Runners are often comfortably warm running in shorts when less active people feel cold while wearing sweaters or coats. Then as the day warms up, runners may feel that it is too hot to run when the more sedentary are finally feeling comfortably warm. On marathon runs runners often hit the wall at about 20 miles. On even longer ultramarathon runs, runners must refuel to keep running Why?
The answers stem from thermal physics and the three laws of thermodynamics.
Three Laws of Thermodynamics
Physicists have identified three fundamental laws that govern thermal physics. These thermodynamic laws apply to anything, but physicists usually apply these laws to what they call heat engines. A heat engine is anything that uses energy to do work, including both mechanical engines and the muscles of humans or other living creatures.
The first law of thermodynamics is simply the law of conservation of energy applied to heat engines. This law states that a heat engine, including a human runner, can not put out more work than its energy input.
The second law of thermodynamics further limits the work output of a heat engine. There are multiple statements of this law, but one statement is that a heat engine, again including human muscles, will always have an efficiency of less than 100%. The extra energy dissapates as waste heat.
The third law of thermodynamics states that it is impossible to reach a temperature of absolute zero. This law is important to physics but does not directly affect running.
Running and the First Law of Thermodynamics
From the first law of thermodynamics, any heat engine must have an energy input in order to do useful work. A runner's muscles act as a heat engine so they require an energy source. The easiest energy source for the human body to use is the body's stored glycogen. For most runners the stored glycogen runs out at about 20 miles. That is why marathon runners often hit the wall at about 20 miles. It is not a matter of willpower. It is physics. The runner is simply out of fuel.
Runners' bodies can learn to burn fat as fuel, but this process is less efficient. Runners who have not yet run enough long runs for their bodies to have learned to efficiently burn stored fat as fuel can only shuffle along very slowly.
In ultramarathon runs longer than 26.2 miles, runners must eat to keep moving. Their muscles need fuel. They cannot violate the laws of physics by continuing to work with no energy input.
Running and the Second Law of Thermodynamics
Owing to the second law of thermodynamics, runners' muscles, like all heat engines, are less than 100% efficient. When muscles are working they generate waste heat.
In cold weather this waste heat keeps runners warm even if they seem lightly clad. Runners generate most of the waste heat in their legs. Hence runners can often run in shorts in cold weather. Runners might run in shorts yet need a sweatshirt, hat, and gloves because the upper body muscles are not working and generating waste heat.
In hot weather the runner's waste heat can be dangerous. It increases the risk of heat stroke. Runners sweat profusely because evaporating sweat cools the body. The sweating can however deplete the runner's body of fluids and electrolytes. If the fluids and electrolytes are not replaced, the runner's health is at risk.
The laws of physics apply to running as they do to everything else.
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
