To be fit - "of a suitable quality, standard, or type to meet the required purpose" . So, before we can think about getting fitter, we must first define the required purpose effectively.

As a strength and conditioning coach & sport scientist, I'm well aware that people perceive my main role within a sports team or with a client is to improve 'fitness'. While this is true, the term fitness itself means so much more. Very rarely do people specify which part of fitness they want to improve or how they are going to do it. Getting stronger, faster, leaner, more powerful could also classify as getting fitter. For the most part though, and it's going to make up the majority of this article, we'll be talking about the commonly thought about 'fit'. Being able to run further, being able to run faster for longer, being able to do something for longer before being tired, also known as 'work capacity'. What is it, how do we define it properly but most importantly, how can we train it properly.

For years, during high intensity activity lactate or lactic acid in the blood is blamed for that burning sensation and not being able to maintain those intensities. While there's mild truth to it, lactate is not the enemy. It's a by-product of energy production during glycolysis under anaerobic conditions. Our bodies actually use this formed lactate as energy during this glycolysis cycle. Pyruvate is converted into lactate. This lactate is transported in the blood towards the liver where it is converted again to pyruvate for use in gluconeogenesis - the production of new glucose molecules from non-carbohydrate sources. At this point, glycolysis can start again.

The citric acid cycle, or the tricarboxylic acid cycle (TCA) or the Kreb's cycle as it's also known. The final resting place of the intial glucose molecule from glycolysis. At this point glycolysis has broken down glucose into pyruvate (see below). The citric acid cycle is another series of reactions with the help of oxygen that creates an abundance of products which are then used in the production of ATP molecules. We've seen via The Cori Cycle that in the absence of oxygen, pyruvate is converted to lactate and through gluconeogenisis new glucose molecules are produced for the continuation of glycolysis. When oxygen is present and in abundance, the citric acid cycle is used.