If you’re working out but don’t feel like you’re in any better shape, you might be a “nonresponder.” According to The New York Times, a recent study from Queen’s University in Canada finds that how people respond to exercise regimens varies substantially, and what works for one person may not help another person improve at all.

But that doesn’t mean those nonresponders will never get into shape. They just may need to change up their exercise routine for one that is better suited to their body. The study tested two exercise regimes on 21 active adults. Each of them spent three weeks doing endurance training (like running for an extended period of time) or interval training (doing quick bursts of strenuous exercise, like in CrossFit). After a few months of rest between workout periods, they then switched one routine for the other. Endurance trainees rode a stationary bike four times a week for 30 minutes, while high-intensity interval trainees did 20 seconds of hard pedaling on the bike with a 10 second rest after each interval.

Some of the participants showed improvements in physiological markers of fitness like heart rate and oxygen capacity after one of the workout periods, but others didn’t improve at all. Some were even in worse shape than before they began their assigned regimen. However, each individual responded to one of the workouts, even if they didn’t see results in the other.

To figure out which workout works for you, you’ll need to measure your fitness levels, using your pulse as your baseline number, at the beginning of a new workout routine. Then, after a month of either endurance or interval training, you should check to see if you've made improvements in your heart rate, according to the Times. If you haven't, you should switch to another routine.

[h/t The New York Times]