Humans Are Average At Lots Of Things!
The human body is a complex organism. It’s capacity to adapt and carry out a broad range of tasks means that we have been able to thrive in many climates across varied terrain. This versatility is underpinned by the body’s ability to perform at a wide spectrum intensities and durations. Arguably, our anatomy and physiology’s greatest strength is that, relative to other organisms, we are average at lots of things, rather than being pure specialists in one type of activity. Humans can produce a reasonable sprint performance and our long legs, foot structure and fuel stores make us well-suited for extended journeys over land, even when food is scarce. In addition, we are unique in our ability to create machines to facilitate movement and travel. Amongst our inventions, the bicycle is most efficient human-powered land vehicle. It has allowed us to harness our varied performance capacity and given birth to events ranging from track sprints to the solo Race Across America.
ATP: Life’s Energy Currency
Energy is required to keep us alive, fuel the activities of everyday life and push on the pedals. A high-energy molecule called ATP (Adenosine Triphosphate) has been described as life’s energy currency. When ATP breaks down, energy is released. Every day, an adult human may need a quantity of ATP equivalent to their body weight; more if they undertake intense exercise, but the store of ATP within the cells of humans is very small. Consequently, when ATP is broken down to release energy, it needs to be continually resynthesised. When someone is talking about ‘energy metabolism’ they are referring to the series of chemical reactions that take place within the body to resynthesise ATP. The human body has three ‘energy systems’ to resynthesise ATP, each featuring a different series of reactions. Two of the energy systems depend on the food we eat to fuel them. The other energy system relies on a molecule called phosphocreatine. Aerobic reactions require the presence of oxygen, anaerobic reactions do not.
Energy Systems As Fuel Tanks
It’s not an entirely accurate analogy, but you could imagine the body’s energy systems as three fuel tanks:
- Tank 1: filled with phosphocreatine fuel, feeds the ATP-PC system.
- Tank 2: filled with glucose, feeds Anaerobic Glycolysis
- Tank 3: feeds the Aerobic System.
The tank for Aerobic Metabolism may be seen as the Back To The Future DeLorean of the energy system world, accepting carbohydrate, protein and fat fuels. The body draws on all it’s fuel tanks, regardless of the type of effort, never closing one off completely. Nevertheless, the three energy systems contribute different proportions depending on the duration and intensity of the effort.
Energy System 1: ATP-PC
There are not many steps in the chemical reactions that make up the ATP-PC system. The reactions can take place in the absence of oxygen and phosphocreatine (PC) is a relatively high energy molecule. As a result, the ATP-PC system can provide a lot of energy quickly but only for short periods. If you want to see a highly developed ATP-PC system in action, watch Mark Cavendish’s burst in the final 200m before the finish line.
Energy System 2: Anaerobic Glycolysis
The human body has around 2000 Calories of carbohydrate, stored in the tissues of the liver and muscles in the form of glycogen. This amount of energy would fuel approximately 2000 Kilojoules of mechanical work on the bike, as recorded by a power meter. Regardless of how long an effort is, carbohydrate is always initially broken down through a chemical reaction called anaerobic glycolysis. Oxygen is not required for this reaction and whilst only about 5% (2 ATP molecules) of the energy potential of a glucose molecule can be realised the energy is liberated quickly, so this energy system is well suited to high intensity efforts greater than 10 seconds, up to around 2 minutes. Because anaerobic glycolysis can only supply short efforts, it only makes a small dent in the 2000 Kilojoules of stored carbohydrate available, so the time limitation is related to the chemical processes involved in anaerobic metabolism and their interaction with the body, rather than a lack of availability of carbohydrate. Professional riders such as Philippe Gilbert use their well developed anaerobic energy system to generate spectacular performances in the finales of classic races. The majority of the 1,300 metre climb of the Mur de Huy, the site of the finish line of the Flèche Wallonne, reaches a 26% grade and required Philippe to produce an explosive 2:44 minute effort when he won the race in 2011.
The anaerobic glycolysis reaction produces lactate, the base of lactic acid. During high intensity efforts lactate is produced in greater amounts than can be removed. Contrary to popular belief, fatigue may not simply be the result of lactic acid accumulation – energy metabolism has turned out to be much more complicated! However, measuring blood lactate concentration is a useful way to gauge a rider’s effort.
Energy System 3: Aerobic Metabolism
In the presence of oxygen, the chemical reaction can progress beyond anaerobic glycolysis. When oxygen enters the equation, energy can be liberated much more efficiently, offering 34 ATP molecules from a molecule of glucose, in addition to the 2 resynthesised through anaerobic glycolysis, but it takes longer. In addition, the fuel tank for aerobic metabolism will also accept fat and protein. The high energy nature of fat molecules means that they offer the potential to resynthesise even more ATP molecules than glucose. Aerobic metabolism provides energy for all types of cycling effort, regardless of duration. When an effort intensity increases, anaerobic metabolism makes a bigger contribution, which limits the amount of time that the particular effort intensity can be sustained. At lower intensities, aerobic metabolism is the primary source of energy and provides the energy that can not be supplied by fuel consumed during riding, such as in gels and drinks.
A 75 kilogram rider with 15% body fat has 11.25 kilograms of stored fat, representing 101,250 Calories of energy potential. If the rider tried to use it all, they would die, but it is possible to access a huge proportion of this fuel store during cycling, providing the intensity is low enough. When a rider’s aerobic system is better developed and adapted through training, the athlete will be able to ride at higher intensities whilst still using fat as principle fuel. Consequently, the limited stores of carbohydrate are preserved for higher intensity efforts later in the ride. For example, in Stage 18 of the 2013 Tour de France, Jens Voigt escaped in a breakaway group. The route covered 172.5km. According to his power meter, he recorded a 309 watt average power for the duration of the stage, expending over 1000 Calories per hour for over 5 hours, much of which would have been supplied by fat metabolism. Consequently, he had reserves of carbohydrate available to fuel the higher intensity efforts on the stage, such as when he averaged 390 watts on the climb of the Col de Manse.
What Does All This Mean For Cyclists?
The road cyclist must use all their energy systems to ride well on routes ranging from pan-flat plateaus to the high mountains of the Alps. Our individual genetic make-up means that we will have a predisposition towards being better at some durations and intensities relative to others. Most cyclists will find early on that they are better a sprints than long climbs, or find they prefer short-steep hills to time-trials but everyone’s energy systems can be improved through training.
Training should address all the rider’s energy systems, combining efforts from sprints to long rides over multiple hours. Beginning training with a large volume of relatively long (90 minutes or more), low-intensity rides creates adaptations in the aerobic system.
You could define low intensity as:
- Riding at an intensity that results in blood lactate concentrations of less than 2.0 millimoles (but not many riders test blood lactate when riding!)
- Riding at less than 76% of your Functional Threshold Power (FTP is sometimes described as 95% of your best power output over 20 minutes)
- Riding at an intensity to keep you heart rate below 80% of your Functional Threshold Heart Rate (make a 30 minute effort as fast as you can and record your average heart rate for the last 20 minutes) for the majority of the ride.
This low intensity ‘base training’ provides the foundation for specialisation as the rider’s experience develops and strengths and weaknesses become apparent. In time, it may be that the rider chooses to emphasise a particular type of training to address some of their strengths and weaknesses. A rider who struggles to hold consistent high power outputs on long climbs may aim to develop their aerobic energy system at higher intensities, where a rider who always gets beat in the town-sign sprint may work on 10 second efforts fueled by their ATP-PC system. The rider may not think of their training in these terms, but there is a general understanding that training at particular intensities and durations stimulates the body to improve over these periods.
Essentially, training zones are a way of categorising how intense an effort is, which dictates how long the effort will last and which systems will predominantly supply energy for the effort. Training should stress an energy system enough that it is challenging for the system to supply energy for the effort. This creates an ‘adaptive stimulus’ which encourages the body to change and make the same effort easier next time it is attempted. For example, long relatively low intensity training rides help to develop the body’s aerobic energy system and its capacity to use fat as a fuel. There are multiple methods to define training zones. Some are described relative to heart rate, others to power measured in watts, blood lactate concentrations or even perceived intensity. The consistent factor is that if zones are used to prescribe specific training which provides a stimulus by becoming progressively harder, if there is enough time to rest so the body adapts in response to this stimulus, the rider will improve.