Running power represents the intensity you’re running at,” says Nelson, and getting the intensity right is key when following a training plan.
“For pure runners, using pace as a training metric works fine if you’re running on a treadmill, track, or other very flat surface with no wind or temperature changes. You can keep an even pace and your pace represents your intensity. But when you run outside and you hit hills, or it’s windy, or the temperature or humidity changes, you’re going to be working a lot harder to keep a consistent pace.”
On those occasions, keeping your power output steady means your effort will be consistent, whereas trying to hold a certain pace could mean the training run is too hard, because you’re overdoing it up hills, or even too easy, if you have a tailwind pushing you along all the way. Either way, you won’t be getting as many benefits from that run as if you had stuck to the effort specified in your plan, and if you have worked too hard to hold a pace in unfavourable conditions it could affect the quality of the rest of your week’s training.
“Instead of trying to guess the right pace value as the conditions change, it’s easier to run on your energy expenditure directly – to run based on a power number,” Nelson says. “So you’re not trying to keep to seven minutes per mile, you’re trying to keep the energy expenditure that correlates with that speed. You might be given a power target of 300 watts, which correlates to seven-minute paces on flat ground, but when you start running up hill the pace changes. The power target doesn’t.