Each workout, tweak how long you do your speed intervals.
Researchers from McMaster University in Canada, who have spent the past decade tinkering with the timing of HIIT sessions to measure their potency, recently found that one minute of intense work - three 20-second all-out sprints alternated with two-minute recoveries - is as effective as 45 minutes of steady cardio in boosting your cardiovascular fitness. So have they finally found the HIIT recipe that delivers the biggest body payoff in record time? “There actually isn't a single best way to do intervals,” says lead study author Martin Gibala, Ph.D. “The variety is infinite with these routines, and that's empowering.“ His suggestion: Switch up the duration of your sprints from one workout to the next to throw your body a different challenge each time.
Gillen JB, Martin BJ, MacInnis MJ, Skelly LE, Tarnopolsky MA, Gibala MJ (2016) "Twelve Weeks of Sprint Interval Training Improves Indices of Cardiometabolic Health Similar to Traditional Endurance Training despite a Five-Fold Lower Exercise Volume and Time Commitment." PLoS ONE 11(4): e0154075.