The Suunto Smart Heart Rate Belt provides accurate heart rate data without compromising comfort. It provides high performance in sports where hand movement can disturb wrist heart rate measurements. It also allows measuring heart rate while wearing your compatible smartwatch on top of the sleeve, or attached to the bike handlebar, or even not carrying the watch at all during exercise. Memory function with compatible watches It has an in-built memory that's used when the watch is not within reach of the compatible smartphone. That's ideal in team sports where it allows recording without having to wear the smart watch. This memory function is compatible with the Suunto 3, 5, 9, Spartan, Ambit3 and Traverse watches (sold separately). Multi-sport The Suunto Smart Heart Rate Belt is ideal for a huge range of sports such as biking, team sports, martial arts, climbing, swimming, water-sports, triathlon, ski mountaineering, cross country skiing, skating and gym exercise. Sports Tracker app The Suunto Smart Heart Rate Belt is also a great companion for the Sports Tracker app (available on Android, iOS and Watch OS devices). The belt included is size M. Suunto Chest Straps are available separately in S, M & L size.