#6 [MedBlog] - Exhaustion: I feel exhausted... but why actually?
Hello Hivers!
I guess every single human ever witnessed exhaustion – after a long day, exhausting work shift or workout. In this episode we want to deal with “exhaustion”. How is it defined? Which factors lead to exhaustion? Are there any differences between male and female? Why are older people faster exhausted? Why am I unable to keep the weight up? Why do I get exhausted of studying all day long?
In this episode you will find out more!
Exhaustion: Definition & Differentiation
To keep it concise in the beginning: exhaustion describes the inability to maintain a given physical strain.
In theory you can differentiate between peripheral and central exhaustion that influence each other.
The extent of exhaustion and the time until exhaustion occurs depend mainly on…
• the type of stress (intensity & duration) ⚡️🤸♀
• the training and nutritional status 💪 🍎
• age & gender ♂️ ♀️
• environmental factors (temperature ☀️, humidity 💧)
• psychological factors 🧠 (especially motivation)
Periphal exhaustion includes the energy reserves, depletion & accumulation of different metabolites. The central exhaustion refers to the oxygen supply, thermoregulation & psychological factors
Peripheral Exhaustion
Energy Reserves
In order to avoid the need for a permanent supply of energy, the body has different energy stores that are used up more or less quickly.
ATP Pool
ATP is the energy currency of our body – it is a molecule that is easy to transport and can store energy efficiently. In our muscle cells the amount of ATP available is enough for 2 – 4 seconds of muscle contraction.
• Energy content: 10 kJ (2 kCal) per kilogram of lean muscle mass.
Creatine phosphate (PCr)
This molecule can restore ATP out of ADP by donating the bound phosphate of the Creatine.
• Energy content: 35 kJ (8 kCal) per kilogram of lean muscle mass
Glycogen Reserves
Glycogen is the storage of glucose in our body in form of a long-branched chain of glucose molecules. Trough Glycogenolysis, splitting single glucose molecules from glycogen, and Glycolysis, breaking down the glucose to generate energy in from of ATP, our body can provide energy in a quick manner.
• Energy content: 4,800 – 8,000 kJ (1,200 – 2,000 kCal)
Fat
An average male has 10 – 20 % body fat and an average female 20 – 30 % - this would approx. mean 10 – 20 kg of fat for the average Joe, which is the biggest energy storage of our body.
To transform fatty acids to energy during the Beta Oxidation Oxygen is essential.
• Energy content: 400,000 – 800,000 kJ (100,000 – 200,000 kCal)
Protein
In theory e.g. the proteins of our muscles can be broken down and used to provide energy – and average Joe has 10 kg of lean muscle mass.
• Energy content: 80,000 – 160,000 kJ (20,000 – 40,000 kcal)
Metabolite depletion
Depending on the level of intensity our body recruits different types of muscle fibers that have an impact on the depletion and accumulation of metabolites.
Simply put, there are 2 types of muscle fibers:
Type 1 muscle fibers: designed for endurance exercise with low intensity.
• Usage of fat as their energy source (slow energy reserves) ➡️ aerobic energy
• smaller diameter
• slow fatigue
Type 2 muscle fibers: designed for intensive work with high power development in a short time.
• Usage of PCr & Glucose as their energy sources (fast energy reserves) ➡️ anaerobic energy
• Thicker diameter with more muscle fibers
• Quick fatigue
Different muscle fibers work at different intensities and use up different type of energy resources of our body. This is the reason why you can walk a mile without a problem (assuming you are a healthy human being) but need to rest after a 100 meter sprint.
An elite endurance athlete is able to maintain a higher intensity of work without depleting its ATP pool by providing constantly energy due to an optimized metabolism. Nevertheless it is recommended to consume Carbohydrate drinks during endurance exercises.
Metabolite Accumulation
During the anaerobic energy supply an accumulation of different metabolites is inevitable – the pH shift is the most performance-limiting factor. The acidosis is caused by lactate, a product of anaerobic glycolysis. Due to the lack of perfusion and oxygen supply the muscle cells have to switch to a way to generate energy without the need of oxygen, which is ultimately the anaerobic glycolysis. Due to the pH shift the enzymes work out of their optimal range.
An acidic environment means an excess of protons (H+) – those protons e.g. compete with the calcium on the troponin of the muscle fibers, which influences the contractility of the filaments, or activate pain receptors inducing nausea and vomiting. There are a lot of other interactions which, however, would go beyond the scope here.
The anaerobic threshold is the performance intensity from which a further increase in intensity leads to a performance-limiting lactate accumulation, therefore it can also be interpreted as a maximum continuous power limit or endurance limit.
So, the lactate accumulation is caused by the increased recruitment of Type 2 muscle fibers, a decrease of the lactate clearance and decrease in the supply of oxygen to the muscle.
In addition to that the calcium homeostasis is disturbed leading to a less efficient muscle contraction cycle. Due to the mechanical and metabolic load damage is being done to the muscle cells – they are heavier to activate, their metabolism on a cellular level is less efficient and therefore it is much more difficult to work with the same load.
If we compare man to woman there is no difference in the accumulation of metabolites during performance of work – the higher average cardiac output of men leads to a better lactate clearance. In old age we see a decrease in metabolism & repair processes and the more likely lower cardiac output impacts the clearance of metabolites during/after work.
Central Exhaustion
The central regulation mechanisms, like thermoregulation and increase in oxygen supply, counteract the exhaustion until they hit their maximal capacities.
Oxygen Supply
A sufficient supply of oxygen is necessary for maximum endurance capacity. The V(O2 max) describes the maximum oxygen capacity meaning the oxygen volume the body can utilize per minute under maximum load. The V(O2 max) depends on the breathing limit, oxygen content of arterial blood, cardiac output and the arteriovenous oxygen difference.
The breathing limit is the maximum product of respiratory rate and the tidal volume. The respiratory time volume can reach up to 120 - 170 litres per minute.
The oxygen content of arterial blood depends on the concentration of haemoglobin, therefore erythrocytes. The more erythrocytes you have, the more oxygen you can transport from the lungs into other parts of the body.
At rest, the skeletal muscles receive approx. 20% of the cardiac output, which increases up to 80% under load. This is achieved by the constriction of the blood vessels in the abdominal organs and the dilatation of blood vessels in the muscles.
The cardiac output is increased by the higher performance of the hearth provided by the effect of e.g. adrenalin and a higher blood pressure. An untrained person can increase its cardiac output up to approx. 20 litres per minute, while a trained person pump approx. 30 – 40 litres per minute.
More muscles, higher haemoglobin concentrations in the blood and a higher cardiac output increase the V(O2 max) – on average, this is what is naturally given to a man, but no reason to jump to conclusions: training effects clearly exceed gender effects, so that sporty women easily outperform untrained men.
In old age or from the third decade of life, the V(O2 max) decreases – the cardiac output, lung capacity, muscle mass and the haemoglobin level decreases over time naturally. Likewise the training effect outweighs the age effect. Without training the V(O2 max) decreases about 1 % per year – however a 70 year old man, who trains regularly, has the same maximum oxygen capacity as an untrained 40 year old man.
Thermoregulation
Bad news for all of us – our body generates energy in an inefficient way. Only 20 % of the released energy is given off as mechanical power, while 80 % is released as heat. So, it is imaginable that during load our body needs to get rid of all the excess heat to prevent overheating.
Therefore, our body needs to increase the skin perfusion & evaporation to cool down. Logically the perfusion of the other organs goes down which leads into lower oxygen and nutrient supply for the muscles.
The maximum sweating rate is 3 litres per hour. Dehydration of about 4 % water of the body weight leads to a huge decline of performance. A heat collapse threatens from dehydration of approx. 7% of the body weight. Trough dehydration the total volume of your blood system decreases leading to a higher heart rate trying to compensate.
It is important to know that you should drink max. 1 litre per hour for rehydration, because that is approx. the max. absorption performance – approx. 6 g per litre of sodium chloride and appropriate amounts of kalium and magnesium is crucial to keep a healthy electrolyte balance.
By getting older your body water percentage, sweat production and skin perfusion goes down. In addition to that you feel less often thirsty leading to a lower water intake. Many factors that limit thermoregulation in old age.
There is no significant difference in gender.
Mental Exhaustion
Mental exhaustion is often the result of monotonous endurance work due to the transmitter depletion and increase in adenosine in the Reticular formation, a big autonomous area in the brainstem, and in the Ncl. Preaopticus, which is the main control centre for tiredness and wakefulness. The result is the impairment of information transfer, sensory perception, coordination of movements, motivation & pain tolerance.
Checklist
By now ...
✅ you know how exhaustion is defined
✅ you are familiar with the crucial factors leading to exhaustion
✅ you know the components of central and peripheral exhaustion
✅ you got to know some basics of exercise physiology
Do not forget to follow in order to not miss out on new content!
See you soon!
Noogie 👨⚕
Disclaimer
The content shown here is no alternative to consulting a doctor – if you have any kind of health issues bothering you, firstly consider visiting a health-expert. This is just meant to feed your personal interests. All the information given are related to the German standards