The more time you spend doing steady-state cardio training, both in terms of session duration and session frequency, the greater your risk of losing lean muscle mass, says Jacob Wilson and Ryan Lowery
One of the biggest problems with cardio training is that you lose muscle on a dose-dependant ratio, so the longer you do, and the more frequently you do it, the more muscle tissue you lose.
Our lab compared high-intensity Wingate bike sprints against 60-minute steady-start cardio. The second group lost muscle mass and gained fat. The first group gained muscle and lost more fat.
Repeated bouts of low-intensity steady-state cardio causes neural adaptations so not all of your muscle fibers are recruited at once, which you want to happen when resistance training.