Duration limits for exposure for the whole body and extremities with a military extreme cold protection clothing ensemble at an ambient temperature of -40°C

ABSTRACT

Manual performance and body temperature responses were assessed in a 1-h trial at an ambient temperature (T_AMB) of −40°C for 7 male participants (32 ± 14 (mean ± SD) years) wearing a typical military extreme cold protection clothing ensemble. The purpose was to establish duration limited exposure (D_LIM) for these conditions, and it was hypothesized that (i) core temperature (T_CORE) would remain normothermic, whereas extremity skin temperature (T_SK) would decrease; (ii) decrements of manual performance would be in proportion to decreases of hand T_SK; and (iii) D_LIM would be determined by the hand or foot T_SK responses. Linear regression was employed to assess associations of manual performance scores and body temperatures with D_LIM assessed using the Required Clothing Insulation (I_REQ) model and extremity temperatures in ISO 11079–2007. Results showed T_CORE remained at ~37.3°C, whereas there were significant (0.0001 < p < 0.05) decreases in extremity T_SK. Associations between manual performance and hand T_SK showed coefficients of determination (R²) ranging from 0.48 < R² < 0.98; 0.00005 ≤ p ≤ 0.08. The D_LIM for the whole-body ensemble ranged from 2.2 h to > 8 h, whereas the D_LIM for the extremities was 0.56 ± 0.20 h for T_SK decreasing 15°C. In conclusion, the hypotheses of a stable core temperature and decreases of extremity skin temperature giving decrements in manual performance were accepted as was the hypothesis that duration limits for exposure would be determined by extremity skin temperatures of the hand and foot.

List of Abbreviations

Ambient temperature (T_AMB, °C)

Body heat gain or loss (Q, kJ m⁻²)

Body Mass Index (BMI, kg m⁻²)

Clothing insulation (clo)

Core temperature (T_CORE, °C)

Coefficient of determination (R², unitless)

Correlation (r, unitless)

Duration Limited Exposure (D_LIM, h)

Heart rate (HR, beat min⁻¹)

International Standards Organization (ISO)

Limit value of Q (Q_LIM kJ m⁻²)

Metabolic Rate (M, W m⁻²)

Minimal Required Clothing Insulation (I_{REQ minimal} clo or m² °C W⁻¹)

Movement Time (MT, ms)

Neutral Required Clothing Insulation (I_{REQ neutral} clo or m² °C W⁻¹)

Physical Activity Readiness Questionnaire Plus 2019 (PAR-Q+ 2019)

Pill core temperature (T_PILL, °C)

Rate of Heat Storage (S, W m⁻²)

Reaction and Movement Timer (RMT)

Reaction Time (RT, ms)

Rectal temperature (T_RE)

Relative Humidity (RH, %)

Required Clothing Insulation (I_REQ, clo or m² °C W⁻¹)

Resultant clothing insulation (Icl, r clo or m² °C W⁻¹)

Skin temperature (T_SK, °C)

Introduction

It is well established that manual performance is impaired as ambient and extremity skin temperatures decrease [1–8]. With skin temperatures of ~15-20°C, changes are evident for hand/upper limb blood flow [9], skeletal muscle function [10], motor-unit function [11], and synovial fluid viscosity [3] with each of these purported to contribute to physiological mechanism(s) of these cold-induced decrements in manual performance. Impaired manual performance in the cold is also from physical constraints from thick and restrictive thermal protection mitts and/or gloves [12–15]. Collectively, these outcomes [1–15] suggest that there is an optimal range of hand skin temperatures and design of mitts and/or gloves to give the best manual performance and protection from peripheral cold injuries with low ambient temperatures.

A recent report on a joint Canada-USA military arctic deployment [16], as well as other similar recent reports [17,18], show that frostbite in the periphery rather than whole-body hypothermia is a more common cold injury. This evidence [16–18] supports that technical apparel employed by troops provides adequate thermal protection for the torso but not so for the periphery including the upper limbs. It follows for operations in low-temperature arctic environments the focus has been on reducing peripheral cold injuries [19] and assessing manual performance [12,20,21] with varying glove/mitt weights, sizing, and designs [4,8,12,20,22–24]. Significant contributions to the understanding of duration limits for exposure (D_LIM) in the cold for the whole body alone have been made using thermal manikins [25], however, it remains to be established what are D_LIM for the whole body and extremities for humans performing manual tasks in these arctic environments.

For participants clad in a typical military extreme cold protection clothing ensemble [25] at an ambient temperature of −40°C, the purpose of this study was to establish D_LIM [26] values for both the whole body and the extremities. To support the applicability of the D_LIM values in these conditions, manual performance was assessed at 15-min intervals during the −40°C exposure. It was hypothesized that for this extreme cold exposure, (i) core temperature would remain normothermic, whereas there would be decreases in extremity skin temperatures; (ii) decrements in manual performance would be in proportion to the reductions in hand skin temperature; and (iii) duration limits for this exposure would be determined by hand or foot skin temperatures.

Materials and methods

Participants

Seven male participants with an age of 32 ± 14 years (mean ± SD), who were 1.75 ± 0.05 m tall, weighed 75.7 ± 14.2 kg, and had a Body Mass Index (BMI) of 24.7 ± 4.4 kg m⁻², volunteered to partake in this study. The sample size for each outcome variable was justified with a power calculation, using means from a previous study of manual dexterity and hand grip strength test decrements following upper limb cooling [27]. Each participant was first familiarized with study procedures during an orientation session that was a minimum of 24 h prior to their first trial. The orientation included an explanation of what the participant would be asked to do and a demonstration of all study tasks. Each participant completed a health screening questionnaire, a Physical Activity Readiness Questionnaire Plus 2019 (PAR-Q + 2019) form, and a consent form prior to their study participation. The study was approved by the SFU Office of Research Ethics and the Ethics committee at Center for Medical Ethics, Southeast University, China.

Instrumentation

Technical apparel

Mittens: A large size, used pair of Hy-Arctic mittens was employed in the study. These Hy-Arctic mittens are commercially available and have an outer shell and an inner liner. The mitt's outer shell was made from tanned goat skin with a shearling patch on the posterior side of the mitt, while its cuff was made from nylon reinforced with a cotton canvas and 6 leather strips. The inner liner was made of quilted nylon with thermal insulation and stitched with cotton. The inner liner was lined with wool, and its cuff was made of “duck face” that features a wool serge. The total thermal insulation of this pair of used mittens and liners employed in the study was assessed at 2.19 clo or 0.339 m²°CW⁻¹, which was measured on an 8-zone heated hand manikin (Thermetrics LLC, Seattle, WA, USA) at an ambient temperature of 10°C, a Relative Humidity (RH) of 85%, and a wind velocity of 0.40 m s⁻¹. As a point of comparison, a new, unused large size Hy-Arctic Shell mitt and liner were also assessed under the same conditions on the same 8-zone heated hand manikin to give a total thermal insulation of 2.42 clo or 0.375 m² °C W⁻¹.

Clothing Ensemble Insulation: Each participant wore the same large size extreme cold protective clothing ensemble. The ensemble included base layers of a thermal undershirt, long johns, and a fleece jacket as well as bib pant plus a winter parka with an insulation of 3.109 clo or 0.482 m² °C W⁻¹ [25]. Total insulation of the ensemble with addition of the used mittens was calculated using the parallel method [28] to give a I,_T of 3.08 clo or 0.477 m² °C W⁻¹. Each participant also wore a pair of Smith Optics OV-1 lens ski goggles (Ketchum, Idaho, USA) to complete the coverage of all body skin surfaces.

Whole body and extremity duration limits for cold exposure

For these conditions, the whole-body Required Clothing insulation (I_REQ) model given in International Standards Organization (ISO)11079: 2007 [26] was employed to determine D_LIM for the whole-body clothing ensemble for its insulation value given in the preceding paragraph. Duration limits for exposures were calculated [26] from the ratio of the limit value of Q (Q_LIM) set at 144 kJm⁻² and the rate of heat storage (S), where D_LIM = Q_LIMS⁻¹. The metabolic rate (M) was set at a moderate rate of 165 Wm⁻² following the criteria in ISO 11079: 2007 [26]. The whole-body heat balance model was assessed for both a low physiological strain with a starting T_SK = 33.34–0.0285 M to give D_{LIM neutral} and I_{REQ neutral}, which corresponds to a thermal sensation of “neutral,” and a high physiological strain with a starting mean T_SK = 33.34–0.0354∙M to give D_{LIM minimal} and I_{REQ minimal}, which corresponds to a thermal sensation of “cold.” Duration limit exposure (D_LIM) values provided a means to quantify the expected permissible period for resultant clothing insulation (I_{cl, r}), for each level of physiological strain. If I_cl, _r is < I_{REQ minimal}, then D_LIM values are calculated to give the period for the decrease of body heat content by 144 kJ m⁻² for both levels of physiological strain with I_{REQ neutral} and I_{REQ minimal}, giving the required clothing insulation for each level of physiological strain. As given in ISO 11079 2007 [26], D_LIM was assessed as the time to reach an extremity T_SK of 15°C. In addition, the D_LIM for the time to reach an extremity T_SK of 10°C is given.

Manual dexterity and upper limb strength tasks

Reaction and Movement Time: An in-house Reaction and Movement Timer (RMT) was employed that included two buttons each approximately 2 cm in diameter separated by 60 cm, a stimulus light and a digital display screen. The device records and reports the time taken for the first depressed button to be released to give the Reaction Time (RT) and the time between lifting the finger from the first button on the left and the depression of the second button on the right to give the Movement Time (MT). The RMT was placed so that the midpoint between the two buttons is in line with the sagittal plane of the participant. The participant depressed the left button with their index finger and focused their attention on the stimulus light, which was initially turned off. Upon seeing the light turn on, the participant lifted their finger from the left button and compressed the right button as fast as possible to allow assessment of reaction and movement times.

Grip Strength: Grip strength was measured using a Sammons Preston Jamar hydraulic hand dynamometer (Clifton, NJ, USA) The participant held the hand grip dynamometer at the width setting they felt would give their best grip strength performance. Each participant completed a maximum voluntary hand grip contraction within a time frame of ~3 s.

Tracing Maze: An in-house tracing maze was constructed, and it has three parts:

1. The tracing maze included a wired cylindrical plastic and metal stylus consisting of two parts, a handle, and a tip. The plastic handle was 15.3 cm in length with a grip portion of 10 cm in length. The grip was roughly knurled and had a diameter of 20 mm, with the remaining portion of the handle being smooth. The metal tip of the stylus consisted of two portions: the top portion that was 6-mm length and the same diameter as the handle and the bottom portion that was 26-mm length with a 3 mm width “pointer” portion, designed to be traced through the narrow maze track.

2. The maze track was in a 34 cm by 45 cm aluminum plate where the tracing track was approximately 9 mm in width and 216 cm in length with 33 straight segments of varying lengths from 1.8 cm to 16.8 cm with each segment connected at right angles.

3. The tracing maze included a DC-powered counter with a digital screen that registered the number of touches of the metal tip to the sides of the maze track in each trial.

The participant was asked to hold the stylus like holding a pen and to hold it against a rubber stop at the start of the maze. Upon being given the instruction to begin, the participant traced the path, making as few errors or “touches” as possible and in as short a time as possible. Maze tracing performance was assessed with the time to trace the maze in seconds, the number of errors, and the error rate in errors/second.

Nut and Bolt Test: Four M16 size bolts with matching M16 nuts were anchored on a 46 cm by 46 cm plexiglass board. The bolts faced vertically and were affixed on the corners of an 8 cm by 6 cm rectangle at the center of the board. Each bolt had a diameter of 35 mm, and the nuts had an external diameter of 23 mm and a width of 14 mm. The nuts were stored in a 25 cm by 16 cm by 7 cm plastic container affixed to the top of the board. In each trial, using only their dominant hand, the participant was asked to remove four nuts from a bowl, one at a time, and thread the nut onto a separate bolt as fast as possible. The bolts were numbered, and the participant was instructed to thread each nut onto the bolt before moving to the next bolt, starting from the upper left-hand corner, followed by the lower left, upper right, and lower right bolts. Once all nuts were threaded, the participant was instructed to reverse the task and remove each nut in the reverse order to which they were threaded before placing them back into the container, in a similar procedure to the one outlined by Chen et al. [29]. The performance on the nut and bold task was the time to complete the task.

Peg and Ring Test: The peg and ring task consisted of 7 cylindrical pegs mounted on a 30 cm by 30 cm plexiglass board. On the board, a grouping of 6 pegs, numbered 1 to 6 that were 3.5 cm in height and 2.0 cm in diameter, were arranged in a 2 × 3 pattern spaced with pegs separated by 10 cm. A seventh, taller peg of the same diameter was 10.5 cm above the grouped pegs on the board. In this timed task, a total of 6 rings, with an external diameter of 41 mm, an internal diameter of 27 mm, and a thickness of 0.7 mm, were individually taken from the taller peg, sequentially placed on the 6 shorter pegs, and then returned to the taller peg. The participant completed the test with their dominant hand, and the task completion time was recorded for each trial.

Disc Test: A series of 18 metal discs 25 mm in diameter of varying thicknesses from 0.25 mm to 30 mm were employed to assess fine dexterity. The discs were arranged in a line in order of decreasing thickness. Using only their dominant hand, the participant was instructed to pick up the largest disc first and then to pick up progressively smaller discs until failure. The participant was given up to three attempts to pick up any disc. The final smallest disc successfully lifted was recorded as the score for the task.

Whole Body Cooling: Cooling was achieved in a climatic chamber (Thermotron Model WP-1290-THCM2-705-705-AC, Thermotron Industries, Holland, MI, USA) capable of regulating its ambient temperature from −65°C and to 85°C. The chamber’s environmental conditions were controlled at −40°C and ~50% RH with a wind speed of 0.4 ms⁻¹ using a Thermotron® 8800 controller with an integral data acquisition system.

Surface Skin and Core Temperature Measurements: Skin temperature (T_SK) was assessed with thermistors (HSRTD-3-100-1-240E, OMEGA, Norwalk, CT, USA) placed on the nose, each ear, the dorsal side of the first digit/thumb, and fourth digit/finger for each hand, and on the dorsal side of the first digit/toe and fifth digit/toe, the heel of each foot plus a thermistor was placed on the lumbar skin surface. In addition, core temperature measures included rectal temperature (T_RE) that was measured with a rectal thermistor probe (DeRoyal,12 Fr 400 Series, Powell, TN, USA) inserted 15 cm past the external anal sphincter and an ingestible BodyCap e-Celsius pill for core temperature (e-Celsius Capsule, BodyCap, Caen, France) in the gastrointestinal tract that was ingested 2 h before each trial [30]. The skin thermistor and rectal probe thermistor data were collected using a National Instruments NI PXIe-1078 data acquisition system controlled by LabVIEW software on a personal computer. Data from the onboard memory of each ingestible BodyCap e-Celsius core temperature pill were retrieved wirelessly after each trial by an e-Temp monitor and ePerformance Manager Software (BodyCap, Caen, France).

Heart Rate: Continuous heart rate (HR) in beats per minute (beats min⁻¹) was recorded using a Polar V800 (Polar, Electro, Oy) heart rate monitor.

Protocol

To negate any learning effect the participant was trained on all manual performance tasks, under thermoneutral conditions with a T_AMB of 21°C, before the −40°C trial. Ten tests were completed with the bare dominant hand, and a further 10 tests were completed with the mitt on the same hand within approximately 1 week prior to the whole-body −40°C trial. The success of this precold trial exposure training, irrespective of mitt use, was evidenced by improvements and subsequent plateauing of nut and bolt completion time (F_TRIAL# = 3.0, p = 0.007), peg and ring completion time (F_TRIAL# = 5.9, p = 0.0001), maze tests errors (F_TRIAL# = 4.6, p < 0.0001), maze test error rate (F_TRIAL# = 2.6, p = 0.016), and movement time (F_TRIAL# = 1.81, p = 0.016) performances.

For each trial at a T_AMB of −40°C, with the participant wearing only upper body cold protection garment, preceding any cooling that started at time 0, manual performance tests were made exterior to the climatic chamber at an ambient temperature of ~21°C. Next the participant donned the full extreme cold clothing ensemble for a 5 min rest prior to entry into the climatic chamber with a T_AMB of −40°C. The participant was then seated after entering the chamber when a single trial of all manual performance tests was completed at 15, 30, 45, and 60 min. The criterion for termination of any trial with a T_AMB of −40°C was when any of the T_SK measurement sites decreased to 10°C.

Statistical analysis and outcome measures

The manual performance and physiological outcome variables are given in the above instrumentation section. Head skin temperature was calculated as the mean of the nose and earlobe temperatures, and each hand skin temperature was calculated as the mean of the distal thumb and 5^th finger temperatures, whereas each foot skin temperature was calculated as the mean of the heel, first toe, and fifth toe temperatures. Two-way repeated measures ANOVAs were employed for each manual performance test with factors of Hand Cover (Levels: No Mitt, Mitt) and Trial Number (Levels: Trial 1–10) during normothermic training at a T_AMB of 21°C, for Skin Temperatures with factors of Time (Levels: 0, 15, 30, 45, and 60 min) and Measurement Location (Levels: Head, Left Hand, Right Hand, Left Foot, and Right Foot), and for T_CORE responses for the −40°C trial with factors of Time (Levels: 0, 15, 30, 45, and 60 min) and Temperature Site (Levels: T_RE & T_PILL). Also, for the manual performance on each test and for heart rate responses for the −40°C trial, a one-way ANOVA model was employed with a factor of time (Levels: 0, 15, 30, 45, and 60 min). Post hoc tests for significant main effects or interaction terms from ANOVA models, as applicable, were completed with paired, two-tailed t-tests. For the −40°C trials, each measure of manual performance was regressed as a function of dominant hand skin and core temperatures with a first-order linear regression model to give correlation coefficients (r). The degree of fit of the regression line was assessed by coefficients of determination (R²). The p value was set at 0.05.

Results

In the normothermic conditions at T_AMB = 21°C (Table 1), the extreme cold protective mitt gave manual performance that was significantly less versus the no mitt condition for fine motor tasks including the disc (F_{HAND_COVER} = 174.2, p < 0.001), the nut and bolt (F_{HAND_COVER} = 25.2 p < 0.001) the peg and ring (F_{HAND_COVER} = 65.3, p < 0.001) tasks and grip strength (F_{HAND_COVER} = 48.1, p < 0.001). There was no effect of mitt use on gross motor tasks.

Table 1.

Comparisons of performance on manual dexterity tasks with and without an extreme cold protective clothing ensemble mitt at an T_AMB of 21°C. Values in the table are the mean (SD) of the last 3 trials from each of the training sessions (n = 7).

Tasks Category	Task Name		Units	No Mitt	Mitt	p
	Disc test		mm	0.43 (0.31)	6.19 (1.20)	p < 0.001
Fine Motor	Nut and bolt		s	46.1 (9.0)	57.3 (8.7)	p < 0.001
	Peg and ring		s	11.9 (0.8)	39.6 (9.7)	p < 0.001
	Maze time		s	30.9 (11.6)	30.9 (11.0)	NS
	Maze errors		#	10.1 (5.2)	11.6 (5.4)	NS
Gross Motor	MER1		#/s	0.39 (0.27)	0.44 (0.24)	NS
	RT1		ms	242.4 (25.8)	241.5 (12.8)	NS
	MT1		ms	205.7 (32.0)	208.6 (52.0)	NS
Strength	Grip strength		kg	44.59 (6.51)	34.24 (7.83)	p < 0.001

¹Maze Error Rate (MER), Reaction Time (RT), and Movement Time (MT)

Mean manual performance in the climatic chamber at T_AMB = −40°C relative to the prechamber values at T_AMB = 21°C was unchanged as a function of time across the measurement points at 0, 15, 30, 45, and 60 min (Table 2). For these manual performance assessments, there were 7 participants for manual performance tests at 0, 15, and 30 min, whereas there were 6 remaining at 45 min and 1 remaining at 60 min. These decreasing participant numbers were on account of at least one T_sk value dropping to a trial-ending value of 10°C for each of the 6 participants who had a trial of less than 1 h.

Table 2.

(A) Five time points for manual performance tests with corresponding ambient temperature (T_AMB) outside and inside the climatic chamber plus the number of participants (n) at each testing time. (B) Performance outcomes on fine and gross motor tasks as well as grip strength for the extreme cold protective clothing ensemble mitt. Values in the table are means (SD).

(A)
Time (min)			0	15	30	45	60
TAMB (°C)			21	−40	−40	−40	−40
(n)			7	7	7	6	1
(B)
Task Category	Task Name		Mitt	Mitt	Mitt	Mitt	Mitt
	Disc test	mm	4.29	6.29	4.86	5.67	4.00
			(8.42)	(9.12)	(10.63)	(9.29)	(3.54)
	Nut and bolt	s	82.40	111.71	94.25	98.04	86.88
Fine Motor			(29.48)	(34.96)	(14.39)	(30.40)	(26.28)
	Peg and ring	s	80.62	104.69	93.39	109.87	151.55
			(66.67)	(25.12)	(32.92)	(74.33)	(96.80)
	Maze Time	s	27.29	29.05	29.79	29.42	27.72
			(7.10)	(6.14)	(5.01)	(5.37)	(4.69)
	MazeErrors	#	17.14	27.00	22.00	22.67	16.50
			(6.36)	(8.35)	(4.16)	(11.69)	(2.12)
Gross	MER1	#/s	0.63	0.99	0.76	0.79	0.61
Motor			(0.23)	(0.43)	(0.22)	(0.41)	(0.18)
	RT1	ms	244.14	287.00	285.50	280.83	372.00
			(24.94)	(53.57)	(47.17)	(52.58)	(148.49)
	MT1	ms	243.57	309.60	362.67	328.33	177.00
			(129.70)	(173.43)	(208.72)	(174.89)	(18.38)
Strength	Grip strength	kg	36.07	30.29	31.43	29.50	40.50
			(8.42)	(9.12)	(10.63)	(9.29)	(3.54)

¹Maze Error Rate (MER), Reaction Time (RT), and Movement Time (MT)

For a representative participant during the −40°C trial (Figure 1), there were progressive decreases in peripheral T_sk until ~50 min when a T_sk measurement site reached a trial-ending value of 10°C, whereas concurrently, the lumbar and ear T_SK remained greater than 25°C. For the group (Figure 2) at T_AMB = −40°C, mean T_SK on the head (F_TIME = 24.2, p < 0.0001), right hand (F_TIME = 11.0, p < 0.0001), left hand (F_TIME = 15.3, p < 0.0001), right foot (F_TIME = 9.6, p <0.0001), and left foot (F_TIME = 11.0, p < 0.0001) showed significant decreases from the precooling values. From all T_SK sites, the largest decrements were from 0 to 15 min and the greatest T_SK changes were by ~15 to ~16°C, on the left hand that decreased from ~34.9 to ~18.4°C, and on the right hand that decreased from ~34.2 to ~18.9°C.

Figure 1.

Sample skin temperature (T_SK, n=1) responses at 14 surface locations as a function of time during a whole-body exposure to an ambient temperature of -40°C in a climatic chamber. The trial was ended at 50 min when T_SK on L. Finger 4 decreased to 10°C. Footnote: The T_SK threshold for cessation of each scheduled 60 min trial was 10°C.

Figure 2.

Head, right and left hand plus right and left foot skin temperatures (mean ± SD) at the manual performance time points during exposure in a climatic chamber to T_AMB = −40°C. Values for each body surface measurement location are means of 2 to 3 values, as given in the methods section. Footnote: At 0, 15 and 30 min n = 7, at 45 min n = 6 and at 60 min n = 1 *p<0.05, †p<0.01, ‡p<0.001, §p<0.0001, each vs. Time 0

Linear regression for scatterplots of mean T_SK of the dominant hand and the percentage change for fine and gross motor task performance in the cold trial (Table 3) gave coefficients of determination (R²) from 0.48 < R² < 0.98 (0.00005 ≤ p ≤ 0.08) with corresponding correlation coefficients (r) from −0.69 to −0.99 (0.00005 < p < 0.08). For the percentage change of grip strength and mean hand T_SK, the R² was 0.92, with p = 0.003, and r was 0.85, with p = 0.003 (Table 3).

Table 3.

Correlations (r) and coefficients of determination (R²) for mean hand skin temperature (°C) and percentage change in manual performance during an exposure T_AMB = −40°C for participants wearing an extreme cold protection clothing ensemble:

Task Category	Task Name	Correlation Coefficients (r)	Coefficients of Determination (R2)	p
Fine Motor	Disc test	−0.69	0.48	0.08
	Nut and bolt	−0.70	0.49	0.08
	Peg and ring	−0.85	0.73	0.014
	Maze time	−0.98	0.96	0.0001
	Maze errors	−0.88	0.78	0.008
Gross Motor	MER1	−0.80	0.64	0.03
	RT1	−0.76	0.58	0.05
	MT1	−0.99	0.98	0.00005
Strength	Grip strength	+0.92	0.85	0.003

¹Maze Error Rate (MER), Reaction Time (RT), and Movement Time (MT)

Gastrointestinal T_PILL and T_RE remained stable (Figure 3). with no significant differences from precooling values (F_TIME = 1.45, p = 0.36) nor between the two core temperature measurement sites (F_{CORE_SITE} = 0.04, p = 0.87). Mean T_RE had a high of ~37.4 ± 0.3°C at min 20 and a low of ~37.3 ± 0.3°C, at min 60, whereas T_PILL had a high value of ~37.6 ± 0.3°C at min 45 and a low of ~37.2 ± 0.3°C at min 1.

Figure 3.

Gastrointestinal pill core temperature (T_PILL) and rectal temperature (T_RE) responses (mean ± SD) as a function of time during exposure to -40°C. The percentage of the sample of 7 participants remaining across the cold trial is given in the plot.

No significant differences were found for the heart rate compared to (F_TIME = 0.76, p = 0.56) the precooling rate (Figure 4). The heart rate ranged from 73.2 ± 14.0 beats min⁻¹at min 5 to 95.7 ± 12.5 beats min⁻¹ at min 45. The heart rate appeared to increase during and decrease following manual performance trials at 0, 15, 30, and 45 min (Figure 4).

Figure 4.

Heart rate responses (mean ± SD) during a 60 min exposure to T_AMB = −40°C in a climatic chamber. The percentage of the sample of 7 participants remaining across the cold trial is given in the plot.

All participants had at least one a finger or toe T_SK reach 15°C with the D_LIM for the extremities occurring at 0.56 ± 0.20 h (Table 4A). Six of seven participants’ cold trial ended prior to 1 h due to an extremity T_SK decreasing to the trial cessation threshold of 10°C; the group’s (n = 7) mean trial duration was 0.80 ± 0.17 h at T_AMB = −40°C. For the whole-body ensemble including mittens and mukluks, the D_{LIM minimal} was > 8 h with an I_{REQ minimal} of 2.78 clo or 0.431 m² °C W⁻¹, whereas D_{LIM neutral} was 2.24 h with an I_{REQ neutral} of 3.09 clo or 0.479 m² °C W⁻¹. The T_SK measurement sites for each participant for the D_LIM value of 15°C and the trial cessation threshold of 10°C are given in Table 4B.

Table 4.

A: Duration time limits (D_LIM) for the extremities and the whole body; required insulation (I_REQ) is given for the whole body for minimal and neutral physiological stresses. B: Participants separated by the first extremity skin temperature (T_SK) measurement site to reach the D_LIM of 15°C and D_LIM of 10°C*.

A
	Whole Body	Extremities
DLIM = 15°C (h)	-	0.56 ± 0.20 (n=7)
DLIM = 10°C (h)	-	0.80 ± 0.17 (n=6)
DLIM minimal (h)	> 8	-
DLIM neutral (h)	2.24	-
IREQ minimal (clo & m2°CW−1)	2.78 & 0.431	-
IREQ neutral (clo & m2°CW−1)	3.09 & 0.479	-
B	Extremities
TAMB (°C)	15	10
Right or Left 4th Finger/5th Digit (n)	4	3
Right or Left Thumb/1st Digit (n)	2	2
Right or Left 5th Toe/Digit (n)	1	1
Right or Left 1st Toe/Digit (n)	1	1
Totals (n)	8	7

*If different Tsk sites reached D_LIM values of 10°C or 15°C simultaneously both were counted in Table 4B; 1 clo = 0.155 m²•°C•W⁻¹

Discussion

The evidence supports that the thermal protection provided by the typical military extreme cold protection clothing ensemble employed was sufficient in maintaining core temperature but insufficient in maintaining hand and foot skin temperatures. The D_LIM value for this simulated arctic exposure was determined by extremity skin temperatures of the digits on both the hands and feet that took 0.56 ± 0.20 h (n = 7) to decrease to 15°C and 0.80 ± 0.17 h (n = 6) to decrease to 10°C. In contrast, for the whole body, the D_{LIM minimal} was greater than 8 h with an I_{REQ minimal} of 2.78 clo or 0.431 m² °C W⁻¹ and D_{LIM neutral} was 2.24 h with I_{REQ neutral} of 3.09 or 0.479 m² °C W⁻¹. The ensemble’s total insulation of 3.08 clo or 0.478 m² °C W⁻¹ exceeded I_{REQ minimal} and was similar to I_{REQ neutral}(Table 4A), which suggests using these whole-body ISO 11079: 2007²⁶ predictions that deployments would be possible with thermal sensations of “cold” for > 8 h and “neutral” for 2.24 h. With this typical military extreme cold protection clothing ensemble [25], however, deployed troops could be expected to effectively conduct manual tasks for the extremity D_LIM duration of only 0.56 h or ~34 min.

The results confirm that for these conditions, with a typical military extreme cold protection clothing ensemble, the decrements in manual performance were a consequence of insufficient thermal protection provided by the mittens. This was evidenced by moderate to high associations between hand skin temperature and manual performance scores (Table 3). For the D_LIM of 0.56 h for an extremity T_SK of 15°C, a skin temperature when manual performance declines [31] was 4 times shorter in duration than for the D_{LIM neutral} of 2.24 h for the whole-body ensemble and ~14 times shorter for the 8 h exposure permitted by D_{LIM minimal} for the whole-body ensemble. The extremity D_LIM values help to demonstrate by how much the duration of an effective deployment in these arctic conditions is reduced relative to D_LIM for the whole body. They also highlight the need to improve hand thermal protection in that a T_SK measurement site on the hand digits was the first for 6 of 7 participants to reach the D_LIM of 15°C as well as the first for 5 of 6 participants to reach the D_LIM of 10°C.

The physiological mechanism(s) of cold-induced decrements of manual performance remain unresolved with contributions suggested to arise from reductions of upper limb blood flow [9], skeletal muscle function [10], motor-unit function [11], and increase in synovial fluid viscosity [3]. Temperature thresholds for onsets of decreases of manual performance as a result of hand and arm cooling are reported with both air [1,4,5,6,7,8,31] and water [2,6,27,31] cooling. Clark and colleagues [1] identified a skin temperature for the onset of a “severe cold effect” on manual performance of 55 ± 4°F (≈12.8 ± 0.9°C), and Heus et al. [31], in a review of physiological criteria for hand function in the cold, state that with hand and finger skin temperatures of 20°C, there is a minimal decrease in finger dexterity and there are “important” losses of dexterity at hand skin temperatures ≤15°C. Currently, the mean dominant hand T_SK at the end point of the −40°C trial was 14.93 ± 3.73°C, a value comparable to these previous reports where there were reductions in manual performance. For D_LIM extremity and thermal protection needed to maintain manual performance, a practical contribution from the current outcomes is to ensure that these two values are chosen as best as possible to circumvent the decline of upper limb musculoskeletal function that begins at skin temperatures of 15–20°C [31]. As reported previously [23,32], however, employing sufficient thermal protection for the hands is at the cost of a reduction in manual performance and dexterity.

Manual performance with [4,8] and without mitts and/or gloves [8,27] has been previously assessed, and even thin nitrile gloves result in decrements to manual performance without cooling of the upper limb [4,8,33]. It follows that currently, the use of extreme cold protective clothing ensemble mitts, which are much larger and bulkier than nitrile gloves, also gave in decrements on manual dexterity tasks (Table 1). At an ambient temperature of −10°C, Geng and colleagues [4] found lower manual performance for heavy insulated gloves than for lighter less insulated gloves. With and without hand cooling [34], the reduced grip strength may also be explained by the bulk and extra cushioning of the extreme cold protective clothing ensemble mitts on the hand dynamometer. There does appear to be a tradeoff between increased manual performance with tight fitting vs. increased insulation with looser fitting handwear in the cold [23,32]. As such, a possible source of variability in the current study was the fit or sizing of the mittens. With this consideration, the current results are consistent with previous outcomes showing that glove design, fit, and low hand as well as finger skin temperatures each impair manual performance.

What might be viewed as unexpected outcome was the group’s manual performance did not progressively decrease across the 60 min at −40°C (Table 2). Explanation of this difference from previous reports appears to follow in part from methodological differences and in part due to participant attrition in the latter half of the −40°C trial as their extremity T_SK reached the cessation threshold of 10°C. An example of a previous report is the study by Giesbrecht et al. [27] who aggressively cooled the entire unprotected arm in 8°C water and reported large decrements of ~70 to 85% for manual performance. Their cold immersion elicited a greater heat loss relative to the −40°C air cooling employed presently that in addition was for a thermally protected arm giving higher upper limb temperatures. Consistent with previous reports of progressively greater decrements in manual performance with lower skin and/or upper limb muscle temperatures [1,27,31,34], however, manual performance did show decrements of ~8 to ~34% (Table 2) and moderate to high associations between the percentage decrement of manual performance and hand skin temperature (Table 3).

Possible limitations of the current study design include that the participants were seated for the cold trial, moving only their upper body every 15 min for approximately 5 min of manual performance testing. This was to simulate conditions for deployments of troops stationed at fixed locations in the arctic during winter months. An extension on this outcome of manual performance with the extreme cold protective clothing ensemble mitts can be with active individuals marching or hiking with suitable technical apparel [35,36]. Also, a single thorax surface thermistor was employed, leaving the possibility of core surface cooling. It has been shown previously that [27], thorax surface and core temperature cooling did not influence manual performance, which was due to upper limb cooling alone [27]; this together with the low heart rate and thermoneutral core temperature responses supports any thorax surface cooling gave a negligible influence on the study outcomes. In addition to limb cooling, the possibilities of the participant having a loss of manual dexterity training as well as “task fatigue” during the −40°C trial need to be considered as contributors to the variability in the manual dexterity outcomes.

The current evidence collectively supports that whole body apparel employed under these conditions was adequate, whereas there is a need for extreme cold weather mitts and footwear to give better thermal protection. As upper limb frostbite is reported as prevalent for Canadian as well as USA military troops deployed in extreme low temperature arctic environments [16,17,18], this indicates that a solution is urgently needed to help reduce the incidence of these debilitating cold weather injuries.

Conclusion

The evidence supported the hypotheses that core temperatures would remain normothermic, where there would be significant decreases in extremity skin temperatures and decrements in manual performance would be in proportion to the decreases of hand skin temperature. The duration limit for this −40°C exposure was determined by hand or foot skin temperatures that decreased to 15°C in 0.56 h and to 10°C in 0.80 h, whereas the whole-body duration limit for these conditions was from 2.2 h to greater than 8 h.

Funding Statement

This study received support from the Canadian Foundation for Innovation and from the Innovation for Defence Excellence and Security (IDEaS) program, Canada's Department of National Defence.

Disclosure statement

No potential conflict of interest was reported by the author(s).