Understanding training adaptations: how exercise transforms your body inside and out (part 1)
Training adaptations refers to the physiological changes that occur within our body systems in response to the physical stresses placed upon it through training and exercise.
These adaptations can improve future physical performance and positively impact life expectancy, mood, sleep quality, concentration levels and reduce the rate of injury.
There can be an obvious transformation that occurs in a person’s body when they begin exercising, such as weight loss and/or muscle growth.
However, many subtle changes are occurring beneath the surface to create these aesthetic shifts, and there are a host of adaptative responses that result in improved fitness, health and performance.
Below, we will explore how exercise and training results in different changes in our body.
What happens during exercise?
Exercise means we are moving, and our muscles are contracting. Exercise stimulates the sympathetic nervous system, causing the body to respond as a whole.
This response helps maintain homeostasis to meet the higher physical, metabolic, respiratory and cardiovascular demands placed on our body through exercise.
Most movement and contraction during exercise involves skeletal muscles, and due to the increased workload, skeletal muscles need more oxygen.
But, there are some other muscle types involved in exercise too; let’s have a quick look at those.
The three types of muscles involved during exercise
When we exercise, our body's systems are intricately involved to support the increased demand for energy and oxygen.
Three types of muscles play a crucial role in this process - skeletal, smooth and cardiac muscles.
Skeletal muscles: these are the muscles we consciously control during strength workouts, such as the hamstrings, pecs and biceps. They enable movement by contracting and working in tandem with our tendons and bones.
Smooth muscles: smooth muscles are involuntary and function automatically without conscious effort. Their presence within our blood vessels enables control of vascular dilation and constriction i.e. blood flow.
Cardiac muscles: are only found within our heart and are involuntary muscles. These are what create the pumping action of our heart.
Respiration: working muscles need oxygen
Working muscles need oxygen to continue working. Our intake of oxygen (through the lungs) must increase above what we bring in under normal circumstances.
De-oxygenated blood arrives at our lungs via the pulmonary artery. The lungs are made of hundreds of millions of small sacs called alveoli. This is where gas exchange takes place.
When we breath in these tiny sacs ‘inflate’ with oxygen and exchange it with the carbon dioxide in the de-oxygenated blood.
As we breath out, carbon-dioxide is exhaled, and the oxygenated blood travels through the pulmonary vein to the heart, where it will be dispersed throughout the body.
In terms of respiratory mechanics, our lungs inflate and deflate due to a change in the volume (and therefore pressure) within our rib cage.
This is achieved by the expansion and compression of the rib cage through the action of the intercostal muscles, as well as contraction and relaxation of the diaphragm muscle which sits beneath the lungs.
Ventilation during exercise
Ventilation increases abruptly in the initial stages of exercise and is then followed by a more gradual increase.
This increase is as large as from 5 – 6 litres per minute at rest, to over 100 litres per minute.
The respiratory rate may even stay elevated for 1 – 2 hours after intense exercise, due to the body’s ongoing need for oxygen as recovery processes commence.
The cardiac system during exercise
To distribute oxygen (and fuel) to the working muscles, we must also pump this oxygenated blood more quickly through the body and open up passageways to working muscle.
During exercise, blood is moving through your body at an increased rate. Your heart is beating faster, which consequently increases your blood pressure.
How much your blood pressure rises during exercise is proportional to how hard you're working.
Limitations to oxygenation of working muscles is due to cardiovascular function rather than lung function.
This means that oxygen utilisation by the body can never be more than the rate at which the cardiovascular system can transport oxygen to the tissues.
For this reason, our heart can deliver oxygen more efficiently by increasing the rate of contraction (heart rate) and the volume of blood per contraction (stroke volume).
Heart rate (HR) and stroke volume (SV) increase to about 90% of their maximum values during strenuous exercise.
These individual metrics (HR + SV) combine to give us a measurement known as cardiac output.
The increase in blood flow to muscles requires an increase in the cardiac output, which is in direct proportion to the increase in oxygen consumption.
How cardiac output is controlled
This change in our hearts’ activity - in responses to oxygen demand - is carefully controlled by our autonomic nervous system, which regulates all the involuntary functions in our body.
This automatic part of our nervous system has two key sub-systems: the sympathetic and parasympathetic systems.
The sympathetic system is responsible for our ‘flight or fight’ responses, and so upregulates activity i.e. increases heart rate and dilates blood vessels.
The parasympathetic nerves that innervate our heart, lungs and blood vessels downregulate activity.
All the automatic functions in our body require input from these two branches of the nervous system and the timing and degree of their input is governed by the feedback being received about the conditions throughout our body i.e. oxygenation, blood glucose, pain, etc.
What happens to our body temperature during exercise
Only 20 - 25% of energy nutrients convert to muscular work - the rest is released as heat, raising body temperature.
To get rid of this extra heat, more blood flows to the skin. This happens through the widening of blood vessels in the skin by reducing vasoconstriction.
Evaporation of sweat is also a major pathway for heat loss, and further heat is lost in the expired air with ventilation.
Fuel utilisation in the body
Our muscles need fuel to continue contracting, and so certain hormones begin mobilising stored fat and glycogen into glucose.
Adenosine triphosphate (ATP)
Adenosine triphosphate (ATP) is an essential molecule found in cells and is known as the ‘energy currency’ of life because it's the universal energy source for all living cells.
ATP is made by converting food into energy. Without ATP, cells wouldn't have the fuel or power to perform functions necessary to stay alive.
Muscle contraction is a necessary function of everyday life and could not occur without ATP.
Fuel sources: glucose/glycogen
During exercise, skeletal muscle relies on various fuel sources to meet its energy demands. Glucose, lipids and to a lesser extent, amino acids, are the key contributors to this energy production.
Certain processes exist within cells that allow us to use glucose (carbs) and lipids (fats) to regenerate ATP.
Muscle glycogen is particularly critical during high intensity activities as it provides an immediate source of glucose, allowing for quick bursts of energy.
Glycogen is also stored in your liver and muscles and comes from carbohydrates in the foods you eat and drink.
The liver plays a crucial role in maintaining blood glucose levels by releasing glucose into the bloodstream, ensuring a stable supply of this essential energy source.
While glucose and glycogen are chemically similar, they serve distinct roles in providing your body with energy.
Glucose is a simple sugar and the primary energy source for your body's cells. It's always present in your bloodstream, ready to be used for immediate energy needs.
Glycogen, on the other hand, is the stored form of glucose – it is a complex carbohydrate that is primarily stored in the liver and muscles.
It serves as a backup energy reserve and is tapped into when available glucose levels start to decline, such as during fasting or with high intensity or prolonged physical activity.
Other fuel sources
The body is also able to use fat (lipids) as a fuel supply, which is stored in within adipose tissue throughout our body.
Fat is a very rich supply of energy relative to its mass, which is why our body is good at storing a lot of it!
Like glycogen, lipids need to be broken down from their stored form into simpler molecules to be used for energy.
This is known as lipolysis. It takes longer for our body to perform this, which is why it is a more suitable fuel supply for much longer duration activities.
Note: with insulin resistance, your storage capacity for glycogen becomes maxed out. Instead of excess glucose being stored as glycogen in your liver and muscles, glucose gets converted and stored as metabolically unhealthy visceral fat.
Metabolic flexibility
The ability to smoothly transition between fuel sources, especially glucose to lipids, is known as ‘metabolic flexibility’.
Being able to tap into both glycogen and fat during workouts means better endurance and sustained energy levels. This flexibility allows for improved performance and recovery.
Enhancing metabolic flexibility can help in managing weight, reducing the risk of metabolic diseases, and improving overall metabolic health.
Fuel utilisation within skeletal muscle
Fuel utilisation withing working muscles doesn’t occur in isolation. It is its interaction with other tissues and organs that sustains their ability to keep working during exercise.
The body’s dynamic requirements during exercise result in constant changes in blood flow to the muscle, shifts between the pathways of energy production within muscle cells, and the support of substrates (fats and carbohydrates) from other areas of the body.
The intensity and duration of exercise will affect the balance between carbohydrates and fats as energy sources.
After exercise, there is a significant shift toward lipid oxidation during the recovery period, which is vital for muscle glycogen replenishment.
The physiological response to exercise is dependent on the intensity, duration and frequency of the exercise, as well as environmental conditions, and is unique to every individual.
Stay tuned for part two which focuses on the muscle adaptations and the different types of training.