'Threshold' is a word you can't avoid in endurance sport. But what does it mean? And how do newer views of exercise physiology improve our understanding of where our barriers lie?
Most serious endurance athletes will have an idea of where their 'threshold' lies - whether FTP for cyclists, HM pace for runners, CSS for swimmers. The usual understanding is that above 'threshold', exercise is associated with a gradual rise in the evil lactic acid, which eventually causes the athlete to have to stop. Below threshold, lactate stays steady. Some models appear to allow the athlete to go on forever, but others suggest (and the real world demonstrates) that exhaustion still occurs, and the athlete must stop for what is presumably an entirely different reason.
I had this planned as one post, then two, then more - I'm not sure now, but I think breaking it up into bits is going to help avoid head-spinning.
I'll start with a quick review of energy systems:
|Simplified metabolism - click to enlarge|
ATPThe 'currency' of energy in the cell is phosphate, stuck onto other molecules in the form of ATP, GTP etc. (I'll call them all ATP for simplicity). As a muscle fibre contracts, ATP is broken up into ADP and phosphate - which must be recombined elsewhere if the fibre is to continue contracting. There is enough ATP in our muscles to sustain a couple of seconds' exercise at most.
Creatine phosphate (PCr)Together with ATP this is the 'phosphocreatine system'. It's of less interest to endurance athletes, but explains why power athletes and bodybuilders often take creatine supplements. Creatine phosphate acts as a buffer by giving up its phosphate to turn ADP back into ATP, and is fast enough to consider the two as a single system. However, we only have enough PCr to exercise for about 10 seconds.
Glycolysis ("anaerobic")The bane of an endurance athlete's existence, apparently: "Don't go anaerobic". Glycolysis is the process by which glucose is turned into something that our mitochondria can use in the Krebs cycle (namely Acetyl-CoA, via pyruvate). Handily, it can produce ATP without using oxygen while it's doing so - but in the process it uses up the cell's supply of NAD. This can be regenerated either by the electron transport chain in the mitochondria (which needs oxygen), or by turning the pyruvate into the dread lactate (boo, hiss).
Glycolysis has three main advantages over aerobic metabolism, which vary in importance depending on who you believe about lactate production - personally I'm a lactate-believer:
- Can occur without oxygen (but produces evil lactate)
- Much faster ATP production than aerobic metabolism (lactate allows 'uncoupling' from aerobic metabolism)
- Frankly, and people keep forgetting, it's the only way to get glucose ready to burn anyway!
Beta-oxidation (glycolysis for fats)As a long-distance athlete it's vital to burn fat as much as possible, yet most descriptions of metabolism targetted at athletes talk mainly about sugars. Beta-oxidation is a sort-of-equivalent to glycolysis, for fats. The end result is the same (Acetyl-CoA) and it also doesn't require oxygen - but it doesn't produce ATP directly, only via NAD which can't be recycled without oxygen. Lactate production is unfortunately not an option here as Acetyl-CoA is produced directly, and unlike plants we can't turn it back into pyruvate without going through some complicated shenanigans. So fat burning should be considered purely 'aerobic'.
Krebs cycle ("aerobic")This is vitally important for distance athletes, and is ultimately responsible for burning all our fuels. Strictly speaking, it doesn't use oxygen (but does make CO2) - and makes ATP. However, it also uses up NAD and from here the only way to regenerate it is via the electron transport chain which uses oxygen. As a process its maximum speed is much slower than glycolysis, but it can extract much more of the available energy from what's fed in.
Electron Transport ChainThis is rarely considered separately from the Krebs cycle but is actually the only real 'aerobic' part of metabolism, in that it is where oxygen is used to regenerate NAD. The real reason that other processes are considered aerobic is that the ETC is the only way to regenerate the NAD - the exception being glycolysis where lactate production provides an alternative.
NAD regeneration provides a lot of energy, and this is used here to produce a large amount of ATP - compared with the much smaller amounts produced elsewehere.