Cardio training is a special load aimed at burning fat. However, many athletes notice that multiple cardio sessions may also burn your hard gained muscle tissue, so according to them, while on the mass-period you should avoid treadmill and include it in your training regimen delicately while you are getting lean.
Bulking up is a crucial period of every bodybuilder so they would rather wholly avoid cardio to protect themselves from a catabolic state.
It is quite an exciting misconception that from aside looks like a real theory. That's why researchers from the Department of Health Sciences at Mid Sweden University decided to investigate this problem.
In fact, burning hard work you put in is far from cardio sessions, cardio can actually increase muscle size.
The study involved ten male participants aged 25-30 who were put through a 5-week training programme. Their leg day was divided: one leg regularly performed a 45-minute cycle and seven reps of knee extensions, while the other just performed the knee extensions.
The results were shocking: "An MRI of both legs post-programme revealed the muscles in legs subjected to extra cardio work increased in volume by 14% and 17%, compared to 8% and 9% in the legs put through just strength work."
Quite interesting remembering that cardio burns extra calories which in its turn unlikely can lead to muscle gain.
But this case was also sorted out by specialists. Provided you are in a calorie surplus that covers both cardio and lifting work out your mass will increase. As far as running and other activities can increase the number of capillaries (small blood vessels) that network through your muscles, transporting oxygen, nutrients and hormones would be more efficient. Thus your body would be able to work and recover muscles.
Another reason is that cardio improves insulin sensitivity, which governs how effectively your body processes and uses carbs.