The Forerunner 45S from Garmin is the smaller (39mm) version of the Forerunner 45 fitness tracker. The entire Forerunner line is geared for runners, although other athletes may still find it useful due to the many other activities the Forerunners are capable of tracking. The 45 and 45S are both on the cheaper end of the Forerunner line, and allow casual runners and other casual athletes to track their workouts in depth without breaking a buck.