We will start backwards. When we hike (or run) we move our limbs in desired motions and speeds. To get the desired movements, we need our muscles to contract. Muscles cannot contract without having energy available. So we need to get energy to the muscles.
Where does the energy come from, you say? Well, it depends on the energy demands of the muscle. Compare walking at 3 mph on a flat terrain to walking 3pmh on a steep trail and running 6 mph on a steep trail. Wouldn't you agree the energy demands are different? In the 6 mph case, there is a high energy demand and it is probable that this pace could not be kept up for long. High energy demand exercise is considered anaerobic exercise. This basically means that the energy for the muscles is produced without using oxygen, and that the energy supply is quite limited - only lasting for a few minutes. But because the energy can be created without oxygen, a lot of it can be produced quickly, so fast movements that last up to 3 minutes will rely on this source.
If you try to run up a 25% grade, you'll be quickly using up your anaerobic energy
