http://www.everydayhealth.com/multiple-sclerosis/treatment/wearable-activity-trackers-improve-ms-care?xid=nl_EverydayHealthLivingWithMultipleSclerosis_20160206
I use a fitbit which gives me my heart rate when i ride my incumbent bike. That way i know when to push faster or use my arms to increase the rate. One day i plan to get one of the more expensive ones that you can wear in the pool, and it measures a lot more than the one I have.
It also shows your daily levels of activities and goes back a week so you can see where you are currently in the week.