Is it useful/safe to do most of your workouts at 80% to 90% of your maximum heart rate? I'm trying to get back in shape as quickly as possible. My body is good at recovering quickly after an intense workout and my heart rate drops back to normal very quickly when I stop. The thing is that I sometimes feel a bit dizzy when I'm working out and for the rest of the day, my breathing becomes shallow. Aside from the difficulty of working out consistently when pushing your body like this, are there any dangers in high heart rate workouts? Is this the fastest way to get in shape? I also feel like I'm building up my mental toughness by doing this.