According to which theory you refer to there are many limitations to exercise performance. I will outline some simple things that may help you understand exercise and how the body works. I will also relate this to the types of sessions that we do in the studio to help give you a better understanding of why the sessions run the way they do.
Limitations to exercise performance
Limits to exercise are task dependent, this means that under different conditions the body may stop by different means. Before I delve into this I will just give you a quick overview of the body, the mechanisms that helps us exercise and then the limitations to each. I will not go into great detail, so if you would like more information you can email me at firstname.lastname@example.org. When you exercise there are some fundamental things that need to happen, the muscle needs energy, the body needs to supply the energy in the form of different substrates (e.g. fats, carbohydrates, phosphocreatine), you need oxygen to turn this into movement, and you need to remove the waste or by products of this process. There are some important components to all this happening, the lungs to get oxygen in the body, the heart and circulatory system to transport the oxygen and substrates to the muscle, the nervous system to stimulate the muscle and assist with increasing the rate of the above, and the same systems drive removal of exercise by-products. So naturally limitations to exercise performance can be part of any or all of these systems. There are a few more to this list but I will keep it simple.
The cardiovascular supply theory suggests exercise can be limited by the amount of oxygen supplied to the model. Once a threshold is reached, the body must work harder and harder without additional oxygen, this creates an increase in exercise by-products and eventually exercise will have to cease. So the model works in two ways, limit supply or inability to buffer/tolerate exercise metabolites (by-products). Although an upper limit exists for oxygen delivery there are arguments that not all oxygen in the blood is used even at maximal intensities, so something other then this may limit exercise.
Fuel is import for exercise and dependent on the exercise task, the duration, the environment, and the intensity different fuels are required. The substrate supply/demand theory has a few components. Exercise may be limited by limited cellular fuel sources (Adenosine Triphosphate), you do not need to know what this is, just know that it is very important. The other component is limited fuel availability i.e. low carbohydrate or low phosphocreatine (two limited fuel sources). In context to the situation both these can be correct. The evidence suggests that exercise may be limited by the amount of fuel available or by the rate at which it can be consumed or metabolised by the muscle.
These two theories focus on the metabolism limitations to exercise, however we also have some nervous limitations to exercise. All these theories are in context to a situation, and under certain conditions exercise can be limited by the central nervous system (brain to the nervous junction at the muscle) and peripheral nervous system (nervous junction through to action). When a muscle activates a signal is sent from the brain, down the spinal cord, along peripheral nerves to a junction at the muscle. Here the signal is converted from a electrical signal to a chemical signal and eventuates as an action i.e. picking up a cup. There a many limitations to both of these areas, however the simplest way to put it would be central limitations are any that stop or reduce the signal from the brain, peripheral limitations are any that limit the mechanical movement. To complicate it more, one of these may act to influence the other. However, I will keep it simple. If you exercise at a very high intensity for a very short period of time (<30 seconds) there may be lots of metabolites (by-products) that build up in the muscle and limit the mechanical movement. In a different scenario, long duration exercise may slowly build up fatigue related factors and limit the drive to the muscle reducing force/power it can produce. Its not always time related, however these are simple ways to view fatigue or the nervous system.
The body is not a simple machine. The above limitations to exercise are therefore true in isolated tasks, however, when we look at exercise or movement as a whole it is most likely that all these factors contribute differently. There is a few complex models of performance, however these are yet to be proven. The most contemporary is the inclusion of the brain in the conscious control of exercise based on feedback from all these systems interpreted in the mind as a perception of effort. How hard you think you are working is always going to dictate how much effort you decide to put into a training session when you get to control or regulate the intensity. So this is where training comes into play, if you improve the body's ability to cope with stress you can change the perception of the exercise task and therefore hopefully work harder. One catch to this, you may get fitter and better at working under harder conditions but perception is very hard to change. It may only change at what intensity you think is hard. Confused? Imagine you are on the bike, spinning 90rpm and at resistance 8. If you do this for 10 minutes and decide that is 10/10 on a effort scale (10 being I can't do anymore) so you stop. After a few weeks, you might be at 90 rpm, resistance 12, after 10 minutes you decide that effort is again 10/10. You have not changed your perception of the exercise, it feels the same, what you have done is increased your ability to tolerate exercise so that a 10/10 effort is at a greater intensity. So lets look at how we can change the above limitations to exercise to help us do just that.
Exercise Training, How does it help?
Cardiovascular/supply model, when we exercise it is natural our heart rate goes up, our breathing rate goes up and our blood is pumped around the body a lot faster. There is also some other things happening at the muscle level, oxygen and substrates are being transported into the muscle. If its your first time, there is a limited amount of blood in the body, so the heart has to work faster to supply the muscle with oxygen and substrate so your heart rate is often very fast, over time as you adapt to the exercise you get an increased volume of blood, this allows your heart to pump less and get more benefit. If you have a really high heart rate, you normally think the exercise is harder (i.e. rate it 10/10). Same goes with breathing, when you first start exercising your breathing rate can be very shallow, very quick and very inefficient. Overtime you develop stronger breathing muscles, better control and this allows you to slow your breathing down and take in more oxygen. Like before higher breathing rates are normally correspond to higher effort. This is where our threshold classes are really powerful, they challenge the body under maximal heart rates and breathing rates which forces it to get better, but also forces you to get used to working under those conditions. Interval training is also great for this as you can work very hard for short duration yet still maximally stimulate the breathing and heart rates.
The central and nervous system theories are based around limitations to either nerve conduction to the muscle or mechanical disruption at the muscle. Both these can be affected by high metabolite production in the muscle that may eventually spill out and disrupt other systems, this is normally when you feel pretty sick (i.e. end up in the garden). Improving the body's ability to tolerate high concentrations of metabolites forces the body to develop better buffering systems. Without going into detail we have safeguards that protect change or disruption, once we go beyond these the body has to reduce the exercise intensity or you risk further problems. If we systematically and appropriately challenge the body in these environments then we can develop better safe guards and we can also get better at working through the barrier. High intensity sprints, repeated sprints, repeated hill climbs and high intensity time trials or intervals are all great ways to test and develop this system. All these modes of exercise work on generating high force, which normally means high metabolic loads and lots of metabolites.
I often refer to some sessions as mixed. These sessions normally look to improve all these factors, whereby we do some high energy sprint work, tolerance work in the middle and then heart/lung orientated training at the end. So the session covers all the basis. I hope this helps you understand a little bit more about exercise, if you have a specific event coming up feel free to chat to me to work out what the best form of training may be for you. We can also manipulate a lot of these to help with weight loss goals.