Believe it or not, this is a big deal. Freedive training enhances the diving reflex significantly, while I had assumed that if anything, UWH training would do the opposite. The fact that UHPs still had a strong diving reflex suggests that heartrate under the water is lower than expected for the activity undertaken, which means hockey players are trading longer apneas for higher lactic loads and slightly longer recoveries. The higher ppCO2 means that like many top freedivers, you've (probably accidentally) discovered that higher aterial CO2 concentrations enhance the diving reflex, allowing longer apneas with a lower chance of sudden blackout. It probably also explains why Stella kicked arse when she did some serious freediving training.

If anyone's interested, I can post the DAN report on breath-hold diving. It's a bit of a monster, but there's some amazing stuff in there, like Umberto's recorded R-R heart rate frequency of 9 bpm @ -50m and Tanya Streeter's experience with narcossis at -160m.