The next time you want to work out but are short on time, saddle up. New research shows that doing small—like, seriously small—amounts of high-intensity interval training (HIIT) on an exercise bike might actually make you fitter than longer sessions. Just be prepared to work hard.

Led by Niels Vollaard, Ph.D., of The University of Stirling in Scotland, researchers reviewed 34 studies that examined how different HIIT workouts affect cyclists’ VO2 max, the gold-standard measurement of cardiovascular fitness. They found that VO2 max started to diminish after just two all-out sprints, with additional intervals reducing overall improvements by an average of five percent.

The researchers are currently performing studies to figure out why doing more sprints would result in worse results, but they believe it’s connected to the “rapid glycogen depletion” that occurs with all-out exercise, says Vollard.

Here’s why: During exercise, muscles break down glycogen (or sugar) for energy. Researchers believe this leads to increases in blood volume and mitochondria density—two things associated with improved VO2 max. However, the highest rates of glycogen breakdown occur during the first 15 seconds of a sprint and have typically tapered off by the third, hence why extra efforts many not necessarily result in further increases of VO2 max.

VO2 Max—Simplified
Unless you’re an elite or professional athlete, chances are you don’t know your VO2max. That’s ok. If you own a Fitbit Charge 2 or Fitbit Blaze, you have access to your Cardio Fitness Score, a unique Fitbit feature that uses your your resting heart rate, age, gender, weight, and other personal information to estimate your VO2 max. To get a more precise score, you can also do a flat, 10-minute run with GPS. (For more information, see What is my cardio fitness score?)

Once you know what your cardio fitness score is, try to improve it by doing a cycling HIIT workout inspired by the study above. Note: The following workout is designed to be demanding, so if you don’t have a solid fitness base, consult your doctor first or consider starting with something simpler, like one of these 5 Simple Ways to Boost Your Cardio Fitness. Repeat it two to three times a week, never on consecutive days.

The HIIT Workout That Can Improve Your Cardio Fitness Score
Warm-up
Pedal gently against low resistance—keeping your cadence above 90—for at least 10 minutes or until you start to sweat. If you’re using a Fitbit device with heart rate tracking, you should be in the Fat Burn Zone by the end of this warm-up.

Intervals
Increase the resistance until it’s at a level you can firmly pedal against for the full interval. Stay seated and pedal as fast as you can for 30 seconds. If you start spinning out during the interval, increase the resistance; if you’re not able to finish the interval, lower it. Finding the right amount of resistance may take some trial and error.

Remove all resistance and recover by pedaling gently for 4 minutes.

Add an appropriate amount of resistance back on and pedal as hard as you can for 30 seconds.

Remove all resistance and recover by pedaling gently for 4 minutes.

Cooldown
Pedal gently against low resistance for 10 minutes or until your heart rate returns to the Fat Burn Zone or lower.