Top 3 easiest-to-use activity trackers

Looking for an activity tracker to measure your heart rate or estimate your VO2 max score? This blog post isn’t for you. But if you want a simple, straightforward way to track steps and a few other activities — and you don't know what a VO2 max score is — you’re in luck. Here are three basic activity trackers worth buying.Fitbit ZipWith its retro smiley face, easy-to-read screen and affordable price ($60), Zip is Fitbit’s entry-level activity tracker. Plus, its batt