Magnus Bak Klaris, Claes Cubel, Tim Ravn Bruun, Daniel Stampe, Stian Rørvik, Mads Fischer, Thomas Bonne, Peter M. Christensen, Jacob Feder Piil, Lars Nybo

Funding: The study was supported by Team Danmark through the Novo Nordisk Foundation grant to Team Danmark (Grant NNF.22SA0078293).

Please read the original article on: https://onlinelibrary.wiley.com/doi/10.1111/sms.14699

Abstract

Fatigue resistance is vital for success in elite road cycling, as repeated, intense efforts challenge the athletes' ability to sustain peak performance throughout prolonged races. The present study combined recurrent performance testing and physiological measures during 6 h simulated racing with laboratory testing to investigate factors influencing fatigue resistance.

Discussion

The present study reveals physiological responses and fatigue patterns in national elite cyclists during a novel 6 h simulated race that included paced peloton riding, intense rolling turns, peak sprints, and four maximal 5 km TT efforts—in total, covering a distance of ~220 km and accumulating an external workload of ~5000 kJ, representing a physiological load comparable to major professional road cycling races [9].

Peak sprint power declined by ~7% after the first 2 h of racing but with no further reduction in the subsequent 4 h of the field test. In contrast, the loss of TT power was modest from 0 to 2 h and from 2 to 4 h of racing, with an average loss of 7 W in each 2 h block. However, the loss of TT power aggravated from the fourth to sixth hour, with a doubling of the mean power loss to 14 W, corresponding to a mean 7% total decline in mean power output from 0 to 6 h. The loss of power during the last 2 h block was also associated with an increased differentiation between athletes. The most fatigue-resistant individuals were able to confine the decline in TT power to less than 3% over the entire field test, and for half of the present group of elite cyclists, it was below 5%.

The physiological measurement revealed increased fat oxidation from pre- to posttest at the intensity eliciting MFO in fasted conditions, although the riders ingested high amounts of carbohydrates. Interestingly, the fatmax intensity was similar to the mean power output during the periods with paced peloton riding, indicating that fat metabolism, even in conditions with very high glucose ingestion, as practiced in competitions, may cover a substantial part of the energy turnover in certain phases of prolonged races. We also observed a gradual lowering of the peak lactate levels following all-out efforts, indicating that fatigue was accompanied by reduced glycolysis, resulting in lower lactate spillover.

Overall Fatigue Responses

In opposition to our initial hypothesis, peak sprint power was already lowered after the first 2 h of racing, but with no further decline from 2 to 6 h, suggesting that early effects on muscle homeostasis or neural drive significantly affected force production. In contrast, the similar peak power in the second, fourth, and sixth hour sprint tests indicates that further alteration in muscle homeostasis—for example, glycogen depletion was not critical for neuromuscular function under these conditions. However, the lowered availability of muscle glycogen may have impacted the 5 km TT performances and explained the progressive decrease in TT power. The short 5 km TT test was selected to tax maximal anaerobic and aerobic capacities, and the noticeable decrease in post-TT blood lactate concentrations and reduced heart rate during the TTs is indicative of reduced anaerobic energy contribution as well as diminished capacity to achieve maximal heart rate, indicating lower aerobic energy contribution. Considering that urination contributed to the overall weight loss (1.3 kg on average, i.e., less than 2% of their body mass) and in reflection of the ambient temperature between 2°C and 5°C during the field test, the influence from dehydration and potential carry-over effects on thermal stress is considered as negligible.

Individual Fatigue Patterns

When the individual TT performances are analyzed in relation to accumulated work (kJ/kg) or time spent above the lactate threshold (see Figure 5A,B), it appears that both factors may have influenced fatigue. However, considered in the context of recent work by Leo et al. [11] and Spragg et al. [13] and supported by correlation analysis on the individual data, showing a moderate correlation between the individual loss of TT power and time spent above LT1, it appears that intense work (time above LT1) rather than accumulated work is indicative of individual fatigue patterns [11-13, 15]. It should be noted that statistical analyses also accounting for time above FTP or including an integrated Training Stress Score (TSS) index did not significantly strengthen the predicting power of individual fatigue patterns. However, this may be attributed to the absence of formal FTP, leading to a limited sensitivity in detecting FTP. Furthermore, when examining potential predictors of fatigue development, including laboratory-derived physiological measurements, fresh-state performance metrics, and training data from previous months, we found no correlations that could explain the individual declines in TT power from 0 to 6 h. This included analyses accounting for the rider's absolute and relative intensity during race simulations (power normalized to iPPO, LT or as a percentage of mean power output during the first TT). Furthermore, no correlations were observed between the decline in TT power and carbohydrate intake during the entire field test or intake during the last 2 h of testing.

Physiological Attributes and Metabolic Responses

A novel finding in our field study was that fat oxidation at the submaximal intensity, eliciting MFO in fasted conditions, markedly increased from baseline to values above fasting conditions postexercise despite very high carbohydrate intake before and during exercise. Notably, the ability to oxidize fat in a rested state, as indicated by MFO fasted and MFO at 0 h, did not correlate with the ability to oxidize fat after 6 h of race simulation, in agreement with recent findings from laboratory experiments [22]. This suggests that a high MFO in a fasted state does not necessarily translate into a high MFO during prolonged exercise, and the practical value of fatmax testing in a fasted and recovered state thus can be questioned, as it may not accurately reflect MFO throughout an entire workout or race involving carbohydrate consumption. Although the correlation between declines in TT power and MFO after 6 h at the individual level only tended to be significant (as our study has limited power for detecting such correlations), the measured fat oxidation rate of 1.1 ± 0.1 g/min at 238 ± 26 W after 6 h implies that fat oxidation may account for approximately 60% of the total EE during lower-intensity phases. The high-fat oxidation rates may, combined with the consumption of ~90 g carbohydrates per hour, be sufficient to fuel most of the metabolic needs during the less intense periods of professional races (as reported by e.g., Teun van Erp, 2021) and here simulated by the phases with peloton riding. Therefore, enhanced fat oxidation capacity during prolonged exercise, as indicated by the high MFO at 6 h, may be relevant for durability and resilience to fatigue in elite cycling races with a duration equal to or exceeding the present study.

Limitations

It should be noted that the present study, conducted on a flat course with riders specialized in flat racing, may not cover all types of road race competitions or rider profiles. Tactics and pacing strategies in different races may be influenced by environmental factors like heat or hypoxic stress or the terrain with an associated impact on power distribution and fatigue patterns. Furthermore, different rider types, such as sprinters versus climbers, might be affected differently by the same protocol [7].

Furthermore, the present study involved semi-professional cyclists, and it could be considered that the upper quartile of world tour riders participating in major international World Tour one-day races and grand tours may display even higher fatigue resistance. However, the most fatigue-resistant riders in the present study demonstrated a ~2% decline in the 5 km TT power output comparable to observations in top grand tour contenders [10]. Additionally, categorizing riders by functional power output metrics like peak sprint power, FTP, or critical power [33] rather than VO2max measurements, the subjects in the present study would be ranked around the 50th percentile among professional cyclists [34], confirming their competitive elite status.

Perspectives

The present findings may impact both future research in the durability field and provide knowledge relevant to practitioners. For example, our findings indicate that high-fat oxidation can be achieved without nutritional strategies seeking to compromise carbohydrate availability or training strategies, where cyclists avoid high-intensity intervals during prolonged rides to stay in a certain fat oxidation zone. Training of peak aerobic power and metabolic fitness to achieve high LT-power, respectively, “performance power” in the fresh state may still be considered as the top priority, as it both benefits performance in the initial phases of races and makes riders less vulnerable to accumulate time in the high-intense domain.

In addition to a better understanding of specific physiological factors relevant for resilience to fatigue in prolonged endurance events, future studies could explore how different tapering strategies affect durability. Tapering with a reduction in training volume is known to benefit performance in the fresh state but is not consequently practiced by professional cyclists. Perhaps because the tapering effect is different in terms of preventing fatigue in stage races or single-day competitions with duration matching the present study. Studies including biopsies for the assessment of muscle glycogen utilization, as well as evaluations of enzymatic activity levels, may, in this context, be relevant for further exploration.

Conclusion

In this study of national elite cyclists, we observed fatigue patterns in 5 km TT power outputs and sprint peak power, alongside physiological measures, during 6 h of simulated road racing. Peak sprint power was reduced after 2 h of racing, with no further declines observed from 2 to 6 h. In contrast, TT power steadily decreased over the entire 6 h protocol, with increased differentiation between moderate and high fatigue-resistant athletes from four to 6 h. This gradual decrease in TT power was associated with reductions in maximal and average heart rate, as well as lower blood lactate concentrations. Fat oxidation rates increased during the 6 h of exercise despite the high intake of carbohydrates before and during the field test.