If I’m reading that abstract correctly though, doesn’t it just say that water immersion (be it cold or neutral temperature) has the same effects at affecting HRR and HRV? So really it’s just immersion that helps, and that the temperature is less significant?
Exactly. That’s all you can say about that study. This meta-analysis (reviewing multiple studies) aligns with my gut practitioner feel. There’s something about the hydrostatic pressure effect but certainly also the water temperature and speed of heat transfer away from the body that’s effective at inducing this ‘feel’ of being recovered. In the meta-analysis the key variable they used was the participant’s perception of soreness, as less was appreciated about sympathovagal (CNS) balance at that time. But I am certain from my own experience doing long plunges in the Columbia River this winter that they are aligned (arthritic pain relieved to almost zero, HRV up, HR down). Study concluded that: “CWI with a water temperature of between 11 and 15 °C and an immersion time of 11-15 min can provide the best results”. Practically though, do what you can, and especially if it feels good and is getting you the results you’re after.