Here's something that will make you think twice before getting into the ice bath after leg day. Milan Betz spent his PhD studying the smallest blood vessels in muscle tissue — the microvasculature — and what he found upends the simple story most people tell about cold water immersion.
The core finding is this: submerging in 8 degree water reduces blood flow to cooled muscles by 60 to 70 percent compared to non-cooled muscles. Not a modest reduction. Two-thirds of the flow, gone. And since blood flow is how your muscles receive amino acids — the raw material for repair and growth — that reduction matters. Betz measured amino acid uptake directly, using labeled tracers incorporated into muscle tissue, and the data was clear. Cold water immersion can impair muscle protein synthesis, not just temporarily, but during the recovery window that matters most.
The picture that emerges from the broader research in the knowledge base is more complicated than any single study suggests. A 2015 paper on hemodynamics and recovery found that active recovery — light movement after training — actually increased blood flow markers by 17 percent. Blood volume rose significantly. Isometric strength recovery was more efficient. That's the opposite of what Betz found with cold immersion. Movement after training appears to support the very mechanisms that cold inhibits.
And yet the 2018 paper on CWI and athlete wellness found a 1.36-point improvement in sleep quality among athletes using cold immersion therapy. The 2025 scoping review confirms genuine psychological benefits — reduced anxiety, improved mood, a sense of rejuvenation. These effects are real. The question isn't whether cold immersion does anything. It clearly does. The question is what, exactly, it's doing — and whether that's what you actually need.
The honest answer is that researchers largely agree on the mechanism but disagree about its significance. Betz is careful here: the placebo effect on next-day performance is real and meaningful. Athletes who believe the ice bath works often perform better the next day — not because their muscles synthesized more protein, but because their perception of recovery shifted. Pain is reduced. Confidence is elevated. Those things translate into performance.
The tension is between in-season and off-season goals. If you're a team sport athlete playing three times a week, the ability to feel recovered tomorrow matters more than maximizing hypertrophy over twelve weeks. Cold immersion serves that athlete. If you're a strength athlete in an eight-week hypertrophy block, the same protocol may be quietly working against you.
Separate your goals by phase. During training blocks focused on building muscle and strength adaptation, avoid cold immersion in the hours after your resistance sessions. Let the inflammatory cascade run. That discomfort is the signal. It's your body doing exactly what you trained it to do.
Save cold immersion for competition periods, travel recovery, and general wellbeing maintenance. The sleep quality benefits, the mood effects, the parasympathetic reset — those are genuine. Use them when they serve you.
What strikes me most about Betz's work is what it reveals about the placebo response as a physiological mechanism, not just a psychological one. The belief that you've recovered — the felt sense of readiness — actually changes how you perform. It changes your pain threshold, your willingness to push, your nervous system readiness. That's not "just placebo." That's biology responding to a mental state.
We have 69,000 vectors in this knowledge base pointing in the same direction: your body adapts to signals. Cold is a signal. Warmth is a signal. Movement is a signal. The art is learning which signal to send, and when.