Immediate Load Carriage Improves Core Temperature but Not Skin Temperatures When Wearing Wet Clothing in Mild Cold Air

Abstract

Introduction

Members of the military are regularly exposed to austere environmental conditions that may create clothing saturation in cold conditions. This study sought to determine whether immediate ruck initiation is superior in maintaining rectal temperature (T_re), improving skin temperatures, and augmenting thermal comfort compared with remaining static and/or delayed ruck initiation when wet in cold air.

Methods

Eleven healthy adults (10 males and 1 female, 26±9 y old, 22.5±8.6% body fat) participated in this study. Volunteers were immersed in warm water (34.0±0.3°C) before entering the cold chamber (5.3±0.4°C) and began immediately rucking (IR) or stood statically for 60 min (Static) before rucking (delayed rucking [DR]) for 60 min.

Results

IR initiation was superior in elevating T_re compared with Static at 20 min (37.6±0.4 vs 37.3±0.2°C, P=0.033), 40 min (37.9±0.4 vs 37.5±0.2°C, P=0.012), and 60 min (37.9±0.4 vs 37.6±0.3°C, P=0.016). IR T_re was also greater than DR at 40 min (37.9±0.4 vs 37.4±0.5°C, P=0.002) and 60 min of wet-cold exposure (37.9±0.4 vs 37.5±0.5°C, P<0.001). Consequently, IR tended to improve thermal sensation ratings compared with both DR and Static. Yet, IR was no more sufficient in overcoming wet-cold decrements in skin temperatures than Static, especially at the finger (P>0.05).

Conclusions

Compared with DR and Static, IR is effective at elevating deep body temperature and improving thermal perceptions but is significantly less effective at improving wet-cold skin temperatures compared with Static.

Keywords

thermoregulation military rucking exercise temperature

Introduction

Military operations and trainings regularly expose members of the military to rain, water crossings, and sweat-inducing heavy work capable of wetting or saturating their uniforms. Particularly when paired with cold air, wet clothing exhibits reduced yet dynamic insulative properties related to the extent of drying and promotes greater conductive, evaporative, and condensation-driven heat loss.^1–3 It is therefore unsurprising that wet-cold exposures drive meaningful reductions in skin temperature⁴ and increased cold-induced thermogenesis and encourage behavioral thermoregulation (ie, removal of wet clothing, increased physical activity) to defend deep body temperature.

Wet clothing is a notable risk factor for cold injury,⁵ especially when individuals are static and rely on low levels of metabolic heat production (MHP) insufficient to maintain thermal balance.⁶ When tactical necessity and/or pack weight considerations render removal of wet clothing less than ideal, an increase in MHP via load carriage with a military-issued backpack (ruck) may be employed to balance body heat loss and improve body temperature. However, operationally relevant terrain features including wooded wetlands and dense forests/woodlands can reduce rucking speeds significantly, creating a greater concern for adequately opposed body heat loss.

Importantly, walking with a loaded ruck (ie, rucking) may not be able to be initiated immediately due to tactical limitations. Thus, time spent wet, cold, and at a relatively low level of MHP before ruck initiation is increasingly important; delaying exercise could facilitate a reduced ability to achieve thermal gain and therefore elevation of body temperatures and sensations. Yet, little investigation of the influence of ruck initiation timing on wet-cold thermoregulation has been pursued.

This study aimed to determine whether immediate ruck initiation is superior in maintaining deep body temperature and improving skin temperatures as well as thermal comfort when compared with remaining static and/or delayed ruck initiation while wet in cold air. We hypothesized that immediate ruck initiation would cause higher skin and core temperatures as well as greater thermal comfort when compared with either remaining static or delaying ruck initiation.

Methods

All study procedures were approved by the Institutional Review Board of the US Army Medical Research and Development Command (Protocol No. M-10920). Prior to any testing, subjects were briefed on the purpose, procedures, and risks of the study and provided with informed and written consent. Investigators adhered to policies for the protection of human subjects as prescribed in US Army Regulation 70–25 and US Army Medical Research and Development Command Regulation 7–25. The research was conducted in adherence with the provisions of Code 45 of Federal Regulations, Part 46. This study conformed to the principles in the Declaration of Helsinki and was registered at clinicaltrials.gov (NCT05409937).

Eleven healthy adults (10 males, 1 female) participated in this study (age of 26±9 y [range 18–45 y], weight of 77.9±15.7 kg [range, 61.5–114.7 kg], height of 175.6±10.5 cm [range, 158.4–191.8 kg], body fat of 22.5±8.6% [range, 12.9–37.7%], and body surface area of 1.93±0.22 m² [range 1.63–2.37 m²]). Body fat percentage was measured via dual-energy x-ray absorptiometry for all volunteers. Given the low total and subcutaneous body fat of select military populations,⁷ study recruitment prioritized recruiting a subset of volunteers with a measured body fat of <18% for males and <24% for females to best represent these military populations. Five of the 11 volunteers fell within this range.

Volunteers were without a history of cold injuries or cold-induced syndromes (eg, Raynaud's syndrome or cold-induced asthma/bronchospasm); known disorders affecting the heart, lungs, kidneys, muscles, or nerves; and not taking medications or dietary supplements that could alter cardiovascular, thermoregulatory, or vascular control. Female volunteers were scheduled during “lower hormone” states to include the follicular phase (Days 0–7 using a self-report of menses initiation) or the “active phase” of hormonal contraceptive use to reduce variance in thermoregulatory responses.

Study Design

After medical clearance through the US Army Research Institute of Environmental Medicine Office of Medical Support and Oversight, volunteers reported to the laboratory on 3 occasions: once to complete a preliminary loaded-walking task (ie, familiarization) and two experimental trials. Experimental trials consisted of a total of 2 h of cold air exposure wearing a wet uniform, with ruck initiation either immediately on cold entry or after 1 h remaining static. A randomized crossover design trial order was determined for the 2 trials using a computer random generator (randomizer.org). Experimental trials were separated by a minimum of 48 h. Data collection occurred in Natick, MA, only from April to November (mean average outdoor temperature 16.3±6.1°C [range, 6.3–23.4°C], mean minimum outdoor temperature 15.1±6.0°C [range, 4.9–21.9°C]) to avoid the influence of seasonal cold exposure. Each trial for a given volunteer was conducted at the same time of day (ie, either early morning or early afternoon) to avoid circadian influence in core temperature and thermoregulatory adjustments. Timeline details for each study visit can be found in Figure 1.

Uniform

All volunteers were fit for an appropriately sized Army Combat Uniform (ACU) during the first laboratory visit, and uniform size was consistent for all subsequent visits. Volunteer self-reported comfort and a visual assessment by study staff minimized clothing bunching and ensured complete limb coverage. Undergarments consisted of compression shorts (and sports bra for the female volunteer). The ACU included a moisture wicking polyester t-shirt, ACU long-sleeve blouse, and ACU trousers (thermal resistance: dry, 1.40 clo; wet, 0.92 clo). The ensemble also included mid-calf boot socks (100% wool; Fox River Socks, Osage, IA), combat boots (Burma 901 V2; Belleville Boot Co, Belleville, IL), and a patrol cap.

Load-Carriage Metabolic Matching

To reduce postural muscle soreness, discomfort from prolonged pack wear, and overall study attrition, the metabolic cost of walking at 0.89 m·s⁻¹ (2.0 mph) with a 35-kg load was matched to that with a 17.5-kg load by adjusting treadmill grade for most volunteers (n=9). Metabolic matching asked each volunteer to don a 35-kg pack before walking for 10 min at 0.89 m·s⁻¹, 0% grade, wearing the complete ACU uniform. Oxygen consumption (VO₂; TrueOne 2400, ParvoMedics, Salt Lake City, UT) was captured during the last 5 min of the 10-min walk. Volunteers then replaced the 35-kg pack with the 17.5-kg pack before walking with grade. Starting treadmill grade with the 17.5-kg pack was estimated using volunteer body mass, target pack masses (17.5 vs 35 kg), and target walking speed (0.89 m·s⁻¹) input into the US Army Load Carriage Decision Aid.⁸ If average VO₂ across the last 3 min of metabolic collection with the 17.5-kg pack was not within ±2 mL·kg⁻¹·min⁻¹ of that collected with the 35-kg pack, grade was adjusted in 1% increments until agreement. Lin's concordance correlation coefficient indicates excellent agreement (0.89 m·s⁻¹; concordance correlation coefficient=0.914) between the oxygen cost of walking with the 17.5-kg pack and the 35-kg pack in thermoneutral air (21.0±1.8°C). The first enrolled volunteer walked with a 35-kg pack during each experimental trial, and 1 volunteer was unable to tolerate the 35-kg load, and therefore, the 17.5-kg estimated grades were not able to be adjusted or verified.

Experimental Trials

Following baseline data collection, volunteers completed a 2-min head-out immersion in warm water (34.0±0.3°C) followed by a 2-min static drip-dry period. Thereafter, volunteers entered the cold chamber (5.3±0.4°C, 54±7% relative humidity, wind speed 0.8 m·s⁻¹), where they randomly completed the delayed ruck (DR) or the immediate ruck (IR) trial. Environmental conditions were selected to replicate January-March ambient air temperatures experienced during the swamp phase of Army Ranger School.⁷ DR had volunteers stand statically for 60 min before donning the 17.5-kg rucksack and completing 60 min of loaded treadmill walking at 0.89 m·s⁻¹ with a metabolically matched grade. Volunteers were instructed to remain standing and not engage in voluntary thermoregulatory behavior (eg, cupping of hands, placing hands in armpits, or excess fidgeting) while static to reduce extraneous increases in MHP. IR had volunteers don the 17.5-kg rucksack immediately on cold entry and complete 60 min of loaded treadmill walking at 0.89 m·s⁻¹ with the metabolically matched grade. During each trial, volunteers were able to watch movies or engage in conversation with study personnel during cold exposure as a means of cognitive distraction and to pass the time. Water ingestion was ad libitum during the first experimental trial and matched for the following trial within volunteers. Drying time of the ACU, referred to as estimated water mass loss, was calculated using change in uniform-clothed body mass (CPWplus 150 Digital Bench Scale; Adam Equipment, Inc, Oxford, CT) measured from dry to wet and every 20 min exposed to the cold air. Estimated water loss was corrected for water ingestion and urine excretion across both trials.

Measures

Deep Body and Skin Temperatures

Rectal temperature (T_re), as a measure of deep body temperature, was collected every 15 s (n=9) or 30 s (n=2) during experimental trials using a temperature capsule (e-Celsius Performance Capsule; BodyCap, Saint-Clair, France) inserted as a rectal suppository on arrival to the laboratory.⁹ Volunteers were provided lubrication and asked to self-insert the capsule past the anal sphincter to the level of the second knuckle in private. Skin temperatures were continuously collected across each trial every 15 s. Skin temperatures were measured using thermistor sensors (YSI 400 Adult Temperature Probe, YSI Inc, Yellow Springs, OH, or EUS-UU Skin Probe Thermistor, Grant Instruments, Beaver Falls, PA) attached to skin surfaces using medical adhesive (Tegaderm; 3M, Saint Paul, MN) at 11 sites: dorsal middle finger, dorsal hand, ventral forearm, posterior upper arm, chest (pectoralis), abdomen, back (subscapular), anterior thigh, posterior calf, dorsal foot, and forehead. The large number of skin sites was chosen to be representative of the whole body, especially given the clothed nature of the investigation. A thermoregulatory finite-element mesh model, which was developed using medical imaging to determine the body size and proportion of an average US male and female, was used in determining the weighting for each site.^10,11 Mean skin temperature ( $\bar{T}$ _sk) was calculated using the following sex-specific equations based on this model⁴:

{\bar{T}}_{sk} (m a l e) = (0.074 T_{h e a d}) + (0.087 T_{c h e s t}) + (0.174 T_{s u b s c a p}) + (0.096 T_{a b d o m e n}) + (0.083 T_{t r i c e p}) + (0.059 T_{a r m}) + (0.026 T_{h a n d}) + (0.023 T_{f i n g e r}) + (0.189 T_{t h i g h}) + (0.117 T_{c a l f}) + (0.072 T_{f o o t})

{\bar{T}}_{sk} (f e m a l e) = (0.074 T_{h e a d}) + (0.094 T_{c h e s t}) + (0.154 T_{s u b s c a p}) + (0.077 T_{abdomen}) + (0.079 T_{tricep}) + (0.062 T_{arm}) + (0.027 T_{hand}) + (0.028 T_{finger}) + (0.214 T_{thigh}) + (0.115 T_{calf}) + (0.076 T_{foot})

Metabolic Heat Production

Metabolic data were captured in 5-min collection blocks before cold exposure and every 20 min during cold exposure (TrueOne 2400, ParvoMedics). Metabolic heat production (MHP, W·m⁻²) was calculated from the oxygen uptake (VO₂, L·min⁻¹) and respiratory exchange ratio (RER) using the following equation,¹² where A_D is estimated body surface area (m²) using body mass (kg) and height (cm)¹³:

M H P = (0.23 RER + 0.77) (5.873 V O_{2}) (60 / A_{D})

A_{D} = [71.84 (b o d y m a s s^{0.425}) (h e i g h t^{0.725})] / 10, 000

where RER≥1.0, the following equation was used to reflect an energy equivalent for carbohydrate use only^14,15:

M H P = (5.873 V O_{2}) (60 / A_{D})

Volunteers were asked to refrain from eating 3 h prior to laboratory arrival and were provided with a standardized meal ∼1.5 h before pre-exposure metabolic collections were performed.

Perception: Thermal Sensation and Comfort

Perceptual scale ratings were prompted before and after every 20 min of cold exposure. Thermal sensation reporting had the volunteer rate how warm or cold they felt using an adapted American Society of Heating, Refrigerating and Air-Conditioning Engineers 9-point analog scale¹⁶ ranging from −4 (unbearably cold) to +4 (unbearably hot). Thermal comfort was evaluated using a 5-point analog scale¹⁷ ranging from 0 (comfortable) to 4 (intolerable). Volunteers were prompted to gauge their sensations and comfort considering their entire body, only their hands, and only their feet.

Statistical Analysis

Minute 0 for the Static and IR conditions is reported as an average of the last 5 min of a 20-min thermoneutral passive sit. Minute 0 for the DR condition consists of a 5-min average sampled from the end of the 60-min Static cold exposure. A one-way repeated-measures analysis of variance was used to evaluate baseline (Minute 0) differences for all outcome metrics between Static, DR, and IR conditions.

Cold-exposure data for T_re, skin temperature, and metabolic data were averaged over representative 5-min blocks at each 20 min of exposure time. Water-mass data and perceptual data were collected at the end of each 20-min block. Differences were analyzed using a 3×3 (condition×time) repeated-measures analysis of variance. When sphericity was violated (P<0.05), the Greenhouse-Geisser correction was used. Post hoc comparisons were made, when applicable, using a Bonferroni correction. Unless otherwise specified, the level of significance for differences reported is P<0.05. Each outcome figure or table notes the individual analysis sample size. Data are presented as mean±SD or mean difference [95% CI] throughout the paper unless noted otherwise. Data analysis was conducted using IBM SPSS Statistics version 30 for Windows (SPSS Inc, Chicago, IL). Data visualization was completed using GraphPad Prism version 10.3.1 (GraphPad PRISM, La Jolla, CA).

Results

Estimated Water Mass

Uniforms during Static (mean difference [95% CI], +0.81 kg [0.71–0.90], P<0.001) and IR (+0.89 kg [0.72–1.05], P<0.001) conditions held more water compared with DR at Minute 0. A main effect of condition (P=0.003) for Static (0.45 kg [0.19–0.71], P=0.002) and IR (0.44 kg [0.05–0.84], P=0.027) indicates that both conditions tended to hold more absolute water mass than the DR condition across the compared 60-min blocks (Figure 2). No differences in estimated water mass existed between Static and IR conditions at Minute 0 or throughout the cold exposure (P>0.05).

Rectal Temperature

The DR condition was preceded by 60 min of cold exposure that elicited a higher starting T_re (Figure 3A) than the Static condition (0.36°C [0.18–0.53], P<0.001) and IR condition (0.35°C [0.18–0.52], P<0.001) at Minute 0. IR showed an elevated T_re at Minute 20 of exposure compared with Static (0.30°C [0.02–0.57], P=0.033) but not DR (0.19°C [−0.04–0.42], P=0.125). IR continued to demonstrate greater T_re than Static (0.36°C [0.08–0.63], P=0.012) as well as DR (0.41°C [0.18–0.65], P=0.002) at 40 and 60 min (Static: 0.31°C [0.06–0.56], P=0.016; DR: 0.40°C [0.19–0.60], P<0.001). No differences were present for T_re between Static and DR at any measured cold air exposure timepoint (P>0.05).

Mean, Foot, and Finger Skin Temperatures

Skin temperature data from 8 volunteers were analyzed due to measurement difficulties. Missing data resulted from clothing- and ambulation-induced connectivity issues with at least a single measurement site either at the data-acquisition system or at the skin (n=2) or failure to appropriately initiate data logging (n=1). When data were not available for a single skin temperature measurement site and mean weighted skin temperature (MWST) was unable to be calculated, hand and foot data also were removed from the analysis. Starting MWST (Figure 3B) for both Static (+9.17°C [8.27–10.07], P<0.001) and IR (+9.10°C [8.42–9.78], P<0.001) were higher than precooled DR. MWST demonstrated a significant condition×time interaction (P<0.001). MWST for Static was greater than DR at 20 min (+2.98°C [1.54–4.43], P=0.001) with no continued difference between the 2 conditions at 40 or 60 min (P>0.05). IR was greater than DR at 20 min (+2.39°C [1.95–2.82], P<0.001), 40 min (+1.27°C [1.00–1.55], P<0.001), and 60 min (+0.69°C [0.35–1.04], P=0.001). Static and IR MWST did not differ throughout the wet-cold exposure (P>0.05).

Foot temperatures (Figure 3C) while Static (+11.60°C [7.97–15.23], P<0.001) and with IR (+10.47°C [8.16–12.79], P<0.001) were both higher than DR at Minute 0 with no difference between Static and IR (P>0.05). A significant interaction (P<.001) indicated that Static and IR foot temperatures continued to decrease over the 60-min exposure, with both remaining higher than DR at 20 min (Static: +8.20°C [7.03–9.38], P<0.001; IR: +7.03°C [6.32–7.74], P<0.001), 40 min (Static: +5.95°C [4.43–7.45], P<0.001; IR: +5.37°C [3.96–6.77], P<0.001), and 60 min (Static: +3.54°C [1.62–5.46], P=0.002; IR: +3.91°C [2.19–5.63], P<0.001).

Starting Static (+20.90°C [17.75–24.05], P<0.001) and IR (+21.07°C [18.38–23.77], P<0.001) finger temperatures (Figure 3D) were higher than DR. During subsequent cold exposure, a significant interaction (P=0.026) presented for finger temperatures. Static and IR finger temperatures decreased, with Static eliciting a higher finger temperature than DR at 20 min (+2.82°C [0.46–5.19], P=0.022). No differences were seen between conditions at 40 or 60 min of exposure.

Metabolic Heat Production and Oxygen Cost

Minute 0 of DR demonstrated a higher starting MHP (Figure 4) than Static (+35.76 W·m⁻² [18.62–52.90], P<0.001) and IR (+35.61 W·m⁻² [17.49–53.73], P<0.001). With ruck onset, MHP increased in both DR and IR at 20 min, with DR producing more metabolic heat than IR (+17.20 W·m⁻² [1.99–32.40, P=0.026). Both rucking conditions, DR (+141.97 W·m⁻² [116.71–167.22], P<0.001) and IR (+124.77 W·m⁻² [109.07–140.47], P<0.001), were higher than Static. The remaining wet-cold exposure had both DR and IR remain higher than Static at 40 min (DR: 116.83 W·m⁻² [97.76–135.89], P<0.001; IR: 112.74 W·m⁻² [92.69–132.78], P<0.001) and 60 min (DR: 105.08 W·m⁻² [86.14–124.02], P<0.001; IR: 95.49 W·m⁻² [80.09–110.89], P<0.001). DR and IR were similar at 40 min (P>0.99) and 60 min (P=0.102). MHP in the Static condition trended upward over the course of the 60-min exposure but tended to remain significantly below both rucking conditions.

DR had a higher VO₂ than both Static and IR (mean±SD; DR: 0.50±0.10 L·min⁻¹ vs Static: 0.29±0.04 L·min⁻¹, P<0.001 vs IR: 0.29±0.05 L·min⁻¹, P<0.001) at Minute 0. Expectedly, conditions with rucking (IR and DR) demanded greater VO₂. DR had the greatest VO₂ at 20 min compared with Static (DR: 1.16±0.17 L·min⁻¹ vs Static: 0.36±0.06 L·min⁻¹, P<0.001) and IR (IR: 1.06±0.12 L·min⁻¹, P=0.024). Furthermore, IR had a higher VO₂ than Static (IR: 1.06±0.12 L·min⁻¹ vs Static: 0.36±0.06 L·min⁻¹, P<0.001) at 20 min. A similar trend followed as rucking continued with DR and IR higher than Static at 40 min (Static: 0.41±0.09 L·min⁻¹ vs DR: 1.07±0.13 L·min⁻¹, P<0.001, vs IR: 1.04±0.11 L·min⁻¹, P<0.001) and 60 min (Static: 0.49±0.10 L·min⁻¹ vs DR: 1.09±0.13 L·min⁻¹, P<0.001, vs IR: 1.04±0.11 L·min⁻¹, P<0.001). DR and IR did not differ at 40 min (P=0.934) or 60 min (P=0.175).

Thermal Perception

DR thermal perceptions began lower than Static and IR due to the prior 60-min cold-air exposure. With exposure to cold air, IR improved whole-body and foot thermal sensations (volunteers felt less cold) compared with Static and DR, whereas thermal comfort was lower (volunteers felt more comfortable) for IR compared with Static, but not DR (Table 1).

Table 1.

Thermal sensation (TS) of whole body, hands, and feet and thermal comfort (TC) of whole during wet-cold exposure (n=11).

	0:00	20:00	40:00	60:00	Condition mean
TS, whole body
Static	0.4 [−0.3 to 1.0]	−2.7 [−3.3 to −2.2]	−3.0 [−3.3 to −2.7]	−3.0 [−3.3 to −2.7]	−2.9 [−3.3 to −2.5]	Condition: P<0.001
IR	0.5 [0.0 to 0.9]	−1.9 [−2.5 to −1.4]	−1.6 [−2.4 to −0.9]	−1.5 [−2.5 to −0.6]	−1.7 [−2.3 to −1.1] ^b ^, ^c	Time: P=0.542
DR	−3.0 [−3.3 to −2.7] ^a ^, ^b	−2.5 [−2.9 to −2.0]	−2.2 [−3.0 to −1.4]	−1.8 [−2.6 to −1.0]	−2.2 [−2.6 to 1.7]	Condition×time: P=0.242
TS, hands
Static	0.3 [−0.4 to 1.0]	−2.6 [−3.1 to −2.2]	−3.0 [−3.5 to −2.5]	−3.4 [−3.7 to −3.0]	−3.0 [−3.4 to −2.6]	Condition: P=0.032
IR	0.3 [−0.3 to 0.9]	−2.5 [−3.1 to −2.0]	−2.5 [−3.6 to −1.3]	−2.2 [−3.3 to −1.0]	−2.4 [−3.2 to −1.8]	Time: P=0.719
DR	−3.4 [−3.7 to −3.0] ^a ^, ^b	−2.9 [−3.4 to −2.4]	−2.5 [−3.5 to −1.6]	−2.1 [−3.0 to −1.2]^a	−2.5 [−3.2 to −1.8]	Condition×time: P<0.001
TS, feet
Static	0.5 [−0.2 to 1.2]	−1.6 [−2.5 to −0.8]	−2.2 [−2.6 to −1.8]	−2.4 [−2.8 to −1.9]	−2.1 [−2.6 to −1.6]	Condition: P=0.004
IR	0.6 [−0.1 to 1.3]	−1.4 [−1.9 to −0.8]	−1.2 [−2.1 to −0.2]	−1.3 [−2.3 to −0.2]	−1.3 [−2.0 to −0.5] ^b ^, ^c	Time: P=0.936
DR	−2.4 [−2.8 to −1.9] ^a ^, ^b	−2.5 [−3.1 to −2.0]	−2.1 [−2.9 to −1.3]	−1.8 [−2.9 to −0.7]	−2.2 [−2.9 to −1.4]	Condition×time: P=0.075
TC, whole body
Static	0.1 [−0.1 to 0.3]	1.5 [1.1 to 1.8]	1.9 [1.7 to 2.1]	2.1 [1.9 to 2.3]	1.8 [1.7 to 2.0]	Condition: P<0.001
IR	0 [0 to 0]	1.2 [0.9 to 1.5]	1.3 [1.0 to 1.6]	1.2 [0.9 to 1.5]	1.2 [1.0 to 1.4]^c	Time: P=0.106
DR	2.1 [1.9 to 2.3] ^a ^, ^b	1.6 [1.3 to 2.0]	1.5 [1.2 to 1.9]	1.5 [1.2 to 1.9]	1.6 [1.3 to 1.9]	Condition×time: P=0.050

DR, delayed ruck; IR, immediate ruck; Static, standing statically for 60 min. Time 0:00 for DR reflects values after standing for 60 min in the cold with wet clothing.

Note: Data are represented as mean [95% CI]. “Condition mean” contains condition estimated means from the 3×3 RM analysis of variance. TS, hands post hoc testing indicates no significant pairwise comparisons for the condition main effect.

P<0.05 Static vs DR.

P<0.05 DR vs IR.

P<0.05 Static vs IR.

Discussion

This study provides new insight regarding wet-cold thermoregulation in the context of ruck initiation timing. IR initiation was superior in elevating T_re than Static after 20 min of wet-cold exposure. After 40 min, IR T_re values exceeded those of both Static and DR conditions. Consequently, volunteers reported feeling warmer when evaluating thermal sensation of their whole body and feet with IR than with both Static and DR conditions. Yet, IR initiation was no more sufficient in overcoming decrements in skin temperatures than Static, especially at the finger. Delaying IR by 60 min allowed skin temperatures to drop, creating a precooled start to the rucking task, contributed to lower foot temperatures that were unable to improve over 60 min toward Static and IR temperatures.

T_re was elevated more with the IR. Static showed a slight increase in T_re but remained lower than IR throughout the exposure. Both conditions elicited T_re values well above hypothermia (T_re<35°C). IR was not sufficient to prevent skin temperatures from falling at a similar rate as Static. Interestingly, the total estimated water mass shed over 60 min was comparable between the IR and Static conditions (0.28–0.34 kg), indicating that the extent of clothing saturation may play a role in the dampened skin temperature recovery. Moreover, the ambulatory movement with exercise did not appear to contribute to greater shedding of clothing water mass.

The wet-cold exposure in this study exhibited a main effect for whole-body and foot thermal sensations (Table 1), indicating that volunteers felt warmer (yet still cold) with the immediate initiation of exercise compared with remaining static. Skin temperature is the primary driver of thermal perception¹⁸ and likely the main contributing sensory input within this study. However, sensory signals originating from the spinal cord, abdominal viscera, and brain also relay information to the preoptic area of the hypothalamus, which is then relayed to the thalamus to contribute to perception and behavioral thermoregulation.^19,20 MWST and foot temperatures did not differ throughout the 60-min exposure. Yet, T_re was significantly higher in IR at 40 and 60 min of wet-cold exposure. Thus, the rise in deep body temperature seen with IR may have lessened the feeling of cold compared with Static to a mild extent.

Lack of statistical findings between the IR and DR conditions within the first 20 min suggests that neither exercise initiation order was better than the other in defending T_re in the short term, despite higher starting T_re with DR. However, IR showed a greater gain in T_re over the remaining 40 min. Perceptually, greater whole-body and foot thermal sensations with IR than with DR align with the greater skin temperatures seen with IR for those regions. Unchanged rucking hand thermal sensations corroborate the similarity of between-condition finger temperatures. Lastly, equivalent thermal comfort ratings between IR and DR suggest that neither condition was superior at improving cold comfort (Table 1).

Notably, finger temperatures for all conditions fell below 15°C, a temperature linked to dexterity decrement.^21–23 Such a decrement would negatively influence military-relevant dexterous tasks such as intravenous catheter placement and weapons use. Mäkinen et al²⁴ investigated a similar exercise intensity (Mäkinen at 195 W/m² vs this study at ∼194 W/m²) in cold air (−10°C) with low wind (1.0 m·s⁻¹) in volunteers wearing fiber pile inner mittens and nylon outer mittens. Finger temperatures within these conditions remained >15°C (22.5±3.8°C), suggesting mittens as a potential option to defend finger temperatures while exercising,²⁴ although if mittens were to become water saturated prior to being donned or after being donned due to dripping, their insulative value may be compromised.

DR elicited the greatest MHP after 20 min (205±27 W/m²), being significantly higher than both IR (188±17 W/m²) and Static (64±9.1 W/m²), suggesting a potential small additive influence of shivering thermogenesis atop that of exercise. The additive effect of shivering thermogenesis disappears from DR after 40 min, indicating that delayed exercise initiation MHP is comparable with IR after 40 min of exercise. Although the oxygen cost associated with DR was higher within the first 20 min of rucking compared with IR, the overall effect on caloric expenditure (∼10 kcal) in the first 20 min is estimated to be negligible.²⁵

Despite DR having the highest MHP after 20 min of exercise and then a comparable MHP with IR, T_re did not increase similarly to IR over the 60-min exercise bout. The lack of T_re increase likely was a result of the precooled skin temperatures causing a decrease in deep body temperature typically seen with cold rewarming.²⁶ The onset of exercise in DR likely allowed blood from the cooled tissue to more rapidly enter the circulation to the warmer core resulting in a reduction in T_re. Furthermore, the ambulation from exercise could have increased convective and conductive heat loss as the wet and cool uniform repeatedly made contact with the skin and created more air movement with the environment. The difference in MHP between DR and IR at 20 min, while statistically significant, was quite small (∼17 W·m⁻²) and likely not enough to compensate for the heat-loss effect of exercise-induced increases in blood flow. The lack of significance between DR and IR following 40 and 60 min of exercise also indicates the minor nature of this increase in MHP with DR.

Limitations

Volunteers enrolled in this study were well rested, well fed, and nonfatigued, thus limiting the generalizability of these results to military training paradigms featuring high physical and mental stress. Volunteers were instructed not to deviate from their typical daily routines but were not specifically instructed to avoid other voluntary thermal stressors (eg, cold showers, sauna bathing). While unlikely, some volunteers may have been exposed to such thermal stressors between study visits. Additionally, only total MHP was assessed in this study with no direct measures of shivering (eg, electromyography). Therefore, our proposed explanation of why DR elicited an elevated MHP compared with IR at 20 min cannot be directly verified. Such measurements would be a useful tool in future studies examining wet-cold scenarios.

The rucking intensity of 0.89 m·s⁻¹ is relatively slow compared with specialist-school/badge speed requirements (eg, Air Assault, Expert Field Medical Badge, and Expert Infantryman Badge) of 1.79 m·s⁻¹. The rucking speed from this study was used to mimic speeds common to stream/swamp crossings during US Army Ranger School.²⁷ Notably, exposure to warm 34°C water would be rare in tactical cold-air scenarios, although in a laboratory setting the relatively thermoneutral water served to saturate clothing without decreasing skin temperatures rapidly prior to cold exposure. The use of cold water for the 2-min saturation immersion likely would have led to a rapid upward skin temperature fluctuation while in transit to the cold chamber, before the cold air reduced skin temperatures once more. Finally, more severe cold and windy conditions certainly would warrant higher exercise intensities to compensate for the increased body heat loss and therefore warrant further investigation.

Conclusions

In conclusion, immediate ruck initiation elicited the largest gain in rectal temperature and improved whole-body and foot thermal sensations while wet-cold compared with delayed ruck initiation or remaining static. MWST and foot temperatures fell at a similar rate on cold entry whether volunteers remained static or began immediately rucking. Delayed ruck initiation was not able to meaningfully increase precooled MWST or foot temperatures. Finger skin temperatures quickly fell below 15°C for both Static and IR conditions, whereas DR began at <15°C, and finger temperatures remained relatively steady throughout the 60-min blocks compared in all 3 conditions. The addition of low-speed rucking (MHP ∼194 W·m⁻²) did not thwart the threat of deteriorating manual dexterity and potential peripheral nonfreezing cold injury, if prolonged.

These findings suggest that immediately rucking, when possible, can help defend rectal temperature while also allowing an individual to feel warmer than remaining static or delaying rucking. It is important to note that this recommendation assumes that the 60-min ruck endpoint provides a low-heat-loss environment (ie, a tented or higher-air-temperature environment) to otherwise prevent a potentially large heat dump, capable of reducing core temperature, attributable to postexercise heightened skin perfusion, perspiration, and remaining clothing water content. If this reprieve is not available, remaining static remains a core temperature safe option in a well-rested and well-fed population at the expense of thermal perceptions. Regardless of the movement pattern (static or ruck) selection, efforts should be made to improve the insulative protection provided to the periphery (hands and feet). Further research should focus on strategies to improve wet-worn uniform drying times and elevate compromised skin temperatures, especially at the hand, in mild cold air scenarios.

Footnotes

Acknowledgments

The authors extend thanks to Claudia Toussaint for her invaluable help with thermal chamber support as well as Thomas Mayer and Benjamin Fry for their technical assistance. Lastly, the authors are indebted to the willing participants who made the investigation possible.

Author Contribution(s)

Phillip O. Bodurtha: Data curation; Formal analysis; Investigation; Project administration; Visualization; Writing – original draft; Writing – review & editing.

MariaLena A. Shaw: Investigation; Writing – review & editing.

Andrew M. Greenfield: Investigation; Writing – review & editing.

Dina M. Pitsas: Investigation; Project administration; Writing – review & editing.

Billie K. Alba: Conceptualization; Methodology; Writing – review & editing.

John W. Castellani: Conceptualization; Methodology; Writing – review & editing.

Catherine O’Brien: Conceptualization; Funding acquisition; Methodology; Writing – review & editing.

Afton D. Seeley: Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Project administration; Visualization; Writing – original draft; Writing – review & editing.

Consent for publication

All participants provided informed and written consent to publication prior to beginning data collection.

Consent to Participate

All participants provided informed and written consent prior to beginning data collection.

Data Availability

Data generated or analyzed during this study are available from the corresponding author on reasonable request.

Ethical Considerations

All study procedures were approved by the Institutional Review Board of the US Army Medical Research and Development Command (Protocol No. M-10920). Prior to any testing, subjects were briefed on the purpose, procedures, and risks of the study and provided informed and written consent. Investigators adhered to policies for the protection of human subjects as prescribed in US Army Regulation 70–25 and US Army Medical Research and Development Command Regulation 7–25. The research was conducted in adherence with the provisions of Code 45 of Federal Regulations, Part 46. This study conformed to the principles in the Declaration of Helsinki and was registered at clinicaltrials.gov (NCT05409937).

Financial/Material Support

This work was supported by the Joint Program Committee-5/Defense Health Program (JPC-5/DHP).

Disclaimers

The views, opinions, and/or findings contained in this article are those of the authors and should not be construed as an official US Department of the Army position, or decision, unless so designated by other official documentation. This article is approved for public release, and distribution is unlimited. Citations of commercial organizations and trade names in this paper do not constitute an official Department of the Army endorsement or approval of the products or services of these organizations.

This research was supported in part by an appointment to the Department of Defense (DOD) Research Participation Program administered by the Oak Ridge Institute for Science and Education (ORISE) through an interagency agreement between the US Department of Energy (DOE) and the DOD. ORISE is managed by ORAU under DOE contract number DE-SC0014664. All opinions expressed in this paper are the authors’ and do not necessarily reflect the policies and views of the US Army, DOD, DOE, or ORAU/ORISE.

ORCID iDs

Phillip O. Bodurtha

Andrew M. Greenfield

Billie K. Alba

John W. Castellani

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