Field Testing with XCO MTB Racer Carter Hall (part 1)
Testing in a ‘lab-setting’ provides many benefits such as controlled environmental conditions (temperature, humidity, altitude, etc.) and precise power control using an ergometer (ERG). However, pushing watts at a fixed resistance on an ergometer is simply different from outdoor riding. This is especially true as the workload gets higher. If you’ve tried to replicate your outdoor power numbers on an ERG (i.e. indoor trainer), you will know that you cannot produce nearly as many watts as outside on your bike. In this article, you will learn about how we are using field testing in elite-level sport.
Carter Hall is a professional cross-country mountain bike racer and coach who recently competed in his first World Cup in Snowshoe, West Virginia. For an athlete of this caliber, the planning and execution of his training is of utmost importance to be competitive against the best riders in the world. To understand how his physiology is operating during his peak-fitness of the year, we put together a day of field testing to ‘profile’ several key variables of performance including VO2max, VO2 kinetics, Lactate Threshold, and Cycling Economy.
Going into testing, we knew Carter favored ‘VO2max’ efforts in the 3-5 min range and ‘Anaerobic’ sprints < 1 min, however, was relatively weaker at ‘Threshold’ efforts in the 15-30 min range. Using testing, we had three key questions we aimed to answer:
Can we measure a cyclists VO2max using field testing? And how does this compare to his competitors?
Does the ratio of a cyclist’s metabolic rate at ‘threshold’ in relation to VO2max provide insight on future training strategies?
Are VO2 uptake kinetics important for XCO MTB racing?
Figure 1a: Critical Power Calculator
Test protocol and application of Critical Power
We first compiled data from his power-duration curve over the last 6 months and plugged it into our Critical Power calculator. Critical Power (CP) estimates a cyclist’s maximal metabolic steady state, which can be calculated using best output for 3-durations in the 2-15 minute range. For example, best 3 min, 5 min, & 12 min power (W). This model theorizes that steady state conditions can be reached at power outputs < CP, but when exercising at intensities >CP, the onset of fatigue will occur due to the depletion of the anaerobic energy reserve, accumulation of metabolites, and attainment of VO2max. With this information, we determined 4 power targets to be completed on a steady-hill climb:
200 - 220 W (Moderate Domain - Endurance Pace)
250 - 280 W (Heavy Domain - Tempo Pace)
320 - 350 W (Heavy Domain - Subthreshold Pace)
> 400 W (Severe Domain - VO2max Pace)
Figure 2a: Critical Power pacing calculator used in example.
So in his case, his estimated CP was 353 W and his Watt Prime (W’) was 22.91 kj. W’ represents anaerobic energy reserve and is expressed as the amount of work (kj) that he can expend at intensities > CP. With this information, we can predict an athlete’s maximal power over durations between 3 and 30 minutes or the length of time one could hold a specific power.
Example : To be competitive during a specific event, he needs to hold at least 380 W to stay on the wheel of his competition. We can predict that he can hold this for 14 minutes before his anaerobic reserve is depleted and he will have to drop his pace. Since the climb will only take 5-minutes, we then know the top end of what he can push is 429 W. From here, we can build some pacing strategies into his race plan.
Question 1: Can we measure a cyclists VO2max using field testing? And how does this compare to his competitors?
During the 4th trial of testing, Carter was instructed to maintain a steady maximal power output that he could maintain for the entire climbing segment. By exercising in the severe domain, VO2max can be reached at a similar time frame to W’ depletion estimates. In figure 2, his VO2peak of 80.5 mL/kg/min occurred 3.44 mins into the effort, which aligns well with the CP calculator estimate of 4.14 mins riding at 448 W (30-s avg maximal W).
The traditional Ramp Test format of increasing +25 W every minute or 12.5 W every 30 seconds on an ERG usually elicits VO2max values in 8-12 minutes whereas this format is a 4-5 minute all-out test. The advantage of the Ramp Test format is getting a VO2max value much faster than the multiple steps we used in this test. However, this format lacks accuracy in estimating threshold values since constant load exercise for at least 3-minutes is needed for valid lactate measures.
Figure 2: Determination of VO2max using 4-5 minute ‘all-out’ test format. VO2max: 80.5 mL/kg/min Powermax (Pmax): 448 W.
VO2max values have been reported to be within 70 - 86 mL/kg/min for males and 57 - 70 mL/kg/min for females in professional level cross country mountain bikers (Arriel, 2022). In a study comparing performance variables of 12 professional level male XC racers, the average VO2max within the group was 76.9 mL/kg/min (Impellizzeri, 2005). The group performed a race simulation on a 33 km (20 mi) course with 1362 m (4468 ft.) and six total laps. Interestingly, VO2max was not positively correlated with XC race performance indicating that a VO2max is more-so a “ticket to entry” to compete at this level (Impellizzeri, 2005). In other words, you need a high VO2max to compete, but there are other variables within the group that were more predictive of race success. The only variables that were positively correlated with race performance in that study were power output (W) and VO2 at the respiratory compensation point (RCP), which is synonymous for CP & the 2nd Lactate Threshold (LT2).
Question 2: Does the ratio of a cyclist’s metabolic rate at ‘threshold’ in relation to VO2max provide insight on future training strategies?
During field testing, we first wanted to validate that riding at a steady effort near CP elicited a lactate response that was similar to riding at threshold. The instruction was to hold a power output on the hill climb between 320-350 W while making sure not to exceed CP (353 W). The lactate value at the end of this stage was 4.4 mmol/L, which coincides with what we would expect for setting LT2 if we were following a normal step-test protocol in the lab. The agreement between the CP estimate and the lactate response allows us to trust using his power data in this testing scenario.
When analyzing his VO2 response to this testing stage, we see that his highest 30-s average VO2 was 66 mL/kg/min and his highest 60-s average power was 348 W. This means that his threshold occurs at 82% of his VO2max, which indicates that he has room to increase his threshold as this metric tends to range between 75-90% of VO2max. If we compare this to the cyclists in the study by Impellizzeri et al. (2005), the average VO2 at threshold was 67.3 and ranged from 61 - 75 mL/kg/min within the group. The average power output at threshold was 360 W and ranged from 323 to 432 W.
Figure 3: VO2 response to riding at an intensity just below Critical Power. Highest 60-s avg power: 348 W
With this information, we can see that Carter fits right in with other professional level cross country racers when it comes to VO2max and threshold variables. With such a high VO2max within normative ranges, it is promising to see that there is some room to increase variables such as power and VO2 at threshold. For instance, if his VO2max was 74 mL/kg/min, his threshold would then be at 89% of VO2max and would have less room to improve.
Question 3: Are VO2 uptake kinetics important for XCO MTB racing?
To our knowledge, there are no reported normative values for VO2 uptake kinetics in professional cross-country mountain bike racers. In cross country olympic (XCO) mountain bike race format, there is typically a course with varying climbs, downhills, and technical features that athletes complete laps on for a period of 90 mins. The start of each race begins with an ‘all-out’ sprint effort where we see power numbers exceeding 1000 W in the first 20-30 seconds to gain a race position that limits the degree of traffic with other riders on course. After this initial sprint, the intensity is intermittent throughout with many surges above threshold. The nature of this race format requires athletes to repeatedly produce high power output with short recoveries on downhill terrain. This means that athletes need a high glycolytic capacity to produce such power output but also the ability to quickly access the aerobic system to recover and repeat these efforts.
Figure 4: VO2 kinetics of Trial 2 analyzed using: https://mgreene.shinyapps.io/kinetic-vo2/
Our best data for VO2 kinetics during field testing came from trial 2 with a power target of 250-280 W (for more information on VO2 kinetics, refer to earlier blogs on the topic). We observed the time constant, tau, of the primary phase to be 21 s, meaning it would take 84 s (21 x 4) to reach steady state conditions at this power. There was a noticeable slow component rise following the primary phase, which is to be expected for at athlete with a high anaerobic capacity needed for mountain biking.
Faster VO2 kinetics may be observed in endurance sport disciplines that are more “slow-twitch” in nature such as marathon running or cross country skiing (refer to blog on nordic roller ski testing), where we may see tau values closer to 10 s. However, if we compare tau values to “trained” cyclists, an average value 28 s has been reported (Caputo, 2003) in the heavy domain, indicating that at a professional level kinetics tend to be be faster.
Further testing will be needed to understand the utility of VO2 kinetics in XCO mountain bike racing. Of particular interest is the role of VO2 kinetics when mountain bikers transition between downhill and uphill segments of a course. Transitioning into an uphill climb requires a sudden increase in the need for energy from both the aerobic and anaerobic energy systems to produce the necessary power output. Faster VO2 uptake kinetics implies less energy contribution from the anaerobic system, which preserves the energy stores (i.e. glycogen, phosphocreatine) needed for repeating these transitions throughout the entire 90 mins of the race. This repeatability of producing high power outputs will require the right balance between anaerobic and aerobic capacity to optimally meet energy demands.
Carter racing his 2nd World Cup in ‘Mont Sainte Anne’, Canada.
References:
Impellizzeri FM, Marcora SM, Rampinini E, Mognoni P, Sassi A. Correlations between physiological variables and performance in high level cross country off road cyclists. Br J Sports Med. 2005 Oct;39(10):747-51. doi: 10.1136/bjsm.2004.017236. PMID: 16183772; PMCID: PMC1725050.
Arriel RA, Souza HLR, Sasaki JE, Marocolo M. Current Perspectives of Cross-Country Mountain Biking: Physiological and Mechanical Aspects, Evolution of Bikes, Accidents and Injuries. Int J Environ Res Public Health. 2022 Oct 1;19(19):12552. doi: 10.3390/ijerph191912552. PMID: 36231848; PMCID: PMC9565958.
Caputo F, Mello MT, Denadai BS. Oxygen uptake kinetics and time to exhaustion in cycling and running: a comparison between trained and untrained subjects. Arch Physiol Biochem. 2003 Dec;111(5):461-6. doi: 10.3109/13813450312331342337. PMID: 16026035.
