Effect of arterial oxygenation on quadriceps fatigability during isolated muscle exercise

The effect of various levels of oxygenation on quadriceps muscle fatigability during isolated muscle exercise was assessed in six male subjects. Twitch force (Q(tw)) was assessed using supramaximal magnetic femoral nerve stimulation. In experiment 1, maximal voluntary contraction (MVC) and Q(tw) of resting quadriceps muscle were measured in normoxia [inspired O(2) fraction (Fi(O(2))) = 0.21, percent arterial O(2) saturation (Sp(O(2))) = 98.4%, estimated arterial O(2) content (Ca(O(2))) = 20.8 ml/dl], acute hypoxia (Fi(O(2)) = 0.11, Sp(O(2)) = 74.6%, Ca(O(2)) = 15.7 ml/dl), and acute hyperoxia (Fi(O(2)) = 1.0, Sp(O(2)) = 100%, Ca(O(2)) = 22.6 ml/dl). No significant differences were found for MVC and Q(tw) among the three Fi(O(2)) levels. In experiment 2, the subjects performed three sets of nine, intermittent, isometric, unilateral, submaximal quadriceps contractions (62% MVC followed by 1 MVC in each set) while breathing each Fi(O(2)). Q(tw) was assessed before and after exercise, and myoelectrical activity of the vastus lateralis was obtained during exercise. The percent reduction of twitch force (potentiated Q(tw)) in hypoxia (-27.0%) was significantly (P < 0.05) greater than in normoxia (-21.4%) and hyperoxia (-19.9%), as were the changes in intratwitch measures of contractile properties. The increase in integrated electromyogram over the course of the nine contractions in hypoxia (15.4%) was higher (P < 0.05) than in normoxia (7.2%) or hyperoxia (6.7%). These results demonstrate that quadriceps muscle fatigability during isolated muscle exercise is exacerbated in acute hypoxia, and these effects are independent of the relative exercise intensity.     

Response of genioglossus muscle activity to nasal airway occlusion in normal sleeping adults

To determine the combined effect of increased subatmospheric upper airway pressure and withdrawal of phasic volume feedback from the lung on genioglossus muscle activity, the response of this muscle to intermittent nasal airway occlusion was studied in 12 normal adult males during sleep. Nasal occlusion at end expiration was achieved by inflating balloon-tipped catheters located within the portals of a nose mask. No seal was placed over the mouth. During nose breathing in non-rapid-eye-movement (NREM) sleep, nasal airway occlusion resulted in multiple respiratory efforts before arousal. Mouth breathing was not initiated until arousal. Phasic inspiratory genioglossus activity was present in eight subjects during NREM sleep. In these subjects, comparison of peak genioglossus inspiratory activity on the first three occluded efforts to the value just before occlusion showed an increase of 4.7, 16.1, and 28.0%, respectively. The relative increases in peak genioglossus activity were very similar to respective increases in peak diaphragm activity. Arousal was associated with a large burst in genioglossus activity. During airway occlusion in rapid-eye-movement (REM) sleep, mouth breathing could occur without a change in sleep state. In general, genioglossus responses to airway occlusion in REM sleep were similar in pattern to those in NREM sleep. A relatively small reflex activation of upper airway muscles associated with a sudden increase in subatmospheric pressure in the potentially collapsible segment of the upper airway may help compromise upper airway patency during sleep.     

Effects of sleep-wake state on the genioglossus vs.diaphragm muscle response to CO(2) in rats.

The effects of sleep on the ventilatory responses to hypercapnia have been well described in animals and in humans. In contrast, there is little information for genioglossus (GG) responses to a range of CO(2) stimuli across all sleep-wake states. Given the notion that sleep, especially rapid eye movement (REM) sleep, may cause greater suppression of muscles with both respiratory and nonrespiratory functions, this study tests the hypothesis that GG activity will be differentially affected by sleep-wake states with major suppression in REM sleep despite excitation by CO(2). Seven rats were chronically implanted with electroencephalogram, neck, GG, and diaphragm electrodes, and responses to 0, 1, 3, 5, 7, and 9% CO(2) were recorded. Diaphragm activity and respiratory rate increased with CO(2) (P < 0.001) across sleep-wake states with significant increases at 3-5% CO(2) compared with 0% CO(2) controls (P < 0.05). Phasic GG activity also increased in hypercapnia but required higher CO(2) (7-9%) for significant activation (P < 0.05). Further studies in 15 urethane-anesthetized rats with the vagi intact (n = 6) and cut (n = 9) showed that intact vagi delayed GG recruitment with hypercapnia but did not affect diaphragm responses. In the naturally sleeping rats, we also showed that GG activity was significantly reduced in non-REM and REM sleep (P < 0.04) and was almost abolished in REM even with stimulation by 9% CO(2) (decrease = 80.4% vs. wakefulness). Such major suppression of GG activity in REM, even with significant respiratory stimulation, may explain why obstructive apneas are more common in REM sleep.     

Effect of circulatory occlusion on human muscle metabolism during exercise and recovery

To assess muscle metabolism and inorganic phosphate (P_i) peak splitting during exercise, 31-phosphorus nuclear magnetic resonance spectroscopy was performed during ramp incremental and submaximal step exercise with and without circulatory occlusion. Seven healthy men performed calf flexion in a superconducting magnet. There was no P_i splitting during ramp incremental exercise with the circulation present and phosphocreatine (PCr) decreased linearly by 0.07 (SEM 0.01) mmol · l⁻¹ · s⁻¹, while exercise with the circulation occluded caused the P_i peak to split into a high and a low pH peak. The rate of PCr decrease during exercise with the circulation occluded was 0.15 (SEM 0.03) mmol · l⁻¹ · s⁻¹ which with the efficiency of the adenosine 5′-triphosphate (ATP) hydrolysis reaction corresponded well to the mechanical energy. Both with and without occlusion of the circulation PCr decreased with some time lag which may reflect the consumption of residual oxygen. In submaximal step exercise PCr decreased exponentially at the onset of exercise with the circulation open whereas it decreased linearly by 0.15␣mmol · l⁻¹ · s⁻¹ when the circulation was occluded. After exercise, occlusion of the circulation was maintained for 1 min more and there was no PCr resynthesis. It is suggested that ATP synthesis was limited by the availability of oxygen. Accepted: 14 August 1996     

Skeletal muscle oxygenation during incremental exercise

The purpose of this study was to investigate the relationship between muscle oxygenation level at exhaustion and maximal oxygen uptake (VO2max) in an incremental cycling exercise. Nine male subjects took part in an incremental exhaustive cycling exercise, and then cuff occlusion was performed. Changes in oxy-(deltaHbO2) and deoxy-(deltaHb) hemoglobin concentrations in the vastus lateralis muscle were measured with a near infrared spectroscopy (NIRS). Muscle oxygenation during incremental exercise was expressed as a percentage (%Moxy) of the maximal range observed during an arterial occlusion as the lower reference point. A systematic decrease was observed in %Moxy with increasing intensity. A significant relationship was observed between %Moxy at exhaustion and VO2max (p < 0.01). We concluded that the one of the limiting factor of VO2max is the muscle oxygen diffusion capacity, and %Moxy during exercise could be one of the indexes of muscle oxygen diffusion capacity.     

Effects of acute hypoxia on cerebral and muscle oxygenation during incremental exercise.

To determine if fatigue at maximal aerobic power output was associated with a critical decrease in cerebral oxygenation, 13 male cyclists performed incremental maximal exercise tests (25 W/min ramp) under normoxic (Norm: 21% Fi(O2)) and acute hypoxic (Hypox: 12% Fi(O2)) conditions. Near-infrared spectroscopy (NIRS) was used to monitor concentration (microM) changes of oxy- and deoxyhemoglobin (Delta[O2Hb], Delta[HHb]) in the left vastus lateralis muscle and frontal cerebral cortex. Changes in total Hb were calculated (Delta[THb] = Delta[O2Hb] + Delta[HHb]) and used as an index of change in regional blood volume. Repeated-measures ANOVA were performed across treatments and work rates (alpha = 0.05). During Norm, cerebral oxygenation rose between 25 and 75% peak power output {Power(peak); increased (inc) Delta[O2Hb], inc. Delta[HHb], inc. Delta[THb]}, but fell from 75 to 100% Power(peak) {decreased (dec) Delta[O2Hb], inc. Delta[HHb], no change Delta[THb]}. In contrast, during Hypox, cerebral oxygenation dropped progressively across all work rates (dec. Delta[O2Hb], inc. Delta[HHb]), whereas Delta[THb] again rose up to 75% Power(peak) and remained constant thereafter. Changes in cerebral oxygenation during Hypox were larger than Norm. In muscle, oxygenation decreased progressively throughout exercise in both Norm and Hypox (dec. Delta[O2Hb], inc. Delta [HHb], inc. Delta[THb]), although Delta[O2Hb] was unchanged between 75 and 100% Power peak. Changes in muscle oxygenation were also greater in Hypox compared with Norm. On the basis of these findings, it is unlikely that changes in cerebral oxygenation limit incremental exercise performance in normoxia, yet it is possible that such changes play a more pivotal role in hypoxia.     

Changes in muscle oxygenation during weight-lifting exercise

The quantitative analysis of haemoglobin oxygenation of contracting human muscle during weight-lifting exercise was studied noninvasively and directly using near-infrared spectroscopy. This method was developed as a three-wavelength method which confirmed the volume changes in oxygenated haemoglobin (oxy-Hb), deoxygenated haemoglobin (deoxy-Hb) and blood volume (total-Hb; Oxy-Hb + deoxy-Hb). Nine healthy adult men with various levels of training experience took part in the study. Ten repetition maximum (10 RM) one-arm curl exercise was performed by all the subjects. Results showed that at the beginning of the 10-RM exercise, rapid increases of deoxy-Hb and decreases of oxy-Hb were observed. In addition, total-Hb gradually increased during exercise. These results corresponded to the condition of arm blood flow experimentally restricted using a tourniquet in contact with the shoulder joint, and they showed the restriction of venous blood flow and an anoxic state occurring in the dynamically contracted muscle. In three sets of lifting exercise with short rest periods, these tendencies were accelerated in each set, while total-Hb volume did not return to the resting state after the third set for more than 90 s. These results would suggest that a training regimen emphasizing a moderately high load and a high number of repetitions, and a serial set with short rest periods such as usually performed by bodybuilders, caused a relatively long-term anoxic state in the muscle.     

Recruitment and rate-coding strategies of the human genioglossus muscle

Single motor unit (SMU) analysis provides a means to examine the motor control of a muscle. SMUs in the genioglossus show considerable complexity, with several different firing patterns. Two of the primary stimuli that contribute to genioglossal activation are carbon dioxide (CO(2)) and negative pressure, which act through chemoreceptor and mechanoreceptor activation, respectively. We sought to determine how these stimuli affect the behavior of genioglossus SMUs. We quantified genioglossus SMU discharge activity during periods of quiet breathing, elevated CO(2) (facilitation), and continuous positive airway pressure (CPAP) administration (inhibition). CPAP was applied in 2-cmH(2)O increments until 10 cmH(2)O during hypercapnia. Five hundred ninety-one periods (each ～ 3 breaths) of genioglossus SMU data were recorded using wire electrodes(n = 96 units) from 15 awake, supine subjects. Overall hypercapnic stimulation increased the discharge rate of genioglossus units (20.9 ± 1.0 vs. 22.7 ± 0.9 Hz). Inspiratory units were activated ～ 13% earlier in the inspiratory cycle, and the units fired for a longer duration (80.6 ± 5.1 vs. 105.3 ± 4.2% inspiratory time; P < 0.05). Compared with baseline, an additional 32% of distinguishable SMUs within the selective electrode recording area were recruited with hypercapnia. CPAP led to progressive SMU inhibition; at ～ 6 cmH(2)O, there were similar numbers of SMUs active compared with baseline, with peak frequencies of inspiratory units close to baseline, despite elevated CO(2) levels. At 10 cmH(2)O, the number of units was 36% less than baseline. Genioglossus inspiratory phasic SMUs respond to hypercapnic stimulation with changes in recruitment and rate coding. The SMUs respond to CPAP with derecruitment as a homogeneous population, and inspiratory phasic units show slower discharge rates. Understanding upper airway muscle recruitment/derecruitment may yield therapeutic targets for maintenance of pharyngeal patency.     

Inspiratory-resistive loading increases the ventilatory response to arousal but does not reduce genioglossus muscle activity on the return to sleep

Arousals from sleep are thought to predispose to obstructive sleep apnea by causing hyperventilation and hypocapnia, which reduce airway dilator muscle activity on the return to sleep. However, prior studies of auditory arousals have not resulted in reduced genioglossus muscle activity [GG-electromyogram (EMG)], potentially because airway resistance prior to arousal was low, leading to a small ventilatory response to arousal and minimal hypocapnia. Thus we aimed to increase the ventilatory response to arousal by resistive loading prior to auditory arousal and determine whether reduced GG-EMG occurred on the return to sleep. Eighteen healthy young men and women were recruited. Subjects were instrumented with a nasal mask with a pneumotachograph, an epiglottic pressure catheter, and intramuscular GG-EMG electrodes. Mask CO(2) levels were monitored. Three- to 15-s arousals from sleep were induced with auditory tones after resting breathing (No-Load) or inspiratory-resistive loading (Load; average 8.4 cmH(2)O·l(-1)·s(-1)). Peak minute ventilation following arousal was greater after Load than No-Load (mean ± SE; 8.0 ± 0.6 vs. 7.4 ± 0.6 l/min, respectively). However, the nadir end tidal partial pressure of CO(2) did not differ between Load conditions (43.1 ± 0.6 and 42.8 ± 0.5 mmHg, respectively), and no period of reduced GG activity occurred following the return to sleep (GG-EMG baseline, minimum after Load and No-Load = 2.9 ± 1.2%, 3.1 ± 1.3%, and 3.0 ± 1.3% max, respectively). These findings indicate that the hyperventilation, which occurs following tone-induced arousal, is appropriate for the prevailing level of respiratory drive, because loading did not induce marked hypocapnia or lower GG muscle activity on the return to sleep. Whether similar findings occur following obstructive events in patients remains to be determined.     

Effect of work and recovery duration on skeletal muscle oxygenation and fuel use during sustained intermittent exercise

The purpose of this study was to compare rates of substrate oxidation in two protocols of intermittent exercise, with identical treadmill speed and total work duration, to reduce the effect of differences in factors such as muscle fibre type activation, hormonal responses, muscle glucose uptake and non-esterified fatty acid (NEFA) availability on the comparison of substrate utilisation. Subjects (n = 7) completed 40 min of intermittent intense running requiring a work:recovery ratio of either 6 s:9 s (short-interval exercise, SE) or 24 s:36 s (long-interval exercise, LE), on separate days. Another experiment compared O(2) availability in the vastus lateralis muscle across SE (10 min) and LE (10 min) exercise using near-infrared spectroscopy (RunMan, NIM. Philadelphia, USA). Overall (i.e. work and recovery) O(2) consumption (VO(2)) and energy expenditure were lower during LE (P < 0.01, P < 0.05, respectively). Overall exercise intensity, represented as a proportion of peak aerobic power (VO2(peak)), was [mean (SEM)] 64.9 (2.7)% VO2(peak) (LE) and 71.4 (2.4)% VO2(peak) (SE). Fat oxidation was three times lower (P < 0.01) and carbohydrate oxidation 1.3 times higher (P < 0. 01) during LE, despite the lower overall exercise intensity. Plasma lactate was constant and was higher throughout exercise in LE [mean (SEM) 5.33 (0.53) mM, LE; 3.28 (0.31) mM, SE; P < 0.001)]. Plasma pyruvate was higher and glycerol was lower in LE [215 (17) microM, 151 (13) microM, P < 0.05, pyruvate; 197 (19) microM, 246 (19) microM, P < 0.05, glycerol]. There was no difference between protocols for plasma NEFA concentration (n = 4) or plasma noradrenaline and adrenaline. Muscle oxygenation declined in both protocols (P < 0.001), but the nadir during LE was lower [52.04 (0. 60)%] compared to SE [61.85 (0.51)%; P < 0.001]. The decline in muscle oxygenation during work was correlated with mean lactate concentration (r = 0.68; P < 0.05; n = 12). Lower levels of fat oxidation occurred concurrent with accelerated carbohydrate metabolism, increases in lactate and pyruvate and reduced muscle O(2) availability. These changes were associated with proportionately longer work and recovery periods, despite identical treadmill speed and total work duration. The proposal that a metabolic regulatory factor within the muscle fibre retards fat oxidation under these conditions is supported by the current findings.     

Effect of inspired air temperature on genioglossus activity during nose breathing in awake humans

Experimental data suggest the presence of sensory receptors specific to the nasopharynx that may reflexly influence respiratory activity. To investigate the effects of inspired air temperature on upper airway dilator muscle activity during nose breathing, we compared phasic genioglossus electromyograms (EMGgg) in eight normal awake adults breathing cold dry or warm humidified air through the nose. EMGgg was measured with peroral bipolar electrodes during successive trials of cold air (less than or equal to 15 degrees C) and warm air (greater than or equal to 34 degrees C) nasal breathing and quantified for each condition as percent activity at baseline (room temperature). In four of the subjects, the protocol was repeated after topical nasal anesthesia. For all eight subjects, mean EMGgg was greater during cold air breathing than during baseline (P less than 0.005) or warm air breathing (P less than 0.01); mean EMGgg during warm air breathing was not significantly changed from baseline. Nasal anesthesia significantly decreased the mean EMGgg response to cold air breathing. Nasal airway inspiratory resistance, measured by posterior rhinomanometry in six subjects under similar conditions, was no different for cold or warm air nose breathing [cold 1.4 +/- 0.7 vs. warm 1.4 +/- 1.1 (SD) cmH2O.l-1.s at 0.4 l/s flow]. These data suggest the presence of superficially located nasal cold receptors that may reflexly influence upper airway dilating muscle activity independently of pressure changes in awake normal humans.     

Heterogeneous activity of the human genioglossus muscle assessed by multiple bipolar fine-wire electrodes.

Genioglossus (GG) electrical activity [measured by electromyogram (EMGgg)] is best measured by intramuscular electrodes; however, the homogeneity of EMGgg is undefined. We investigated the relationships between EMGgg and the site from which activity was measured to determine whether and to what extent inhomogeneity in activity occurred. Eight healthy human volunteers underwent ultrasound to determine GG depth and width. Four pairs of electrodes were then inserted percutaneously into the left and right GG muscle, anteriorly and posteriorly. Additional configurations were obtained by connecting electrodes across the midline and along each muscle belly. EMGgg activity was simultaneously recorded from these 10 configurations at rest and during various respiratory maneuvers. Heterogeneous behavior of the GG was evidenced by 1) the variable presence of phasic EMGgg at rest, which was undetectable in two subjects but evident in 65% of configurations in six subjects and present in all configurations in all subjects during voluntary hyperventilation; 2) a greater amplitude of EMGgg response to pharyngeal square-wave negative pressure in anterior than posterior configurations (14.1 +/- 7.1 vs. 8.5 +/- 5.1% of maximum, P < 0.05); and 3) variable (linear and alinear) relationships between EMGgg and lingual force within and between subjects. We hypothesize that regional differences in density and type of muscle fiber are the most likely sources of heterogeneity in these responses.     

Control of respiratory activity of the genioglossus muscle in micrognathic infants

The genioglossus (GG) muscle activity of four infants with micrognathia and obstructive sleep apnea was recorded to assess the role of this tongue muscle in upper airway maintenance. Respiratory air flow, esophageal pressure, and intramuscular GG electromyograms (EMG) were recorded during wakefulness and sleep. Both tonic and phasic inspiratory GG-EMG activity was recorded in each of the infants. On occasion, no phasic GG activity could be recorded; these silent periods were unassociated with respiratory embarrassment. GG activity increased during sigh breaths. GG activity also increased when the infants spontaneously changed from oral to nasal breathing and, in two infants, with neck flexion associated with complete upper airway obstruction, suggesting that GG-EMG activity is influenced by sudden changes in upper airway resistance. During sleep, the GG-EMG activity significantly increased with 5% CO2 breathing (P less than or equal to 0.001). With nasal airway occlusion during sleep, the GG-EMG activity increased with the first occluded breath and progressively increased during the subsequent occluded breaths, indicating mechanoreceptor and suggesting chemoreceptor modulation. During nasal occlusion trials, there was a progressive increase in phasic inspiratory activity of the GG-EMG that was greater than that of the diaphragm activity (as reflected by esophageal pressure excursions). When pharyngeal airway closure occurred during a nasal occlusion trial, the negative pressure at which the pharyngeal airway closed (upper airway closing pressure) correlated with the GG-EMG activity at the time of closure, suggesting that the GG muscle contributes to maintaining pharyngeal airway patency in the micrognathic infant.     

Force production of the genioglossus as a function of muscle length in normal humans.

Resting muscle length affects both maximum force production and force maintenance. The strength and force maintenance characteristics of the genioglossus as a function of resting muscle length have not been described. We hypothesized that genioglossus optimum length (L(o)) could be defined in vivo and that the ability of the genioglossus to sustain a given workload would decrease as resting length deviated from L(o). To test this, 11 normal men repeated maximal isometric genioglossus protrusions at different muscle lengths to determine L(o). L(o) was also obtained by using submaximal efforts while simultaneously recording electromyographic activity of the genioglossus, with L(o) defined as the length at which the force-to-genioglossus electromyographic activity ratio was maximum. Both methods provided similar results. Force maintenance was measured at four muscle lengths on separate days. Target efforts representing 60% of each subject's maximum at L(o) and lasting 5 s were performed at 12-s intervals. Time limit of endurance of the genioglossus was defined as the time from trial onset at which 90% of the target could not be maintained for three consecutive efforts. Time limit of endurance was greatest at L(o) and fell to 47.5% at L(o) + 1 cm, 53.8% at L(o) - 1 cm, and 47.4% at L(o) - 1.5 cm. We conclude that L(o) of the genioglossus can be determined in vivo and that force maintenance of the genioglossus is decreased when operating length deviates from L(o).     

Upper airway response to electrical stimulation of the genioglossus in obstructive sleep apnea.

Contraction of the genioglossus (GG) has been shown to improve upper airway patency. In the present study, we evaluated responses in upper airway pressure-flow relationships during sleep to electrical stimulation (ES) of the GG in patients with obstructive sleep apnea. Five patients with chronically implanted hypoglossal nerve (HG) electrodes and nine patients with fine-wire electrodes inserted into the GG were studied. Airflow was measured at multiple levels of nasal pressure, and upper airway collapsibility was defined by the nasal pressure below which airflow ceased ["critical" pressure (Pcrit)]. ES shifted the pressure-flow relationships toward higher flow levels in all patients over the entire range of nasal pressure applied. Pcrit decreased similarly during both HG-ES and GG-ES (deltaPcrit was 3.98 +/- 2.31 and 3.18 +/- 1.70 cmH2O, respectively) without a significant change in upstream resistance. The site of collapse (velo- vs. oropharynx) did not influence the response to GG-ES. Moreover, ES-induced reductions in the apnea-hypopnea index of the HG-ES patients were associated with substantial decreases in Pcrit. Our findings imply that responses in apnea severity to HG-ES can be predicted by characterizing the patient's baseline pressure-flow relationships and response to GG-ES.     

Effects of assuming constant optical scattering on measurements of muscle oxygenation by near-infrared spectroscopy during exercise.

The aim of this study was to examine the effects of assuming constant reduced scattering coefficient (mu'(s)) on the muscle oxygenation response to incremental exercise and its recovery kinetics. Fifteen subjects (age: 24 +/- 5 yr) underwent incremental cycling exercise. Frequency domain near-infrared spectroscopy (NIRS) was used to estimate deoxyhemoglobin concentration {[deoxy(Hb+Mb)]} (where Mb is myoglobin), oxyhemoglobin concentration {[oxy(Hb+Mb)]}, total Hb concentration (Total[Hb+Mb]), and tissue O(2) saturation (Sti(O(2))), incorporating both continuous measurements of mu'(s) and assuming constant mu'(s). When measuring mu'(s), we observed significant changes in NIRS variables at peak work rate Delta[deoxy(Hb+Mb)] (15.0 +/- 7.8 microM), Delta[oxy(Hb+Mb)] (-4.8 +/- 5.8 microM), DeltaTotal[Hb+Mb] (10.9 +/- 8.4 microM), and DeltaSti(O(2))(-11.8 +/- 4.1%). Assuming constant mu'(s) resulted in greater (P < 0.01 vs. measured mu'(s)) changes in the NIRS variables at peak work rate, where Delta[deoxy(Hb+Mb)] = 24.5 +/- 15.6 microM, Delta[oxy(Hb+Mb)] = -9.7 +/- 8.2 microM, DeltaTotal[Hb+Mb] = 14.8 +/- 8.7 microM, and DeltaSti(O(2))= -18.7 +/- 8.4%. Regarding the recovery kinetics, the large 95% confidence intervals (CI) for the difference between those determine measuring mu'(s) and assuming constant mu'(s) suggested poor agreement between methods. For the mean response time (MRT), which describes the overall kinetics, the 95% confidence intervals were MRT - [deoxy(Hb+Mb)] = 26.7 s; MRT - [oxy(Hb+Mb)] = 11.8 s, and MRT - Sti(O(2))= 11.8 s. In conclusion, mu'(s) changed from light to peak exercise. Furthermore, assuming a constant mu'(s) led to an overestimation of the changes in NIRS variables during exercise and distortion of the recovery kinetics.