HIIT me harder

When working out hard in this manner, you're putting a lot more strain and long term difficulties on your body. While you may be adding more muscle and losing more calories, when you're 60 years old and can't bench 250 anymore, or sprint 30 times in an hour, you'll regret all this painful exercise. Think of all the NFL players that are now in wheelchairs 25-30 years after their career ended. That could be you.