Physiology of Performance and Periodization
Physiology is the science of how the body works: How muscles contract; how the heart, lungs and blood vessels deliver oxygen and fuel to the body; how the energy in food is converted into muscle power; how training affects muscles and so on. Understanding physiology is not essential to racing success. Many top pros have no knowledge of how the training they do affects them on a microscopic or biochemical level. They leave science to the coaches and doctors, follow their training plans and race to glory. Still, in a world where people are pushing a variety of training methods and gadgets, a basic understanding of how muscles work and respond to training can help a rider sort solid advice from hokum. Physiological knowledge can help you choose training methods that will benefit you and avoid wasting time on those that aren’t.
Three Energy Systems
Muscles draw on three interrelated energy systems to drive the pedals. The first is the aerobic pathway, which includes the Kreb’s citric acid cycle and oxidative phosphorylation. The second system is anaerobic glycolysis whose major byproduct is lactate. The third is the anaerobic, alactic or phosphagen pathway. The three systems are distinguished by power produced, length of time that power can be sustained, energy source, by-products and response to training.
Sprinting Power Comes From The Phosphagen System
Ultimately every muscle contraction is fueled by the energy carrier adenosine tri-phosphate, usually known by its acronym, ATP, but a muscle at rest or during exercise contains only enough ATP to fuel a few seconds of extremely hard work. ATP is like fuel in your car’s cylinders, ready to burn and just waiting for a spark. Once the charge of ATP is depleted, it must be replenished before the muscle fiber can contract again. Creatine phosphate (CP) can deliver its energy almost instantly to replenish the depleted ATP supply. ATP and CP, together called phosphagens, can fuel about 20 seconds of hard work. The phosphagens provide the energy for sprint type efforts. Full recovery from a maximal, phosphagen-fueled effort takes a few minutes.
Hill Cresting, Attacks and Breakaways Are Powered by Anaerobic Glycolysis
When ATP and CP are depleted the muscles suddenly become much weaker and power decreases despite continued effort. This is what happens when you unexpectedly find yourself sitting during a long sprint. At this point the muscle must replace ATP by using energy sources that take longer to mobilize and won’t support as high a power output as the phosphagens. Anaerobic glycolysis fires up as the phosphagen system runs down. Anaerobic glycolysis relies on glucose as its principal fuel and produces lactate as its principle by-product. Depending on training and natural talent, anaerobic glycolysis will support between two and six minutes of all out effort, after which several minutes to several hours of recovery are required before the effort can be repeated. Two indicators of reliance on anaerobic glycolysis are heavier breathing, and a burning feeling in working muscles such as a racer might experience during sustained attacks, fighting for position in the final laps of a crit, near summits, and other places where the pace causes extended suffering.
Aerobic Power Drives Sustained Breakaways, Long Climbs and Time Trials, as Well as Keeping Up
When a muscle is delivering maximal effort and deriving the majority of it’s energy from anaerobic glycolysis, it fatigues rapidly and weakens suddenly within a few minutes. The old thinking was that this weakening resulted from a buildup of lactic acid in the muscle or blood. Recent research suggests that phosphate, a breakdown product of ATP, is the true biochemical culprit. Either way, independent of training and talent, the phosphagen and anaerobic glycolysis pathways cannot be made to provide maximal power for more than a few minutes. To increase power for the few seconds of a maximal sprint, train the phosphagen system with short maximal efforts or strength training. To improve maximal power for efforts lasting a few minutes, or extend those efforts a bit but still well under ten minutes, train to enhance anaerobic glycolysis with gut busting efforts of similar length. If you need to sustain for more than a few minutes output levels that you currently generate from anaerobic glycolysis or phosphagen, or that you can’t currently generate at all, no amount of training these two systems is going to help. In that case you need to train to increase aerobic power.
Once the lactate producing anaerobic glycolysis pathway has been running for a few minutes, whether or not it is pushed to its maximum, lactate begins to accumulate in the muscles and in the blood and the aerobic energy system becomes the dominant mode of regenerating ATP. The main fuels for aerobic metabolism are fats and the lactate produced by anaerobic glycolysis. Since anaerobic glycolysis can generate lactate from free glucose or from glucose released from glycogen, these substances are considered fuel for the aerobic system. A well-fed body contains enough of these for one to several hours of sustained effort as well as for all-day low intensity rides. The main by-products of aerobic power production are water, carbon-dioxide and heat, all of which your body can usually get rid of as fast as they are produced. Since fuel for the aerobic system is plentiful and the byproducts are easily disposed of, aerobic energy sources can power efforts too long to be sustained by anaerobic glycolysis. Aerobic capacity is closely related to endurance.
The aerobic energy system is highly trainable to increase both the power that can be produced aerobically and the time that power can be sustained. Saying a rider is “fit”, means that rider can sustain a high power output relying only on the aerobic pathway. Remember that one of the signs of anaerobic glycolysis is heavy breathing, and that the length of that sort of effort is limited. If two riders roll along together and one of them is breathing hard while the other is not, we say that the easy-breathing rider is more fit, meaning that he can sustain a higher speed while relying only on aerobic energy sources than can his partner. He will be able to sustain that speed after his partner blows up. If his partner improves in aerobic ability, the partner will be able to keep up again.
Many bike racing strategies take advantage of this effect, trying to force others to tap anaerobic energy stores while the strategizing rider does not, such as when a strong rider tows a string of riders along the gutter in a cross wind, or a rider sits in and does not pull with the other riders in his break-away group.
All Three Energy Systems Important and Trainable
A high level of development of all three energy systems is essential to racing success, but its difficult or impossible to effectively train them all at the same time. You improve each with training that specifically challenges that system. The most effective training of a system involves maximally challenging that system, whether that is with short sprints to challenge the phosphagen system, gasping all-out intervals for anaerobic glycolysis, or longer, steady efforts below LT to challenge the aerobic system. Aerobic training causes an increase in the ability of the muscle to use oxygen by actually increasing the infiltration of the muscle by capillaries and by increasing the mass of the mitochondria, the parts of the cells in which aerobic metabolism actually occurs.
Unfortunately training that benefits one system won’t always benefit the others, or may hinder them by using up training time or by causing fatigue that interferes with other training. Some coaches have argued that harder efforts directly reverse capillary and mitochondrial development, but this has not been proven. In any case, mixing up training for all three systems all year long is not the best way to prepare for racing. Due to differences in genetic talent and exercise history, different riders need different amount of training of each of the three systems. Each system also has its own time course for response to training, so one can introduce them at different times to have top levels of all three coincide with the important races. That coincidence of good condition in all three energy systems is the physical part of peaking. The timing of training to make it happen is called periodization.
Which System to Train and When
The aerobic energy system is clearly and simply the most important of the three when it comes to road racing success at a high level. If one can’t go as fast as other riders over the length of a race, it doesn’t matter how fast one can go for a few seconds or a few minutes. It’s not that the rider with the highest aerobic power wins, but that there is a minimum sustainable aerobic power required to get to those points where strength for shorter efforts can lead to victory. As one move up in the categories, that minimum power required to be in the game increases to levels near the limits of human capacity. The aerobic system responds slowly to training, with aerobic ability and efficiency increasing gradually over many years. This is why a good road-training program includes sub-LT, aerobic training almost year round, and why it takes patience to reach one’s potential.
Only when you keep up with the main group through the majority of the race and get intro trouble only when the strongest riders are dropping the hammer, is the capacity for anaerobic glycolysis a possible limiter to racing success. There are riders with such great aerobic capacity that they don’t often need to call on anaerobic energy sources to counter the efforts of competitors (think Lance). For the rest of us, training to enhance anaerobic glycolysis requires exhausting, painful efforts that detract from the ability to continue other training. Fortunately, moderate doses of such training a day or two per week will generate adequate development within a few weeks to a few months, so you don’t need to do this sort of training year round. It can start just a month or two before the most important races of the year. Many racers will use their first, less important races to improve anaerobic glycolysis. When you can generally keep up to the ends of races and get position for the finish, you may benefit from training your phosphagen system and sprint itself. While the ability to win sprints can be improved through practice of techniques and tactics, the actual power output possibilities of the phosphagen system can’t be changed much, except perhaps by building bigger muscles. Many top riders won’t specifically train this system at all outside of races. Of course they will also do as many as a hundred races a year. If you race less than this, include some sprinting in your training, but mostly in or close to your racing season. Focus your time and energy on aerobic development the whole year and anaerobic development in the final months before the important races.
A little knowledge of muscle physiology helps us understand why it makes sense to do adopt a periodized approach to training, doing a lot of aerobic training and very little anaerobic volume many months before the race season, and then shifting toward more work at higher intensities as the season approaches.