Just an observation, but...
tommee_hawk raises an interesting point.
If several people are wearing heart monitors, which are transmitting to the receiver on their wrist, the signals from one individuals chest strap may interfere with the person's next to them. I have experience of this in the gym myself, so feel free to substitute 'may interfere' with 'will interfere'.
So if a group of people are being clocked within, say, 10 feet of each other, you may end up with someone else's heart reading.
There is a strong probability that incorrect readings will be taken.
That'll be an MAA, then.