Final answer:
During exercise, both heart rate and stroke volume increase linearly with work until reaching 50 to 80% of VO2 max, leading to an increase in cardiac output which can be 4-5 times resting rate for healthy individuals, and even higher for athletes.
Step-by-step explanation:
The component that linearly increases with work until reaching 50 to 80% of VO2 max is heart rate, while stroke volume also increases. In healthy young individuals, heart rate (HR) may increase to 150 beats per minute (bpm) during exercise, and stroke volume (SV) can also increase from 70 to approximately 130 mL. This effectively increases cardiac output (CO) to approximately 19.5 liters per minute, which is 4-5 times the resting rate. Furthermore, top cardiovascular athletes can achieve even higher levels and may increase their resting CO by 7-8 times during peak performance.
For example, if a person is generating power on a Stairmaster at the same rate as climbing 116 stairs per minute with an efficiency of 20% and a mass of 76.0 kg, their body temperature would eventually rise, considering that all other forms of heat transfer in and out of the body are balanced. This response to exercise underlines the relationship between physical exertion, energy expenditure, and thermal regulation during workouts.