This graph shows physiological responses of trained and untrained athletes at rest and during maximum VO2 exercise - HSC - SSCE Personal Development and Physical Education - Question 19 - 2003 - Paper 1

To understand what the dotted line represents, we need to analyze the graph and the physiological responses depicted for both trained and untrained athletes.

1. Understand the Variables: The graph depicts 'Blood pumped (mL)' at 'Rest' and during 'Maximum VO2 Exercise'. This indicates the output of blood pumped by the heart.

2. Interpret the Dotted Line: The dotted line typically indicates a specific metric being compared against the solid lines which represent the physiological data of the two groups.

3. Consider the Options:

   • A refers to the cardiac output of a trained athlete.
   • B refers to the cardiac output of an untrained athlete.
   • C refers to the stroke volume of a trained athlete.
   • D refers to the stroke volume of an untrained athlete.

4. Draw Conclusions: In this context, the stroke volume (the amount of blood pumped per heartbeat) would be represented as a parameter that could vary significantly between trained and untrained individuals. Since the question asks about the dotted line specifically in relation to exercise, we assess that it likely indicates the stroke volume in this scenario.

Thus, the most accurate answer given the analysis is D: The stroke volume of an untrained athlete.