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.
While still depending on fitness level, the two 3 mph cases are more likely to utilization of aerobic energy. We can broadly classify exercise into 2 forms: 1) Anaerobic & 2) Aerobic exercise. "Aerobic" indicates that energy used for exercise requires oxygen, while anerobic exercise does not use oxygen. Now, you might be able to guess which one is going to be important in endurance exercise - aerobic. Energy derived from chemical reactions without oxygen will give you energy more rapidly - but cannot last very long.
We can break down energy stores to 4 sources. We'll look at the 2 anaerobic sources in this post.
1. ATP-CP - Anaerobic
ATP-CP uses creatine phosphate to produce energy (ATP). It is known as the "immediate" energy source, and will basically depleted after 6-10 seconds of the highest energy exertion. So activities such as sprinting 100 m will use mostly this energy pathway. You'll notice in an all-out effort that you quickly cannot go as fast as when you started. This is because after 10 seconds, you run out of this energy and have to utilize the other energy pathways which cannot give as high of an energy output.
In Hiking Terms
Ideally, you will not be focusing on this energy system when hiking, but it can happen. Take, for instance, a really steep, but short climb that you want to ascend as fast as possible. You may push as hard as possible to get to the top, and you will be using ATP-CP to do it. But obviously, you cannot do that for long, and if you are doing a long hike, you wouldn't want to try that and then wear yourself out.
This energy system will only be utilized during interval training. Intervals from 5 seconds to 5 minutes will at least partially tap the ATP-CP system, and for hiking you would only want to focus on the longer duration intervals. As you will be hiking for several hours, you will want to train with longer intervals that also heavily utilize oxygen. More on this later.
2. Glycolysis - Anaerobic
Glycolysis is the generic term for the breaking down of glycogen. Glycogen is basically a storage of carbohydrates in the muscles. It can be broken anaerobically (without oxygen) or aerobically. Anerobically, the process cannot last as long as lactic acid is produced and glycogen is consumed. This acid is the burning sensation you feel in muscles when doing high intensity exercise...and so the fact that this energy lasts from 30 seconds to a few minutes makes sense.
The rate of ATP production is high, behind ATP-CP. However, the efficiency is low, with
1 molecule glucose --> 2 ATP
In comparison to the yield from aerobic glycolysis (more on this later), it's paltry. So it's fast, but inefficient.
In Hiking Terms
An important system to consider when hiking - if only because when doing steady state hiking, its something you want to avoid. If you are planning on doing several hours of hiking, you do not want to feel the burn in your muscles at the beginning - unless you are doing it for training. You will be able to go further if avoiding this intensity of exercise (you can only do for a few minutes). So when the hikes get steep, trying to keep the same pace will require more energy for your muscles, and they'll start using anaerobic glycolysis. To avoid it, slow your pace down!
Very important. One determinant of cardiovascular fitness is your aerobic/anaerobic threshold (AT). The level of intensity at the AT is when the body can flush out the lactate as quickly as it is created. At higher intensity, the body cannot remove it quickly enough. By training above this threshold, you can train your body to better flush out the lactate, and increase the intensity at which AT occurs. (This is not to say that lactate is the cause of fatigue, but it is correlated).
Bouts of exercise from 30 seconds to 10 minutes will tax the anaerobic glycolysis system. Training in this range will be an important part to improve hiking performance, more on this later.
This is a basic overview of the anaerobic systems. Next up will be aerobic energy.