The human body relies on three different energy-producing pathways to fuel our muscles during activity. While most endurance sports will rely heavily on aerobic respiration, it takes all three systems to create varying efforts that you incur during a race or training sessions. High-intensity efforts will utilize a different pathway, as will long-endurance base rides.
To begin, we will look at one of the key players that is responsible for muscular contractions: ATP. ATP or Adenosine Triphosphate is our body’s immediate source for energy. So how do we produce ATP? There are three different pathways:
- Phosphocreatine Breakdown
- Aerobic Respiration
Phosphocreatine Breakdown: Anaerobic ATP Production
Anaerobic ATP Production can also be called your Neuromuscular Energy System. Our muscles’ cells contain very little ATP themselves. With small amounts of ATP storage, the muscle cells run out of energy very quickly, approximately 3 seconds. After initial depletion, the cells rely on our three energy systems to produce ATP in order to sustain a given effort.
Phosphocreatine breakdown is an anaerobic process, meaning that it occurs “without oxygen.” *Although nothing is truly occurring in the body without the presence of oxygen to some level. This pathway’s process is the simplest and most rapid method of producing ATP involving the donation of a phosphate group. This donation is enabled by the enzyme Creatine Kinase. Creatine Kinase is activated within our bodies through the increase of ADP (Adenosine Di-Phosphate) in our cells due to the onset of exercise (muscle contraction.)
This system provides energy for short bouts of high-intensity exercise. This system maxes out its contributions within approximately 20-30 seconds of maximal flux. With continued exercise, this system becomes inhibited with continual increases in ATP.
When you think of a sprint attack or crit racing, your phosphocreatine system works in the background to give you immediate energy when called upon. While performing the classic 2 x 20 minute session is a great way to get into aerobic fitness, don’t forget to stress your short term anaerobic system during training as well. This system requires rest and low intensity in order to produce its best results. When you perform repeated 20 seconds sprints with limited rest, you are depleting the system faster than it can recharge itself. So if your cycling prowess is in the final sprint, it is best to save as much energy as possible in the pack in order to maximize your efforts in the final straightaway throwdown.
Glycolysis: Anaerobic ATP Production
The next energy system to contribute during high-intensity exercise will be Glycolysis. Glycolysis is the breakdown of glucose or glycogen which occurs in two phases. Glycolysis yields 2 molecules of NADH+ (nicotinamide adenine dinucleotide, which plays a role in the transfer of hydrogen and electrons), 2 molecules of ATP, and 2 molecules of Pyruvate.
The fate of Pyruvate is dependent upon the conditions it exists in:
- If oxygen is present: begins the process of aerobic respiration (Krebs cycle)
- If oxygen is not present: converted into lactate
The process of conversion to lactate occurs when glycolysis can no longer continue, and the electron transport chain is saturated due to bouts of high-intensity exercise. We are able to synthesize glycogen through high energy states and increased amounts of glucose. This process is inhibited by low energy states. We can think of this process being used in events such as a 400 meter run or kilometer on the track, lasting from 1-2 minutes. We also often see this energy production pathway turned on and then off frequently during non-steady-state activities like in team sports or during a criterium or cyclocross race with repeating high-intensity energy demand interspersed with limited recovery. It is sometimes helpful to think about this process for producing energy quickly like a battery that can be quickly discharged, and then slowly recharged. Individuals with a high anaerobic capacity have a bigger battery (car battery) versus athletes with a low anaerobic capacity (AA or AAA battery).
Workouts that can help stress your anaerobic system would be repeated sprints with limited rest. For example:
- 3 sets of the following:
- Set 1: 10 x 40 seconds on 20 seconds off; 10 minutes recovery
- Set 2: 10 x 30 seconds on 30 seconds off; 10 minutes recovery
- Set 3: 10 x 20 seconds on 40 seconds off; 10 minutes recovery
Krebs Cycle: Aerobic Respiration
The primary function of this system is to completely oxidize carbohydrates, fats, or proteins using the hydrogen carrier molecules of NAD and FAD (flavin adenine diphosphate). This means that we are trying to attach oxygen molecules to our sources of energy that we derive from carbohydrates, fats, and proteins. We can look at this as a three-step process:
- Generation of a critical two carbon molecule: Acetyl – CoA
- Oxidation of Acetyl – CoA in Krebs Cycle
- Oxidative phosphorylation in the Electron Transport Chain
We can use carbohydrates in this cycle through glycolysis (entering in as pyruvate), fats, which enter through beta-oxidation (enter in as Acetyl – CoA), and protein which is converted into its constituent amino acids and synthesized to glucose in the liver (then enters into glycolysis).
During each turn of the Krebs cycle, we form 3 molecules of NADH, 1 molecule of FADH, and 1 molecule of GTP (guanosine triphosphate – a high energy compound that can transfer a phosphate group to ADP). These NADH and FADH molecules transfer their electrons to molecules near the beginning of the Electron Transport Chain. The ETC moves electrons down a series of proteins to provide energy that drives ATP production in the mitochondria of the cell. These electrons are passed down moving from high energy to low energy and release energy along the way. When oxygen is present, the last enzyme (ATP Synthase) can be oxidized into ATP and re-enter into aerobic respiration, thus allowing for continued energy production. Other byproducts of the ETC are the creation of water and CO2.
Total ATP Tally from 1 molecule of Glucose:
- 2 ATP directly via glucose
- 5 ATP by the energy contained in 2 molecules of NADH
- Krebs Cycle:
- 5 ATP from pyruvate to Acetyl- CoA
- 15 ATP from NADH via Krebs cycle
- 3 ATP from 2 FADH
- Grand Total: 32 ATP
This type of energy system is used for long-distance events and lower intensity training as it can last for hours while glucose is still available.
While all of this can seem a bit overwhelming at a glance, it is important to keep in mind key takeaways. While cycling may seem like it functions solely in our aerobic energy systems, at different points of workouts, races or endurance rides our bodies are cycling through all of our energy systems dependent upon the demands of the sport. If you are looking to gain an advantage over your competition or to beat your next personal goal, understanding what specific needs your body has to perform will help you to better prepare for the demands ahead.
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Mac Cassin is the Chief Cycling Physiologist at Wahoo Sports Science. He holds a degree in Integrative Physiology from the University of Colorado-Boulder and has won multiple National Championships. The experience of juggling athletic goals with collegiate and career responsibilities has taught Mac that peak performance is achievable even for those who cannot focus exclusively on training. While concentrating on exercise physiology in an academic setting, Mac competed at the World Championships, Pan American Championships, and World Cups on both the road and track.