It’s commonly believed that combining cardio and weights will interfere with muscle growth—and rightfully so, I might add, considering the number of studies that have reported signaling interference when these modes of exercise are performed concurrently.
By “signaling interference” I mean that, because cellular signals created by aerobic exercise are dominated by energy-deficit pathways, but signals created by resistance exercise are dominated by anabolic pathways, performing these exercises together creates an energy deficit within the muscle cell, activating pathways that inhibit or greatly reduce energy-requiring processes such as protein synthesis. Indeed, numerous studies have shown that when the muscle is exposed to both aerobic and anaerobic exercise, reduced muscle hypertrophy results.
So you can understand my surprise when I came across a study claiming that combining aerobic and resistance exercise may actually enhance muscle growth.
A group in Sweden recently conducted a study in which they found that anabolic signaling pathways were not compromised when aerobic exercise was performed six hours before strength training. In addition, they discovered that the muscle-growth inhibitor myostatin was suppressed to a greater extent in the group that performed both aerobic and strength training on the same day.
To see if these changes resulted in tangible gains in muscle size, researchers tested participants for five weeks, during which one group performed resistance exercise only, and the other performed aerobic exercise early in the day, then resistance exercise six hours later. Resistance exercise consisted of four sets of seven maximal single-leg reps on a flywheel-based leg-extension machine, while cardio consisted of 45 minutes on a one-leg cycle ergometer. This was done three times a week on nonconsecutive days. All subjects trained one leg using resistance exercise only, and the other combining both resistance exercise and cardio.
After five weeks, changes in quadriceps-muscle volume were measured using MRI to compare one leg to the other. Individual fiber growth was measured using biopsies. Result: In the resistance-only leg, muscle volume increased 8%, while in the combined cardio-and-weights leg, muscle volume increased 14%. Individual fiber growth in the resistance-only leg increased 9%, while muscle fiber in the combined leg increased 17%.
Contrary to a lot of research, this group found that workouts consisting of 45 minutes of cardio six hours before weight training led to greater increases in muscle growth than weights-only workouts. These results need to be replicated before we can say with certainty that cardio doesn’t necessarily interfere with muscle growth, but the prospect is intriguing.