Running wearing a heart-rate monitor strapped to your chest can feel a little bit like having a cold, hard tentacle giving you a squeeze. Wrist-worn optical sensors like the ones you find in smartwatches aren’t always the most accurate, but what Polar doesn’t know about timing your ticker would fit on the back of, well, a smartwatch. The OH1 (£69.50) is designed to be worn on your upper- or forearm, with an internal memory that can store up to 200 hours of heart-rate data, although its 12-hour battery life means you won’t be able to log it all in one go. It’s also waterproof up to 30m, so you can wear it while you swim. If you still think you can feel a tentacle wrapping itself around you then, get out of the water. Quickly.