A 50 year old client has a resting heart rate of 70 beats per minute. Calculate the exercise heart rate for this client using 50% of heart rate reserve.

85 bpm results from incorrectly taking 50% of resting HR alone (70 x 0.50 = 35, then adding 50), not applying the Karvonen formula correctly.

100 bpm would result from taking 50% of max HR only (170 x 0.50 = 85) without adding resting HR, or from miscalculating the reserve.

110 bpm does not correspond to any correct application of the Karvonen formula with the values given and reflects a calculation error.

Using the Karvonen formula: max HR = 220 - 50 = 170 bpm; heart rate reserve (HRR) = 170 - 70 = 100 bpm; target HR = (HRR x 0.50) + resting HR = (100 x 0.50) + 70 = 120 bpm. This formula accounts for resting HR, making it more individualized than simple percentage-of-max-HR methods.