When we start exercising, numerous biological and physiological changes occur in our body that promote both short-term and long-term health benefits. These changes are the result of complex interactions between our muscles, heart, lungs, hormones, and nervous system. Let’s break down the key scientific processes that happen when we begin to engage in physical activity, supported by current research.
Muscle Contraction and Energy Use
When you exercise, your muscles contract to perform movement, and this process requires energy. The body primarily relies on adenosine triphosphate (ATP), the energy currency of the cell, to fuel muscle contractions. During exercise, your body uses ATP, and as ATP stores deplete, your cells switch to other energy sources, such as glycogen (stored carbohydrate) and fat.
Glycolysis (breaking down glucose into pyruvate) happens in the absence of oxygen (anaerobic metabolism), providing a quick but limited energy source during intense exercise (e.g., sprinting). Because of the quick and urgent requirement the food that we eat are looked at as the major source of energy here.
Oxidative phosphorylation (aerobic metabolism) kicks in during more moderate-intensity or prolonged exercise. Here, fat and glycogen are used to produce ATP in the presence of oxygen, providing a longer-lasting energy source.
Increased Heart Rate and Blood Circulation
As exercise begins, the heart rate increases to supply more oxygen and nutrients to the working muscles. The autonomic nervous system (ANS) controls this process. When you exercise, your body releases catecholamines (adrenaline and noradrenaline), which signal the heart to pump faster and more forcefully.
Research shows that aerobic exercise (such as running, swimming, or cycling) improves cardiovascular efficiency by increasing the size of the heart’s chambers and enhancing the ability of the heart to pump blood (stroke volume). Studies have shown that regular exercise can lead to lower resting heart rates and improved heart health over time. This adaptation is called cardiac remodeling.
Increased Breathing Rate and Oxygen Delivery
The lungs also adapt to physical activity. As exercise intensity increases, your body needs more oxygen. Breathing rate and tidal volume (the amount of air inhaled per breath) increase to meet this demand. The respiratory muscles become more efficient, and there is an increase in pulmonary ventilation (the movement of air in and out of the lungs).
Oxygen uptake (VO2) is a key measure of aerobic fitness. As you begin exercising, your VO2 increases because the cardiovascular and respiratory systems work harder to deliver oxygen to the muscles. Over time, your VO2 max (the maximum capacity to consume oxygen during exercise) increases, which is a sign of improved aerobic fitness.
Hormonal Response and Adaptation
Exercise triggers a cascade of hormonal responses that support energy production, muscle growth, and fat loss. Endorphins, often referred to as “feel-good” hormones, are released during exercise and help improve mood and reduce pain perception. This is one reason why exercise is often associated with improved mental health.
Cortisol, a stress hormone, is released in response to physical stress, including exercise. Cortisol helps break down glycogen and fat to fuel the body during intense physical activity. However, chronically elevated cortisol levels (due to overtraining or stress) can lead to negative health effects, such as muscle breakdown and fat accumulation. Proper recovery and balanced exercise can help regulate cortisol levels.
Insulin sensitivity improves with regular physical activity. Research shows that exercise helps muscles use insulin more effectively, reducing the risk of type 2 diabetes by promoting better glucose uptake in muscle cells.
Muscle Adaptation and Hypertrophy
When you engage in strength training or resistance exercises, tiny tears (microtears) occur in the muscle fibers. The body repairs these microtears, and in the process, the muscle fibers become thicker and stronger. This is known as muscle hypertrophy. Over time, your muscles adapt to the increased workload, allowing you to lift heavier weights or perform more repetitions.
Protein synthesis is a key part of this process. After exercise, your body synthesizes proteins to rebuild the damaged muscle fibers. Consuming adequate protein (amino acids) after exercise is essential to support this muscle repair and growth.
Fat Loss and Metabolic Changes
One of the primary reasons people exercise is to burn fat and lose weight. When you start exercising, your body burns calories during the activity itself, and in the long term, your metabolism can become more efficient at using stored fat for energy.
Fat oxidation increases during aerobic exercise, as fat becomes the primary fuel source once glycogen stores begin to deplete. In the long term, regular exercise improves your body’s ability to burn fat at rest, contributing to overall fat loss.
EPOC (Excess Post-Exercise Oxygen Consumption), also known as the “afterburn effect,” occurs after high-intensity workouts. This refers to the increased rate of calorie burn following exercise as the body works to return to its pre-exercise state. High-intensity interval training (HIIT) has been shown to induce a significant EPOC, leading to greater fat loss even after the workout is over.
Increased Blood Flow and Vascular Health
Exercise stimulates the growth of new blood vessels, a process known as angiogenesis. This is particularly important for improving cardiovascular health and enhancing the delivery of oxygen and nutrients to tissues. Regular physical activity increases blood flow to muscles, helping them function more efficiently during exercise.
Endothelial function, which refers to the health of the blood vessels lining, improves with regular exercise. Better endothelial function leads to greater vascular dilation and improved circulation, which can lower the risk of hypertension and other cardiovascular diseases.
Neuroplasticity and Brain Function
Exercise has profound effects on the brain. Physical activity stimulates the release of brain-derived neurotrophic factor (BDNF), a protein that supports the growth of new neurons and strengthens existing neural connections. This is crucial for learning, memory, and overall cognitive function.
Exercise also increases blood flow to the brain, providing it with more oxygen and nutrients to support brain health. Research shows that regular exercise can reduce the risk of neurodegenerative diseases like Alzheimer’s and Parkinson’s by promoting brain plasticity.
The Immediate and Long-Term Effects of Exercise
The science of exercise shows that the benefits of physical activity go far beyond just burning calories. Immediately, exercise increases heart rate, respiration, and energy expenditure. Over time, consistent exercise leads to improved cardiovascular function, increased muscle mass, enhanced fat oxidation, and improved mental health. Regular physical activity promotes a healthier, more efficient metabolism, reduces the risk of chronic diseases, and can even help improve brain function and cognitive performance.
By understanding the science behind what happens when we start exercising, we can better appreciate the profound impact that physical activity has on our overall health. Whether you’re just beginning your fitness journey or are already an avid exerciser, the physiological adaptations that occur as a result of exercise help your body become stronger, more resilient, and healthier over time.