In the transition from rest to constant-load exercise below the lactate threshold (LT), the pulmonary oxygen uptake (V̇O2) response can be partitioned into three distinct phases. After the cardiodynamic phase (phase I), V̇O2 rises in an approximately monoexponential fashion (phase II or primary component) to attain a new steady state (phase III) within 2–3 min (1). However, during exercise above the LT, the attainment of a steady state is delayed or absent due to the development of an additional V̇O2 slow component, which is superimposed on the primary component (1).
The mechanism responsible for the development of the V̇O2 slow component remains elusive. Attention has been given to the influences of lactate, adrenaline, the cost of ventilatory work and rises in core or muscle temperature (10). Poole et al. (21) demonstrated that approximately 86% of the V̇O2 slow component originates in the exercising limb, and therefore the recruitment of Type II muscle fibers has been considered to be a possible mediator of the phenomenon. Indeed, it has been shown that the proportion of Type II fibers in the vastus lateralis is significantly correlated with the relative magnitude of the V̇O2 slow component (1,22). Furthermore, several studies (but not all, e.g., 25) have reported changes in neuromuscular activity in line with development of the V̇O2 slow component (4,20). This may represent the recruitment, in some manner, of the Type II muscle fibers which are thought to be of lower mechanical efficiency (19). The percent Type I muscle fibers also appears to be associated with a greater increase of the V̇O2 primary component relative to power output (referred to as the “gain” of the primary component) during constant-load exercise (1,22) and with a greater ΔV O2/Δ work rate slope during incremental (2) exercise. Seemingly in direct contrast to these studies, Coyle et al. (8) found a lower exercise V̇O2 for individuals with a high proportion of Type I fibers. However, because V̇O2 was measured some 5 min into exercise, individuals with a high proportion of Type II fibers will be seen to be less efficient due to inclusion of the developing V̇O2 slow component. Direct comparison between this work and that of Barstow and colleagues (1,2) is difficult because the primary component gain is not reported.
The studies to date have not established the causality of the relationship between muscle fiber type and the V̇O2 response. It would be preferable to manipulate the independent variable, i.e., fiber type recruitment, and observe the effects on the V̇O2 response. Therefore, the purpose of the present study was to induce glycogen depletion in discrete fiber pools in an attempt to alter the motor unit recruitment patterns during constant-load exercise. The assumption is that metabolism in these glycogen depleted fibers will be disturbed and recruitment during subsequent exercise therefore impaired (27). It has been shown that prolonged low intensity exercise (~30% V̇O2max) depletes glycogen concentration exclusively in the Type I muscle fibers (11). We hypothesized that this condition would necessitate a greater reliance of the Type II fibers during subsequent square-wave exercise, resulting in a reduction in the gain of the V̇O2 primary component and a greater amplitude of the V̇O2 slow component. In contrast, after high-intensity exercise that would deplete the glycogen content of the Type IIa and IIx fibers (28), we hypothesized there would be an increased reliance on Type I fibers resulting in an increased gain of the V̇O2 primary component and a decreased amplitude of the V̇O2 slow component.
Experimental design and participants.
There were two stages to this experiment, which was approved by the University’s Ethics Committee. The participants gave written informed consent after the experimental procedures, and the associated risks and the benefits of participation were explained. Fourteen recreationally active participants (age 25.5 ± 4.5 yr; mass 76.0 ± 10.1 kg; V̇O2max 0.5 2.95 ± L·min−1) volunteered to take part in stage 1. The aim of this stage was to validate the glycogen depletion protocols to be used in stage 2. This stage involved a muscle biopsy before and after exercise. Nineteen participants (age 24.8 ± 4.0 yr; mass 74.9 ± 9.2 kg; V̇O2max 3.07 ± 0.71 L·min−1) participated in stage 2 of the study which investigated the effect of glycogen depletion on the V̇O2 kinetic response. Twelve participants completed both stages of the study. The participants were all fully familiar with laboratory exercise testing procedures. The participants were instructed to arrive at the laboratory after an overnight fast, to be rested and in a fully hydrated state having abstained from the consumption of caffeine for the previous 4 h. The participants were also asked to avoid strenuous exercise in the 48 h preceding a test session and to record food intake for this period. Subjects kept a food diary before the first lab visit and then the same diet was replicated before each session.
All tests were conducted on an electrically braked cycle ergometer (Jaeger ER800, Germany), with seat and handlebar height kept constant over the sessions for each participant. Pedal frequency was maintained at 60 ± 5 rev·min−1 for all tests. Pulmonary gas exchange was determined breath-by-breath using standard algorithms, allowing for the time delay between gas concentration and volume signals. Individuals breathed through a low dead space (90 mL), low resistance (0.65 mm H2O·L−1·s−1 at 8 L·s−1) mouthpiece and turbine assembly. Gases were continuously drawn from the mouthpiece through a 2-m capillary line of small bore (0.5 mm) at a rate of 60 mL·min−1 and analyzed for O2, CO2, and N2 concentrations by a quadrupole mass spectrometer (CaSE EX670, Gillingham, Kent, UK), which was calibrated before each test using gases of known concentration. Expiratory volumes were determined using a turbine volume transducer (Interface Associates, CA). The volume and concentration signals were integrated by computer after analog-to-digital conversion. Respiratory gas exchange variables (V̇O2, V̇CO2, V̇E) were calculated and displayed for every breath. Heart rate was recorded telemetrically throughout the exercise tests (Polar Electro Oy, Kempele, Finland).
At the beginning of the study, all participants performed a ramp exercise test to volitional exhaustion in order to determine the ventilatory threshold (VT) and the V̇O2max. This test was performed at the same time of day in all individuals (~1000 h). The initial power output was 20 W, which was then increased by 5 W every 10 s (equating to 30 W·min−1).
The highest 30-s average of the breath-by-breath V̇O2 data was taken to be the V̇O2max. Attainment of V̇O2max was confirmed by the incidence of a plateau phenomenon in V̇O2, RER values above 1.10, and heart rates within 5 beats·min−1 of age-predicted maximum. In all subjects, at least two of the three criteria were met. The VT was determined by two investigators as the point above which there was a nonlinear increase in minute ventilation (V̇E) plotted against V̇O2 and an increase in V̇E/V̇O2 against V̇O2 with no increase in V̇E/V̇O2 against V̇O2 (30).
Extrapolation of the relationship between V̇O2 and power output (correction was made for the lag in V̇O2) for exercise <VT was used to estimate the power output at V̇O2max. Subsequently, a series of power outputs were calculated: power outputs requiring 30% and 120% of V̇O2max for the glycogen depletion protocols; and power outputs requiring 80% of the V̇O2 at VT (moderate-intensity exercise), and 50% of the difference in V̇O2 between VT and V̇O2max (heavy-intensity exercise, 50%Δ) for the performance of “square-wave” exercise in stage 2 of the experimentation.
Stage 1: validation of protocols to deplete glycogen stores in Type I and II muscle fibers.
The group of 14 participants were randomly assigned to one of two groups. Seven participants undertook low intensity exercise (30% V̇O2max) for 3 h, with the aim of depleting the Type I muscle fibers (11). The remaining seven participants performed 10, 1-min bouts of high-intensity exercise (at 120% V̇O2max), separated by 5-min rest periods. This protocol has been shown previously to preferentially deplete the Type II fibers (28). A muscle biopsy was taken before and immediately postexercise in both conditions (see below). During the low-intensity depletion protocol, pulmonary gas exchange, heart rate, and blood lactate concentration were recorded at 10 min, 30 min, and then every 30 min until the end of exercise. During the high-intensity depletion protocol, blood [lactate] and heart rate were measured at rest, and after the 5th and 10th sprints.
Muscle biopsies were taken under local anesthesia (2 mL, 1% lidocaine) from the lateral portion of the vastus lateralis muscle. The biopsies were obtained by a percutaneous needle biopsy technique (3) with suction, thus allowing ~100–150 mg of tissue to be extracted. Muscle biopsies were taken within 15 s of exercise cessation with a portion of the tissue being immediately frozen in liquid nitrogen. The rest of the muscle sample was mounted on a specimen holder in optimum cutting temperature embedding medium (Ames Tissue-Tek) and frozen in isopentane cooled in liquid nitrogen. The muscle specimens were then moved to storage at −80°C until analysis. Serial cross sections (10 μm thick) were cut in a cryostat at −20°C and mounted on slides for histochemical analysis. Muscle fiber type was determined by estimation of myofibrillar ATPase activity at pH 9.4 (after preincubation at pH 4.7) after the methods of Brooke and Kaiser (5), with fibers being classified as Type I, IIa, and IIx. A qualitative assessment of glycogen concentration in these fibers was made using the periodic acid-Schiff (PAS) reaction (29). Serial sections could then be matched to allow the glycogen content of each fiber pool to be determined pre- and postexercise (see Fig. 1). To determine the quantitative reduction in muscle glycogen with exercise, the unmounted portion of the tissue was acid hydrolyzed allowing glucose residues to be measured enzymatically (F200 fluorometer, Wheaton Scientific, UK) as described by Lowry and Passonneau (18). Glycogen concentrations are expressed “wet weight” (w.w.).
Stage 2: effect of glycogen depletion on the V̇O2 kinetic response during constant load exercise.
Nineteen participants took part in this stage of the study. A period of at least two weeks elapsed before retesting of the 12 subjects taking part in both stages of the study to prevent any impact on muscle glycogen storage by the muscle biopsy procedure, as previously reported (7). On three separate occasions participants entered the laboratory and performed a series of “square-wave” transitions of 6-min duration (preceded by 3 min of baseline) in order to determine V̇O2 kinetics: two “moderate” (80% VT) intensity and one “heavy” (50% Δ) intensity transitions, separated by 6-min recovery. Pulmonary gas exchange was measured breath-by-breath throughout the square-wave tests. Fingertip capillary blood samples were taken immediately before and after exercise. The difference between the end-exercise [lactate] and the resting [lactate] was expressed as a delta value (Δ [lactate]). After 1 h of rest, a further blood sample was taken to ensure that blood [lactate] had returned to resting levels. The subjects then performed an identical set of square wave transitions. In total, the subjects performed a total of four moderate and two heavy transitions for each of the three conditions.
On two of the visits, the square-wave exercise was preceded by a glycogen depleting protocol (as described in stage 1). No biopsies were taken in this stage of the experimentation, but measures of V̇O2, heart rate and blood lactate were taken as previously described. At least an hour of rest was given after the depletion protocols before commencing the square-wave exercise bouts to allow muscle temperature, blood metabolites, and V̇O2 to return to resting levels (12,14,15). During this time, no food was consumed, but water was permitted ad libitum, with a minimum of 150% of the volume lost during the depletion regimen (based on pre- and postexercise body mass) being consumed. The order in which the subjects performed the control (CON), low-intensity depletion condition (LOW), and the high-intensity depletion condition (HIGH) was randomly assigned.
For each exercise transition, the breath-by-breath data were interpolated to give second-by-second values. The data from the equivalent transitions were then time aligned to the start of exercise and averaged in order to enhance the underlying response characteristics. Nonlinear regression techniques were used to fit the V̇O2 data after the onset of exercise with an exponential function. The time course of the V̇O2 response after the onset of exercise was described in terms of a two- (moderate intensity) or three-(heavy intensity) component exponential function. Each exponential curve was used to describe one phase of the response. The first phase began at the onset of exercise, whereas the other terms began after independent time delays (1):
where V̇O2 (b) is the baseline V̇O2 measured in the 3 min preceding the onset of exercise; Ac, Ap, and As are the asymptotic amplitudes for the exponential curves; τc, τp, and τs are the time constants; and TDp and TDs are the time delays. The cardiodynamic response was terminated at the onset of the primary component (at TDp), and given the value for that time (defined Ac′). The amplitude of the primary response (Ap′) was defined as the increase in V̇O2 from baseline to the end of the primary component (i.e., Ac′ +Ap′). The amplitude of the V̇O2 slow component was determined as the increase in V̇O2 above the asymptotic primary component at the end of exercise (defined Ap′), rather than from the asymptotic value (As), which may lie beyond physiological limits. Initial estimates for the time-based parameters used in the modeling were: TDp 20 s, TDs 120 s, τc 12 s, τp 25 s, and τs 260 s, based upon values presented in the literature (1). An iterative process ensured the residual sum of squares (RSS) was minimized. To ensure that the minimized residuals were not due to localized minimized least squares residuals, further iterations with both under and overestimated parameters were performed.
Once the data were checked for normal distribution and homogeneity of variation, one-way repeated measures ANOVA and Bonferroni corrected paired t-tests were used to test for differences with significance accepted at P < 0.05. The 95% confidence intervals for the time-based parameters were calculated using procedures outlined previously (17). Results are presented as mean ± SEM.
Stage 1: validation of protocols to deplete glycogen stores in Type I and II muscle fibers.
Data collected during the low-intensity depletion protocol indicated that subjects were working at 31.9 ± 2.4% V̇O2max; RER decreased during the 3 h (from 0.88 ± 0.02 at 10 min to 0.79 ± 0.02 at 3 h); heart rate increased (from 86 ± 5 beats·min−1 at 10 min to 93 ± 6 beats·min−1 at 3 h); and blood [lactate] decreased (from 1.2 ± 0.1 mM at 10 min to 0.8 ± 0.1 mM at 3 h). Immediately after the high-intensity depletion protocol, heart rate was markedly elevated (167 ± 4 beats·min−1), as was blood [lactate] (an increase from 0.8 ± 0.2 mM at rest to 7.7 ± 0.1 mM).
The percentage distribution of Type I, IIa, and IIx fibers in the vastus lateralis muscle averaged 52.1 ± 11.3%, 31.7 ± 5.7%, and 16.2 ± 10.6%, respectively. As can be seen in Figure 2 (A and C), there was no differential PAS staining pattern before exercise, with all fiber types stained dark. On examination of panels B and D it can be seen that: a) there was an impact on the muscle fiber glycogen content with exercise and b) the depletion pattern was dependent on the type of exercise performed. After the low-intensity depletion exercise (see panel B) only ~6% of Type I fibers were PAS dark, whereas the majority of Type IIa and IIx fibers remained dark (~70 and 96%, respectively). The opposite differentiation was seen after the high-intensity depletion exercise (see panel D). Approximately 85% of Type I fibers counted remained PAS dark, whereas the Type IIa and IIx fibers were discernibly depleted, with only 1% of both fiber types remaining PAS dark.
Muscle glycogen concentration was similar at rest in both the low- (108.1 ± 14.2 mmol·kg−1 w.w.) and high-intensity (101.3 ± 12.9 mmol·kg−1 w.w.) depletion groups. The absolute glycogen concentration dropped significantly after both the high- and the low-intensity depletion protocols. However, the reduction was somewhat greater after the high-intensity exercise protocol (by 55% to 46.1 ± 16.3 mmol·kg−1 w.w.) than after the low-intensity exercise protocol (by 44% to 61.0 ± 21.3 mmol·kg−1 w.w.).
Stage 2: effect of glycogen depletion on the V̇O2 kinetic response during constant-load exercise.
The data collected during the depletion protocols in stage 2 of the experiment were remarkably similar to those observed in stage 1. During the low intensity depletion protocol (calculated as 31.4 ± 4.1% V̇O2max, RER and blood [lactate] decreased during the 3 h (from 0.91 ± 0.01 to 0.78 ± 0.04 and from 1.0 ± 0.07 to 0.7 ± 0.03 mM, respectively) and heart rate increased (87 ± 3 beats·min−1 to 92 ± 2 beats·min−1). After the high-intensity depletion protocol heart rate was 163 ± 2 beats·min−1, whereas blood [lactate] had risen to 6.0 ± 0.3 mM (from 0.9 ± 0.1 mM at rest).
Table 1 shows the impact of the depletion protocols on the V̇O2 kinetic parameters and other measures during moderate exercise, as compared with the control condition. The 95% confidence intervals for the time based parameters were: TDp, 0.9, 0.8, and 0.8 s; τp, 3.9, 2.8, and 4.2 s (for CON, LOW, and HIGH, respectively). Although there were no significant differences in the kinetic parameters across the three conditions, differences were seen in end exercise heart rate and end exercise RER. Heart rate was significantly higher in HIGH (t18 = −3.8, P = 0.001) and in LOW (t18 = 4.4, P < 0.01) when compared with CON. The RER was significantly lower in both HIGH (t18 = 4.7, P < 0.01) and in LOW (t18 = 4.6, P < 0.01) conditions when compared with CON.
Table 2 outlines the parameters of the V̇O2 kinetics and other measures taken during heavy exercise under the three experimental conditions. The 95% confidence intervals for the time based parameters were: TDp, 1.6, 2.9, and 2.9 s; TDs, 14.4, 11.3, and 16.5 s;τp, 2.9, 3.8, and 3.4 s; and τs, 21.1, 26.1, and 26.2 s (for CON, LOW, and HIGH, respectively). There were differences again in end exercise heart rate (LOW vs CON, t18 = 4.0, P = 0.001), end exercise RER (HIGH vs CON, t18 = 3.1, P = 0.02, LOW vs CON, t18 = 4.4, P < 0.001) and the blood [lactate] (HIGH vs CON, t18 = 3.8, P = 0.001) across the experimental conditions. There were no differences in the V̇O2 kinetic responses after LOW. However, there were differences after HIGH, most notably in the amplitudes of the V̇O2 primary and V̇O2 slow components. The gain of the V̇O2 primary component (Gp) was significantly higher after HIGH (9.4 ± 0.2 mL·min−1·W−1) compared with CON (8.8 ± 0.2 mL·min−1·W−1, t18 = −4.3, P = 0.001). The increase in the V̇O2 primary component was accompanied by a decrease in the amplitude of the V̇O2 slow component (As′) after HIGH both in absolute terms (0.18 ± 0.03 L·min−1 compared with 0.24 ± 0.04 L·min−1 in CON, t 18 = 3.5, P = 0.003) and when expressed relative to the net increase in V̇O2 at end exercise (8.9 ± 1.3% vs 12.5 ± 1.7 in CON). The onset of the V̇O2 slow component (TDs) was also later in exercise after HIGH (127.4 ± 7.9 s compared with 110.9 ± 6.9 s in CON, t18 = −2.8, P = 0.0015). Though there were differences in the V̇O2 primary and V̇O2 slow components across conditions, EE V̇O2 was unchanged (~1.9L·min−1).
The results of this study indicate that altering the muscle glycogen stores of the exercising muscles leads to changes in the V̇O2 kinetic response during heavy (but not moderate) exercise. Specifically, glycogen depletion of the Type II muscle fibers resulted in an increase in the amplitude of the primary component, and a decrease in the amplitude and a delay in the onset of the V̇O2 slow component compared with the control condition. However, there were no differences after glycogen depletion of the Type I fibers compared with the control condition.
Validation of the glycogen depletion regimes.
On examination of the PAS staining of the muscle sections (Fig. 2), it can be clearly seen that the exercise depletion protocols were successful in targeting the specific muscle fiber pools. The pattern of glycogen depletion across the Type I, IIa, and IIx muscle fibers was similar to that presented in the studies on which the current protocols were based (11,28). The absolute reduction in glycogen concentration was also similar to previous work during short duration, supramaximal exercise (28), and prolonged submaximal exercise (29).
Though biopsies were not taken in stage 2, we are confident that similar patterns of muscle glycogen storage were present in the participants before the square-wave exercise bouts. The data collected during the depletion protocols in stage 1 and stage 2 were very similar, showing the expected trends in heart rate, RER, and blood lactate. Also, diet was carefully recorded and monitored over the duration of the study and records indicate that subjects maintained a consistent diet between trials. Furthermore, the measures taken during the square-wave exercise support differences in fuel supply and metabolism across conditions. During the square-waves after both LOW and HIGH, RER and Δ [lactate] were significantly lower, and heart rate was significantly higher indicating a shift toward fat metabolism (13).
Oxygen uptake kinetic response after HIGH.
The impact of the HIGH condition on the V̇O2 response followed the hypothesized pattern. The amplitude of the V̇O2 primary component was significantly larger, and the amplitude of the V̇O2 slow component significantly smaller after HIGH compared with CON. This type of repeated, high-intensity exercise is known to recruit the Type II muscle fiber pools (11,28), and depletion of these fibers was hypothesized to shift recruitment toward Type I fibers during subsequent exercise.
Glycogen depletion of the Type II muscle fibers leading to a higher primary gain during exercise draws interesting parallels to the work of Barstow and coworkers. Barstow et al. have reported the percent Type I fibers in the vastus lateralis to be significantly related to the primary component gain in constant-load (1) and to the Δ V̇O2/Δ work rate slope in ramp (2) exercise. Indeed, a similar relationship between percent Type I muscle fibers and the primary gain term was found in the present data set (N = 12, r = 0.73, P = 0.03). Barstow and coworkers suggest that in subjects with a high proportion of Type II fibers, who have a lower primary component V̇O2, this reflects an initial underestimation of the necessary motor units to sustain power output beyond a couple of minutes. This in turn leads to recruitment of more Type II fibers and development of the V̇O2 slow component (1). It must be considered that an individual’s fiber type proportion provides no information on whether or not those fibers are being recruited during exercise. However, it seems reasonable to assume that a person is more likely to recruit a specific fiber pool if those fibers exist in abundance.
In the present study, the V̇O2 slow component was significantly reduced after HIGH. The development of the V̇O2 slow component has been attributed to the recruitment of Type II muscle fibers as exercise progresses (10). Glycogen depletion studies show the Type I fibers to be recruited first during exercise, with Type II fibers being recruited as exercise intensity increases (11). In vitro, Type II fibers appear to be less efficient than Type I fibers. Type II fibers produce more heat and consume more oxygen for the same tension development (31). Calcium pump activity is 5–10 times greater, as is actin myosin turnover (9). Isolated mitochondria from Type II fibers exhibit an 18% lower phosphate produced per unit of oxygen consumed (32), thus predicting a higher V̇O2 for a given ATP resynthesis rate. It was therefore hypothesized in the present study that depletion of Type II fibers after the high-intensity exercise protocol would lead to a reduced reliance early in exercise on recruitment of Type II fibers and this would in turn decrease the amplitude of the V̇O2 slow component. Indeed, this may explain the significant delay in the onset of the V̇O2 slow component (TDs) after HIGH.
Drawing parallels with the present study, the performance of prior heavy exercise also results in an increase in the V̇O2 primary gain during subsequent heavy exercise concomitant with a trend of an increase in integrated EMG activity during the first 2 min of the second bout of exercise (6). However, other EMG studies have provided equivocal data on the relationship between neuromuscular activity and V̇O2 kinetics, with some authors reporting increases in EMG concomitant with the development of the V̇O2 slow component (4,20,24), and others reporting no clear relationship (25,26). Although the exact mechanism by which motor unit recruitment patterns influence the V̇O2 response remains unclear, further evidence in support of a causal relationship was recently provided by Pringle et al. (23). These authors demonstrated a high pedal rate (which the authors hypothesized would increase Type II fiber recruitment) resulted in a lower primary component gain and an increased amplitude of the V̇O2 slow component compared with the control condition (23).
Although it is impossible to rule out the possibility that factors other than glycogen depletion, per se, were partly responsible for the differences after the HIGH glycogen depletion protocol, we consider this to be unlikely. The concentrations of metabolites that might affect respiration, for example, ammonia (12), lactate, and potassium (14), are known to return to baseline values after an hour of recovery in both the muscle and the blood. Muscle temperature has also been documented to return to resting values in this time period (15). The similarity of the baseline V̇O2 (see Table 1) before all conditions adds support to this notion.
Oxygen uptake kinetic response after LOW.
After LOW and the depletion of the Type I muscle fibers in the present study, the amplitude of the V̇O2 slow component tended to be greater than in CON, but this difference was not statistically significant. The primary component amplitude was not different from that seen in CON. It is difficult to explain why the LOW condition was not successful in significantly altering the V̇O2 response. It is possible that due to the capacity of the Type I fibers to resist fatigue, the depletion protocol, though reducing the glycogen content of the muscle fibers did not render them “unrecruitable.” Because muscle fiber recruitment cannot be measured directly, it can only be inferred that the reduction in glycogen stores led to alternative fibers being recruited during exercise. Type I muscle fibers are better suited to fat oxidation than the Type II fibers (29). Fat oxidation will also lead to a higher V̇O2 for a given power output than carbohydrate metabolism (13). Therefore, it is possible that any reduction in the V̇O2 primary component due to greater Type II recruitment may have been counterbalanced by the increased O2 cost of fat metabolism in the glycogen depleted Type I fibers. This would potentially lead to no net effect. It should also be noted that the LOW depletion protocol did not lower the muscle glycogen concentrations to the same extent as in the HIGH condition (a 44% decrease in LOW compared with 55% for HIGH).
As stated previously, this study design makes the assumption that glycogen depleted fibers will not be recruited during subsequent exercise. It is not known whether a depleted fiber would be unusable during exercise below a “threshold” of glycogen concentration or whether the fiber would still be recruited but not contribute significantly to the power production. Indeed, it is possible that other factors present in a “fatigued” muscle partially account for the proposed changes in muscle fiber recruitment rather than, or in addition to, the muscle glycogen concentration per se. As suggested by Lacombe et al. (16), decreased availability of muscle glycogen stores likely reduces the rate of anaerobic glycogenolysis and, therefore, limits anaerobic ATP synthesis and prevents ADP homeostasis at the contractile site. Therefore, in the present study, all of these mechanisms of interaction between muscular fatigue and glycogen availability may contribute to the changes in the oxygen uptake response observed in the glycogen-depleted state.
Although fiber type depletion influenced the primary and slow components of the V̇O2 response during heavy exercise, the end exercise V̇O2 (at 6 min) was not significantly different between conditions (see Table 2). This may simply be a coincidence, and, indeed, it is possible that the end-exercise V̇O2 would have been different if the exercise bouts had been extended. However, although we did not measure V̇O2 beyond 6 min, we note from the modeled response that the extrapolated asymptote of the EEV̇O2 was the same across conditions (2.04 ± 0.58 in CON, 2.08 ± 0.67 in LOW and 2.05 ± 0.61 in HIGH).
It was interesting to note that the magnitude and direction of effect was dependent on the absolute change in muscle glycogen concentration from the prior exercise (Fig. 3). In the group of 12 participants that completed stages 1 and 2, those individuals with a greater depletion tended to show a greater reduction in the V̇O2 primary gain (panel A) and increase in the V̇O2 slow component after LOW (panel B); and a higher V̇O2 primary gain (panel C) and smaller V̇O2 slow component after HIGH (panel D). This relationship supports our assumption that glycogen depletion does impact on muscle fiber recruitment: the greater the reduction in the substrate, the greater was the experimental effect.
In conclusion, the present study has shown an alteration in the V̇O2 kinetic response subsequent to muscle fiber pool specific glycogen depletion. The increase in the gain of the V̇O2 primary component and reduction in the V̇O2 slow component after glycogen depletion of the Type II muscle fibers supports the contention that motor unit recruitment patterns influence the V̇O2 response during heavy constant-load exercise.
The authors would like to thank Alison Carlisle for invaluable laboratory assistance during the muscle tissue analysis.
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