Physiology Explained: Energy Systems & VO2 Kinetics of MTB Training
What Makes Mountain Biking Different from Road Cycling?
Mountain biking is a sport involving climbing and descending trails with varied incline, technicality, and difficulty. Steady-state consistent efforts do exist such as riding evenly graded 2-track road climbs; however, it is predominantly an endurance sport involving intermittent variations of intensity based on the demands of the trail. For example, a common ride for a mountain biker may include a single-track hill climb that involves moderate-intensity pedaling interspersed with downhills and variable short sprint-efforts and long threshold efforts. On particularly steep trails, completing sprint and long threshold efforts may be unavoidable.
A steep uphill technical move in St. George, UT.
Compared to road cycling, intensity control is less precise; rather than intentionally selecting a specific power and cadence, a mountain biker is responding to the demands of the trail with whatever power and cadence is necessary for success.
How is Cycling Studied in the Scientific Literature?
In the scientific literature, cycling is primarily studied using controlled power and durations. For instance we might look at someone’s power profile including their best 1 min, 5 min, 20 min, & 60 min power. Other common research variables may include a time-to-exhaustion test at a specific power, or an interval training protocol with set work rest ratios (i.e. what is the response of 4 minute intervals at 90% of VO2max power and 4 min recoveries at 50% of VO2max power).
"Steady-state" conditions are also a major focus in cycling research. Steady state conditions exist when there is a balance between the energy required by working muscles and the production of our energy currency - adenosine triphosphate (ATP). In steady state conditions we observe that the rate of the oxygen delivery and consumption (VO2) is matched to the required metabolic demand of the workload (i.e., power). Steady state may be achieved at a set power as long as the individual is exercising below their Critical Power (CP), a mechanical power threshold separating intensities that can be maintained versus intensities that cannot be maintained due to the onset of fatigue. In other words, steady state conditions can be achieved below CP but not above.
This brings us to mountain biking where the intensity is purely stochastic. Constant intermittent changes in intensity challenges our physiology much differently than other disciplines where intensity is more consistent. To implement effective MTB training, we need to understand the physiology of how we produce our energy currency, adenosine triphosphate (ATP), to meet the metabolic demand of riding. Considering that MTB involves a wide range of intensities, all 3 of our primary energy systems are required in important ways.
Primary Energy Systems
(1) ATP & Creatine-Phosphate (ATP-PCr) energy systems for very short efforts
used for short powerful motions that require immediate energy supply; i.e. technical rock/root move, short punchy section of trail, or any short acceleration.
ATP is stored within muscle cells in small quantities and can be used for very short durations (<2 seconds)
breaking down Creatine Phosphate leads to an immediate supply of energy and subsequent build up of inorganic phosphates. This system lasts <10 seconds and is restored aerobically.
(2) Glycolytic energy system for power demands in the heavy and severe domains (see figure 1 below)
used when the demand for ATP is not being met by aerobic mitochondrial respiration. This system is used in isolation only when an effort is all-out and lasting ~30 seconds.
at an intensity >LT1 (1st Lactate Threshold), this system is recruited increasingly based on the speed of metabolism. It is associated with fast twitch muscle fiber recruitment and subsequent build up of hydrogen ions (metabolic acidosis) during exercise.
system also used to produce ATP during phase II (primary phase) of VO2 kinetics (more on this later).
(3) Aerobic mitochondrial respiration
this energy system is in most cases always being used by your body when cycling. This is a slower-working energy system but produces a bulk of the energy (ATP) required for metabolic function.
Substrates for our aerobic system could be fats, glucose, and even lactate.
Classifying Training Status and Individual Physiology
Based on the training status and physiologic profile of a cyclist, a different % of each system is required at a set-power. To have an understanding of how one’s physiology responds to increasing power output, we typically have a cyclist complete a physiological test measuring variables like blood lactate accumulation, heart rate, oxygen consumption (VO2), ventilation, breathing frequency etc. in response to increasing power. The 1st & 2nd Lactate Threshold are determined which set the boundaries between moderate, heavy, severe, and extreme intensity domains (see figure 1).
Figure 1: Training Intensity Domains - Moderate, Heavy, Severe, & Extreme.
During an incremental step-test, we progress the cyclist through stages of increasing intensity while measuring physiology variables such as blood lactate, heart rate, VO2, minute ventilation, breathing frequency, etc. In this test, workload increased by 25 W every 3 mins until volitional fatigue. The first Lactate Threshold (LT1) is identified where there is a sudden rise in blood lactate accumulation in the blood. This threshold separates zone 2 and zone 3 in a 5-zone HR model. The second Lactate Threshold (LT2) is identified where the lactate curve begins exponentially. This represents the metabolic intensity where lactate disposal has reached its physiological limit while lactate continues to be produced at fast rates. LT2 separates zone 4 and zone 5 in a 5-zone HR model. In the severe & extreme domains, anaerobic capacity is a limiting factor to repeated high-intensity performance. Each domain implies certain physiological characteristics are working whereas thresholds represent the capacity of certain physiologic systems.
For a trained rider with a well-developed cardio-metabolic system, there will be a higher % of energy supplied by aerobic mitochondrial respiration. Whereas an untrained cyclist will require a higher reliance on the glycolytic system.
For instance, compare a recreational cyclist who's LT1 power is 160 W to a trained cyclist who's LT1 is 220 W (figure 2). When riding the same trail at 200 W, the recreational cyclist will be exercising in the heavy domain whereas the trained cyclist will be exercising in the moderate domain. This means that the recreational cyclist will be depleting glycogen rapidly, fatiguing fast twitch muscle fiber, and facing the burden of neutralizing metabolic acidosis. The trained rider, however, will be preserving glycogen by burning fat at high rates and maintaining pH balance.
You may also notice that the peak Lactate between each individual varied significantly. The recreational rider finishing their test at a lactate level of 7.8 mmol/L whereas the trained rider finishing their test at 12.1 mmol/L. While keeping lactate low at low intensities is advantageous, the ability to produce high levels of lactate at maximal intensities is also very important indicating a well-developed glycolytic system.
Figure 2: Comparing Lactate Profile of Trained vs. Recreational Cyclists
While developing a strong aerobic base in important, the use of the ATP-PCr and glycolytic systems is unavoidable for the well-trained cyclist. The process of oxidizing the main substrates of aerobic metabolism - glucose & free fatty acids simple takes too long when the speed at which energy is needed immediately, such as riding a steep climb or technical-powerful move. Simply put, if you cannot produce high levels of lactate during a sprint, your ability to produce power will be diminished.
VO2 Kinetics
VO2 Kinetics refer to the "responsiveness" of your aerobic system when there is a rapid change in exercise intensity. In other words, how quickly can aerobic metabolism respond and meet the energy requirement of a given workload.
Let’s look into what happens when there is a sudden change in intensity. This scenario represents what occurs when a cyclist is pedaling at an easy rate and all of a sudden has to pedal harder such as when there is a steep climb requiring higher power output. In figure 3 you will notice that the VO2 kinetics of moderate, heavy, and severe domains are unique. This is due to the contributions of the each of the primary energy systems in each domain.
Figure 3: The following test measured the VO2 response of transitioning between easy cycling at 150 W then transitioning to 5-min periods in the moderate (180 W), heavy (240 W), and severe (300 W) domains.
Let’s look specifically at the VO2 kinetics of the 5-min interval in the heavy domain (see figure 4). To properly analyze VO2 kinetics, we need to use computing software to clean our data, analyze the curve, etc. A great resource for this can be found on ExPhysLab.com.
When transitioning from rest or low intensity to an intensity in the heavy domain, we observe the primary (fast) component as how quickly VO2 uptake rises to the VO2 “cost” of that power. In this specific scenario, let’s assume the VO2 cost of 240 W was 3600 mL/min. The area below the curve is highly dependent on the % of slow-twitch muscle fiber of the individual and how well they can transport environment oxygen to working muscle. The area above the curve is known as the oxygen deficit because non-oxidative energy systems are producing the energy needed UNTIL steady state conditions exist. So this is when the ATP-PCr and Glycolytic Systems are relied on.
Figure 4: VO2 Kinetics of the Heavy Domain (240 W).
After steady-state has been reached at 3600 mL/min, we observe the slow component where VO2 continues to rise gradually above the O2 cost of the intensity. There are many speculations of why VO2 continues to rise. One theory is that the slow component represents the degree in which fast twitch fibers are used at this intensity. Furthermore, the slow component may also represent lactate oxidation due to the reliance of the glycolytic system.
If you look at the VO2 kinetics in the severe domain (figure 3), you will notice that the VO2 slow component is much steeper and rises until VO2max. In this case a VO2max somewhere in the 70-75 mL/kg/min range. This steeper rise compared to the heavy domain indicates an even greater reliance on fast-twitch muscle fiber.
Is there variability in the VO2 kinetics response to exercise in the heavy domain?
In work from Barstow, Jones, et al., VO2 kinetics was strongly associated with the % of slow twitch vs. fast twitch muscle fiber. As you can see, the individual with a high % of slow twitch fiber (67%) displayed a much quicker primary component compared to the individual with low % slow twitch fiber (18%). This individual will therefore have a much higher reliance on the non-oxidative energy systems described earlier.
What this means for MTB performance:
When the non-oxidative energy systems are repetitively being used for energy production, there are consequences such as:
·accumulation of fatigue-related metabolites
metabolic acidosis
heat-build up
glycogen depletion
So, developing an aerobic engine is absolutely necessary for MTB performance. However, mountain bikers are faced with the demand of constant changes in exercise intensity, which requires the development of the ATP-PCr and glycolytic systems. Therefore, the goal with training is to develop the aerobic system enough for optimal energy efficiency but also not lose track of the fact that intermittent intensity does require well developed non-oxidative energy systems.
