Kima is an athlete and weighs more than her sister, who is the same way. what could be the reason?

The very obvious answer could be that Kima has more muscle weight because she is an athlete. Athlete's have, on average, more muscles than an average person, because they need the extra muscles to help them support their physical activities. However, it is also possible that Kima is heavier than her sister due to the fact that she is overweight. However, I would say the answer because she has more muscles is the correct one. 

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Pe.dometer would be the most useful to you for monitoring heart rate.