Long-term high-intensity training can produce several positive effects on the heart, including increased cardiac output, improved stroke volume, and enhanced overall cardiovascular efficiency. High-intensity exercise stimulates adaptations in the heart muscle, making it stronger and more efficient in pumping blood. Regular intense workouts can also lower resting heart rate and improve the body's ability to utilize oxygen, contributing to better endurance performance. However, overtraining without adequate recovery can lead to negative effects, so balance is crucial for maintaining heart health.