When it comes to cardio causing issues with building muscle, coaches often refer to something called the interference effect as the reason, due to certain situations where cardio causes problems with the pathways involved in recovery.

There were a few reasons: cardio uses energy, and building muscle requires energy. So too much cardio could lead to too little energy being available. But this shouldn’t be a major issue as long as you eat enough calories to compensate for the energy used in cardio.

The interference effect is real, but it is only an issue if you do long or hard sessions and these sessions are close to your strength training. So, as long as you keep your cardio session short (under 60 mins) and move them away from your strength sessions, especially leg sessions, this will mitigate most of the issues.

But, do appreciate running comes with a large neuromuscular demand due to impact and ground reaction force. So there is an argument for switching some cardio to low-impact forms like cycling, swimming, rowing, battling ropes, sled work etc. This would reduce the recovery demands running places on the body 

Finally, it’s important to point out that measuring muscle gain is hard. Adding muscle takes time. So it can be hard to gauge progress.

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