But short workouts can be just as effective as longer ones, and, in some cases, more effective. It all depends on how you use that time. "Interval training, if you're truly going to your all-out maximum during the working intervals, is incredibly effective in terms of training your heart — because you're demanding a lot of work from it and giving it just a short amount of time to recover, it becomes so much stronger and more efficient," says Anna Maltby, Refinery29's deputy editor of health & wellness and an ACE-certified personal trainer. "And it's great that she's combining cardio with strength-training during these interval workouts.