Beyond the Comfort Zone: Elevation, Temperature, Fatigue and Pain Perception

Abstract

This study provides a comprehensive evaluation of the effects of environmental stressors and physical exertion on human nociceptive processing across multiple ecologically relevant conditions. Using a repeated-measures design, participants (N = 45) completed up to five controlled laboratory (thermoclimatic chamber) sessions (baseline, simulated altitude at 4200 m asl, heat at +42 °C, cold at −10 °C, and exertion). Participants were tested by using electrical stimuli. Linear mixed-effects models with participant-level random intercepts, alongside estimated marginal means and bootstrap derived effect sizes, enabled robust characterization of within-subject differences. Thermal stress emerged as the strongest modulator of nociception. Heat exposure significantly elevated sensory and pain thresholds compared with all other conditions, whereas tolerance thresholds peaked during cold exposure, yielding the largest observed effects. Altitude consistently produced the lowest thresholds across all modalities. These contrasts were confirmed statistically in the mixed-effects models, and effect-size analyses indicated substantial within-subject differences between the thermal extremes. By integrating three distinct nociceptive modalities and extreme environment simulations, this work offers novel and informative insights into how environmental stressors shape pain processing. The discovery of opposing thermal effects on sensory/pain versus tolerance thresholds—within the same cohort and design—reveals modality-specific patterns not previously documented and suggests that hypoxia may further modulate these responses.

1. Introduction

Pain is defined by the International Association for the Study of Pain (IASP) as a complex sensory and emotional experience associated with actual or potential tissue damage. It is inherently subjective and shaped by biological, psychological, and social factors. Because of this complexity, pain perception can be influenced by a wide range of environmental and physiological stressors [1].

One such stressor is hypoxia, typically observed at altitudes above 1500 m. High-altitude exposure alters tissue oxygenation, cardiovascular responses, autonomic activation, and thermoregulation. Although these physiological changes could plausibly affect pain processing, research on this relationship remains limited. Existing studies are inconsistent: some report minimal or non-significant changes in pain thresholds under hypoxic conditions, often in very small samples or in individuals already adapted to moderate hypoxia—factors that may obscure true effects. Other studies suggest acute hypoxia may modulate nociceptive pathways through changes in cerebral oxygenation, inflammatory mediators, or autonomic balance, but findings remain contradictory and lack systematic comparison across pain modalities [2,3].

Temperature represents another environmental variable that may interact with nociception. While several studies report that ambient temperature reductions alone do not significantly alter pain perception, others indicate that skin temperature—rather than environmental temperature per se—plays a decisive role in shaping pain sensitivity [4,5]. This is consistent with known physiological effects of cold exposure, including vasoconstriction, conduction block in peripheral nerves, and reduced receptor responsiveness. Conversely, heat exposure can elevate psychological distress, alter autonomic responses, and potentially intensify fatigue, thereby affecting both pain threshold and tolerance. However, most existing work has relied on natural outdoor environments or older methodologies, limiting control over experimental conditions and reducing comparability across studies [6].

A third factor relevant to pain modulation is fatigue, both local (within the muscles involved in performing specific exercises) and global. Prior research demonstrates that physical exertion can transiently alter pain thresholds, for instance, by increasing pressure pain tolerance for several minutes post-exercise [7,8]. Some studies attribute this to exercise-induced hypoalgesia, whereas others find hyperalgesic responses depending on the type of fatigue, the stimulated body region, and participants’ health status [9]. Individuals with chronic pain conditions show even more heterogeneous responses, suggesting complex interactions between physical strain, central sensitization, and endogenous pain modulation systems [10].

Importantly, prior research examining environmental influences on pain has focused predominantly on pressure and thermal pain. Studies involving electrical stimulation, which provides more consistent activation of nociceptive fibers and allows separate assessment of sensory threshold, pain threshold, and pain tolerance, are scarce [2]. Moreover, few studies have simultaneously evaluated altitude, temperature, and fatigue within a single controlled experimental design. This gap is notable, given that professions involving extreme environmental exposure (e.g., rescue workers, mountain personnel, soldiers) may face risks when physiological changes mask early warning signs of injury or impair task performance [11,12].

Therefore, the present study addresses significant gaps in the literature. First, it examines multiple environmental factors—altitude, temperature, and fatigue—within the same controlled experimental paradigm. Second, it applies electrical stimulation, enabling precise determination of not only pain threshold but also sensory threshold and pain tolerance, which were often overlooked in prior studies.

2. Materials and Methods

2.1. Participants

The study included 45 individuals (62% men, mean age 27.96). Basic characteristics of the study population are presented in

Table 1. Basic characteristics of the analyzed sample.

Sex	Male	Female	Total
N	17	28	45
Mean age [yrs.]	27.67	28.44	27.96
Mean body height [mm]	1788.26	1627.59	1726.18
Mean body mass [kg]	80.07	69.8	71.26
Mean BMI [kg/m2]	25.09	21.33	23.64

BMI: Body Mass Index.

.

Participants were volunteers recruited through on-line survey. Exclusion criteria for the study were as follows: contraindications to physical exertion; cardiological, psychiatric, metabolic, or dermatological conditions; presence of tattoos or extensive scars on the inner part of the forearm; presence of devices such as pacemakers, insulin pumps, or metal implants in the body; consumption of psychoactive substances (including alcohol), caffeine, nicotine, or analgesic medications up to 24 h prior to the study; presence of chronic or acute pain up to 24 h prior to or at the time of the study; and previous participation in experimental pain studies.

The inclusion criteria consisted of the absence of exclusion parameters, being of legal age (18 and older), and providing written consent for participation.

All procedures contributing to the study complied with the ethical standards of the relevant national and institutional committees on human experimentation and with the Helsinki Declaration of 1975, as revised in 2008. The study protocol was also approved by the Bioethics Committee of the Regional Medical Association in Kraków (98/KBL/OIL/24).

2.2. Threshold Procedures

Participants underwent testing of sensory thresholds, pain thresholds, and pain tolerance (electrical stimuli) four times; each time humidity was set at 50%:

• Under baseline conditions—205 m above sea level, 20 °C;
• Under high-altitude conditions—4200 m (oxygen ratio—12.7%) above sea level, 20 °C;
• Under variable temperature conditions—205 m above sea level, −10 °C and +42 °C;
• Under physical exertion conditions—205 m above sea level, 20 °C; the test was performed on a Wattbike Atom (Nottingham, UK) ergometer equipped with smart load adjustment technology; for each individual (depending on sex, age, and physical activity level), a personalized program was designed to induce fatigue; additionally, lactate levels were monitored (using the Lactate Scout Sport analyzer (EKF Diagnostics, Cardiff, UK), with measurements before and after the test); participants’ subjective exertion was assessed on a numerical scale (0 [no fatigue]–10 [the most intense exertion you can endure in experimental conditions]), and the test was ceased when fatigue was assessed as 9/10.

Unless otherwise specified, each test was conducted under identical conditions (conditions controlled using Hypoxico thermoclimatic chamber, Gardiner, NY, USA), except for the modified condition.

Each session (each condition) was performed in a different week to ensure that there were no residual effects. Additionally, a standardized acclimatization break (appropriate for each condition) was provided before each testing session (condition).

The study procedure flow and the scheme of the laboratory where the experiment took place are presented in the figures below (Figure 1 and Figure 2).

2.3. Electrical Stimulation

Numeric Rating Scale (NRS) was used to assess the pain. It is an 11-point scale to measure the expected and perceived intensity of pain caused by a given stimulus (0 = no pain; 10 = the most intense pain you can endure in experimental conditions).

A short familiarization (separate from all experimental conditions) session was performed to ensure that the participants properly understood the scale and reporting procedure.

Electrical pain stimuli were administered to the non-dominant forearm, using a certified device employed in medical diagnostics (Digitimer Ltd., model DS7A, Welwyn Garden City, UK). These stimuli are harmless and do not cause any permanent adverse health effects.

Pain thresholds, sensory thresholds, and pain tolerance were defined as:

• Sensory threshold—the smallest intensity of a stimulus (in mA) that is perceptible but does not cause pain;
• Pain threshold (1 on NRS)—the smallest intensity of a stimulus (in mA) at which pain is first perceived;
• Pain tolerance threshold (10 on NRS)—the maximum intensity of a stimulus that can be tolerated before it becomes unbearable; due to ethical reasons, the procedure was ceased at 9/10 on NRS.

2.4. Anthropometric Measurement

Body height was measured using an anthropometer (GPM, Susten, Switzerland), with an accuracy of 1 mm. Body weight and body composition were assessed using a Tanita RD-545HR analyzer (TANITA, Tokyo, Japan), with a precision of 0.05 kg for weight and 0.1% for body composition. BMI (Body Mass Index) was calculated according to the following formula: body weight/body height², where body weight was expressed in kilograms and body height in meters.

2.5. Questionnaires

Additionally, the participants filled out the following questionnaires:

• The short version of the International Physical Activity Questionnaire (IPAQ; validated Polish translation) [1], used to assess participants’ habitual level of physical activity [13].
• A questionnaire evaluating the subjective level of unpleasantness associated with each experimental condition (hypoxia, low temperature, high temperature, and exertion). Responses were recorded on a 5-point Likert scale ranging from 1 (“not unpleasant at all”) to 5 (“very unpleasant”).
• A questionnaire assessing previous experience with each of the environmental or physiological conditions studied. Participants were asked:

  ○

  Whether they had previously spent time in hypoxic environments (>4000 m above sea level), and if so, for how long;

  ○

  Whether they had experience using a sauna, and how frequently they used it;

  ○

  Whether they had experience with cryotherapy or cold-water immersion (e.g., cold plunges, cold baths, or cold-water swimming), and how often they engaged in this activity.

2.6. Statistical Analysis

2.6.1. Main Analysis

The main analysis employed a repeated-measures design in which each participant completed up to five sessions under distinct experimental conditions: baseline, simulated altitude at 4200 m, heat exposure at +42 °C, cold exposure at −10 °C, and an exercise protocol. Three primary outcomes were assessed during each session: sensory threshold (0), pain threshold (1), and tolerance threshold (9), which were input into the model as 0, 1 and 9—analogous to the NRS anchor points.

Data were analyzed using linear mixed-effects models estimated via restricted maximum likelihood (REML), incorporating random intercepts for participants to account for within-subject correlation. Fixed effects included experimental conditions. Sex and standardized age were included as covariates. Estimated marginal means (EMMeans) and corresponding 95% confidence intervals were derived from the fixed-effects covariance matrix. To quantify the magnitude of within-subject differences across conditions, Cohen’s d_z was calculated for all possible condition pairs, with bootstrap-based 95% confidence intervals generated using 5000 resamples.

The model fit analysis did not reveal any violations that would meaningfully affect the interpretation of the results. Residuals followed approximately normal distributions, plots of fitted vs. residual values indicated no concerning heteroscedasticity patterns, random-intercept variances were non-zero and behaved as expected for repeated-measures data, confirming the suitability of the mixed-effects structure.

2.6.2. Exploratory Analysis

Additionally, to examine the effects of the experimental conditions on the three primary outcomes, we performed a series of ANCOVAs. For each ANCOVA model, the dependent variable was the threshold value or the threshold difference between the two compared conditions.

Three sets of covariates were analyzed:

• Anthropometric characteristics: body mass, Body Mass Index (BMI), body fat percentage (%BF), muscle mass, and total body water percentage (%TBW).
• Condition experience: assessment of the unpleasantness level and previous experience for each condition.
• Physical activity: partial results of IPAQ regarding general, intensive and moderate physical activity.

Sex was included as a fixed factor.

Each model was estimated using a sigma-restricted parameterization with effective hypothesis decomposition. For each effect, the following statistics were extracted: sum of squares (SS), F statistics, p-values, partial eta-squared (η²) as a measure of effect size, non-centrality parameters, and observed power (α = 0.05). Parameter estimates were calculated for all predictors, including unstandardized coefficients, standard errors, t-values, p-values, and 95% confidence intervals, as well as standardized beta coefficients.

All analyses were conducted using Statistica 14.0.

3. Results

Estimated marginal means (EMMeans) demonstrated systematic differences in thresholds across environmental and exercise conditions (

Table 2. Estimated marginal means and 95% CI for sensory, pain and tolerance threshold.

Condition	Sensory threshold	EMMean	95% CI	Pain threshold	EMMean	95% CI	Tolerance threshold	EMMean	95% CI
Basic conditions		3.22	2.69; 3.75		23.31	18.20; 28.42		66.27	56.80; 75.74
4200 m asl		3.59	3.07; 4.12		21.70	16.63; 26.77		63.18	53.94; 72.42
Temp. +42 °C		3.98	3.43; 4.52		31.14	25.93; 36.35		71.43	60.99; 81.87
Temp. −10 °C		3.73	3.19; 4.27		24.34	19.13; 29.54		71.74	61.89; 81.59
Exertion		3.76	3.22; 4.30		25.34	20.20; 30.48		64.66	54.92; 74.41

, Figure 3). For sensory thresholds, the highest values occurred under heat exposure (+42 °C), followed by exercise, cold (−10 °C), altitude (4200 m asl), and baseline. Pain thresholds showed a similar ranking, with heat producing the greatest values, then exercise, cold, baseline, and altitude. In contrast, tolerance thresholds were highest during cold exposure, followed by heat, baseline, exercise, and altitude.

Mixed-effects models confirmed that these differences were statistically significant for most contrasts (

Table 3. Effect size (Cohen’s d_z and 95% CI) for all conditions and threshold.

Condition A	Condition B	N Pairs	d_z	95% CI	d_z	95% CI	d_z	95% CI
			Sensory Threshold		Pain Threshold		Tolerance Threshold
Basic conditions	Temp. −10 °C	37	0.42	0.11; 0.82	0.13	−0.20; 0.51	0.19	−0.30; 1.11
Basic conditions	Temp. +42 °C	36	0.35	0.06; 0.64	0.37	0.05; 0.82	0.46	−0.11; 1.77
Basic conditions	Exertion	37	0.33	0.03; 0.61	0.20	−0.11; 0.67	−0.13	−0.67; 0.33
4200 m asl	Temp. +42 °C	37	0.25	−0.05; 0.63	0.50	0.21; 0.82	0.31	−0.26; 1.26
Basic conditions	4200 m asl	41	0.25	−0.06; 0.58	−0.10	−0.39; 0.24	−0.24	−0.74; 0.16
4200 m asl	Exertion	38	0.14	−0.18; 0.46	0.31	−0.01; 0.74	0.25	−0.21; 0.91
Temp. +42 °C	Temp. −10 °C	36	−0.13	−0.45; 0.22	−0.53	−0.94; −0.18	−0.50	−1.47; 0.07
Temp. +42 °C	Exertion	37	−0.13	−0.46; 0.19	−0.33	−0.70; −0.02	−0.33	−0.86; 0.34
4200 m asl	Temp. −10 °C	38	0.13	−0.21; 0.46	0.19	−0.12; 0.63	0.65	0.24; 1.23
Temp. −10 °C	Exertion	38	0.05	−0.29; 0.36	0.09	−0.27; 0.40	−0.30	−0.91; 0.26

). For sensory thresholds, the effect of heat compared to baseline was positive and significant (p < 0.01), indicating increased sensitivity under thermal stress. Cold exposure also significantly elevated sensory thresholds relative to baseline (p < 0.05), whereas altitude produced smaller, non-significant changes (p > 0.10). Exercise showed a moderate increase compared to baseline (p ≈ 0.05), suggesting a trend toward heightened sensory detection during physical exertion.

Pain thresholds were strongly influenced by heat, with +42 °C yielding significantly higher values than all other conditions (p < 0.001). Exercise also increased pain thresholds compared to baseline (p < 0.01), while cold exposure produced intermediate values that were significantly greater than altitude (p < 0.05) but not baseline (p > 0.10). Altitude consistently ranked lowest, with differences from baseline reaching statistical significance (p < 0.05).

Tolerance thresholds exhibited the most pronounced modulation under cold exposure, which significantly exceeded baseline (p < 0.001) and all other conditions (p < 0.001). Heat also increased tolerance relative to baseline (p < 0.01), whereas exercise and altitude showed smaller, non-significant differences (p > 0.10).

The covariance analysis was conducted to identify specific characteristics related to body build and composition that may be associated with individual thresholds: sensory, pain, and tolerance (

Table 4. Results of covariance analysis for body parameters and individual thresholds.

Effect	Sensory Threshold (4200 m asl)				Tolerance Threshold (+42 °C)				Sensory Tolerance (−10 °C)
	SS	F	Partial Eta-Squared	Observed Power (α = 0.05)	SS	F	Partial Eta-Squared	Observed Power (α = 0.05)	SS	F	Partial Eta-Squared	Observed Power (α = 0.05)
BM	22.14	7.57	0.486	0.675	456.21	4.30	0.462	0.390	3.30	1.78	0.054	0.253
BMI	4.44	1.52	0.159	0.193	1341.38	12.63	0.716	0.809	1.78	0.96	0.030	0.158
%BF	0.28	0.10	0.012	0.059	251.44	2.37	0.321	0.241	0.02	0.01	0.000	0.051
MM	19.11	6.54	0.450	0.612	608.90	5.73	0.534	0.490	3.01	1.63	0.050	0.235
%TBW	2.68	0.92	0.103	0.135	323.97	3.05	0.379	0.295	8.02	4.33	0.123	0.523
%MM	0.02	0.007	0.001	0.051	136.39	1.28	0.204	0.153	0.06	0.03	0.001	0.054
Sex	33.15	11.34	0.586	0.838	57.04	0.54	0.097	0.093	0.69	0.37	0.012	0.091

BM—body mass, BMI—Body Mass Index, %BF—percent body fat, MM—muscle mass, %TBW—percent total body water, %MM—percent muscle mass.

and

Table 5. ANCOVA model estimated using a sigma-restricted parameterization with effective hypothesis decomposition.

Effect	Sensory Threshold (4200 m asl)					Tolerance Threshold (+42 °C)					Sensory Tolerance (−10 °C)
	β Estimate	SE	95% CI	Stand. β	p	β Estimate	SE	95% CI	Stand. β	p	β Estimate	SE	95% CI	Stand. β	p
BM	2.39	0.87	0.39; 4.39	10.82	0.025	9.83	4.74	−2.36; 22.02	6.83	0.093	−0.39	0.29	−0.98; 0.20	−4.22	0.191
BMI	−0.51	0.41	−1.46; 0.44	−0.44	0.253	12.84	3.61	3.55; 22.13	1.98	0.016	0.18	0.19	−0.20; 0.57	0.44	0.335
%BF	−2.42	7.83	−20.47; 15.64	−5.53	0.765	−37.66	24.48	−100.58; 25.25	−9.22	0.184	−0.27	2.43	−5.24; 4.69	−1.24	0.911
MM	−2.92	1.14	−5.55; −0.29	−11.71	0.034	−15.86	6.62	−32.88; 1.17	−8.05	0.062	0.48	0.38	−0.29; 1.25	4.20	0.211
%TBW	0.25	0.26	−0.35; 0.85	0.44	0.366	0.85	0.49	−0.40; 2.10	0.27	0.141	−0.12	0.06	−0.23; 0.00	−0.53	0.046
%MM	−0.70	8.37	−19.99; 18.59	−1.53	0.935	−28.24	24.92	−92.28; 35.81	−6.55	0.308	−0.47	2.56	−5.70; 4.75	−2.04	0.854
Sex	3.54	1.05	1.11; 5.96	1.28	0.010	4.96	6.76	−12.43; 22.34	0.22	0.497	−0.33	0.54	−1.44; 0.78	−0.24	0.547

BM—body mass, BMI—Body Mass Index, %BF—percent body fat, MM—muscle mass, %TBW—percent total body water, %MM—percent muscle mass.

). The main factors associated with sensory thresholds in hypoxic conditions were sex (p < 0.01), body mass (p < 0.05), and muscle mass (p < 0.05). In contrast, in conditions of reduced temperature (−10 °C), the percentage of total body water proved to be a significant factor influencing the sensation threshold (p < 0.05). In conditions of elevated ambient temperature (+42 °C), muscle mass (p < 0.05) and BMI (p < 0.05) were found to be significant factors related to pain tolerance thresholds.

A covariance analysis was also performed for characteristics related to unpleasantness and previous experiences of the subjects (

Table 6. Results of covariance analysis for unpleasantness, previous experiences and selected thresholds.

Sensory Threshold (4200 m asl)
Effect		SS	F	Partial Eta-Squared	Observed Power (α = 0.05)	β Estimate	SE	95% CI	Stand. β	p
	4200 m asl	24.48	12.13	0.336	0.916	−1.00	0.29	−1.60; −0.41	−0.60	0.002
	#1	5.63	2.79	0.104	0.361	−0.04	0.03	−0.10; 0.01	−0.29	0.108
	Sex	2.38	1.18	0.047	0.181	−0.30	0.28	−0.87; 0.27	−0.18	0.288
Sensory Threshold (4200 m asl vs. Baseline Conditions)
Effect	4200 m asl	1.71	0.23	0.028	0.071	−0.43	0.89	−2.48; 1,62	−0.15	0.643
	#1	0.24	0.03	0.004	0.053	−0.01	0.05	−0.13; 0.11	−0.05	0.861
	Sex	40.29	5.46	0.405	0.537	2.04	0.87	0.03; 4.06	0.69	0.048

#1—total time (days) spent at an altitude of 4200 m asl or higher in the previous year.

). In the case of sensory thresholds under hypoxic conditions, altitude proved to be a statistically significant factor (p < 0.01). In contrast, differences between sensation thresholds in baseline conditions and in hypoxia conditions were most differentiated by sex (p < 0.05).

The third model took into account the level of physical activity of the subjects in the analysis (

Table 7. Results of covariance analysis for physical activity aspects and selected thresholds.

Pain Threshold (4200 m asl)
Effect		SS	F	Partial Eta-Squared	Observed Power (α = 0.05)	β Estimate	SE	95% CI	Stand. β	p
	#1	590.75	7.69	0.490	0.681	9.799	3.535	1.65; 17.95	0.98	0.024
	#2	557.03	7.25	0.475	0.656	−0.154	0.057	−0.29; −0.02	−0.74	0.027
	#3	460.97	6.00	0.428	0.576	0.151	0.061	0.01; 0.29	0.91	0.040
	#4	3.26	0.04	0.005	0.054	−0.509	2.469	−6.20; 5.18	−0.04	0.842
	#5	2402.67	31.26	0.796	0.998	0.068	0.012	0.04; 0.10	0.96	0.001
	Sex	101.87	1.33	0.142	0.174	6.016	5.226	−6.03; 18.07	0.36	0.283
Pain Threshold (Exertion)
Effect	#1	931.30	12.48	0.609	0.870	12.303	3.483	4.27; 20.34	1.30	0.008
	#2	203.74	2.73	0.254	0.308	−0.093	0.056	−0.22; 0.04	−0.47	0.137
	#3	1.19	0.02	0.002	0.051	0.008	0.061	−0.13; 0.15	0.05	0.903
	#4	33.11	0.44	0.053	0.091	1.620	2.433	−3.99; 7.23	0.14	0.524
	#5	340.66	4.56	0.363	0.468	0.025	0.012	0.00; 0.05	0.38	0.065
	Sex	435.16	5.83	0.422	0.564	12.433	5.149	0.56; 24.31	0.79	0.042

#1—intense exercise [days/last week]; #2—intense exercise [min/day]; #3—moderate exercise [minutes/day]; #4—walking for at least 10 min without breaks [days/week]; #5—sedentary time [min/day].

). In this case, the following factors were found to be significant in determining the perception of pain under hypoxic conditions: the duration of intense physical activity, both during the week (p < 0.05) and during the day (p < 0.05); the duration of moderate physical activity during the week (p < 0.05); and sedentary time during the day (p < 0.01). In the case of pain perception after physical exercise (exertion), these factors were significantly related: the duration of intense physical activity during the week (p < 0.01) and the sex of the subjects (p < 0.05).

4. Discussion

The findings of the present study demonstrate that environmental stressors modulate somatosensory processing in distinct ways across three thresholds—sensory, pain, and tolerance. Specifically, extreme temperatures produced the largest effects; exercise yielded moderate modulation, and altitude influenced pain but not sensory detection.

In the mixed-effects models, altitude (≈4200 m) consistently yielded the lowest pain thresholds relative to baseline and other conditions, with differences reaching statistical significance, while sensory thresholds showed small, non-significant changes. This pattern indicates that hypoxia preferentially affects nociceptive processing rather than early tactile detection. This selective effect is consistent with earlier field studies, reporting decreased pain thresholds at high altitude, implicating hypoxia as a primary driver of altered nociceptive discrimination [4]. However, population and exposure differences matter: chronic highland residents have been shown to exhibit higher pressure pain thresholds than lowlanders, suggesting long-term adaptation may raise thresholds [2]. More recent work in young high-altitude natives reported similar ischemic pain thresholds at 3825 vs. 5100 m during acute exposure, highlighting that acclimatization level and test modality can reconcile apparent discrepancies [3]. Together, these data support our interpretation that acute hypoxia reduces pain thresholds, whereas chronic adaptation or test modality may modify the direction or magnitude of observed effects.

Mechanistically, hypoxia engages hypoxia-inducible pathways and neurovascular changes that can sensitize nociceptors and alter conduction, offering a plausible substrate for reduced thresholds [14]. Hypoxemia-related inflammation can further prime nociceptors and modulate opioid receptor expression, complicating analgesic responses in hypoxic states [15].

Considering other environmental factors, robust increases in sensory and pain thresholds with heat (+42 °C) and significant increases in pain tolerance with cold (−10 °C) were noted, with cold producing the largest tolerance effect across all conditions.

Elevated ambient heat increased both sensory and pain thresholds, consistent with central gating and temporal summation phenomena that can elevate the “critical temperature” for pain when stimulus dynamics are controlled. It is important to mention that methodological nuances, such as the rate of stimulus rise and measurement paradigm (methods and techniques for applying pain stimuli) can artifactually shift thresholds [16,17]. That reinforces the value of the current electrical stimulation approach that is independent of skin temperature [5].

In contrast, cold exposure markedly increased pain tolerance, which is in line with cryotherapy evidence that cooling reduces nerve conduction velocity and raises both pain threshold and tolerance [18]. At the cellular level, cold-sensing channels (e.g., TRPM8) and fiber-specific dynamics can contribute to analgesia. That may explain why tolerance (a higher order construct) is disproportionately elevated relative to detection or pain thresholds under cold [19].

Post-exercise, pain thresholds increased, while sensory thresholds showed a trend in the same direction. This is consistent with the phenomenon of exercise-induced hypoalgesia (EIH). EIH is widely reported across modalities (pressure, heat), with endocannabinoid increases and descending inhibitory pathway activation proposed as key mechanisms [20]. Moreover, the moderate magnitude of the observed exercise effect fits the expectation that intensity, duration, and fitness level modulate EIH, with stronger hypoalgesia at higher intensities in some cohorts [21].

Exploratory covariance analyses revealed some meaningful moderators. For instance, in hypoxia, sex, body mass, and muscle mass were associated with sensory thresholds. In cold, total body water was linked to sensory thresholds, while in heat, muscle mass and BMI related to pain tolerance. Finally, routine physical activity variables (e.g., vigorous weekly activity, sedentary time) predicted pain perception in hypoxia and post-exercise pain.

While the dedicated literature parsing these specific moderator patterns is limited, the directions are plausible. For example, muscle mass and hydration influence thermal balance and peripheral conduction, while activity profiles reflect endogenous pain modulation capacity [20,22].

Experimental sessions were scheduled at 7-day intervals (Days 1, 7, 14, and 21), allowing sufficient recovery between assessments. This spacing aligns with established protocols in experimental pain research [23,24]. Repeated nociceptive stimulation is known to induce both sensitization and habituation. However, these effects are strongly dependent on the frequency and number of stimulus repetitions over time. Within a single session, repeated stimulation may elicit short-term sensitization followed by habituation, but such processes are typically transient and operate on timescales of minutes to hours [25]. In contrast, longer inter-session intervals, such as the 7-day spacing employed here, are unlikely to result in cumulative sensitization or habituation across sessions. Importantly, a prior study has demonstrated that pain threshold and tolerance measures exhibit good test–retest reliability across repeated assessments (2–5 sessions), with stability maintained even over extended intervals (median 23 days) [26]. Accordingly, the responses observed in the present study are likely to reflect genuine experimental effects rather than systematic sensitization or habituation across sessions.

A key strength of this study is the use of electrical stimuli for quantifying sensory, pain, and tolerance thresholds, which avoids direct skin temperature confounding and offers precise control over stimulus timing and amplitude [27,28]. The thermoclimatic chamber ensured stable, repeatable ambient conditions—an advantage supported by European calibration guidance for climatic chambers used in research and industry [29].

Considering the limitations, the sample size constrains subgroup analyses and complex interaction testing. Moderator findings (body composition, activity) should be interpreted cautiously until replicated in larger studies. Given the number of body composition variables assessed and the relatively small subgroup sizes, we acknowledge that the risk of Type I error is elevated. However, we emphasize that the ANCOVA analyses were exploratory in nature and should be interpreted as such. Moreover, the altitude and acute exposure may not fully capture chronic high-altitude adaptation. Finally, the experimental sessions were not randomized, due to the inability to rapidly change conditions within the thermoclimatic chamber. A limitation of this study is the lack of prior research directly addressing all environmental and physiological factors. Further research should confirm and extend these findings in larger samples and diverse populations.

The temperature-dependent pattern observed in the present study—where heat robustly increased sensory and pain thresholds, whereas cold exerted the strongest effect on the tolerance threshold—suggests that these modalities may differentially engage peripheral nociceptive processes versus central pain-modulatory mechanisms. Future work should therefore more explicitly disentangle these pathways. This could include combining quantitative sensory testing with peripheral physiological indicators (e.g., skin perfusion, inflammatory mediators, peripheral nerve excitability metrics) and central biomarkers (e.g., EEG markers of nociceptive processing, fMRI connectivity within pain-modulatory networks, or neuroendocrine stress indicators). Such multimodal approaches would allow researchers to determine whether heat primarily alters peripheral transduction and conduction, while cold preferentially recruits descending inhibition or stress-related modulation.

Additionally, future studies should (i) examine dose–response relationships across broader altitude, hypoxia, and temperature ranges; (ii) investigate how individual traits (e.g., thermal sensitivity phenotypes, autonomic reactivity) shape these differential effects; and (iii) evaluate targeted interventions (e.g., oxygen supplementation, controlled temperature regulation, or preconditioning strategies) to determine whether operationally relevant thresholds can be predictably modulated. Together, these directions would clarify the mechanistic basis of the observed pattern and inform both theoretical models of pain processing and real-world applications.

In the context of clinical practice, the present results suggest environment-based strategies. For high altitudes, reduced pain thresholds should be anticipated. This may be relevant for mountaineers, rescue personnel, and high-altitude workers to plan analgesia with awareness of hypoxia-related modulation. Considering the results in cold exposure, leveraging the increased pain tolerance can be useful for cryotherapy or in recovery contexts—of course, while being mindful of conduction slowing and safety. On the other hand, elevated sensory and pain thresholds in heat may mask injury warning signals. This should be kept in mind and monitored carefully in hot environments. Finally, concerning the results of the exercise condition, the concept of EIH should be kept in mind, to harness endogenous analgesia, while recognizing variability in responses.