What's Cool In Road Cycling

Durability and Fatigue with Prolonged Cycling

How does your fitness change over a long ride?

Do you find yourself getting stronger over the course of a long ride, or do you find yourself fading after a couple of hours? What is durability, and is it another major pillar of fitness?

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Durability, an emerging area in cycling science, seeks to unravel the relationship between duration, intensity, and an athlete’s performance. Durability can be defined as “…the time of onset and magnitude of deterioration in physiological characteristics over time during prolonged exercise” (Maunder et al., 2021). Simply put, it’s the measure of how well an athlete maintains performance over extended periods. Picture this: after a multi-hour zone 2 or a grueling interval session, how much would your Functional Threshold Power (FTP) drop compared to when you’re fresh? Theoretically, athletes with greater durability should experience less decline in performance over time.

The study under review from Stevenson et al. (2022) represents an important contribution to this rapidly evolving field. Despite the inherent complexity of cycling ‘durability’, the study’s design is refreshingly straightforward. It aims to explore the impact of prolonged moderate-intensity cycling – think Zone 2 – on the aerobic threshold (AeT), the intensity at which the body begins to rely more on anaerobic metabolism. It’s crucial to note that AeT differs from the anaerobic threshold (AnT), often determined by metrics like FTP or Critical Power (CP). With that clarified, let’s delve into how the researchers tackled their investigation.

What was the primary goal of this study?

This study sought to examine how prolonged cycling at moderate intensity affects the power output and heart rate of the aerobic threshold.

Who participated?

Interestingly, I found that the participants who participated were similar to myself in terms of age, relative VO2max, and training volume. The researchers enlisted 14 athletes for this study, including 13 males & 1 female (34 ± 10 years old). Notably fit, the athletes boasted an average relative VO2max of 59.9 ± 6.8  mL * Kg−1 * min−1. Their dedication to training was evident, with an average weekly commitment of 9 hours (± 3).

What did they do? 

During the study, athletes made two separate visits to the lab, spaced one week apart. The initial visit involved a comprehensive characterization trial, commencing with a maximal, incremental cycling test conducted after an overnight fast to establish baseline parameters.

On the subsequent visit, athletes underwent a prolonged cycling assessment. This session involved estimating the first ventilatory threshold (VT1), synonymous with the aerobic threshold, both before and after two hours of moderate-intensity cycling, again following an overnight fast. Notably, athletes were permitted water intake but refrained from consuming any food during the entire two-hour cycling session.

Figure 1. Schematic of the two visits. Taken from Stevenson, et al. 2022.

What did they measure?

On the initial visit, athletes underwent a two-stage ramp test. The first stage, featuring a 35 W increment every 3 minutes, served to determine their first ventilatory threshold (VT1) along with the corresponding power output. At VT1, athletes sustained a mean power output of 216 ± 45 W. Following this, the second stage of the ramp test involved incremental increases of 35 W per minute until reaching failure, aimed at gauging the athletes’ VO2max.

During the subsequent visit, athletes completed the slow stage of the ramp test twice – once before (PRE) and once after (POST) two hours of riding at a moderate intensity. The workload for the moderate-intensity ride was set at 90% of the power output at VT1 determined during the first visit, averaging 194 ± 41 W for the athletes. To contextualize, this workload corresponds to riding at the upper boundary of ‘Zone 2’ (approximately 75% of FTP) for an athlete with an FTP of 260 W.

Throughout the two-hour moderate-intensity ride, researchers monitored various parameters including heart rate (HR), VO2, whole-body energy expenditure (EE), and respiratory exchange ratio (RER), which can be used to measure carb & fat oxidation. This comprehensive data collection allowed researchers to speculate on the specific changes in substrate utilization and overall metabolic efficiency during prolonged moderate-intensity cycling, potentially influencing alterations in the aerobic threshold.

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What did they find?

The primary outcome revealed a predictable yet significant decrease in the aerobic threshold, as measured by power at VT1, declining from 217 ± 42 W (PRE) to 196 ± 42 W (POST). This translated to a relative decrease of 10.0 ± 5.8%. Additionally, the authors observed a rise in heart rate at VT1, escalating from 142 ± 9 to 151 ± 12 beats per minute, as illustrated in the figure below.

To provide additional context for these results, let’s return to the example mentioned previously. If we consider an average FTP of 260 W for the athletes involved in this study, an individual beginning a ride at the upper boundary of Zone 2 (194 W, or approximately 75% of FTP) with an emphasis on predominantly aerobic efforts might observe a reduction in their effort to around ~68% of FTP (176 W) to sustain that aerobic pace. This adjustment would likely be accompanied by an elevation in their heart rate, particularly if they started the ride in a fasted state.

Figure 2. Change in power output at VT1 from PRE to POST. Taken from Stevenson, et al. 2022.

The decline in the aerobic threshold prompts the question: what factors contribute to this decrease? Here, the additional data provided by the authors proves invaluable, offering insights into the potential drivers behind the observed decline in performance.

Throughout the two-hour period of constant work-rate cycling, athletes experienced a significant increase in heart rate. Although rates of whole-body energy expenditure (EE) and oxygen consumption (VO2) showed an upward trend, they did not reach statistical significance. Notably, respiratory exchange ratio (RER) remained unaffected during the constant work-rate cycling. Taken together, the authors suggest that a decrease in energetic efficiency likely contributes to the decline in the aerobic threshold.

In simpler terms, athletes are expending more oxygen and energy to sustain the same level of effort (194 ± 41 W) during the two hours of moderate-intensity cycling. This inference is supported by the relative increase in both EE and VO2 during the two-hour constant work-rate effort, as depicted in the figure panels B, & C below. The authors attribute this decreased efficiency to the recruitment of less efficient muscle fibers, as outlined in previous studies (Jones et al., 2011).

Figure 3. Changes in HR, EE, VO2, & RER throughout 2 hours of cycling at a constant moderate effort Figure taken from Stevenson, et al. 2022.

The authors further propose that the remaining portion of the reduction in VT1 might also be attributed to carbohydrate depletion. This inference is supported by the observed increase in whole-body fat oxidation rate from PRE to POST, as illustrated in Figure 3 below. Such an increase indicates a heightened reliance on fat as a fuel source after two hours of moderate-intensity cycling. Given that the ride commenced in a fasted state and athletes were prohibited from consuming any food during the ride, this explanation appears plausible.

Figure 4. Whole body fat oxidation rate during the 3 stages of the slow ramp PRE & POST 2 hours moderate intensity cycling. Taken from Stevenson, et al. 2022.

Key Takeaway & Future Directions

In summary, this study underscores the significant reduction in power at VT1 following two hours of moderate-intensity cycling, likely driven by decreased efficiency and glycogen depletion.

Of particular interest is the discussion point regarding peak fat oxidation (PFO) as a marker of an individual’s capacity for fat oxidation during exercise. Despite the intuitive assumption that greater fat oxidation capacity might correlate with enhanced durability, the authors found no evidence supporting this notion. This raises intriguing questions about the interplay between fat metabolism, glycogen depletion, and durability, warranting further investigation. For my own curiosity, I would like to see the results of a similar study if athletes were provided a standardized breakfast, as well as consuming 30, 60, or 100 g/hr of carbohydrates during the 2 hour constant work rate stage… would this increase an athlete’s durability?

It’s worth noting the variability in the decrease of power at VT1 among athletes, ranging from 9 W to 44 W. This highlights the individual differences in durability and underscores the ongoing need for more research in this area.

That concludes this month’s discussion! Stay safe, ride fast, and I’ll see you next month!

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Jones AM, Grassi B, Christensen PM et al (2011) Slow component of VO2 kinetics: mechanistic bases and practical applications. Med Sci Sports Exerc 43:2046–2062

Maunder E, Seiler S, Mildenhall MJ, Kilding AE, Plews DJ. The Importance of ‘Durability’ in the Physiological Profiling of Endurance Athletes. Sports Med. 2021 Aug;51(8):1619-1628.

Maunder E, Plews DJ, Kilding AE (2018) Contextualising maximal fat oxidation during exercise: determinants and normative values. Front Physiol 9:1–13.

Stevenson JD, Kilding AE, Plews DJ, Maunder E. Prolonged cycling reduces power output at the moderate-to-heavy intensity transition. Eur J Appl Physiol. 2022 Dec;122(12):2673-2682.

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