Every time we walk into the gym, we’re faced with “the question”: where do I start? Do I go for 30 minutes on the treadmill before I hit the weights, or do I knock out my dumbbell routine before polishing things off with a light run? Who do you want to look like: Lance Armstrong or Jason Momoa?
Until the last several years, scientists thought the answer was a matter of logistics: if you’re wiped from the cardio session, you’ve got no energy left for weight lifting, and vice versa. That makes sense, of course: we’ve have a limited energy supply during a workout, so we need to invest it wisely. The real answer, though, comes from a “master switch” that’s flipped at the beginning of the workout.
According to Runner’s World columnist Alex Hutchinson, the cellular sequence that creates bigger muscles also causes better endurance. This master switch enzyme, known as AMP kinase, gets set for either adaptation at the beginning of your workout and cannot change quickly during the workout itself. In other words, you can’t get both swole *AND* lean in the same session.
Fortunately, it’s possible to get the best of both worlds. If you’re preparing for both endurance and strength exercise–a Spartan race or Crossfit competition, for example–simply shift your workout’s focus from day to day. Do weight lifting on Mondays, Wednesdays and Fridays, while Tuesdays and Thursdays get the full cardio treatment. The variety will challenge your body to adapt more broadly, working that little master switch for all it’s worth.