Fatigue During High-Intensity Exercise: Relationship to the Critical Power Concept

The hyperbolic power-duration relationship for high-intensity exercise is defined by two parameters: an asymptote (critical power; CP) reflecting the highest sustainable rate of oxidative metabolism, and a curvature constant (W'), which indicates a fixed amount of work that can be completed above CP (W>CP). According to the CP model of bioenergetics, constant work rate exercise above CP depletes the capacity-limited W' with fatigue occurring when W' is completely expended. The complete depletion of W' has been reported to occur when VO2max is attained and a critical degree of muscle metabolic perturbation (decline of finite anaerobic substrates and accumulation of fatigue-related metabolites) is reached. However, while the CP model is effective at predicting metabolic perturbation and the tolerable duration of severe-intensity constant work rate (CWR) exercise, it is unclear if metabolic perturbation and exercise performance can be explained by the CP model when different methods of work rate imposition are applied. Therefore, the purpose of this thesis was to: 1) investigate the efficacy of the CP concept to predict performance in exercise tests using different work rate forcing functions; and 2) explore whether the physiological bases for W' are consistent across different methods of work rate imposition. In study 1, compared to severe-intensity CWR exercise, the tolerable duration of intermittent severe-intensity exercise with heavy- (S-H) moderate- (S-M) and light-intensity (S-L) ‘recovery’ intervals was increased by 47%, 100% and 219%, respectively. W>CP (W') was significantly greater by 46%, 98%, and 220% for S-H, S-M and S-L, respectively, when compared to S-CWR, and the slopes for the increases in VO2 and iEMG were progressively lowered as the recovery work rate was reduced. In study 2, both the VO2max and W>CP were similar across incremental cycling protocols that imposed a fixed ramp rate and cadence (4.33 ± 0.60 L•min-1; 14.8 ± 9.2 kJ), a fixed ramp rate with cadence self-selected by the subjects (4.31 ± 0.62 L•min-1; 15.0 ± 9.9 kJ) and a step incremental test where subjects were instructed to select power output according to prescribed increments in ratings of perceived exertion (4.36 ± 0.59 L•min-1; 13.0 ± 8.4 kJ). In study 3, the VO2max and W>CP were also not different across a 3 min all-out cycling test (4.10 ± 0.79 L•min-1; 16.5 ± 4.0 kJ), cycling at a constant work rate predicted to lead to exhaustion in 3 min until the limit of tolerance (4.20 ± 0.77 L•min-1; 16.6 ± 7.4 kJ) and a self-paced 3 min work-trial (4.14 ± 0.75 L•min-1; 15.3 ± 5.6 kJ). In study 4, after completing severe-intensity exercise (>CP) to exhaustion, muscle homeostasis ([PCr], pH, [ADP] and [Pi]) returned towards baseline and subjects were able to exercise for at least 10 min at a heavy-intensity work rate (CP), muscle metabolites ([PCr], pH, [ADP] and [Pi]) did not recover and exercise tolerance was severely limited (39 ± 31 s). Finally in study 5, during severe-intensity intermittent knee extension exercise, the tolerable duration of exercise was 304 ± 68 s when 18 s recovery was allowed and was increased by ~69% and ~179% when the intermittent recovery periods were extended to 30 s and 48 s, respectively. The increased exercise tolerance with longer recovery periods occurred in concert with increased W>CP (3.8 ± 1.0 kJ, 5.6 ± 1.8 kJ and 7.9 ± 3.1 kJ for the intermittent protocols with 18, 30 and 48 s of recovery, respectively) and a delayed attainment of critical intramuscular metabolite concentrations ([PCr], pH, [ADP] and [Pi]). Therefore, the results of this thesis demonstrate that fatigue during various high-intensity exercise protocols is influenced by the capacity to complete work above the CP (W') and that W' depletion is linked to the attainment of VO2max and the attainment of critical levels of intramuscular [PCr], pH, [ADP] and [Pi]. These findings suggest that the CP model can be adapted to predict the degree of metabolic perturbation and exercise performance across a range of exercise settings in humans

Application of the speed-duration relationship to normalize the intensity of high-intensity interval training

The tolerable duration of continuous high-intensity exercise is determined by the hyperbolic Speed-tolerable duration (S-tLIM) relationship. However, application of the S-tLIM relationship to normalize the intensity of High-Intensity Interval Training (HIIT) has yet to be considered, with this the aim of present study. Subjects completed a ramp-incremental test, and series of 4 constant-speed tests to determine the S-tLIM relationship. A sub-group of subjects (n = 8) then repeated 4 min bouts of exercise at the speeds predicted to induce intolerance at 4 min (WR4), 6 min (WR6) and 8 min (WR8), interspersed with bouts of 4 min recovery, to the point of exercise intolerance (fixed WR HIIT) on different days, with the aim of establishing the work rate that could be sustained for 960 s (i.e. 4×4 min). A sub-group of subjects (n = 6) also completed 4 bouts of exercise interspersed with 4 min recovery, with each bout continued to the point of exercise intolerance (maximal HIIT) to determine the appropriate protocol for maximizing the amount of high-intensity work that can be completed during 4×4 min HIIT. For fixed WR HIIT tLIM of HIIT sessions was 399±81 s for WR4, 892±181 s for WR6 and 1517±346 s for WR8, with total exercise durations all significantly different from each other (P<0.050). For maximal HIIT, there was no difference in tLIM of each of the 4 bouts (Bout 1: 229±27 s; Bout 2: 262±37 s; Bout 3: 235±49 s; Bout 4: 235±53 s; P>0.050). However, there was significantly less high-intensity work completed during bouts 2 (153.5±40. 9 m), 3 (136.9±38.9 m), and 4 (136.7±39.3 m), compared with bout 1 (264.9±58.7 m; P>0.050). These data establish that WR6 provides the appropriate work rate to normalize the intensity of HIIT between subjects. Maximal HIIT provides a protocol which allows the relative contribution of the work rate profile to physiological adaptations to be considered during alternative intensity-matched HIIT protocols

Age differences in physiological responses to self-paced and incremental V˙O2max\dot V O_{2max} testing

Purpose: A self-paced maximal exercise protocol has demonstrated higher $\dot V O_{2max}$ values when compared against traditional tests. The aim was to compare physiological responses to this self-paced $\dot V O_{2max}$ protocol (SPV) in comparison to a traditional ramp $\dot V O_{2max}$ (RAMP) protocol in young (18–30 years) and old (50–75 years) participants. Methods: Forty-four participants (22 young; 22 old) completed both protocols in a randomised, counter-balanced, crossover design. The SPV included 5 × 2 min stages, participants were able to self-regulate their power output (PO) by using incremental ‘clamps’ in ratings of perceived exertion. The RAMP consisted of either 15 or 20 W min$^{−1}$. Results: Expired gases, cardiac output (Q), stroke volume (SV), muscular deoxyhaemoglobin (deoxyHb) and electromyography (EMG) at the vastus lateralis were recorded throughout. Results demonstrated significantly higher $\dot V O_{2max}$ in the SPV (49.68 ± 10.26 ml kg$^{−1}$ min$^{−1}$) vs. the RAMP (47.70 ± 9.98 ml kg$^{−1}$ min$^{−1}$) in the young, but not in the old group (>0.05). Q and SV were significantly higher in the SPV vs. the RAMP in the young (0.05). No differences seen in deoxyHb and EMG for either age groups (>0.05). Peak PO was significantly higher in the SPV vs. the RAMP in both age groups (<0.05). Conclusion: Findings demonstrate that the SPV produces higher $\dot V O_{2max}$, peak Q and SV values in the young group. However, older participants achieved similar $\dot V O_{2max}$ values in both protocols, mostly likely due to age-related differences in cardiovascular responses to incremental exercise, despite them achieving a higher physiological workload in the SPV

Physiological and anthropometric determinants of critical power, W ′ and the reconstitution of W ′ in trained and untrained male cyclists

From Springer Nature via Jisc Publications RouterHistory: received 2020-01-17, accepted 2020-07-31, registration 2020-08-01, pub-electronic 2020-08-09, online 2020-08-09, pub-print 2020-11Publication status: PublishedAbstract: Purpose: This study examined the relationship of physiological and anthropometric characteristics with parameters of the critical power (CP) model, and in particular the reconstitution of W′ following successive bouts of maximal exercise, amongst trained and untrained cyclists. Methods: Twenty male adults (trained nine; untrained 11; age 39 ± 15 year; mass 74.7 ± 8.7 kg; V̇O2max 58.0 ± 8.7 mL kg−1 min−1) completed three incremental ramps (20 W min−1) to exhaustion interspersed with 2-min recoveries. Pearson’s correlation coefficients were used to assess relationships for W′ reconstitution after the first recovery (W′rec1), the delta in W′ reconstituted between recoveries (∆W′rec), CP and W′. Results: CP was strongly related to V̇O2max for both trained (r = 0.82) and untrained participants (r = 0.71), whereas W′ was related to V̇O2max when both groups were considered together (r = 0.54). W′rec1 was strongly related to V̇O2max for the trained (r = 0.81) but not untrained (r = 0.18); similarly, ∆W′rec was strongly related to V̇O2max (r = − 0.85) and CP (r = − 0.71) in the trained group only. Conclusions: Notable physiological relationships between parameters of aerobic fitness and the measurements of W′ reconstitution were observed, which differed among groups. The amount of W′ reconstitution and the maintenance of W′ reconstitution that occurred with repeated bouts of maximal exercise were found to be related to key measures of aerobic fitness such as CP and V̇O2max. This data demonstrates that trained cyclists wishing to improve their rate of W′ reconstitution following repeated efforts should focus training on improving key aspects of aerobic fitness such as V̇O2max and CP

Elevated baseline work rate slows pulmonary oxygen uptake kinetics and decreases critical power during upright cycle exercise

Critical power is a fundamental parameter defining high-intensity exercise tolerance, and is related to the phase II time constant of pulmonary oxygen uptake kinetics (O2). Whether this relationship is causative is presently unclear. The present study determined the impact of raised baseline work rate, which increases O2, on critical power during upright cycle exercise. Critical power was determined via four constant-power exercise tests to exhaustion in two conditions: 1) with exercise initiated from an unloaded cycling baseline (U→S), and 2) with exercise initiated from a baseline work rate of 90% of the gas exchange threshold (M→S). During these exercise transitions, O2 and the time constant of muscle deoxyhaemoglobin kinetics (τ[HHb + Mb]) (the latter via near-infrared spectroscopy) were determined. In M→S, critical power was lower (M→S = 203 ± 44 W vs. U→S = 213 ± 45 W, P = 0.011) and O2 was greater (M→S = 51 ± 14 s vs. U→S = 34 ± 16 s, P = 0.002) when compared to U→S. Additionally, τ[HHb + Mb] was greater in M→S compared to U→S (M→S = 28 ± 7 s vs. U→S = 14 ± 7 s, P = 0.007). The increase inO2 and concomitant reduction in critical power inM→S compared to U→S suggests a causal relationship between these two parameters. However, that τ[HHb + Mb] was greater in M→S exculpates reduced oxygen availability as being a confounding factor. These data therefore provide the first experimental evidence that O2 is an independent determinant of critical power. Keywords critical power, exercise tolerance, oxygen uptake kinetics, power-duration relationship, muscle deoxyhaemoglobin kinetics, work-to-work exercise

Critical Power: An Important Fatigue Threshold in Exercise Physiology

The hyperbolic form of the power-duration relationship is rigorous and highly conserved across species, forms of exercise and individual muscles/muscle groups. For modalities such as cycling, the relationship resolves to two parameters, the asymptote for power (critical power, CP) and the so-called W' (work doable above CP), which together predict the tolerable duration of exercise above CP. Crucially, the CP concept integrates sentinel physiological profiles - respiratory, metabolic and contractile - within a coherent framework that has great scientific and practical utility. Rather than calibrating equivalent exercise intensities relative to metabolically distant parameters such as the lactate threshold or V[spacing dot above]O2 max, setting the exercise intensity relative to CP unifies the profile of systemic and intramuscular responses and, if greater than CP, predicts the tolerable duration of exercise until W' is expended, V[spacing dot above]O2 max is attained, and intolerance is manifested. CP may be regarded as a 'fatigue threshold' in the sense that it separates exercise intensity domains within which the physiological responses to exercise can (CP) be stabilized. The CP concept therefore enables important insights into 1) the principal loci of fatigue development (central vs. peripheral) at different intensities of exercise, and 2) mechanisms of cardiovascular and metabolic control and their modulation by factors such as O2 delivery. Practically, the CP concept has great potential application in optimizing athletic training programs and performance as well as improving the life quality for individuals enduring chronic disease

Fatigue during high-intensity exercise : relationship to the critical power concept

The hyperbolic power-duration relationship for high-intensity exercise is defined by two parameters: an asymptote (critical power; CP) reflecting the highest sustainable rate of oxidative metabolism, and a curvature constant (W'), which indicates a fixed amount of work that can be completed above CP (W>CP). According to the CP model of bioenergetics, constant work rate exercise above CP depletes the capacity-limited W' with fatigue occurring when W' is completely expended. The complete depletion of W' has been reported to occur when VO2max is attained and a critical degree of muscle metabolic perturbation (decline of finite anaerobic substrates and accumulation of fatigue-related metabolites) is reached. However, while the CP model is effective at predicting metabolic perturbation and the tolerable duration of severe-intensity constant work rate (CWR) exercise, it is unclear if metabolic perturbation and exercise performance can be explained by the CP model when different methods of work rate imposition are applied. Therefore, the purpose of this thesis was to: 1) investigate the efficacy of the CP concept to predict performance in exercise tests using different work rate forcing functions; and 2) explore whether the physiological bases for W' are consistent across different methods of work rate imposition. In study 1, compared to severe-intensity CWR exercise, the tolerable duration of intermittent severe-intensity exercise with heavy- (S-H) moderate- (S-M) and light-intensity (S-L) ‘recovery’ intervals was increased by 47%, 100% and 219%, respectively. W>CP (W') was significantly greater by 46%, 98%, and 220% for S-H, S-M and S-L, respectively, when compared to S-CWR, and the slopes for the increases in VO2 and iEMG were progressively lowered as the recovery work rate was reduced. In study 2, both the VO2max and W>CP were similar across incremental cycling protocols that imposed a fixed ramp rate and cadence (4.33 ± 0.60 L•min-1; 14.8 ± 9.2 kJ), a fixed ramp rate with cadence self-selected by the subjects (4.31 ± 0.62 L•min-1; 15.0 ± 9.9 kJ) and a step incremental test where subjects were instructed to select power output according to prescribed increments in ratings of perceived exertion (4.36 ± 0.59 L•min-1; 13.0 ± 8.4 kJ). In study 3, the VO2max and W>CP were also not different across a 3 min all-out cycling test (4.10 ± 0.79 L•min-1; 16.5 ± 4.0 kJ), cycling at a constant work rate predicted to lead to exhaustion in 3 min until the limit of tolerance (4.20 ± 0.77 L•min-1; 16.6 ± 7.4 kJ) and a self-paced 3 min work-trial (4.14 ± 0.75 L•min-1; 15.3 ± 5.6 kJ). In study 4, after completing severe-intensity exercise (>CP) to exhaustion, muscle homeostasis ([PCr], pH, [ADP] and [Pi]) returned towards baseline and subjects were able to exercise for at least 10 min at a heavy-intensity work rate (CP), muscle metabolites ([PCr], pH, [ADP] and [Pi]) did not recover and exercise tolerance was severely limited (39 ± 31 s). Finally in study 5, during severe-intensity intermittent knee extension exercise, the tolerable duration of exercise was 304 ± 68 s when 18 s recovery was allowed and was increased by ~69% and ~179% when the intermittent recovery periods were extended to 30 s and 48 s, respectively. The increased exercise tolerance with longer recovery periods occurred in concert with increased W>CP (3.8 ± 1.0 kJ, 5.6 ± 1.8 kJ and 7.9 ± 3.1 kJ for the intermittent protocols with 18, 30 and 48 s of recovery, respectively) and a delayed attainment of critical intramuscular metabolite concentrations ([PCr], pH, [ADP] and [Pi]). Therefore, the results of this thesis demonstrate that fatigue during various high-intensity exercise protocols is influenced by the capacity to complete work above the CP (W') and that W' depletion is linked to the attainment of VO2max and the attainment of critical levels of intramuscular [PCr], pH, [ADP] and [Pi]. These findings suggest that the CP model can be adapted to predict the degree of metabolic perturbation and exercise performance across a range of exercise settings in humans.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Slowing the Reconstitution of W′ in Recovery With Repeated Bouts of Maximal Exercise

Accepted author manuscript version reprinted, by permission, from International Journal of Sports Physiology and Performance, 2019, 14 (2): 149-155,https://doi.org/10.1123/ijspp.2018-0256. © Human Kinetics, Inc.Purpose: This study examined the partial reconstitution of the work capacity above critical power (W′) following successive bouts of maximal exercise using a new repeated ramp test, against which the fit of an existing W′ balance (W'bal) prediction model was tested. Methods: Twenty active adults, consisting of trained cyclists (n = 9; age 43 [15] y, V˙ O2max 61.9 [8.5] mL·kg−1·min−1) and untrained cyclists (n = 11; age 36 [15] y, V˙ O2max 52.4 [5.8] mL·kg−1·min−1) performed 2 tests 2 to 4 d apart, consisting of 3 incremental ramps (20 W·min−1) to exhaustion interspersed with 2-min recoveries. Results: Intratrial differences between recoveries demonstrated significant reductions in the amount of W′ reconstituted for the group and both subsets (P < .05). The observed minimal detectable changes of 475 J (first recovery) and 368 J (second recovery) can be used to monitor changes in the rate of W′ reconstitution in individual trained cyclists. Intertrial relative reliability of W′ reconstitution was evaluated by intraclass correlation coefficients for the group (≥.859) and the trained (≥.940) and untrained (≥.768) subsets. Absolute reliability was evaluated with typical error (TE) and coefficient of variation (CV) for the group (TE ≤ 559 J, CV ≤ 9.2%), trained (TE ≤ 301 J, CV ≤ 4.7%), and untrained (TE ≤ 720 J, CV ≤ 12.4%). Conclusions: The reconstitution of W′ is subject to a fatiguing effect hitherto unaccounted for in W'bal prediction models. Furthermore, the W'bal model did not provide a good fit for the repeated ramp test, which itself proved to be a reliable test protocol

Effects of high intensity interval training on peak aerobic power output and time trial performance in Thai amateur cyclists

The purpose of this study was to investigate the effects of high-intensity interval training (HIT) on peak aerobic power output (PAP) and time trial (TT) performance of Thai amateur cyclists. Twenty-nine male amateur cyclists were randomly allocated to one of two groups, a moderate-intensity continuous training (MICT) group (n = 14) and a HIT group (n = 15). All subjects performed an incremental exercise test to exhaustion and a 30 km TT to determine the PAP, lactate turnpoint (LTP) and endurance performance before (pre-test) and after the six-week training period (post-test). The HIT group completed ten intervals of 2 min at 120% of LTP with 4 min of rest between intervals, 3 times a week. The MICT group completed three sessions per week of 60 min cycling at 60-75% of the maximum heart rate. Both the HIT and MICT groups also completed one session per week of 120-minute continuous training at 60% LTP. The HIT and MIT training programs were six weeks in duration. Both PAP and performance in the 30 km TT were improved post training in the HIT group (p0.05). The present study suggests that a HIT program was more effective at improving PAP and TT performance of Thai amateur cyclists compared to conventional MICT program