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Cycling performance: the physical demands of cyclo-cross

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Photographs: Benedict Campbell
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Many of the demands of cyclo-cross racing are unique, but the goal is the same as in any bike race; get to the finish line first. At this time of year, we often see a number of professional riders switch asphalt for mud, but what are the similarities and differences between the disciplines?
It’s often said the power meters have changed the language of cycling. While cyclo-cross may look chaotic, the increasing use of power meters in all types of races has provided us with a quantifiable insight which helps to describe the demands of events in more objective terms.
In some ways, the power requirements of a cyclo-cross race are similar to a criterium. Both require riders to sustain high-power outputs for extended periods (typically 40-75 minutes), with frequent bursts of effort. These similarities in power output result in comparable physiological demands.
To be competitive in criteriums, cyclocross and road races, riders must have well-developed energy systems in order to ride close to their maximum sustainable power (sometimes called Functional Threshold Power – or the best power they could maintain for 60 minutes), make repeated efforts above this level and quickly recover from them.
At a cellular level, this capacity is facilitated by the ability of the riders to supply large amounts of ATP (Adenosine Triphosphate) to their working muscles. ATP is a ‘high-energy molecule’ that serves as the basic currency for our all our energy requirements. When ATP breaks down, useful energy is released – in this case for the mechanical work of pedalling.
We need a lot of ATP to get through the day, never mind a cycling event, but the store of ATP within our the cells of our body is very small. Therefore, we have to continually re-synthesise ATP. No ATP = no energy.

When we talk about ‘energy metabolism’ we are referring to the energy systems in our bodies that replenish ATP. There are three such systems, each of which contribute in different proportions depending on the duration and intensity of the effort. The first, ATP-PC, is for high power, short duration efforts. The second, Glycolytic, is  for moderate power, short durations and the last, the oxidative system, is for low power and long durations.
The short and intense efforts which make up cyclo-cross events are supplied by all energy systems, but rely proportionally more on the ATP-PC and glycolytic systems, as opposed to oxidative system which dominates longer road races. In fact, in terms of energy and hydration requirements, cyclo-cross races are similar to long time-trials. Also, the athlete must be able to buffer increasing acidity in the tissues caused by the repeated anaerobic surges.
Despite the course obstacles, cyclo-cross racers still travel at relatively high speeds. The biggest energy cost of a cyclist’s increasing speed is a result of pushing through the air. Aerodynamic drag increases exponentially with increasing speed, so minimising drag is key to preserving energy for the high-power bursts which make or break most bike races, including ‘cross.
This has been recognised in in road-racing for a number of years, and while deep section wheels have long been used in ‘crss, this may have resulted from the more basic desire to accumulate less mud in the spokes. However, we’ve seen increasing use of aerodynamic equipment off-road, with frequent sightings of aero helmets, thoughtfully designed and well-fitting skin-suits, which suggests a growing awareness of the potential for aerodynamic considerations to create a competitive advantage in the ‘cross domain.
The technical aspects of cyclo-cross introduce many new variables which make the sport unique. Riders must jump off and run with their bikes, sometimes for periods of half-a-minute. Tight corners result in much more aggressive braking and accelerations than we see in typical criterium and road-races. Power files reveal relatively large amounts of time spent at low cadences and high power outputs, unlike road-racing which is often characterised by high cadences and high power.

Also, as a consequence of the frequent changes in pace, a rider towards the back of a group will have to use much more energy to get round a cyclo-cross course than a rider closer to the front. At the front, the pace is more even when entering and exiting corners. This means that riders have more choice about which line to take, creating a smoother, more efficient power profile. Consequently, well positioned riders can produce more of their energy using their efficient, oxidative system, accumulating less fatigue and saving their high-power but limited ATP-PC and glycolytic fuelled bursts for race defining moves. In contrast, riders at the back suffer from a ‘concertina effect’. While this is also visible in road races and criteriums, it is exaggerated in ‘cross.
Entering a corner, the effect of riders ahead braking earlier accumulates, so the later riders enter the corner much slower. However, the front riders also exit the corner much earlier and many are already up to maximum speed, whilst the back-markers are still entering or negotiating the bend. This speed differential stretches the groups, requiring riders towards the rear to accelerate hard out of corners and produce a higher sustained power, simply to remain as part of the group.
So why do road racers, who may lack some of the technical prowess of dedicated cross-riders, put themselves through this ordeal? There are many possible benefits. Low cadence, high force efforts may improve muscular strength, power and improve sprint performance. The repeated, short duration surges are powerful stimulus which can be hard to achieve outside of a competitive environment, making a winter spent pounding the pedals in ‘cross races effective training for many riders.
Even if a rider is hanging on the back, efforts hovering around threshold with repeated bursts maintain high concentrations of blood locate for extended periods – creating a powerful stimulus for improvement.

While it’s painful (it’s actually the accumulation of hydrogen ions, associated with increasing lactate concentrations, which cause the burning sensation), this circulating blood lactate may play the role of a signaling hormone, with a range of possible consequences. These include up-regulating the expression of genes, helping the body make better use of lactate as a fuel source, and some studies suggest that the lactate molecules may even stimulate an increase in formation of new mitochondria; an important adaptation in endurance performance as mitochondrion represent the ATP ‘energy factories’ of our cells.
Specific training creates specific ‘tools’. However, whilst it was once believed that the most effective way to develop the aerobic energy system was to do long, steady training rides at relatively low power or heart rate zones, a growing body of evidence suggests that improvements to aerobic performance can be made through short-duration high-power efforts.
While long, low intensity rides still play an important role in the training diet of road cyclists, analysing the training files of world class endurance athletes often reveals a stark polarisation: easy sessions are very easy and hard session are very, very hard. The world’s best endurance athletes seem to spend less time in the mid-threshold zone: the ‘no man’s land’ inhabited by many weekend warriors.
Consequently, the short, bursts of effort which characterise ‘cross racing could provide a potent dose of training stimulus to develop the peak power values which will underpin the New Year’s road racing performance. And even though the low power, oxidative systems provide most of the energy for road races, the high power, short duration ATP-PC and glycolytic systems, emphasised in cyclocross events, are what make the race winning differences in the majority of cycling events. Finally, the technical aspects of the courses likely provide welcome variety, fresh motivation and a chance to hone skills in preparation for another season spent dodging road furniture.
202 is a performance coach at HINTSA Performance, Geneva

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