The increased endurance of skeletal muscles from fitness activities is primarily due to what?

The increase in endurance of skeletal muscles as a result of fitness activities is primarily due to improved blood flow to the working muscles. Enhanced blood flow delivers more oxygen and nutrients necessary for sustained muscle activity and supports the removal of metabolic waste, which helps to maintain performance during prolonged physical activity. This increased vascularization also allows for greater adaptation and efficiency in energy utilization, contributing significantly to endurance over time.

While increased muscle mass plays a role in overall strength and can contribute to endurance indirectly, it is the improved vascular response that directly enhances a muscle's ability to sustain prolonged activity. Improved coordination and enhanced flexibility are certainly beneficial for physical performance, but they do not specifically target the endurance component as effectively as improved blood flow does.