Bridging the Gap: The bodies energy systems

In earlier articles in this series, we have seen that motor training can induce constructive neuroplasticity providing that the tasks are practiced in a particular way and are challenging enough to stimulate positive adaptations. The human body is natural adaptive to the stimulus - or lack of it - that is provided.

Athletes learning new skills and training for peak performance in strength or endurance must draw on the same fundamental body resources as those individuals recovering from a neurological condition. There are many differences however in how to approach training due to the nature of the bodies energy systems and how these are influenced by disability.

When we are encouraged for rehabilitation purposes to train intensively, frequently etc we need to be aware of these energy systems and how they affect fatigue and the perception of effort.

In the case of stroke, for example, it has been proposed that motor training efforts might be adversely affected by fatigue which is a common consequence of the condition. (Sterr & Furlan, 2015)

• Chronic stroke survivors are typically in a state of general deconditioning due to an inactive lifestyle and compromised neural processing caused by the lesion.

• Fatigue is likely to be more pronounced in individuals with poorer motor recovery, due to both a higher likelihood of inactivity and a more severe neural deficit in this population.

• Exacerbated fatigue, in turn, might critically reduce the individual’s engagement with the motor training, and hence interfere with the neuroplasticity driving recovery.

With this model, fatigue status in chronic stroke is made worse by both general deconditioning and neural “inefficiency”.

Chronic stroke individuals are in a cycle of general deconditioning, where their cardiorespiratory and musculoskeletal condition leads to physical inactivity and paretic limbs disuse. Such disuse produces further deterioration in function.

During motor activities, skeletal muscle energy reserves are rapidly depleted causing rapid declines in force production.

The impaired cardiorespiratory and musculoskeletal system further impact on the supply to contracting muscles of oxygen and nutrients – and a deteriorated musculoskeletal function – in terms of reduced capillary density and oxidative capacity limiting muscle energy production

After hemiparetic stroke, primary brain circuits controlling skillful motor behaviours are disrupted to different degrees. The resulting compromised neural processing state manifests itself as increased mental effort for processing movement information during motor tasks. In chronic stroke, this translates into a pattern of widespread brain activation, characterised by enhanced activity in potentially spared primary networks and recruitment of many secondary circuits for movement control. This increased brain activation likely reflects a condition of elevated neuronal metabolism, which favours rapid depletion of brain energy reserves. This, in turn, might elevate perception of fatigue during motor tasks. Besides, an imposed reliance on secondary, less specialised motor control networks is likely to also increase fatigability in chronic stroke individuals owing to deficits in motor task-related neural processing.

The Energy Systems

For all humans, carbohydrates, fats and protein in our diet provide the basic source of energy for the body, but before that energy can be put to work it needs to be converted to something called ‘ATP’.

ATP (adenosine triphosphate) is the primary energy source for the body's cells, and it is made up of an adenosine molecule (the A part) and three phosphate groups (the T part). The phosphate (P part) is known for forming chemical bonds loaded with energy and easily releasing them.

When ATP is split to release energy in the body, it loses one ‘P’ of the three and becomes ADP (adenosine diphosphate).

When the body is challenged to perform work, the ATP available depletes rapidly.

Stored ATP can power only about one and a half seconds of high intensity effort, so ATP must be constantly replenished by the three energy systems of the body which are the phosphagen system, the glycolytic system, and the oxidative system. These systems work together to produce the energy needed for various activities.

1 Phosphagen system: The phosphagen system is the fastest way for the body to produce energy. It uses creatine phosphate (stored in muscle cells) to quickly regenerate ATP (adenosine triphosphate), which is the primary energy source for the body. This system is used for short, explosive bursts of energy, such as sprinting, jumping, or weightlifting.

2 Glycolytic system: The glycolytic system provides energy for activities that last longer than a few seconds, but less than a few minutes. It breaks down glucose (carbohydrates) into ATP, which can be used by the body. This system can produce energy quickly, but it also produces lactic acid, which can cause fatigue and muscle soreness. This system is used during activities such as high-intensity interval training or weightlifting.

3 Oxidative system: The oxidative system provides energy for activities that last longer than a few minutes. It uses oxygen to break down carbohydrates, fats, and proteins to produce ATP. This system is slower to produce energy, but it can produce energy for extended periods of time without fatigue. This system is used during activities such as long-distance running, cycling, or swimming.

As stated above, the energy systems work together to provide the energy needed for various activities. The body constantly adjusts the use of these systems based on the intensity and duration of the activity.

Basically the creatine phosphate and aerobic systems are efficient and “clean burning” whereas the glycolytic process is responsible for the muscles producing acid which has both short and long term negative effects.

The creatine phosphate system is much more powerful than the glycolytic system and the aerobic system but can sustain maximum power for only about five seconds. The more depleted the creatine phosphate system becomes, the more its effect is dialled down.

When the three energy pathways are unable to keep up with the demand for ATP a fourth energy system can take over.

The myokinase system, also known as the adenylate kinase system, is a metabolic pathway that helps replenish ATP. It involves the transfer of a phosphate group from one ADP (adenosine diphosphate) molecule to another, catalyzed by the enzyme myokinase/adenylate kinase.

The myokinase system allows two ADP molecules to be converted back into one ATP molecule and something called AMP; providing a rapid means of replenishing ATP during times of high energy demand. The AMP still has one phosphate group but does not store any more energy.

This system can be especially important during short bursts of high-intensity exercise, such as sprinting or weightlifting, when the body's demand for ATP is increased beyond normal demands. The myokinase system is one of the ways the body maintains energy balance and allows for sustained physical activity beyond about ten to twenty seconds of all out effort.

The AMP biproduct of this energy emergency has an interesting benefit - it triggers mitochondrial growth.

Professor Felix Meerson, a Russian Cardiologist, is credited with the initial discovery that ATP breakdown could bring about mitochondrial growth. There is something called ‘AMPK’ which serves as a master switch or sensor to determine when mitochondrial synthesis is required. It seems that AMPK reacts to very high rates of ATP use - that is when ATP cant be replenished fast enough.

Coaches training athletes might know that a 30 second, all out effort, can increase the AMP/ATP ration by 20 times and lead to significant increases in AMPK, signifying mitochondrial growth. However, if the myokinase emergency continues for longer than 30 to 40 seconds a reaction called ‘Deamination’ kicks in driven by a high concentration of lactic acid. This is bad news as it depletes AMP, deterring mitochondrial growth and producing ammonia which is very toxic for cells.

Exhausting the pool of ATP takes time to restore and until that happens any individual in this state will feel exhausted. Patients suffering from chronic fatigue syndrome have abnormalities in mitochondrial function and reduced ATP resources for example.

Untrained and inactive disabled people, deaminate easier than athletes which means that pushing untrained, deconditioned people too hard is not a healthy thing to do.

Why bother thinking about these energy systems? Well they guide they way we should think about physical training.

Currently, there is no direct method to measure the status of all these energy systems in real-time within the human body. However, several indirect methods can provide useful information about their activation and relative contributions during exercise. These are beyond the scope of this article but include Lactate Threshold Testing, Heart Rate Monitoring, VO2 Max Testing, Metabolic Gas Analysis and Muscle Biopsies

Reference

Sterr, A. & Furlan, L. (2015), A case to be made: theoretical and empirical arguments for the need to consider fatigue in post-stroke motor rehabilitation, Neural Regen Res, 10, 1195-1197.