First the theory - then the reality
Heart rate has and still is the most common guide to controlling an athletes efforts whilst training. If you are doing continuous training then running elevates the heart rate highest, then cycling and then swimming with about around 8-10bpm average drop. This is naturally due to the reduced level of impact.
Heart rates zones. These are calculated by the Karnovan Principles - which is 220 minus your age. This then gives you your Maximum Heart Rate (MHR) from that if you divided by 100 and then times by the percentage of effort you want I.e 50% you will get a Working Heart Rate (WHR)
HOWEVER - heart rate is REACTIVE not proactive. By this I mean that your heart rate can be elevated due to stress, caffeine, illness etc without doing a thing, but this doesn’t mean you we working in the right zone. It’s not uncommon to have a HR that is elevated by simple activity, naturally it’s advisable to be cleared by hour doctor to ensure that the old ticker it healthy. At the same time Some people can also find that they can’t get their HR into the correct zones - this could be due to fitness levels, body’s ability to work at that level.
So what is the knock on affect to this? If you work to HR then you might find you are working too hard or having to slow down to ensure that you are in the correct heart zone.
So what do I do? Use Rate Perceived Exertion RPE. This is the Borg scale of 6-20 but 1-10 is easiest to understand. 1 being sitting down 10 being maximal 10-30 sec effort. You can find a copy of the Internet. You can then see 6 as 60% effort based on time & RPE
Swimming - never wear a HR strap, waste of time, it will annoy you by falling down, your HR will be 16-20 lower due to the activity being low impact. Always refer to the pace needed of RPE.
Cycling - in an ideal world you would use watts but this is expensive. So go RPE, HR can help in situations like hills but RPE will control that over effort.
Running - As above, pace is a great tool to control effort as too can be RPE.
HR should be a guide in HR training and not the definitive.