"It depends on the type of cardio that you're doing," Rondel told POPSUGAR. "If you're doing steady-state cardio, which is a long duration above 30 minutes, that could be detrimental to your muscle-gaining goals." Steady-state cardio can be detrimental to building muscle "because that can put you into a zone where you're developing more stress hormones (cortisol), which can be counterproductive to developing muscle," Rondel explained. The influx of cortisol in your body can potentially break down tissue, he said.
Instead of going for hour-long runs, Rondel recommends getting in your cardio with activities like high-intensity interval training (HIIT). This is because a HIIT workout creates an anabolic response as "it's done above your anaerobic threshold, which elicits a hormonal response (growth hormone and testosterone) that is conducive to gaining muscle," he said.