Energy Systems
The body has several ways by which it fuels the muscles for the production of force during movement. There are two main types of reaction, anabolic (building reactions, requiring energy) and catabolic (deconstructive reactions, consuming energy).
The total amount of energy (anabolic-catabolic reactions) used in the body is known as metabolism.
To facilitate the body’s ability to metabolise, it uses a specific molecule as universal currency when providing the energy required during reactions. This molecule is made up of a single Adenosine molecule and three Phosphate molecules, giving it the name adenosine triphosphate or ATP. This is illustrated below.
The ATP molecule has its bonds with one phosphorus molecule broken using a single molecule of water, in a process known as hydrolysis.
This creates ADP (adenosine diphosphate), and the process can repeat to produce AMP (adenosine monophosphate). This process of severing the phosphate bonds is what releases the energy required for muscle contraction, and therefore movement.
The body then uses one of several mechanisms to restore ATP within the body, depending on the level of exertion being applied during movement;
These are categorised into two main functions.
- Replenishing the ATP stores using oxygen, Aerobic, and
- replenishing the ATP stores without oxygen, Anaerobic
When it comes to weightlifting, it’s massively anerobic when lifting, but this doesn’t mean that’s the end of it. You get out of breath from high intensity training, which is the best clue, that to recovery between sets, and between sessions, there’s a huge aerobic demand as well.
Aerobic (low intensity)
At exercise intensities lower than the anaerobic threshold, the body utilises molecular stores in the presence of oxygen to produce ATP, this is the Aerobic system, also known as ATP-Oxidative.
In this system, the body can utilise macronutrients to generate the required ATP. The nutrients the body utilises are:
- Fat
- Protein
- Carbohydrate
- Alcohol
The primary fuel in the production of ATP in the oxidation system is the metabolisation of fat; in which triglycerides, stored within the fat cells, are broken down by lipase into fatty acids and glycerol. These are then broken down in the mitochondria through the Krebs cycle to produce ATP. During extremely long periods of exercise, protein is broken down into amino acids to undergo this same process.
In essence, the aerobic system, consumes fat to produce ATP for energy, water, and carbon dioxide, which is then exhaled via the lungs.
In a later module, we’ll explore how the body adapts to become more effective at handling oxygen when trained aerobically.
Anaerobic
The Anaerobic system is broken down into 3 further systems.
- ATP Glycotic: the lowest intensity of the three, the gyloctic system kicks in just above the aerobic threshold
- ATP-CP: the highest intensity system that still replaces ATP, where the body utilises creatine within the muscle cell to replace ATP during very high intensity training eg: 200m sprint.
- ATP: The highest intensity training only consumes ATP, it is too high to synthesise replacement molecules. As such this is sometimes not referenced as an energy system at all, as the energy is only consumed, and ATP cannot be synthesised until intensity drops. This would include sports such as olympic weightlifting, where full lifts take less than 2 seconds, as ATP stores can only last for around 4 seconds at most.
Glycotic:
At slightly higher intensity levels, the body uses a process of Glycolysis , breaking glycogen down into glucose, a simple carbohydrate, giving rise to the production of Lactate, more commonly referred to as Lactic Acid. The body can utilise this process for a much longer period of time than ATP-CP and ATP systems below, as the raw materials, simple carbohydrates such as glucose are found in much higher quantities than Creatine Phosphate (CP). When intensity drops back below the anaerobic threshold, the body stops producing lactate, and instead funnels the waste product (pyruvate) into the mitochondria to be recycled with oxygen, this is the Krebs Cycle. If the lactate is transferred into the blood, it is transported to the liver, where it is again converted into glucose (cori cycle). One of the physiological adaptations to exercise, both aerobic and anaerobic is an increased ability of the body to clear lactate and recycle it into usable energy, ATP. one example of training to stimulate this adaptation is interval based training.
ATP-CP:
For short term activities, the body replaces the ATP used through the breakdown of Creatine Phosphate, this reaction creates new ATP molecules by supplying the ADP or AMP molecule with an additional phosphate molecule(s), creating ATP and Creatine as a result. This is known as the phosphagen system, or ATP-CP on the energy system spectrum. While the body will not fully deplete this CP store, as it also requires it for essential function, the CP is stored in the muscle tissue to provide a quick ready supply of energy until intensity levels reduce sufficiently to produce ATP aerobically.
ATP:
During the most explosive movements, eg: Olympic weightlifting or acceleration during sprinting, this replacement of ATP does not occur until the body begins to rest. The body simply uses up the ATP stores, which can last up to around 4 seconds. This occurs at the most explosive and powerful end of the spectrum.
So putting it all together:
When training,
- Warm up – aerobic. moving into some use of ATP only with higher output movements, but still aerobic on recovery between sets.
- High volume eg: starting phases of new programs, muscle mass increase – Hypertrophy focused training etc. Lactate system and ATP-CP
- Lower volume, high intensity (heavy) lifts such as latter stages of a program, with a lot of high percentage lifts, hugely ATP only with later warm ups being ATP-CP
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