Killed the hilly Spartan Race
I love Natural Force and would take all of their stacks if I had the money to spend.  Trained with these for the past month and just ran a 12 mile Spartan Beast using these and I was kicking my training partners ass on the course all day.  Either he was having a bad day, or these definitely helped my cardio and endurance.  I like their Iskiate product a lot too.
Rating: 5 stars