Why moving a little too fast can cost you a lot, especially at altitude

Ever head out trying to follow someone's blistering pace, only to finally slow down and feel depleted? When that happens, you actually can't just slow down a little bit to "make up" for speeding up before. No, basically you've screwed yourself for the rest of the day. Why?

When you hike (or run, bike, etc...), you are using multiple energy pathways. The rate of energy production from each of these pathways is different.

Reference

For the sake of simplicity, we are only going to talk about the two systems that use glycogen (stored sugar). Anaerobic glycolysis can produce ATP (energy) at a faster rate than aerobic glycolysis can. The faster you are moving and harder you are working, the more energy you are going to get anaerobically. Ok sounds great!

But as you would guess, there's a cost to that. The cost is how much glycogen is used.

Energy Systems Part 1 - Anaerobic Exercise

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