Energy production in organisms comes from the controlled breaking of complex molecules into simpler ones. For us humans, we break ATP (adenosine triphosphate) down into ADP (adensoine diphosphate) by popping off one of the phosphate molecules. The energy produced in this process is used by our motor units to produce muscle contractions and, for cyclists, forward propulsion. Then, using fat and sugar, our cells build more ATP from the ADP and the cycle continues. The body has three ways to use these substrates to produce energy to propel us forward, each with their own nuances and efficiencies. Understanding these systems can help us better understand the limitations inherent to cycling power production and explain some of the phenomena we all experience.
Sprint energy system (creatine phosphate cycle)
When a cyclist starts sprinting or an athlete in the gym does an intense, short effort. They are mostly relying on their creatine phosphate cycle. This Sprint energy system has a short window of use before running out of fuel, some 5 to 10 seconds, but can provide power quickly compared to the other energy systems.
In the Sprint system, the following single step chemical reaction occurs:
Since the reaction is only a single step, ATP production can occur quickly. Imagine this example: I just started an all out sprint on the bike, my muscles are using ATP to produce power, but my ATP concentration is dropping dramatically. The muscle cells can use the Sprint system to rapidly create more ATP from the large stock of ADP. The benefit of the Sprint system is that ATP stores can be replaced nearly instantaneously.
A good follow-up question is then: why can’t we sprint forever at full gas? The limiter in the Sprint system’s longevity is the amount of creatine phosphate in our cells. The recycling of creatine back into creatine phosphate occurs slowly and thus our limitation in the amount of time we can sprint is the amount of stored creatine phosphate in our body. Supplementing with creatine has been shown to increase the capacity of the Sprint system, likely due to increases in resting creatine phosphate levels.
The creatine produced as a byproduct during the single stage conversion is absorbed into the mitochondria in the cell and given a phosphate group to convert it back into creatine phosphate. This rephosphorylation occurs slowly and in the presence of oxygen; that’s why cyclists continue to breathe heavily for a while following a sprint effort. A good rule of thumb is that following a sprint, the first minute is spent restoring creatine phosphate levels back to normal values and only at that point does ‘recovery’ from the effort begin. If you are training sprints, make sure you rest at least 2 minutes between efforts or there is no guarantee you are properly recovered for the next effort.
Anaerobic system (Anaerobic glycolysis)
The Anaerobic system is *ahem* anaerobic. As in NOT aerobic, as in without oxygen. Specifically, the Anaerobic system converts ADP back into the energy-storing ATP through a process that doesn’t require oxygen. This is great because one big limiter in cycling performance is the amount of oxygen a cyclist can get into their muscles, but it comes with downsides as well. Lactic acid is a byproduct of anaerobic glycolysis and this acid is responsible for the burning sensation our muscles feel during long, hard efforts.
Compared to the single step chemical reaction of the Sprint system, the process of converting ADP to ATP anaerobically is a massive endeavor. A total of 18 reactions occur and the one reaction that controls the speed of the overall process (the rate limiting step) involves the enzyme PFK. This enzyme slows production of ATP when acidity is higher (more lactic acid in the muscle) and speeds up production when ADP concentrations increase (ATP is being consumed). The overall rate of the Anaerobic system is a balance between these two factors.
There are some serious limitations to the anaerobic system. Firstly, only glucose and glycogen can be used to produce energy in this system. Although humans have ample fat stores, our carbohydrate stores are limited and peak performance involves sparing those carbohydrate stores as much as possible. As a result, efforts heavily relying on the Anaerobic system quickly consumes our carbohydrate stores and leaves us exhausted. Not only is the system reliant on only sugar, the efficiency of the Anaerobic system is horrible. For each glucose molecule burned, our body must consume two ATP to produce four new ATP. We use the ATP that is supposed to be supplying our motor units with fuel in order to make more. This gives an overall efficiency of two ATP molecules per glucose molecule. In contrast, the Aerobic system produces 36!! ATP per glucose molecule.
The main advantage of the Anaerobic system is that it can provide energy for some two minutes, which is much better than the 5 – 10 seconds the Sprint system provides. Although the power lasts longer, ATP stores are replaced more slowly and force production in the muscles is significantly lower.
The most important thing to remember about the Anaerobic system is that it can provide significant power for some short period of time, but the efficiency of energy production is poor, so use it sparingly!
Aerobic system (Oxidative phosphorylation)
The last system is the cyclist’s bread and butter, the Aerobic system. If you haven’t guessed from the naming of the Anaerobic system, the Aerobic system uses oxygen to produce ATP from ADP. This oxidative phosphorylation occurs in the mitochondria as opposed to the two other systems that occur outside. The process by which the Aerobic system converts ADP into ATP is complicated and maybe there will be a few flashbacks to high school or college biology with phrases like “Krebs Cycle” and “Electron Transport Chain”. The total process is 124 reactions and uses significantly more enzymes and chemical compounds, and, as a result, is significantly slower at replacing ATP stores than either Sprint or Anaerobic systems.
Despite the slow speed, the efficiency of the Aerobic system is off the charts. Our bodies can produce 36 ATP from a single glucose molecule and the Aerobic system can use fat as an energy substrate. There are no harmful byproducts and the aerobic system can be used indefinitely as long as energy stores, hydration, electrolytes, etc are maintained.
The Aerobic system is relied on heavily by most cyclists. Any effort greater than two minutes is mostly aerobic, so for every one of us except track sprinters and BMX racers, the value of the Aerobic system cannot be understated. Improving the efficiency of this system is paramount to success in the sport of cycling and is the system most professional cyclists spend their time improving.
The chart above shows the relative use of each system. See how for even a five minute effort both Sprint and Aerobic contributions drop below 10%. Also notice for very short efforts, there is almost no contribution from either Aerobic or Anaerobic systems, but as time of effort increases the Sprint system falls off and for efforts of two minutes the Sprint system accounts for less than 10% of total energy supply.
Each energy system has its place in the performance of a cyclist, but understanding when each system is used can help us focus our training in the right areas. If you want to be a track sprinter, train your Sprint and Anaerobic systems. If you want to be a road racer, train your Aerobic system. If you want to be a crit specialist, you probably need to be competent in all three areas. Regardless of who you are and who you want to be, remember to train with a purpose and use your energy systems to find the results you want.