Continuing the Level 3 Gym Instructor series, this article is about the energy systems muscles use.
For muscles to contract they need a continuous supply of energy in the form of ATP, but as it is too unstable to be stored within muscle it has to be made as needed. Even a resting muscle uses ATP although the amount is relatively low, especially compared to a high-intensity workout which can involve billions of ATP molecules.
When ATP is used in a muscle contraction it breaks down into adenosinediphosphate (ADP) and a single phosphate group (P), both of which are stable and can therefore be stored. All muscles have many of these inside them and by combining the two, ATP can be regenerated. This requires energy that comes from the stored form of protein, fat, but mostly carbohydrates, which is stored in the muscles as glycogen.
Glycogen is a large polymer of individual glucose molecules, which are metabolised to provide the energy to make ATP by enzymes. These are activated by the presence of hormones in the blood such as adrenaline from the adrenal gland and glucagon from the pancreas. These hormones also stimulate the release of fat in the form of fatty acids from adipose cells which are mostly stored under the skin. The fatty acids are transported in the bloodstream to muscle cells, where along with glucose they can be broken down to release energy to drive ATP synthesis from ADP and P.
The production of ATP takes time however, at least 10-20 seconds even by the fastest method, and for fat to be involved in the process there must be sufficient oxygen. Only glucose can do so without it. At the start of an exercise session the capillaries within muscles are not fully dilated and the heart rate is still rising to meet demand, so muscle cells are generally anaerobic at this stage. This is one reason why warming up is such a good idea.
Although carbohydrates, fat and even protein can be used to generate ATP, availability is also a factor. Even the leanest of athletes has many thousands of calories of stored fat on his body, but glycogen stores are limited to about 2000 calories in the muscle and liver. Also, once glucose is stored in a muscle cell, only that muscle cell can use it.
Glucose stored in the liver is intended for the central nervous system (CNS), which includes the brain, and it gets there by being released into the bloodstream. If a muscle uses up its own supply it will take whatever is in the blood, including what was meant for the CNS, then when this runs out we either eat or the liver makes its own glucose by a process known as gluconeogenesis. Unfortunately there is no way for the body to use fat for this as glucose is part of the process, which only leaves protein in the form of amino acids. The only source is muscle.
Energy System 1 – Creatine Phosphate
The ATP stored in muscle is used up within 2 seconds of exercise commencing and although the breakdown of glucose has started, it will be several seconds before this generates sufficient ATP. As there is still insufficient blood getting to the muscles, fat cannot be utilised for anything from 30 seconds to several minutes, depending on fitness levels. This is another reason why warming up before training is such a good idea.
The solution to this is use creatine phosphate (CP), which is a high-energy compound also stored in muscle. While it doesn’t directly cause muscle to contract, it can easily donate its phosphate to ADP to make ATP in just one step.
CP + ADP = ATP + C
The amount of Creatine Phosphate in a muscle is sufficient for about 10 seconds worth of activity, by which time other energy systems have become productive. It can be re-synthesised from available phosphate with the help of the enzyme creatine kinase.
C + P = CP
By the time ATP is generated by other means, Creatine Phosphate might not be required for the remainder of the workout, unless it involves low intensity to phases of maximum effort such as interval training. Footballers who continuously alternate between jogging, walking and sprinting during a game will be depleting and then replenishing Creatine Phosphate throughout.
Creatine phosphate can be obtained from meat but it is also made in the liver, which is then fed into the bloodstream for muscles to use as required. The more that is used the more is made, and the more the Creatine Phosphate system is used the more of it can be stored inside muscle. While it is unlikely that the liver will not be able to meet demand, creatine supplements can help high performance athletes in relevant training. It is obviously of no use to exercise that does not intensively involve the creatine phosphate energy system.
Energy System 2 – Anaerobic Lactic Acid System
When in the early stages of exercise where there is still insufficient oxygen getting to the muscles being worked, glucose is the only thing that can be metabolised for ATP synthesis. This also happens during high intensity phases of exercise at a point called the Anaerobic Threshold, which is dependent on fitness level. For some this can be at more than 90% max heart rate, for others it can be much lower.
A single glucose molecule can generate 38 molecules of ATP, but it requires oxygen and takes time to go through the many steps involved. Without oxygen, 3 molecules can be produced from 1 glucose molecule in a process called Glycolysis, which is a lot less efficient but a lot faster. In a minute the same amount can be produced without oxygen as with oxygen, however it will use up a lot more glucose and rapidly deplete stores. It also generates Lactic Acid.
ADP + P + GLYCOGEN = ATP + LACTIC ACID
Type 2 muscle fibres are designed for glycolysis (without oxygen) but not the presence of lactic acid, which prevents further ATP synthesis when it builds up and therefore muscle contraction. This can only be dealt with when the workout intensity drops to a point where the oxygen level in the bloodstream is sufficient. Then the lactic acid can be flushed out and taken to other cells, such as type 1 muscle which can use it for aerobic ATP production, or the liver, which can convert it to glycogen for storage.
Energy System 3 – Aerobic
Once exercise begins it takes time for maximum oxygen delivery to the working muscles, but depending on fitness levels some will be available after just a few seconds and steadily increase. This allows the most complete processing of glucose for ATP production, generating only carbon dioxide (CO2) and water (H2O) as a by-product. Plus it allows fat to be used to make ATP, preserving glycogen stores for any high-intensity phases that are to come. Fat metabolism can only occur in the presence of glucose or when this runs out, amino acids which would come from muscle catabolism. In other words, continuing to exercise when glucose stores are depleted means breaking down the very muscle being trained.
Aerobic ATP generation happens inside mitochondria, which are found inside type 1 muscle cells. These cells have a good supply of capillaries to bring oxygen and myoglobin to quickly transfer it to the mitochondria. Type 2 cells can do some aerobic work but it is far more limited.
ADP + P + O2 + GLYCOGEN = ATP + CO2 + H20 and FAT
The anaerobic energy system is only used exclusively at extremely high work rates when the other energy systems cannot meet the demands of the exercise, at other times it works together with the aerobic system. It is only the contribution each one makes to ATP production that varies, which is dependent on the intensity of the activity and the fitness of the individual.
As always, any questions or feedback leave a comment below.