In the last few years I would say that the word aerobic has become the most widely used word in the English language and yet the real understanding of what is actually going on in the body is vague to say the least. Fair enough, most people want to do some exercise, not a degree in biochemistry. However, since we have opened the Concept 2 Message Board (http://www.therowingcompany.com/messageboard.htm) it is apparent that some contributors oversimplify what is a complex process and here I will try to explain the way muscles work without the aid of a safety net.When you wake up in the morning and open your eyes the muscles in your eyelids are working anaerobically. You throw back the duvet (anaerobic), swing your legs out of bed (anaerobic) and stand up (anaerobic). It is not how hard something is that determines whether it is aerobic or anaerobic, all initial movement is anaerobic. In the muscles there is a limited supply of fuel known as ATP. The action of the muscle burns one of the phosphate atoms and it becomes ADP. This missing phosphate is replaced by creatine phosphate and this process will last for 10-15 seconds. Beyond this point the missing phosphate can either come from the breakdown of glycogen, which is also stored in the muscle, or the breakdown of free fatty acids which are carried in the blood stream.If the rate at which we need to replace the missing phosphate is high then we will get it from the glycogen. There is a limited supply of glycogen which will last for up to one minute of flat out activity; beyond this we will need to reduce the intensity of the activity so that we can replace the phosphate from fat. There is plenty of fat but its breakdown into a useable form is slow and hence we have to reduce the demand to match the supply.So ATP is always the end fuel and how we recharge it will be done aerobically via the breakdown of fat or anaerobically by the breakdown of glycogen. Now this is a simplified version of a very complex chemical reaction but I'm sure there is a popular belief that somehow the body has three different and independent energy systems and of course it doesn't.The notion that after the initial start of a 2,000 metres race, when you are into your race pace, you are burning a combination of fat and glycogen so that your initial ATP/CP stores are fully replenished and can be used again in a tactical burst is incorrect.Tactical bursts in 2,000 metres on-water rowing races can be effective as your opponents can see you moving ahead. Although this is expensive in energy terms the psychological benefits can outweigh this. With indoor rowing the result of a burst is not apparent to opponents and so there is no psychological gains this only leaves the energy costs.If you watch indoor rowing competitions, changes in position in the final 500 metres are in the majority of cases caused by slowing down rather than a blistering finishing burst of acceleration. This would suggest that the most effective way to race 2,000 metres is even pace at the highest sustainable level.