Rate of Perceived Exertion (RPE)
There is a metric we use in the sports and physiology world called "Rate of Perceived Exertion (RPE)." Many of my athletes know about this because I ask them to record RPE from their various sessions and I often prescribe workouts based off of RPE. The term and methodology for recording RPE was originally coined by a scientist named Gunnar Borg. Borg's original scale started at a rating of 6 and went up to 20. That scale was revised to be on a 10 point system such as the example to the right.
The fascinating thing about RPE is that it is totally subjective and allows the athlete complete control in assessing how they "feel." Whenever we do a workout, we have a pace we are supposed to swim or run, a power number we are trying to hold on the bike, or a specific weight we are trying to lift at the gym. Those numbers are absolutes: you either hit them or you don't. But using an RPE scale while exercising, and matching your RPE with whatever pace or power you are hitting, allows us to see HOW that prescribed intensity level feels to the athlete. These two metrics (pace/power and RPE) work much better when used together as a means of matching the "feeling" to what is actually being done.
If we only rely on how something feels, we may quickly become demotivated when a workout or race "feels" really hard and we're suffering. However, if we match that feeling with the pace we're actually going, it can turn into a motivating factor, because we see that our increased RPE is matching with an increase in speed.