Probably something wrong unless you are a lifelong swimmer. As an adult onset I find it really really hard to get hr up on swim without heat (hot water or wetsuit). My HR zones are about 20bpm lower in the water. Same hr sensor - on form goggles. To finish my first thought: lifelong swimmers seem to be able to get their hr up more for some reason, in my ecperience.

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My Strava | My Instagram | Summerville, SC | 35-39 AG | 4:41 (70.3), 10:05 (140.6) | 3x70.3, 1x140.6 | Cat 2 Cyclist