Physiological effects of cold versus lukewarm shower exposure

This study uses a randomised, crossover, repeated-measures design. Participants will complete two experimental sessions, separated by a one-week washout period, and will be randomly assigned to one of two intervention sequences: cold shower followed by lukewarm shower (sequence AB), or lukewarm shower followed by cold shower (sequence BA). To minimise participant burden, the first experimental session will be conducted immediately after screening and enrolment. All interventions will be performed face to face by the student investigator (LF), whom is a 4th year physiotherapy student completing their Honors degree. Cold shower: Participants will spend three minutes in a cold shower set at the temperature of 15 degrees. Lukewarm shower: Participants will spend three minutes in a lukewarm shower set at the temperature of 35 degrees. Intervention procedure: Screening and enrolment and experimental procedure: For convenience and to minimise participant burden, the experimental procedure will be conducted immediately following the screening and enrolment session. Screening and enrolment Individuals who have completed the online screening will be invited (via email or mobile phone) to attend a screening and enrolment appointment at the School of Physiotherapy building, University of Otago, during which they will be screened against the inclusion and exclusion criteria to determine eligibility for participation in the study. All screening and enrolment sessions will be performed by LF. Prior to this session, participants will be instructed to refrain from additional exercise, alcohol, and caffeine intake on the day of the intervention session, as these factors may influence the primary outcome measure of HRV. Participants will also be asked to ensure they are free for their second experimental session on the same day and time in exactly one week (in case they are eligible and decide to participate). Finally, participants will be advised to bring their togs (swim wear) to the intervention session. During this session, basic demographic information will be collected, including age, sex, and prior experience with cold showers. Once eligibility is confirmed, participants will be given the opportunity to ask further questions and will then be asked to complete a written informed consent form. Informed consent: Following a detailed explanation of the study's purpose, experimental procedures, potential benefits, and possible risks, potential participants will be asked to provide verbal and written informed consent, free from any undue influence. If a potential participant declines to participate, this decision will not result in any disadvantage to them. Experimental procedure: Following the screening and enrolment session (which includes informed consent) the participant will be randomised into their intervention sequence, and the experimental procedure will begin. This will occur in the same room as the screening and enrolment session. The room’s ambient temperature will be recorded. Participants will be asked to lie supine on a plinth for five minutes. During this time, participants will be connected to the PowerLabC via cuffs attached to the wrist and ankle. Participants will be instructed to breathe spontaneously during this period and to avoid talking or using their phone. They will also be asked to take note of their breathing pattern while lying on the plinth, as this is the breathing pattern they will be asked to intentionally replicate during the intervention. At the end of the five-minute period, baseline measurements will be recorded. Following baseline measurement, participants will be escorted to the nearby shower facilities, and provided with a towel and asked to change into their togs. Participants will be advised to wear togs during the intervention session. Wearing togs will mean the results are less generalisable to having a shower at home, where it is unlikely participants will wear togs. However, for participant safety and comfort during this trial, togs will be worn. Participants will be advised that the student investigator will remain outside the shower room in case of an adverse event. The shower door will not be locked; however, LF will ensure that no one enters the room during the intervention. Both cold and lukewarm water interventions will be performed using a standard shower setup. The target temperature for the cold shower will be approximately 15°C, while the target temperature for the lukewarm shower will be approximately 35°C. Prior to each shower intervention, calibration will be performed to account for variability in water temperature. The specific temperature will be achieved by participants adjusting the shower dial to the respective labelled marker. Participants will be instructed to turn the shower on to a comfortable temperature and, within five minutes, gradually adjust the water temperature to the allocated marker (corresponding to the allocated temperature). This five-minute period will serve as an adaptation phase, as recommended in previous literature and supported by the authors’ clinical experience. The researcher will remain outside the shower room and will oversee all timing aspects of the protocol. Once the participant verbally confirms that the shower dial has reached the designated marker corresponding to the allocated temperature, the researcher will commence timing the 3-minute exposure. A verbal cue will be provided at the completion of the 3-minute period, ensuring standardisation of exposure duration while minimising participant cognitive load during cold exposure. During the three-minute shower period, participants will be instructed to alternate water contact between the chest and the mid-thoracic region (around the shoulder blades), while maintaining the same breathing pattern used during baseline measurements. Once the three-minute period is complete, participants will dry themselves with the provided towel, change back into their clothing, and return to the plinth, where they will lie supine for 30 minutes with their legs extended. The timer will start for the post-shower measurements once the participant is lying down, as we expect some variability in the time it takes participants to dry themselves and get changed. This process will be timed. Participants will be instructed not to talk or use their phone during this period, and the rooms ambient temperature will be recorded again. Serial measurements will be collected at five-minute intervals during the 30-minute post-intervention period. Thermal comfort will be measured post shower using a 9-point thermal comfort scale ranging from extremely uncomfortable to extremely comfortable. If the participant is excessively cold or shivering, they will be offered a blanket to warm up. This will be recorded. Following completion of the initial session, participants will be advised that the same procedure will be repeated at the same time one week later and that they will receive a text message reminder the night before the second session. Participants will be advised to bring the same togs to this session. Given that the study involves a healthy population, and that previous research has demonstrated that HRV returns to baseline within approximately 30 minutes following CWI, a one-week washout period was deemed sufficient. Upon arrival at the second session, participants will be screened for any changes in health status or any adverse reactions experienced following the previous intervention. Training: Regarding training of the student investigator. The lead investigator (GF) will train the student investigator (LF) in the appropriate procedures for the screening and enrolment session and experimental sessions. Training will occur over three weeks prior to the start of screening sessions. This will include how to use the equipment, order of measurements, how to manage and adverse event etc. A trial run of all procedures will be performed (multiple times if required) before recruitment. The lead investigator will be present for the first couple of sessions to ensure everything runs to protocol. Duration: -LF will complete all screening and enrolment sessions in the School of Physiotherapy building, University of Otago. Each screening and enrolment session will take approximately 15 minutes. We are aiming to recruit a sample of 15 participants. Conservatively estimating that 20% of people screened will either decline participation or be ineligible, we anticipate requiring 18 screening and enrolment sessions (4.5 hours total). -Immediately following the screening and enrolment session, the participant will have their first intervention session. Each participant will receive two intervention sessions, each lasting one hour. Each intervention will be separated by a one-week washout period. In total, this equates to 30 hours. LF will perform all interventions.