Why I never combine ice bath with hot tub.
Summary
Vasoconstriction is the contraction of smooth muscle tissue to reduce blood flow to the limbs and defend core body temperature.
An ice bath promotes vasoconstriction, which is like a workout for smooth muscle tissue.
Dry sauna promotes vasodilation, which is the opposite of vasoconstriction and helps promote better blood flow.
A phenomenon called hidromeosis prevents the body from sweating when the skin is wet -- as in a hot tub. Without sweat, your body will go into vasoconstriction when immersed in hot water.
For a smooth muscle/vascular workout, combine ice bath with dry sauna -- not hot tub.
Work smooth muscles to strengthen circulation
Every fitness blogger talks about how to build or keep skeletal muscle, because more skeletal muscle looks good and promotes health and longevity.
But almost no one talks about smooth muscle.
Skeletal muscles attach to bones via tendons. They operate under (mostly) voluntary control. They're the muscles that control our limbs and that we can build up by working out.
Smooth muscles attach to our stomach, intestines, bladder, arteries, and veins. They operate under involuntary control, which means there are no workout videos or fitness machines for strengthening smooth muscle tissue.
Smooth muscles perform several important functions in the body, including:
propulsion of food through the digestive tract,
expulsion of urine,
regulation of blood flow and pressure,
contractions during pregnancy & childbirth, and ejaculation,
standing hairs on end.
Smooth Muscle Health
Irregularities in smooth muscle function are linked to asthma, high blood pressure, atherosclerosis, and varicose veins. Moreover, smooth muscle function typically declines with age (Chi et al. 2019, Greaney et al. 2015).
Because smooth muscle control is involuntary, exercising your smooth muscles is different from the activities that exercise your skeletal muscles.
For a smooth muscle work out, you need an ice bath.
Vasoconstriction & vasodilation
When the thermoreceptors in your skin identify cold temperatures, they signal vasoconstriction, which is a smooth muscle contraction in the arteries that control blood flow to the extremities. This is partly why your toes and fingers hurt when submerged in ice water. Although it feels uncomfortable, vasoconstriction is a thermal defense mechanism that automatically protects your core from being exposed to cold blood returning to your heart from your cold limbs.
Afterdrop is a phenomenon that happens when you emerge from the cold water. As your skin rewarms, the smooth muscle tissue in your limbs relaxes to allow blood to return to the cold muscles in your arms and legs. Although your extremities get warmer, your blood gets colder. Then, when cold blood from your limbs returns to your heart, your core temperature may keep dropping as your body temperatures rebalance.
That's why you sometimes don't start really shivering until after you are out of the Forge.
Thermal contrast therapy -- where you go from cold to hot and back again -- is like a workout for your smooth muscle tissue. Cold contracts it. Heat relaxes it (via vasodilation), and every time you switch back and forth is like doing another "rep" in your smooth muscle workout.
Use dry heat for thermal contrast therapy
Ironically, immersion in warm water can also result in vasoconstriction. For example, the wrinkles that appear on the skin of your fingertips when you've been too long in the hot tub are caused by vasoconstriction (Wilder-Smith & Chow 2013). Ordinarily, your warm body would increase blood flow to the skin, where sweat evaporation would help cool the body and prevent overheating. However, when you're in a hot tub, sweat can't evaporate, because you're submerged in water. In a process called hidromeosis, the body senses warm water on the skin and signals smooth muscle tissue to shut down circulation to minimize heat gain.
Thermal contrast therapy works best for your smooth muscle tissue when you go from the wet cold of the ice bath to the dry heat of the sauna, rather than to the hot tub.
Do you need vasoconstriction?
Although there are no studies that relate thermal contrast therapy to indicators of smooth muscle fitness, it stands to reason that a smooth muscle workout might slow ageing and extend health-span, because those who perform thermal contrast therapy typically enjoy fewer adverse outcomes related to markers of smooth muscle dysfunction. For example, elderly subjects who practiced sauna experience better cold intolerance and less constipation and incontinence (Sugie et al. 2020). Moreover, studies in Finland and Japan reinforce the hypothesis that thermal contrast vasoconstriction-vasodilation cycling results in improved cardiovascular outcomes (Heinonen & Laukkanen 2017). Given the critical role that smooth muscle tissue plays in regulation of blood flow, adding a smooth muscle workout to your health regimen is likely to improve digestion and elimination, stabilize blood flows and pressures, improve sexual and reproductive functions, and may slow ageing or wrinkling of your skin.
UPDATE 14 Feb 2025
Last week I was floating on my back in the volcanic hot springs of El Salvador, thinking about nothing in particular when a wave of super hot water splashed over my shoulders from the pool upstream.
That's when my nipples got hard.
Typically, nipple erections happen in the cold, right? In breastfeeding women, sometimes a painful vasoconstriction of the nipple (after nursing) is called vasospasm, and one of the recommended remedies is the application of a warm pack.
I wasn't a breastfeeding Mother and I wasn't cold, so what was going on with my nipples?
It was hot tub vasoconstriction, just like this article describes.
References
Chi C, Li DJ, Jiang YJ, Tong J, Fu H, Wu YH, Shen FM. Vascular smooth muscle cell senescence and age-related diseases: State of the art. Biochimica et Biophysica Acta (BBA)-Molecular Basis of Disease. 2019 Jul 1;1865(7):1810-21.
Greaney JL, Alexander LM, Kenney WL. Sympathetic control of reflex cutaneous vasoconstriction in human aging. Journal of applied physiology. 2015 Oct 1;119(7):771-82.
Heinonen I, Laukkanen JA. Effects of heat and cold on health, with special reference to Finnish sauna bathing. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. 2018 May 1.
Sugie M, Harada K, Takahashi T, Nara M, Fujimoto H, Kyo S, Ito H. Effectiveness of a far‐infrared low‐temperature sauna program on geriatric syndrome and frailty in community‐dwelling older people. Geriatrics & Gerontology International. 2020 Oct;20(10):892-8.
Wilder‐Smith EP, Chow A. Water‐immersion wrinkling is due to vasoconstriction. Muscle & nerve. 2003 Mar;27(3):307-11.
About the Author
Thomas P Seager, PhD is an Associate Professor in the School of Sustainable Engineering at Arizona State University. Seager co-founded the Morozko Forge ice bath company and is an expert in the use of ice baths for building metabolic and psychological resilience.
Made me research this more
https://www.bbc.com/future/article/20220620-why-humans-evolved-to-have-fingers-that-wrinkle-in-the-bath
It happens in colder water too, could be to help with grip in water.
Nevertheless it is a vasoconstriction like you said, interesting, I notice that I do feel more stiff after hot showers actually.
What are the benefits of a hot spring then?
Here's an update on vasoconstriction from my Forge (and why I never combine ice bath with hot tub).