POWERbreathe doesn’t increase VO2 but a result of inspiratory muscle training is that it enables greater blood flow to your other working muscles (legs and arms) so that you can exercise for longer without feeling so fatigued. When your breathing muscles get tired, they can ‘steal’ blood from other exercising limbs to supplement their work, so improving the efficiency of the lungs has the knock-on effect of increasing the performance of your other working muscle groups.