beta blockers will slow your heart rate. therefore when you're swimming or doing any kind of physical activity or exertion your heart rate will not increase much as it did when you were off the meds. it's a whole different experience. now that shouldn't stop you from doing your activities. you just cannot use the heart rate range rules to see if you're getting the caloric or fat burn you want. or if you're an athlete on a beta blocker the same holds. you can't use your heart rate to determine your overall activity efficiency or performance.
does taking beta blockers affect heart rate during exercise?