Feeling calmer after a hot bath sounds ordinary, yet the strongest result in this experiment was measured rather than guessed. After a half hour in 39 degree Celsius water, 20 adults finished with lower anxiety scores and lower salivary cortisol, a hormone often used as a marker of stress.
The same session also raised heart rate and lowered mean arterial blood pressure, which shows that the body was reacting strongly even though the volunteers were simply sitting in water. Researchers were asking a practical question that many spa buyers and hot tub owners might ask on their own: does built in massage add anything meaningful to the effect of hot water immersion itself?
The answer was narrow but clear. The study found no extra reduction in anxiety, cortisol, or enjoyment when the tub used a water massage system. Air jets moved one outcome in the other direction, because they reduced part of the blood flow and shear rate increase that appeared after the control immersion and the hydro jet session.
Readers should keep the size of the trial in view from the start. This was an acute crossover experiment with 20 participants, only four of them female, so the findings are useful as a close look at short term physiology rather than a final rule for every person who uses a spa.
What the researchers tested
The team ran three separate 30 minute sessions for each participant in randomized order. Every volunteer completed plain hot water immersion, hot water with an Air Jet system and hot water with a Hydro Jet system. The water began at 39 C in every condition, which let the researchers compare the massage systems without changing the main heat exposure.
During each visit, the researchers tracked heart rate, blood pressure and blood flow in the superficial femoral artery, a large vessel in the thigh. They also measured shear rate, which is a way to estimate how strongly flowing blood rubs against the vessel wall. That detail matters because previous work has linked repeated passive heating with healthier vascular function over time, including a 2016 PubMed indexed heat therapy study on endothelial adaptation.
Mood and stress markers were measured before and after each session as well. The researchers collected state anxiety ratings, affect measures and salivary cortisol. Using saliva made it possible to track an acute stress related hormone change without drawing blood each time.
The design was strong in one important way: each person served as his or her own comparison. In a crossover format, a naturally tense volunteer is compared with the same volunteer under each tub setting rather than with a different person who may start from another baseline, which lowers some of the noise that often clouds small physiology studies.
What changed after 30 minutes in hot water
Hot water alone produced the broadest set of positive shifts. Across the sessions, post immersion heart rate rose by about 31 beats per minute, while mean arterial pressure fell by about 16 millimeters of mercury. For readers who do not follow cardiovascular terms closely, blood pressure and heart rhythm basics help explain why those two changes can happen together when heat pushes the circulation to move more blood toward the skin.
Blood flow in the superficial femoral artery also climbed sharply after immersion and mean shear rate rose with it. Those increases fit with earlier experimental work showing that 30 minutes of whole body heating can elevate leg artery flow to levels similar to those seen during some forms of exercise, including a 2021 passive heating study that compared hot water immersion with lower body exercise.
The mood results were simpler. Anxiety scores fell after immersion and cortisol fell as well. A broader 2024 systematic review of balneotherapy and cortisol found that bathing based therapies often move stress biomarkers in the same direction, so the new trial fits a growing body of evidence even though it focused on one short laboratory style session.
Enjoyment did not differ across the three conditions. That point helps narrow the story. The added massage systems did not make the experience measurably more pleasant in this dataset, which makes it harder to argue that the null mood result came from people simply preferring one tub setting over another.
Why water jets and air jets diverged
The most surprising split in the data came from the two massage systems themselves. The Hydro Jet condition, which pushes streams of water through the tub, did not beat the control session for anxiety or cortisol. The Air Jet condition, which sends bubbles through the water, went further by blunting part of the increase in blood flow and shear rate.
The paper offers a practical explanation rather than a mysterious one. Air bubbling through the bath appears to lower heat conservation in the water. If the body retains less heat, the circulation may not respond as strongly and that weaker heat load can trim the vascular boost that plain immersion would otherwise create.
Seen that way, the null massage result becomes easier to read. Warm water itself seems to have done most of the work for mood and stress in this experiment, while the massage features changed little or, in the case of the air system, slightly undercut one vascular response. People browsing premium spa features may expect the moving jets to intensify every benefit, yet this study did not support that assumption.
Researchers and clinicians are often careful about how they describe anxiety outside a diagnosed disorder and the National Institute of Mental Health overview makes the same distinction. The participants in this trial were not being treated for a clinical anxiety disorder. They were healthy adults undergoing an acute heat exposure, so the findings speak to short term stress responses rather than mental health treatment.
What the study can and cannot say
The most useful takeaway is modest. A single 30 minute hot water session in this study moved several markers in a favorable direction, including state anxiety, cortisol, blood pressure and leg artery flow. The massage add ons failed to improve the mood response and the air jet setup weakened part of the vascular response.
At the same time, the limits are easy to see. Twenty participants are enough to detect some acute effects in a crossover design, but they do not represent the full range of age, health status, medication use, or heat tolerance found in the public. The study also tested immediate responses after one session per condition, so it cannot tell readers whether daily or weekly hot tub use would build lasting mental health or cardiovascular gains.
Journal of Thermal Biology published the paper in April 2024 and the Coventry University research portal lists the work as open access while stating that Bestway International Limited funded it. The portal also says the company had no input into data collection, interpretation, writing, or the publication decision. That disclosure is worth noting because the study question overlaps with commercial hot tub features.
Even with those caveats, the article adds a concrete result to a field that often leans on broad wellness claims. The evidence here supports a simple message with fewer promises attached: warm water immersion itself produced the clearest short term benefit and the added massage hardware did not improve the anxiety or cortisol outcome that many users would care about most during a typical session.
