A marathon runner reports that caffeine helps them feel less tired late in races but also gives them jitteriness and a faster heart rate. Which of the following explains both effects?

• A. Caffeine increases parasympathetic tone producing calmness and reduced heart rate but at the same time causes muscular hypoxia which the athlete interprets as tremor
• B. Caffeine directly binds muscle oxygen and increases oxygen extraction, which reduces effort perception but causes cardiac oxidative stress manifesting as jitteriness
• C. Caffeine stimulates the central nervous system lowering perceived exertion, while its sympathomimetic action can cause side effects like tremor, palpitations, and anxiety
• D. Caffeine triggers insulin release which both reduces central fatigue and causes sympathetic overdrive and tremor due to hypoglycemia

Answer: (C)