Excessive cardio can negatively affect muscle growth by promoting a catabolic state, where muscle breakdown outpaces muscle synthesis. Overtraining through long-duration cardio can lead to increased levels of the stress hormone cortisol, which can hinder recovery and muscle development. To support muscle growth, it's vital to balance cardio with strength training and ensure adequate nutrition, protein intake, and rest. This approach promotes a healthier body composition and improves overall physical performance.