Haters of aerobic training often cite archaic research suggesting that steady-state cardio interferes with muscle gains, but newer studies show otherwise.
A 2012 report in the International Journal of Sports Medicine found that subjects who performed both strength and endurance training optimized muscle growth and saw strength and endurance gains. Furthermore, the type of aerobic exercise you perform has an impact.
“Running has a significant eccentric component,” says Greg Nuckols, C.S.C.S., a strength coach in Santa Ana, CA. Your muscles have to decelerate your legs as they hit the ground, which causes more muscle damage. “This hinders recovery from lower-body training. Cycling, on the other hand, doesn’t increase muscle damage significantly.” In other words, if you keep your steady-state cardio sessions low-impact (for example, swimming, rowing, or walking on the treadmill at an incline) your lifting shouldn’t suffer.
Assuming you’re eating sufficient calories to grow muscle, and performing only a moderate amount of endurance training (i.e., not training for a marathon), you should have no trouble gaining size with a lifting-meets-aerobics program. Aerobics will actually make you a more efficient lifter. A competitive powerlifter with a 475-lb bench press, Nuckols regularly incorporates steady-state training into his program: “It helps you increase your training density, volume, and frequency by aiding in recovery during your workouts and between your workouts. Being able to work harder and recover from more work is the simplest way to make more progress.”