Using your heart rate to monitor your workout is all about finding the right pace.
You want to work hard enough to get the benefits of exercise, but not so hard that you burn out or hurt yourself.
First find your maximum heart rate--that's 220 minus your age. At 32, Astrid and Gwyneth Terry have a maximum heart rate of 188.
"OK, Astrid what's your pulse? 173. Gwyneth? 142."
Next find your target zone. Fitness expert Eric Harr says there are different zones depending on your fitness goals.
Eric Harr/ Fitness Expert: "IF YOU WANT TO BURN FAT, AND WE ALL WANT TO BURN FAT, IT'S 60-70 % OF THAT 220 MINUS YOUR AGE NUMBER. IF YOU'RE LOOKING TO BOOST YOUR CARDIOVASCULAR FITNESS, IT'S 70-80. IF YOUR LOOKING TO WIN TRIATHLONS LIKE ME, THEN ITS ABOVE 80%."
But you have to mix it up... so you don't plateau.
"IF YOU CONTINUE TO SORT OF EXERCISE AT THAT 60-70% EVENTUALLY YOUR BODY IS GONNA GET WISE TO YOU AND NOT BURN ANYMORE FAT. SO EVERY FEW WEEKS YOU DO WANT THAT HIGHER INTENSITY STUFF WHERE YOU BUST ACROSS THE GOLDEN GATE BRIDGE AT ABOVE 80% YOUR HEART RATE."
Harr says the formulas work well as a guide, but often need to be adjusted for each individual.
Some people are slow beaters, and others, like Astrid, beat fast.
"SO ERIC SAYS YOU'RE A FAST BEATER."
ASTRID TERRY: "UH HUH, YEA."
"A LITTLE HUMMINGBIRD HEART IN THERE WORKING AWAY, RIGHT?"
"WELL I DIDN'T POP SO I MUST BE OK."
Finding her zone has made a big difference for Gwyneth.
Gwyneth Terry/ Polar 100 Participant: "I HAVE THIS BAD REPUTATION OF RUNNING REALLY FAST AT THE BEGINNING AND THEN AFTER 5 MINUTES GOING, WELL, I REALLY DON'T LIKE RUNNING,AND STOPPING. LAST WEEK I RAN 3 TIMES, AND FOR 45 MINUTES AT A POP, SO I'M ACTUALLY STARTING TO LIKE RUNNING, WHICH IS DIFFERENT. THAT'S A SURPRISE."