Resting heart rate
Answers

a.One minute before commencing exercise the heart rate of the trained athlete is approximately 56 beats per minute (bpm), compared with approximately 65 bpm for the untrained athlete.
After 9 minutes of exercise the heart rate of the trained athlete is approximately 135 bpm, compared with approximately 152 bpm for the untrained athlete.
At 3 minutes after exercise the heart rate of the trained athlete is approximately 72 bpm, compared with approximately 92 bpm for the untrained athlete.

b.The graph shows a consistent shape for both trained and untrained athletes as a result of moderate exercise i.e. heart rate (HR) rises with increased exercise and declines after stopping. Rate of increase/decrease may vary between trained and untrained athletes.

The main difference between the two graphs is that:

o for a given workload a trained athlete's HR is lower and the maximum heart rate reached is lower. For example, after 10 minutes of exercise the heart rate of the trained athlete is approximately 135 bpm, compared with approximately 152 bpm for the untrained athlete.

o The resting heart rate of the trained athlete is usually much lower than that of the untrained athlete. For example, the resting heart rate of the trained athlete is approximately 56 bpm, compared with approximately 65 bpm for the untrained athlete.

o The heart rate of the trained athlete will also return to resting levels faster than that of the untrained athlete.

Explanation: The main reason that heart rate is lower in the trained athlete is because training strengthens the heart and allows for an overall increase in stroke volume. As the trained athlete has a lower heart rate, the cardiac output for each individual will be close to the same. 

c. The exercise completed by the two athletes shown was identical. It required the same amount of effort to complete the set amount of work. However, the trained athlete's heart does not need to work as hard as the untrained athlete's because of the higher stroke volume and the heart of the trained athlete is able to pump more blood per beat so it does not have to beat as many times to move the same amount of blood around the body. 

The trained athlete has a lower heart rate for each unit of work. When heart rate is at a maximum level the trained athlete will be able to do a higher level of work, and thus the trained athlete will be able to achieve a higher level of endurance performance.