The debate over limitations in human performance has been waged for decades. In the endurance sports world, the commonly-held perception is that the cardiovascular system is the primary limiting factor. The belief expressed by many endurance sports coaches is that maximum VO₂ and lactate threshold are the primary culprits - and thus, we need to track and scrutinize the appropriate training parameters - namely, heart rate.

But here are some thoughts to refute those claims - to take our focus away from the past and turn it towards the future.

On top of these issues is perhaps a more elemental one - and that is the use of heart rate as a measure of performance and workload. Though this may initially give an athlete a means to assist in the correlation of "effort" and "pace", heart rate on it's own is a tenuous measure. If you are dehydrated, it will be skewed. If you are training in the heat (or cold), it will be skewed. Your target training heart rate ranges are calculated around a moving target - namely, using your age-adjusted (calculated) maximum heart rate. So you have an arbitrary heart rate range - which will be affected significantly by environmental variables - that is calculated from an arbitrary maximum heart rate. From a mathematical point of view, it's not very accurate. From a functional perspective, it is not the best way to establish your level of work in any conditions.

So if the cardiovascular system isn't the primary limiter of endurance sports performance, what is? Well, ladies and gentleman - it's all in your head.

The central nervous system.

Yes, that gray thing between your ears, the one thing that many coaches want you to focus with and many want you to shut off. The same entity that controls your thoughts, your emotions and ... your training?

If you need a measure of workload, then you can use power or velocity in conjunction with perceived effort. Strange as this may sound, perceived effort has been shown in the scientific literature to be a very accurate measure of workload - and is consistently used in exercise physiology and cardiac stress testing labs worldwide.

Of course, in this discussion of the CNS we must also remember that "the brain controls everything". With that in mind (no pun intended), your ability to maintain attentional focus is critical in being able to maintain power output over time. Let's face it - how often do we tell ourselves "I can't" long before the body says "I won't"? How often does our self-talk get in the way of our performance - in an event - or in our everyday lives?

In the grand scheme of human performance, the brain is the primary limiter. Fortunately, it's also the primary resource for success. For that, we can be thankful.