How can a bracelet track your heart rate?
Five minute into his 5 - kilometre ( 3.10 - mile ) discharge , 50 - year - honest-to-goodness Greg can feel his heart poke . Although he ’s been train for a month , this will be his first run without walking interval . Greg wonders whether hisheart ratewill quell in the target zone , which in his case , is 85 to 145 beat per second [ source : American Heart Association ] ....