When we exercise the body needs to convert chemical energy from substrates into mechanical energy in the form of movement. The intensity and type of exercise performed will dictate the substrate and rate at which it is used. We use three major substrates as fuel: phosphocreatine, glycogen and fat. Exercise is a complex task and you will find that all these substrates act at the same time, however for the purpose of this post I will isolate each and describe how each work.
For those in the business world exercise intensity is a factor of supply and demand, there is a demand for energy and a need to supply this energy. When you first start an exercise session your body relies on muscle substrate stores mostly in the form of glycogen and phosphocreatine. Once blood supply to the working muscle is enhanced the body can then regulate the substrate used based on the demand (exercise intensity). We use substrates to form a high energy molecule (ATP) and in combination with oxygen forms mechanical movement. That is about as far as you need to understand that process.
Before I go into more detail about how we use certain substrates for energy I will just highlight some limitations to exercise performance as this is important to understand. There are many limitations to exercise and these are task specific. To help understand energy and fuel I will highlight just a few. Oxygen is key to movement, although we can work without it, once this is limited or maxed out we can only produce a finite amount of work before the system is exhausted, overwhelmed or disrupted. So one limitation to exercise is oxygen use and oxygen supply. To supply oxygen requires blood flow to the working muscle and this requires your heart rate and blood vessels to work to supply enough blood without affecting other important functions (etc. brain). To do this the blood vessels can redirect blood flow from areas that do not require energy while you are working out (i.e. digestion). So a second limitation is blood supply, and this could be affect by heart rate max, blood volume and the nervous system. The last limitations to exercise I will talk about is substrate depletion and metabolic byproduct removal. During exercise we breakdown substrates to perform mechanical work, these produce byproducts and built up enough they can disrupt exercise or provide feedback for us to reduce or down-regulate the intensity. There are limited sources of both glycogen and phosphocreatine in the body and therefore if these become critical during exercise the intensity cannot be sustained. Although there is not an infinite amount of fat in the body it is unlikely to become depleted during any normal exercise event. Substrate depletion is therefore primarily focused on glycogen and phosphocreatine, although depletion can happen, the rate at which these are used may also affect the build up of byproducts and thus reduce the ability to keep exercise at high intensity. Therefore, it could be depletion that stops you or changes in the metabolic environment in the muscle. Substrates are with in the muscle and also external to the muscle so you can also see here that blood supply is important for not only oxygen delivery but also fuel delivery. To recap with can be limited by substrates and oxygen supply, substrate depletion, the rate at which substrates are used, the clearance of metabolic byproducts and blood supply.
Now that you have a little more idea about how we can be limited during exercise it is easy to understand how we use fuel during exercise. I will use a 100m sprint as an example. If we think about Mr Usain running the 100m, oxygen supply and blood flow are not limiting as the exercise is only performed for 9 seconds. The primary fuel here is phosphocreatine as it is readily available in the muscle and lasts about ~10 seconds if your at absolute capacity to produce power/force. So Mr Usain would only be reliant on this fuel to complete the race, high energy and very low duration. If he were to step up to a 400m race where the race goes for 40-60 seconds you can not supply the muscle with enough phosphocreatine to complete the race so intramuscular glycogen becomes important. Phosphocreatine will work until depleted (60-100m) and then intramuscular glycogen would get you to the end of the race (100-400m). Again, oxygen is not a factor here, what is a factor is the rate of fuel use and metabolic build up. So blood blood is very important, if Usain does not remove enough of the substrate byproducts or tolerate their build up then Mr Usain will most likely not be run a world record here. Here is where exercise gets complex. Lets say Mr Usain runs one 100m race, he runs it in under 10 seconds (training run). He then gives him self 5 minutes recovery and runs another sub 10 second casual 100m. The fuel source here is still phosphocreatine, although he probably used it up in the first run it is re-plenshished to a degree. However, if Mr Usain ran a sub 10 100m effort with a short recovery (~20 seconds) and then ran another sub 10 100m effort and another and so on you would see a drop in performance (10 seconds, 12 seconds, 15 seconds) and if you looked the inside of the muscle you would see a shift in fuel and a greater reliance on oxygen. I will put all this together soon. Mr Usain has now stepped up to the big show and is running an 800m or 1500m race. If you think about someone being aerobic these guys are the most aerobic people on the planet. The primary fuel source is still glycogen, however the reliance on oxygen is greater so its an aerobic glycolytic event. Compared to our 400m run before which is anaerobic glycolytic. They are both limited by glycogen more then oxygen, however the rate at which glycogen is used/delivered is the difference. Lets look back at our repeated 100m efforts Mr Usain did before, the first 1-2 might by phosphocreatine dependent, then we start tapping into that anaerobic energy system whereby oxygen is not supplied fast enough to meet demand and the intensity is two high and byproduct build up is inevitable. That may make up the next 2-5 efforts. If he complete 10 sprints, the last 5-10 would be primarily based on aerobic glycolytic metabolism as the intensity would reduce, the rate at which glycogen is used would reduce, the metabolic build up would slow and hopefully start to clear and oxygen supply would have a chance to match demand. The last substrate to cover is fat, limited effects in high energy events, however if Mr Usain jumps into a marathon then it may become more important. Glycogen stores are limited and if you are competing in an event at high intensity for more then 1 hour it is likely you will hit the glycogen wall with our external sources (gels, bananas etc.). Endurance trained athletes such as marathoners have a better ability to use fat at high intensities and spare glycogen for the last sprint to the finish.
To better understand the sessions in the spin studio the above information may help. We want to develop a well rounded system, so high energy power sets are aimed at developing the ability to use the phosphocreatine and cope with metabolic build up. The repeat high intensity sessions aim to develop tolerance for both anaerobic and aerobic systems under conditions whereby the muscle environment is compromised. The longer aerobic sessions are aimed to build a better delivery system i.e. stronger pump and better oxygen/substrate delivery. Then we put it all together during our 10km time trial, under competition this requires you to be able to delivery oxygen, cope with stress and finish strong. I also manipulate the sessions so you may have to do power under fatigue i.e. (power sets at the end) or endurance under fatigue (tolerate work under metabolic build up). I have a few more tricks up my sleeve that I will not fully disclose, hopefully you get an understand of how the body works and how it works during exercise. You can see how training effects your fitness and performance. There are many more limitations and you find that complex mechanisms of control normally mean if one system is disrupted multiple are disrupted. If you have an event coming up or would like more advice on training you can email me at firstname.lastname@example.org