The researchers from Simon Frasier University (SFU) in British Columbia measured aerobic and anaerobic capacity in 10 male mountain marathon runners and found a runner’s pre-race anaerobic fitness capacity is a key factor in determining who will have the fastest finishing times.

Aerobic fitness refers to how the body uses energy when there is enough oxygen, such as the energy burn that occurs when running at a comfortable pace. Anaerobic fitness refers to the body’s ability to exercise when there’s not enough oxygen, such as during a sprint to the finish line at the end of a race.