What is the usual cause of a decrease in heart rate observed in athletes at rest?

A decrease in heart rate observed in athletes at rest is primarily attributed to increased vagal tone. In trained athletes, particularly endurance athletes, the autonomic nervous system adapts to regular exercise, leading to enhanced parasympathetic nervous system activity. This increased vagal tone results in a stronger influence of the vagus nerve on the heart, effectively lowering the resting heart rate.

As athletes' cardiovascular systems become more efficient, their hearts pump a greater volume of blood with each contraction, enabling the heart to work less to maintain adequate blood circulation at rest. Consequently, this adaptation allows for a lower heart rate, which is a hallmark of athletic conditioning.

The other options do not accurately reflect the physiological changes associated with athletic training. While arrhythmia can cause heart rate variations, it is not a physiological adaptation seen in healthy athletes. Decreased blood volume is typically associated with dehydration or other health issues, and anxiety usually leads to an increased heart rate due to heightened sympathetic activity, which contrasts sharply with the findings observed in resting athletes.