A 28-year-old male athlete presents to the sports medicine clinic for routine evaluation. His resting heart rate is 45 bpm, blood pressure 110/70 mmHg, and cardiac output is measured at 5.0 L/min. During a graded exercise test, his heart rate increases to 180 bpm and cardiac output rises to 24 L/min. Which of the following mechanisms BEST explains the increase in cardiac output during exercise in this athlete?