If you’re a runner, it can feel tempting to go above your limits to improve your pace, duration, VO2 max, and other metrics.

But lots of research suggests that going easy is beneficial. Elite athletes only spend about 20% of their time intensively training, for instance – they’re mostly “zone 2” running.

You might think the exception to that is the speedy part of interval training, which sees runners switch between jogs and sprints.

But even in those, researchers think we should only be giving it a seven out of 10 effort.

Researchers wanted to see how running at different intensities affected people’s VO2 max, or their ability to use oxygen efficiently, and their heart rates.

They asked participants to run three times. Each run was three minutes each, with two minutes’ rest in between.

Runners were asked to run at three different levels of perceived intensity: 6/10, 7/10, and 8/10 effort. That was measured by how hard the participants felt they were pushing themselves in their run (they were told to keep the level of difficulty the same throughout).

The longer you spend close to your maximum VO2 and heart rate during exercise, the more your aerobic capacity will increase.

The scientists noticed that people who ran with 6/10 effort in interval training spent less time at 90% of their VO2 max and heart rate levels.

But for those giving it a 7/10 or 8/10, the results were about the same; they spent roughly as long in the 0% or higher zone. And training at 7/10 provided less “ventilatory stress,” or difficulty breathing, than doing so at 8/10.

In other words, 6/10 seemed not to benefit interval runners as much as running at a 7/10 or 8/10. Both of those had the same potential to improve participants’ fitness, but the harder workout was more stressful with no added benefits.

Probably not. Different runs, and even walks, have their own benefits for runners; this test was only done on interval runs, which involve shorter sprints.

The runners only spent nine minutes in total at this higher level of effort, too. Training your aerobic capacity is helpful for more efficient running, but running slowly and long can train your endurance.

But for short bursts of interval training, the study suggests that forcing yourself too far beyond your limit might not necessarily be better. And as anyone who’s struggled with exhausting sprints will know, any mercy is welcome.