycle at 75 rpm. The instantaneous pedal rate was continuously visualized on a display connected to the electromagnetic cycle ergometer, so that the subjects were informed about the cadence throughout the exercise tests. The test was terminated when the subjects could no longer maintain the instructed pedal rate, despite strong verbal encouragement. To test whether regional differences in deoxy[Hb + Mb] response could be related to differences in muscular activity at the measurement sites, surface EMG was performed. To be able to interpret the EMG signal adequately, normalization procedures should be performed as a function of the EMG signal at maximal contraction. At least 10 min before the start of the cycle test, each subject performed three maximal voluntary contractions (MVC) of the musculus vastus lateralis seated upright on a bench with the hips and knees fixed at 90° to obtain a maximum EMG signal. Each MVC trial was 5 s in duration with at least 2 min of rest between trials.
All exercise tests were performed within a period of 2 wk to ensure that the level of physical fitness had not changed. The subjects were asked to abstain from strenuous exercise for at least 24 h before their visit to the laboratory.
During the exercise tests, V˙O2 was measured continuously on a breath-by-breath basis using a computerized O2-CO2 analyzer-flowmeter combination and averaged during 10-s intervals (Jaeger Oxycon Pro, Höchberg, Germany). Before each test, the gas analyzers and volume transducer were calibrated. Deoxy[Hb + Mb] was measured using a frequency-domain multidistance NIRS system (Oxiplex TS; ISS, Champaign, IL). The NIRS probe consisted of eight light-emitting diodes, operating at wavelengths of 750 and 830 nm, and one detector fiber bundle (source-detector distance = 2.0-3.5 cm). The probes were positioned longitudinally on the musculus vastus lateralis of the left leg and secured with Velcro straps (ISS, Inc., Champaign, IL) around the thigh. The first probe was positioned on the proximal section of the respective muscle, whereas the second probe was placed on the distal section. Before the placement of the NIRS probes, the NIRS system was calibrated, and the skin was carefully shaved. Pen marks were made over the skin to indicate the margins of the probe to check for any downward sliding during the cycling exercise and for accurate repositioning of the probe on the subsequent visit to the laboratory. The deoxy[Hb + Mb] was stored at a frequency of 25 Hz and, afterward, digitally averaged into 1-s values. Before the start of the first test, skinfold thickness was measured at the location of the probes (i.e., proximal and distal sections of the musculus vastus lateralis) using a skinfold caliper. The skinfold thickness was 7.9 ± 1.9 and 8.3 ± 2.4 mm at the proximal and the distal sections of the musculus vastus lateralis, respectively, and did not differ significantly.
Surface EMG was recorded using bipolar 34-mm-diameter Ag-AgCl electrodes (Blue Sensor, Danlee Medical Products, Inc., Syracuse, NY) at a sampling frequency of 1000 Hz. The electrodes were placed on the same muscle portions (proximal and distal) of the musculus vastus lateralis as the probes of the NIRS device but on the right leg. Each electrode site was prepared by shaving, abrading, and swabbing the site with diluted ethanol. The reference electrode was placed over the spiny process of a prominent cervical vertebrae. The EMG signal was checked for movement artifacts, and the wires connected to the electrode were taped on the thigh of the subject. To make sure that the electrodes would be placed at the same location of the muscle on the next visit to the laboratory, the electrode sites were pen-marked. Myoelectric signals were relayed from the bipolar electrodes to a Telemyo device (Noraxon, Inc., Scottsdale, AZ). During the ramp protocol, EMG was recorded continuously, whereas during the step protocol, EMG during the final 60 s of each step were recorded.
The peak V˙O2 during each protocol was determined as the highest V˙O2 during an interval of 30 s. The gas exchange threshold (GET) of each ramp exercise test was determined using the V-slope method (2), i.e., the point at which V˙CO2 increases disproportionate to V˙O2.
For the ramp protocol, the 1-s values of the deoxy[Hb + Mb] were averaged by applying a five-point moving average and normalized to the total amplitude of the response. The baseline deoxy[Hb + Mb] was set as the average of the final 30 s of the 3-min cycling period at 40 W, whereas the peak deoxy[Hb + Mb] was determined as the average of the final 15 s of the exercise test. For the step protocol, the deoxy[Hb + Mb] at each work rate was determined as the average of the final 30 s of the step. The deoxy[Hb + Mb] at 40 W was set as baseline (0%), whereas the peak deoxy[Hb + Mb] (100%) was determined as the average of the final 30 s of the final step that was performed by the subject for at least 90 s. The obtained deoxy[Hb + Mb] for each work rate was normalized to the total amplitude of the response.
The normalized deoxy[Hb + Mb] was plotted as functions of both work rate and of percentage of peak power (% peak power). The deoxy[Hb + Mb] data from the ramp exercise were fitted by a sigmoid model, whereas the deoxy[Hb + Mb] from the step exercise was fitted by both a hyperbolic and a sigmoid model:
(a represents the asymptotic value and b is the x value corresponding to 50% of the total amplitude)
(f 0 represents the baseline, A is the amplitude, d is the slope of the sigmoid, and c/d is the x value corresponding to 50% of the total amplitude). For both models (hyperbolic and sigmoid), response curves were compared using Δ Akaike Information Criterion (ΔAIC):
N is the number of data points used in the analysis for that subject, SSsigmoid is the residual sum of squares (RSS) from the sigmoid function, SShyperbolic is the RSS from the hyperbolic function, and K is the number of parameters in the fitted model + 1. The sigmoid function includes four parameters, whereas the hyperbolic model included two parameters, so the ΔAIC equation was reduced to:
A negative ΔAIC suggests that the model in the numerator, in this case, the sigmoid, is the better fit, whereas a positive value favors the denominator model. This technique was chosen because neither model was nested within the other so a standard F-test would not have been appropriate.
The raw EMG signals were rectified, band-pass-filtered (5-2000 Hz), and integrated using commercially available software (MyoResearch 2.10; Noraxon, Inc.). Maximum iEMG was calculated by averaging the highest 1-s iEMG value from each MVC trial. iEMG from the step and ramp protocols were then normalized to the maximum iEMG. For the step protocol, the iEMG was determined at each step up to 320 W (i.e., the final work rate that was completed by all subjects) by averaging the iEMG of the final 60 s in each step. For the ramp protocol, normalized iEMG was determined at the same work rates as for the step protocol, including also 360 and 400 W, by averaging the iEMG 15 s before and after the respective work rates.
The mean values ± SD were determined for peak V˙O2, peak power output (peak power), and GET, and these parameters were compared between the protocols using a paired-samples t-test. The sigmoid and hyperbolic models for the pattern of deoxy[Hb + Mb] were calculated using fit curve in Sigmaplot 10.0. From the output file, the R 2 values, RSS, and SEE were calculated, and afterward, the best-fitting model was determined by solving the ΔAIC equation for each individual subject's response. iEMG was compared between the proximal and distal measurement sites using paired-samples t-test. Significance was declared for P < 0.05.
The ramp protocol lasted on average 695 ± 54 s, whereas the step protocol lasted 1620 ± 135 s. As expected, the subjects reached a significantly higher peak power in the ramp exercise (447 ± 31 W) compared with the incremental step exercise (360 ± 31 W). Peak V˙O2 did not differ significantly between the two protocols (64.8 ± 2.9 and 64.5 ± 3.4 mL·min−1·kg−1 for the ramp and step protocols, respectively). The GET, determined during the ramp exercise, occurred on average at 44.0 ± 5.0 mL·min−1·kg−1.
In Figure 2, the deoxy[Hb + Mb] response for the ramp and step protocols in the proximal and distal sections of the musculus vastus lateralis is presented for a representative subject. In Table 1, the mean values ± SD for R 2, SEE, and RSS are presented for the sigmoid (ramp and step exercises) and the hyperbolic fit (step exercise) to the deoxy[Hb + Mb] response of both the proximal and distal sections of the musculus vastus lateralis. For the step exercise, each subject had a negative ΔAIC, suggesting that the sigmoid model provided the better fit over the hyperbolic fit for the deoxy[Hb + Mb] response (ΔAIC equation: −31.4 ± 8.2 and −25.2 ± 4.8 for the proximal and the distal measurement sites, respectively).
Influence of protocol and measurement site.
Because the sigmoid model provided the best fit to the deoxy[Hb + Mb] response during the incremental step exercise, we could compare the parameters of the sigmoid model between the protocols and measurement sites using a 2 × 2 repeated-measures ANOVA. In Table 2, the mean values for the parameters of the sigmoid models are represented for both protocols and both measurement sites. For the deoxy[Hb + Mb] response expressed as a function of absolute work rate, the statistical analysis revealed that the parameters of the sigmoid model were influenced by the protocol. c/d (W) was significantly higher (P < 0.001) and d (%·W−1) was significantly lower (P = 0.04) in the ramp compared with the step protocol. When deoxy[Hb + Mb] was normalized as a function of percent peak power, however, the protocol did not have an influence on the parameters of the sigmoid, such that c/d (% peak power; P = 0.82) and d (%·%−1 peak power; P = 0.96) did not differ between the two protocols. Further, the sigmoid model did not differ between the proximal and distal portions of the musculus vastus lateralis for either the ramp or the step exercise. Pearson correlation analysis revealed that the c/d values (W) of the proximal and distal sites were correlated both in the ramp (r = 0.92, P < 0.001) and the step protocols (r = 0.79, P = 0.007).
In Figure 3, the normalized iEMG are plotted as a function of work rate both for the proximal and distal portions of the musculus vastus lateralis in both the ramp and step protocols. The statistical analysis revealed no significant differences between the two measurement sites, indicating that the increase in muscular electrical activity was similar at both measurement sites of the musculus vastus lateralis.
In the present study, the pattern of deoxy[Hb + Mb] during incremental exercise was examined, both as a function of protocol (quasi-steady-state step vs -steady-state ramp exercise) and as a function of localization of the NIRS probe (proximal vs distal portion of the musculus vastus lateralis). In contrast to our hypothesis, the sigmoid model provided a better fit compared with the hyperbolic model for the deoxy[Hb + Mb] response to incremental step exercise, similar to the deoxy[Hb + Mb] response to incremental ramp exercise. No differences were observed in the deoxy[Hb + Mb] pattern between the proximal and distal measurement sites in both exercise protocols.
It should be noted that the rate of increase in work rate might have affected the sigmoid pattern of deoxy[Hb + Mb] to the extent that the time course of Q˙m and V˙O2m demonstrate different fundamental kinetics (11,15,39). In our study, two different rates of increase in work rate were used: 35 in the ramp exercise and 40 W·3 min−1 (i.e., ±13 W·min−1) in the step exercise. Although the resulting parameters for the sigmoidal fit were, in fact, different for the two rates of increase in work rate in absolute watts-per-minute terms; nonetheless, when expressed as a function of percent peak power, the differences disappeared. Further, the inherent sigmoidal shape to the response was preserved. Thus, we would argue that, when differences in the kinetics of Q˙m and V˙O2m are accounted for, the response of deoxy[Hb + Mb] during incremental exercise remains fundamentally sigmoidal.
The sigmoid pattern of deoxy[Hb + Mb] during ramp exercise has been reported in two recent studies (5,11). On the basis of the assumptions that (a) deoxy[Hb + Mb] is an expression of microvascular O2 extraction, (b) changes in microvascular O2 extraction are an expression of relative changes in V˙O2m/Q˙m (i.e., Fick principle), and (c) V˙O2m increases linearly as a function of work rate, this sigmoid deoxy[Hb + Mb] pattern is the reflection of an initial rapid increase in Q˙m in the low- to moderate-intensity domain, followed by a less pronounced increase as work rate increases during the ramp exercise (Fig. 1).
Different mechanisms related to changes in Q˙m were proposed to underlie the phenomenon, including two mechanisms related to Q˙m kinetics, i.e., the change in the kinetics of cardiovascular responses inherent to changes in the balance of the sympathetic-parasympathetic activity, and/or different Q˙m and V˙O2m kinetics inherent to the different muscle fiber types (Introduction). The above two mechanisms related to Q˙m kinetics are specific to non-steady-state conditions of the ramp exercise and not to the steady-state conditions of the incremental step exercise. Our observation of a sigmoid increase in deoxy[Hb + Mb] in the step as well as in the ramp exercise suggests that the above-mentioned mechanisms related to Q˙m kinetics are not at the origin of the sigmoid increase in deoxy[Hb + Mb] during incremental exercise. Therefore, other mechanisms related to Q˙m have to be taken into consideration to explain the presence of the sigmoid profile of deoxy[Hb + Mb] in both protocols.
The mechanical effects of muscle contraction (i.e., muscle pump and rapid vasodilation) could induce an initial rapid increase in Q˙m (38) followed by a deceleration in Q˙m with increasing work rate because these effects become less important as work rate increases in the moderate- to heavy-intensity domain (25). However, because the incremental exercises started from baseline cycling at 40 W for 3 min in the present study, the muscle pump and rapid vasodilation probably contributed minimally to the sigmoid pattern of deoxy[Hb + Mb].
The rapid increase followed by a slowing effect of Q˙m could also be related to muscle fiber recruitment. In this context, different profiles of C(a-v)O2 as a function of Q˙m between "slow-twitch" and "fast-twitch" rat muscles have been reported (10). The "fast-twitch" muscles displayed a very steep increase in C(a-v)O2 even at very low metabolic rates, whereas this was not observed in "slow-twitch" muscles. The slope of the Q˙m/V˙O2m relationship did not differ between the muscle types, indicating that the increase in blood flow (O2 delivery) for a given increase in metabolic rate was similar between the muscle types (i.e., a similar slope of the Q˙m/V˙O2m relationship). Therefore, the different profiles of C(a-v)O2 between "slow-twitch" and "fast-twitch" muscles could be related to the lower y-intercept of the Q˙m/V˙O2m relationship and thus the lower resting and submaximal Q˙m values in combination with the higher O2 diffusion capacity in fast-twitch muscles (27). This has been related by the authors to the higher capacity for endothelium-dependent vasodilation and lower sympathetically mediated vasoconstriction in "slow-twitch" muscles (10). Extrapolating these observations in rats to our data, we could hypothesize that at the onset of the incremental exercise, predominantly slow-twitch muscle fibers characterized by a small increase in C(a-v)O2 are recruited. As work rate increases, fast-twitch muscle fibers will be recruited, progressively leading to a substantially greater increase in C(a-v)O2. As peak force and metabolic rate are approached, the C(a-v)O2 tends to plateau in both muscle fiber types, as seen in the deoxy[Hb + Mb] signal. This sequential recruitment of muscle fibers from predominantly slow-twitch to a mixed slow-twitch and fast-twitch fiber recruitment with increasing work rate, which is characteristic to incremental exercise, could, therefore, lead to a sigmoid pattern of C(a-v)O2 and thus of deoxy[Hb + Mb]. The different profiles of C(a-v)O2 in the different muscle fiber types are not related to Q˙m kinetics per se and might therefore explain the observation of a sigmoid pattern of deoxy[Hb + Mb] in the ramp (i.e., non-steady-state conditions) and the incremental step exercises (i.e., steady-state conditions). In addition, because NIRS produces a compound signal composed of several "unit" responses, each of which follows a specific hyperbolic C(a-v)O2 pattern, this might also contribute to the overall sigmoid pattern of the deoxy[Hb + Mb] during incremental exercise.
It cannot be excluded that other mechanisms not related to Q˙m can at least in part help to explain the pattern of deoxy[Hb + Mb] during incremental exercise. The appearance of lactic acidosis during incremental exercise could induce a rightward shift in the O2Hb dissociation curve (i.e., Bohr effect), with an increase of deoxy[Hb + Mb] as a consequence (13,28). However, the first increase in deoxy[Hb + Mb] occurs at a very low work rate (Fig. 1) far below the appearance of the GET. Therefore, the lactic acidosis is probably not the mechanism underlying the sigmoid pattern of deoxy[Hb + Mb]. In a similar fashion, whereas increasing muscle temperature could theoretically shift the O2Hb dissociation curve to the right, the low work rate (and presumably muscle temperature as well) at which the break occurs speaks against this mechanism.
The present study also showed that the relative pattern of deoxy[Hb + Mb] was not influenced by the measurement site under the NIRS probe on the same muscle. It has been reported that regional heterogeneity in muscle fiber composition (24), microvascular structure (blood flow) (17,18,21,31), and motor unit recruitment (12) within a single muscle or among muscle groups, affects muscle perfusion and V˙O2m (32,34) and, as a consequence, tissue (de)oxygenation measured using NIRS (18,20,29,31). Generally, it is accepted that intramuscular pressure is greater in the distal portion of a single muscle (1), inhibiting circulation during muscle contraction (18,30,31). Furthermore, Lexell et al. (24) showed that within a single muscle, the proximal site contains a higher percentage of slow-twitch oxidative fibers, which are characterized by a higher aerobic capacity compared with the distal site. Therefore, it could not be excluded that the pattern of deoxy[Hb + Mb] as an expression of the Q˙m/V˙O2m relationship could be affected by the localization of the NIRS probe. Our results, however, showed no differences in the pattern of deoxy[Hb + Mb] between the proximal and distal portions of the musculus vastus lateralis. This contradiction could at least in part be explained by the qualitative approach of the deoxy[Hb + Mb] pattern in the present study. In accordance with our results, Koga et al. (20) observed no significant differences in the deoxygenation kinetics between the distal and the proximal measurement sites on the musculus quadriceps. Furthermore, because in the present study the test group consisted of highly trained athletes, it is possible that the heterogeneity in muscle architecture in the musculus vastus lateralis was only marginal (16), which would minimize the possible regional differences in tissue deoxygenation.
In conclusion, the present study demonstrated that the deoxy[Hb + Mb] response follows a similar sigmoid pattern both to incremental steady-state (step) and non-steady-state (ramp) exercise in a group of trained cyclists. The sigmoid pattern of deoxy[Hb + Mb] suggests an initial rapid increase followed by a less pronounced increase (i.e., a slowing) of Q˙m as work rate increases. Our results indicate that the change in the rate of increase in Q˙m is not related to changes in Q˙m kinetics per se because the sigmoid profile of the deoxy[Hb + Mb] was observed in the incremental step as well as in the ramp exercise. We hypothesize that changes in the increase in Q˙m during incremental exercise are related to changes in muscle fiber recruitment. Finally, the relative pattern of deoxy[Hb + Mb] was not influenced by the measurement location (i.e., distal vs proximal) on the same muscle.
This research was performed without funding received from any organization.
The authors thank W. Derave for his insightful comments on the manuscript.
The results of the present study do not constitute endorsement by American College of Sports Medicine.
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Keywords:©2010The American College of Sports Medicine
NIRS; SIGMOID MODEL; DEOXYGENATION; MUSCLE BLOOD FLOW