Question

calculate the target heart rate zone (50 - 85 percent) of a 14 - year - old with a resting heart rate of 70 beats per minute.

Explanation:

Step1: Calculate maximum heart rate

The formula for maximum heart rate is $220 - \text{age}$. For a 14 - year - old, the maximum heart rate $M=220 - 14=206$ beats per minute.

Step2: Calculate lower limit of target heart rate zone (50%)

The lower limit $L$ of the target heart rate zone is calculated using the formula $L=(M - \text{resting heart rate})\times0.5+\text{resting heart rate}$. Given a resting heart rate of 70 beats per minute, we have $L=(206 - 70)\times0.5+70=136\times0.5 + 70=68+70 = 138$ beats per minute.

Step3: Calculate upper limit of target heart rate zone (85%)

The upper limit $U$ of the target heart rate zone is calculated using the formula $U=(M - \text{resting heart rate})\times0.85+\text{resting heart rate}$. So $U=(206 - 70)\times0.85+70=136\times0.85+70 = 115.6+70=185.6\approx186$ beats per minute.

Answer:

The target heart rate zone is 138 - 186 beats per minute.