The fear that cardio destroys muscle mass is overblown but not entirely unfounded. Excessive long-duration cardio combined with insufficient calorie intake can indeed break down muscle tissue. However, strategic cardio programming builds cardiovascular fitness without sacrificing muscle mass - in fact, it can enhance recovery and improve training performance.
The keys to maintaining muscle while building cardio fitness are: keeping cardio sessions under 45 minutes for most modalities, prioritising HIIT over long steady-state (HIIT is less catabolic), timing cardio away from strength sessions when possible (morning cardio, evening strength or vice versa), and ensuring you're eating enough protein and total calories. When in a caloric surplus focused on muscle building, even moderate cardio volumes won't cause muscle loss.
For concurrent training (cardio and strength in the same programme), research shows the "interference effect" - cardio impairing strength adaptations - is primarily a problem with high volumes of same-day endurance training. Two to three cardio sessions per week, particularly HIIT and Zone 2, produce negligible interference with strength training when separated by several hours or placed on rest days. In fact, improved cardiovascular fitness often helps performance in strength training by enhancing work capacity, reducing fatigue between sets, and speeding recovery.