When a person runs, their body transforms energy to power each stride. This dynamic activity relies on converting stored chemical energy into various forms of mechanical and thermal energy. This process shows how the human body fuels movement. The body adapts its energy systems to meet the demands of physical exertion.
Fueling the Body: Chemical Energy
The human body gets energy from food, which contains chemical energy. Macronutrients—carbohydrates, fats, and proteins—provide this energy. These compounds are broken down through digestion into simpler forms.
Carbohydrates and proteins each provide approximately 4 kilocalories per gram, while fats are more energy-dense, yielding about 9 kilocalories per gram. This chemical energy, measured in kilocalories (calories), represents the fuel available to the body. This chemical energy fuels all its functions, including muscle contraction during running.
The Energy Currency: ATP Production
Chemical energy from food must be converted into adenosine triphosphate (ATP). ATP is the cell’s “energy currency,” providing readily available energy for various cellular processes, including muscle contraction.
The ATP molecule consists of an adenosine backbone linked to three phosphate groups. Energy is stored in the high-energy bonds between these phosphate groups. When a cell requires energy, the outermost phosphate bond in ATP is broken through a process called hydrolysis, releasing energy and converting ATP into adenosine diphosphate (ADP). The body then regenerates ATP from ADP, ensuring a continuous supply for ongoing activities.
Energy Forms in Motion
As a person runs, the chemical energy converted into ATP is then transformed into several physical forms of energy. A primary form is kinetic energy, which is the energy of motion. The amount of kinetic energy a runner possesses depends on their mass and velocity, calculated by the formula KE = ½ mv². This energy propels the runner forward.
Potential energy, the energy stored due to position, also plays a role, particularly when running on varied terrain. When a runner moves uphill, their body’s gravitational potential energy increases as their height rises. Running downhill involves a decrease in potential energy, some of which can be converted into kinetic energy.
A significant portion of the energy transformations within the body during running results in the production of thermal energy, or heat. The human body is not perfectly efficient at converting chemical energy into mechanical work; 70-80% of the energy expended during exercise is released as heat. This heat explains why runners feel warm and sweat, as sweating is the body’s mechanism to dissipate excess thermal energy and regulate core temperature.
Optimizing Energy: Aerobic vs. Anaerobic Systems
The body employs two main metabolic pathways to produce ATP during running, depending on the intensity and duration of the activity. The aerobic system operates in the presence of oxygen and is the primary energy source for sustained, lower-intensity running, such as long-distance events. This system, primarily occurring in the mitochondria within muscle cells, efficiently breaks down carbohydrates and fats to generate a large amount of ATP.
When running intensity increases to a point where oxygen supply cannot meet the immediate demand, the body relies more heavily on the anaerobic system. This system generates ATP without oxygen, primarily through glycolysis, which rapidly breaks down glucose. While the anaerobic system provides quick bursts of energy for high-intensity activities like sprinting, it produces a smaller amount of ATP per glucose molecule and leads to the formation of lactic acid. Lactic acid accumulation is associated with the burning sensation experienced in muscles during intense exertion, but it is quickly cleared and is not the cause of delayed muscle soreness.