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J Appl Physiol 101: 802-808, 2006. First published May 18, 2006; doi:10.1152/japplphysiol.00197.2006
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Influence of body heat content on hand function during prolonged cold exposures

A. D. Flouris,1 S. S. Cheung,1 J. R. Fowles,2 L. D. Kruisselbrink,2 D. A. Westwood,1 A. E. Carrillo,1 and R. J. L. Murphy2

1School of Health and Human Performance, Dalhousie University, Halifax; and 2School of Recreation Management and Kinesiology, Acadia University, Wolfville, Nova Scotia, Canada

Submitted 16 February 2006 ; accepted in final form 3 May 2006


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
We examined the influence of 1) prior increase [preheating (PHT)], 2) increase throughout [heating (HT)], and 3) no increase [control (Con)] of body heat content (Hb) on neuromuscular function and manual dexterity of the hands during a 130-min exposure to –20°C (coldEx). Ten volunteers randomly underwent three passive coldEx, incorporating a 10-min moderate-exercise period at the 65th min while wearing a liquid conditioning garment (LCG) and military arctic clothing. In PHT, 50°C water was circulated in the LCG before coldEx until core temperature was increased by 0.5°C. In HT, participants regulated the inlet LCG water temperature throughout coldEx to subjective comfort, while the LCG was not operating in Con. Thermal comfort, rectal temperature, mean skin temperature, mean finger temperature (Tfing), change in Hb ({Delta}Hb), rate of body heat storage, Purdue pegboard test, finger tapping, handgrip, maximum voluntary contraction, and evoked twitch force of the first dorsal interosseus muscle were recorded. Results demonstrated that, unlike in HT and PHT, thermal comfort, rectal temperature, mean skin temperature, twitch force, maximum voluntary contraction, and finger tapping declined significantly in Con. In contrast, Tfing and Purdue pegboard test remained constant only in HT. Generalized estimating equations demonstrated that {Delta}Hb and Tfing were associated over time with hand function, whereas no significant association was detected for rate of body heat storage. It is concluded that increasing Hb not only throughout but also before a coldEx is effective in maintaining hand function. In addition, we found that the best indicator of hand function is {Delta}Hb followed by Tfing.

neuromuscular function; manual dexterity; finger temperature; preheating; heat storage

HUMAN THERMOREGULATION IN cold environments has generated substantial interest among physiologists during the past 80 yr. In this light, several studies have demonstrated significant and progressive impairments in fine and gross manual dexterity as early as the initial 5 min of induced local cooling (2, 4, 9), along with rapid impairments of muscular function upon exposure to cold environments (10). Proposed mechanisms involve nerve conduction velocity and finger tactile sensitivity (35), synovial fluid viscosity (18), and blood flow (14), thermal state of the small muscles of the hand (18), as well as between-finger sensory integration (34). These mechanisms have been examined through manipulations of body heat content (Hb) during cold exposure (coldEx) by applying added heat (Q), either directly (i.e., electrically heated gloves or auxiliary heating) or indirectly (i.e., heating another region to increase Hb and augment vasodilation in the extremities) (2–6, 14, 25–27, 30).

Previous experiments applying Q to investigate hand functionality have shown that simple measurements of rectal (Tre) or mean finger temperature (Tfing) are insufficient indicators of extremity function (5). For instance, despite evidence demonstrating that hand function is decreased at Tfing ≤ 16°C (5, 11, 17), impairments are still likely to occur even when Tfing > 16°C, if the body or the forearms are cooled (23, 29). Some authors proposed that, in general, hand function is maintained only when the rate of body heat storage (S) is greater than 0 W (33). In contrast, however, more recently it has been argued that the change in Hb ({Delta}Hb) provides an even better indicator of the relative changes in extremity temperature and hand function during coldEx, compared with either Tre or S (5). Nevertheless, these issues are still a matter of substantial controversy, whereas the precise mechanisms involved as well as the magnitude of Q, the time frame, the location, and the system used to maintain hand function during coldEx remain uncertain (25). In addition, the aforementioned studies have investigated hand function in resting conditions and mainly through manual dexterity tests that involve comparatively complex tasks requiring precise coordination of force in both time and space. However, a comprehensive investigation of the effects of coldEx on hand function should also include the effects of some degree of physical activity (given that individuals exposed to cold generally do not remain completely passive), as well as measurements of neuromuscular function, which is a parameter more general in the sense that it only involves force generation.

The primary objective of the present study was to examine the influence of 1) a prior increase, 2) an increase throughout, and 3) no increase of Hb on neuromuscular function and manual dexterity of the hands during a passive coldEx, interspersed by a short period of submaximal exercise. Increases in Hb were achieved by applying Q either before [i.e., preheating (PHT)] or throughout [i.e., heating (HT)] a 130-min coldEx to –20°C. We selected the 130-min duration to be able to elicit significant decrements on hand function with no serious risk to the participants' thermal integrity. Our secondary objective was to quantify and compare the relationships between either S, {Delta}Hb, or Tfing and hand function during coldEx, to help resolve the existing controversy on the underlying thermoregulatory mechanisms that affect hand function. Based on previous research demonstrating a link between {Delta}Hb and changes in extremity temperature and hand function during coldEx (5), our hypotheses were that increasing Hb not only throughout but also before the coldEx would be effective in maintaining hand function and that the most efficacious indicator of hand function would be the {Delta}Hb.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Participants and procedures.   The experimental protocol conformed to the standards set by the Declaration of Helsinki and was approved by the appropriate ethical review boards at Dalhousie University and Acadia University. Ten healthy, nonsmoking right-handed adults [men: 6; women: 4; age 23.8 yr (SD 2.0); height: 1.8 m (SD 0.1); weight: 71.6 kg (SD 11.4); body surface area: 1.9 m2 (SD 0.1); formula by DuBois and DuBois (13); means (SD)] volunteered to participate in this study. Written, informed consent was obtained from all participants after full explanation of the procedures involved. Certain adaptations to low winter time temperatures may have existed, as measurements were performed during the months of April and May [mean outdoor temperature: 6.7°C (SD 1.5) (Environment Canada)].

Experimental protocol.   During their first visit to the laboratory, participants were given a detailed verbal description of the protocol (depicted in Fig. 1), followed by extensive familiarization with all data collection procedures and instruments. Anthropometric measurements were also performed at this time. Thereafter, using a repeated-measures randomized-block design, participants visited the laboratory on three different occasions, separated by a minimum of 5 days, during a 30-day period. During each visit, participants underwent a coldEx while wearing a liquid conditioning garment (LCG) system (covering the entire body except the face, hands, and feet) and arctic clothing. As illustrated in Fig. 1, in the HT condition, participants regulated the inlet LCG water temperature throughout the coldEx to subjective thermal comfort without any feedback about suit or body temperature. In the PHT condition, 50°C water was circulated in the LCG before coldEx until the participant's Tre was increased by 0.5°C. In the control condition (Con), no water was circulated in the LCG either before or throughout coldEx.


Figure 1
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  		Fig. 1. Experimental protocol.

 
Subjective thermal comfort [from 0 (extremely uncomfortably cold) to 10 (comfortable) to 20 (extremely uncomfortably warm) with increments of 1] and thermal sensation [from 0 (unbearably cold) to 10 (unbearably hot) with 5 as thermoneutral] were recorded using scales modified from Gagge et al. (15). Manual dexterity, neuromuscular function of the hand, and heart rate were recorded before and throughout all conditions in repetitive cycles as illustrated in Fig. 1. To avoid exhausting the participants by prolonging our data collection protocol, we collected baseline measurements for all parameters from each participant only once. This baseline measurement was conducted in normal environmental conditions (room temperature: 25°C; relative humidity: 40%) at the beginning of one of the three conditions (i.e., HT or PHT or Con), chosen at random.

ColdEx and protective clothing.   During coldEx, participants were seated at rest for 130 min, with a 10-min exercise period at half-time (Fig. 1), inside an environmentally controlled chamber with air temperature maintained at –20°C (air velocity 0.05 m/s). During their time inside the chamber, participants wore commercially available clothing simulating all three layers of the Improved Environmental Clothing System Canadian Forces Arctic clothing ensemble (3.6 clo, 0.556 m2·K–1·W–1). The three-layer system included a fleece garment (first layer), an uninsulated inner parka and pants (second layer), and an insulated outer parka and pants (third layer). In addition, standard military mukluks, woolen socks, and a balaclava were worn, together with a thin pair of long, cotton underwear worn under the fleece pants [additional overall clo value of 0.3 (0.05 m2·K–1·W–1)]. The hands were insulated with thin gloves (covering only the proximal phalanx of the fingers) and Arctic mitts (covering the entire hand) for most of the 130-min coldEx, except during manual dexterity testing (see below), when only the thin gloves were worn.

Exercise regimen.   Given that the LCG allowed only for passive increases in Hb, participants were asked to complete a short submaximal exercise protocol during coldEx to examine the effects of increased Hb via endogenous heat on hand function. This paradigm also provides a more realistic real-world scenario, given that individuals exposed to cold generally go through periods of rest interspersed by periods of submaximal physical activity. At the completion of 64 min of coldEx (Fig. 1), participants sat on a stationary Monark bike and exercised for 10 min at a workload requiring 60% of their age-predicted maximum heart rate (220 – age).

Neuromuscular function assessment.   Measurements of evoked twitch force and maximum voluntary contraction force of the first dorsal interosseus muscle were taken in addition to handgrip strength. Evoked force was determined via percutaneous stimulation. After standard skin preparation, two lead stimulating electrodes moistened and covered with electrode gel were used to activate the ulnar nerve. The hand was secured and immobilized using a custom-built myograph (Fig. 2) with the upper arm abducted and the forearm resting prone on a stable base. The myograph was designed to restrict all movement but index finger abduction, and all of its metal surfaces in contact with the skin were covered with insulative material to minimize conductive heat loss. The index finger was secured against a strain gauge force transducer at a height level with the proximal interphalangeal joint, and force data were sampled at 1 kHz and displayed and analyzed using data-acquisition software (Windaq version 1.51, Akron, OH). The muscle stimulation protocol used a supramaximal current to the ulnar nerve using a modified constant current (maximum 1 A) electrical stimulator (Digitimer DS7A, Hertfordshire, UK). Electrodes (used frequently for electrocardiography; diameter: 1 cm) were placed 4 (anode) and 7 cm (cathode), respectively, proximal to the pisiform bone over the ulnar nerve. The intensity of the supramaximal pulse was determined at the beginning of each session by increasing the current of the stimulus in 5-mA increments (beginning at 15 mA) until no further increase in twitch force was observed. Participants were extensively familiarized with this procedure during the initial familiarization session.


Figure 2
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  		Fig. 2. Experimental myograph and configuration of the hand/fingers. The index finger was secured against a strain gauge force transducer at a height equivalent to the proximal interphalangeal joint. Stimulating electrodes were placed 4 and 7 cm proximal from the pisiform bone. All metal parts in contact with the skin were covered with insulating material to minimize conductive heat loss.

 
The maximum voluntary contraction measurements were performed using the aforementioned myograph with the hand secured and immobilized. Although no visual feedback of the force was provided during the contractions, participants were verbally informed of their performance. During the familiarization session, it was ensured that participants were completely accustomed to the required maneuver. Maximum voluntary contraction reliability was assessed with test trials performed at the beginning of each session before the initiation of the experiment.

Tests for hand-grip strength were performed using a hydraulic hand dynamometer (J. A. Preston, Jackson, MI). While seated, participants held the dynamometer in one hand in line with the forearm and hanging by the thigh. Maximum grip strength was then determined without swinging the arm. The better of two trials for each hand was recorded. Participants were extensively familiarized with this procedure during the initial familiarization session.

Manual dexterity assessment.   Two tests were used to assess different aspects of manual dexterity. The standard version of the Purdue Pegboard test (PP) (12) was used to provide a global assessment of dexterity. The test involves picking up metal pegs from a cup and inserting a maximum number of them, one by one, into a row of holes within 30 s. Performance in the task requires the integration of visual and tactile sensations with feedforward and feedback control of hand and finger movements. Participants removed the Arctic mitts from their experimental (dominant) hand for the completion of this test, leaving only the thin gloves.

Repetitive finger tapping (FT) was used to assess temporal coordination of finger movements (22). Performance on this task draws on some similar sensorimotor processes as the PP, but places fewer demands on control of the arm and on visual processing. Participants were instructed to tap down alternatively on two keys placed 4 cm apart with the index and middle fingers of their dominant hand (all participants were right-handed) in rapid oscillations for 10 s. Data recording was initiated automatically when either of the two keys was pressed. The data (open/closed state for each key) were displayed to an investigator outside of the environmental chamber, but the participants were blinded from the results. Performance data were analyzed using custom-made data-acquisition software (Labview version 7, National Instruments, Austin, TX) that returned the mean and the SD of the intertap intervals, which provide information on response speed and consistency, respectively. The software was programmed to detect faults (double tap of the same key) and eliminate them from the computations, allowing only correct sequences to influence the result. Data were sampled at 1 kHz, with the system's timer being calibrated in the beginning of each testing session via a Precision Timer (Lafayette, Indiana, IN).

Thermoregulatory variables.   Core, skin, and finger temperatures were recorded throughout all conditions at 8-s intervals using a data logger (Smartreader 8 Plus, ACR, Vancouver, Canada) interfaced with a computer so that the skin and core temperatures could be monitored continuously. A rectal probe was inserted to a depth of 15 cm beyond the anal sphincter to assess Tre. Four ceramic chip skin thermistors (MA-100, Thermometrics, Edison, NJ) were attached to the chest, upper arm, thigh, and calf to allow the measurement of mean skin temperature (Tsk) using the area-weighting formula of Ramanathan (32). Tfing was measured with thermistors placed on the pad of the fourth finger in both the experimental (i.e., dominant) and the control (nondominant) hand. This method was used to provide an average extremity temperature and compensate for the 2.2°C (SD 4.1) (P < 0.001) lower temperature found in the experimental hand due to removal of the Arctic mitts during manual dexterity testing, as well as conductive heat loss through contact with the different testing devices.

Body heat storage (S; in kJ) for every minute was calculated during all conditions using the thermometric method proposed by Burton (7)

Formula
where 3.47 is the average specific heat of body tissues (in kJ·kg–1·°C–1), mb is the individual's body mass, and {Delta}Tb is the rate of change in mean body temperature (Tb) at time t from the initial Tb at time 0 (in °C/h), calculated as

Formula
Tb was calculated as the weighted sum of Tre and Tsk using the previously described (16) formula

Formula
Following these calculations, S (in W) for every minute was calculated for all conditions as the change in S at time t (in minutes, St) from the change in S at time t – 1 (St – 1), demonstrating, in essence, the magnitude of change per minute in body heat from its initial level

Formula
Concomitantly, Hb (in kJ·kg–1·°C–1) was calculated as follows

Formula
while {Delta}Hb (in kJ) was calculated for every minute as the {Delta}Hb at time t [in minutes, Hb(t)] from the {Delta}Hb at time t – 1 [Hb(t – 1)], demonstrating the change in total body heat per minute

Formula
The added Q of HT and PHT was calculated to quantify and compare the magnitude of the two countermeasures as the residual Hb found at the end of each heating period compared with Con. Q was calculated for HT as the difference in Hb between HT and Con during testing cycle 6, and for PHT as the difference in Hb between PHT and Con during testing cycle 2.

Statistical analyses.   Preliminary analyses included calculation of mean (SD) values of all of the examined parameters for each individual testing cycle (Fig. 1) across all conditions (i.e., HT, PHT, and Con). Consistent with our first objective, mean results for all parameters during all conditions were compared using repeated-measures ANOVA followed by post hoc t-tests incorporating a Bonferroni adjustment. Concurrently, quadratic regression models [Y = b0 + (b1 * t) + (b2 * t ** 2)] were calculated to identify trends in S and {Delta}Hb over time across all conditions. Quadratic was chosen over the linear regression, given its advantage of providing estimates of the local variation, hence demonstrating changes across time more precisely.

Consistent with the second objective, generalized estimating equations (28) models were calculated to examine the relationship between either S, {Delta}Hb, or Tfing (dependent variables in each model) and the assessments of neuromuscular function and manual dexterity [i.e., twitch force, maximum voluntary contraction, handgrip, FT, and PP (independent variables)] using individual data points from Con. The generalized estimating equations approach is highly suitable to examine such hypotheses in research designs that incorporate several repeated measures, because it takes into account that the repeated observations within one individual are not independent and, therefore, is able to provide valid assessments of cross-sectional (between-participants) and longitudinal (within-participants) associations. This is achieved by combining the cross-sectional and longitudinal associations into one regression coefficient that reflects the relationship between the longitudinal development of the outcome variable and the longitudinal development of corresponding predictor variables, using all available data (28). Using an extended approach for the mean structure of generalized estimating equations, Fisher's z transformation was conducted to account for scale differences between the different variables and allow for direct comparisons of the different regression coefficients. Covariates for age and gender were included in each model, while the data were grouped by testing cycle. All statistical analyses were carried out with SPSS (version 13.0.1, SPSS, Chicago, IL) and SAS (version 11.0, SAS Institute, Carry, NC) statistical software packages. The level of significance was set at P < 0.05, except for post hoc tests in which a Bonferroni adjustment was applied.


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Despite the difference in Q between HT and PHT [147.4 (SD 71.0) and 109.3 kJ (SD 76.0), respectively], the mean values did not differ significantly (P > 0.05), probably due to the large amount of variability (evident by the high SD values). This variability was anticipated, given that our sample included both male and female individuals with different body sizes. Thermoregulatory results presented in Table 1 demonstrated significant mean differences in all parameters between HT and Con (P < 0.05). The same was true for comparisons between HT and PHT (P < 0.05), except for Tre, which did not differ significantly between conditions (P > 0.05). Mean comparisons with baseline (resting) conditions in the same data demonstrated that Tre and Tsk remained at similar or higher levels during HT and PHT (P > 0.05), while this was not the case in Con (P < 0.05). In contrast, thermal comfort, thermal sensation, and Tfing were lower than baseline levels in PHT and Con (P < 0.05) but not in HT (P > 0.05). Mean values for maximum voluntary contraction, PP, and FT (Table 2) during HT were significantly different compared with Con (P < 0.05). The equivalent was true only for PP (P < 0.05) in mean comparisons between HT and PHT. Further analyses in the Table 2 data demonstrated that, compared with baseline, significant decrements in mean values were detected across time for twitch force, maximum voluntary contraction, and FT during Con (P < 0.05), whereas this was not the case in HT or PHT (P > 0.05).


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  		Table 1. Thermoregulatory results and comparisons for all conditions and cycles

 

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  		Table 2. Neuromuscular function and manual dexterity results and comparisons for all conditions and cycles

 
The S and {Delta}Hb quadratic regression models (Fig. 3A) for HT demonstrated little variation over time and a general trend for stability with values remaining close to 0, but neither of the models accounted for a significant amount of variation over time (P > 0.05). In contrast, the equivalent models for PHT and Con (Fig. 3, B and C, respectively) revealed negative values with significant trends for increase in {Delta}Hb and decrease in S (P < 0.05) over time, with most curves leveling off after cycle 4 (~64 min into testing). As expected, the PHT model for S demonstrated increased negative values and no leveling off, suggesting increased and uninterrupted negative change per minute in body heat from its initial high level.


Figure 3
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  		Fig. 3. Relationship between rate of body heat storage and change in body heat content across time for all conditions. Values are means per cycle period for each individual; curves represent quadratic regression models [Y = b0 + (b1 * t) + (b2 * t ** 2)], and dotted reference lines are placed at y = 0. Models for heating condition (HT; A) demonstrated little change over time and a general trend for stability, but did not account for a significant amount of variation (P > 0.05), whereas models for preheating condition (PHT; B) and control (Con; C) revealed significant negative values with significant trends for increase in change in body heat content and decrease in rate of body heat storage (P < 0.05) over time, with the curves leveling off after testing cycle 4 (~64 min into testing).

 
The generalized estimating equations model for S (Table 3) revealed no significant relationship with either neuromuscular function or manual dexterity measurements (P > 0.05). In contrast, {Delta}Hb [PP (beta = –0.10; P < 0.05), FT (beta = 0.16; P < 0.001)], and Tfing [PP (beta = 0.09; P < 0.05)] were negatively associated over time with manual dexterity performance (the reader is reminded that FT values represent intertap time; thus increased values suggest worsened performance). Given that the calculated models used only Con data, these results are independent of the different heat manipulations employed in this study. It appears, therefore, that absolute changes in heat content, represented primarily by body heat energy and, to a smaller extent, by extremity temperature, are better indicators of hand function, as assessed through PP and FT, compared with S.


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  		Table 3. Generalized estimating equations standardized (beta) regression coefficients (95% confidence intervals) for the relationship between either S or {Delta}Hb or Tfing and the assessments of neuromuscular function and manual dexterity in the Con data

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
The present results indicated that the increased Hb both before (i.e., PHT) and throughout (i.e., HT) the coldEx maintained twitch force, maximum voluntary contraction, handgrip, as well as FT during a 130-min coldEx to –20°C. Decrements in PP performance were prevented for ~54 min when Hb was increased before the exposure (i.e., PHT), while no decrements were observed when Hb was increased throughout the coldEx (i.e., HT). These results are in line with our primary research hypothesis, demonstrating that increasing Hb not only throughout but also before a coldEx is effective in maintaining hand function. However, it should be noted that the benefits of PHT appear to weaken over time, and the overall advantage was less than that from maintaining Hb throughout coldEx. With regard to our secondary objective, results suggested that the best indicator of hand function during Con was the {Delta}Hb followed by Tfing. These results concur with our research hypothesis and complement a previous investigation by Brajkovic and colleagues (5), which suggested a link between {Delta}Hb and changes in hand function during coldEx. However, the present study provided a more comprehensive investigation of the effects of coldEx on hand function by including the effect of some degree of endogenous heat generation through physical activity as well as measurements of neuromuscular function.

Effects of Q on thermoregulation and hand function.   Our results demonstrated that twitch force, maximum voluntary contraction, hand grip, as well as FT demonstrated no change over time during PHT. This may be attributed to the beneficial effects of Q on Tre and Tsk that were maintained at levels similar to or higher than baseline for the entire coldEx period during PHT. In contrast, the amount of Q in PHT was not effective in maintaining Tfing at baseline levels for the entire coldEx period, but it did result in delaying its decline for ~30 min. Nevertheless, given the high negative values of S and {Delta}Hb, it was inevitable that the beneficial effects of prior increased Hb during PHT on Tfing would only be short term.

It has been previously suggested that finger dexterity can be maintained, despite increased negative {Delta}Hb values as well as low finger blood flow and forearm muscle temperature, if finger skin temperature is maintained relatively high (≥30°C) (2). Our results are partly in line with this notion, demonstrating that PP performance in PHT dropped significantly beyond the second testing cycle (i.e., >54 min), when Tfing fell below 30°C. Regardless, the present results do not support a complete deterioration of manual dexterity in PHT and Con when Tfing < 30°C. Repetitive FT performance, our second measure of manual dexterity, showed only a general increasing trend in mean intertap time in PHT, but no significant impairment (the reader is reminded that FT values represent intertap time; thus increased values suggest worsened performance). Because PP is a complex task requiring precise coordination of force in both time and space, it is largely dependent on visual (feedforward and feedback) and tactile (feedback) information. Thus the linear relationship between PP and Tfing found in our study and others (2) may be explained by the well-known temperature-induced decrements on vibrotactile sensitivity (1), which have been shown to progressively impair performance in functional tests at Tfing ≤ 29°C (24). In contrast, the FT assessment is more general in the sense that it only requires the coordination of force in time (maintenance of timing at the highest possible rate), which is mainly centrally generated, a function effectively preserved by the beneficial effects of Q throughout the entire coldEx period. Given that FT is, for the most part, unaffected by factors external to thermoregulation (e.g., vibrotactile sensitivity), it is tenable that the observed impairments during Con may reflect cold-induced impairments in peripheral neural correlates of motor control. This notion is further strengthened by the significant decrements in neuromuscular function during Con. Previous research has shown a progressively decreased activation of the median nerve at Tfing ≤ 25°C, due to a delay in the generation of action potentials (31). This is in line with the present results, which suggest that the progressive decrements in peripheral motor control (as seen through FT) and neuromuscular function become significant when Tfing ≤ 14°C. Nevertheless, given that our FT and neuromuscular function measurements are only parsimonious indicators of neural correlates of motor control, these notions cannot be fully supported by the present data.

To the best of our knowledge, no previous mechanistic study has directly examined the effects of prior increase in Hb on thermoregulation and hand function during coldEx. Some comparable evidence can be found in the area of anesthesiology, demonstrating that mild skin surface prewarming is effective in avoiding hypothermia caused by general anesthesia for surgeries lasting up to 120 min (8, 19–21, 36). In these settings, preoperative Q has been more efficacious than intraoperative heating in preventing core hypothermia that occurs immediately after induction of anesthesia and persists to a varying extent until the end of surgery. Although generally positive, the beneficial effects of prior increase in Hb in the present study were less pronounced, probably due to the low ambient temperature, which resulted in a large temperature gradient between the body and the environment, with significant effects on S and {Delta}Hb. Indeed, intraoperative {Delta}Hb remains positive, even when no heating is involved (36), whereas in our study {Delta}Hb values in Con reached as low as –183 kJ.

Effects of S, {Delta}Hb, and Tfing on hand function.   Our results demonstrate that {Delta}Hb provided the strongest associations with manual dexterity followed by Tfing, whereas S was not significantly associated with any of the manual dexterity parameters. On the other hand, we found no significant association between any of the thermal variables and the parameters of neuromuscular function. The latter showed a gradual decrease over time, which corresponded to the general thermal body state, but this association did not reach statistical significance. Therefore, it appears that, although thermal body state does influence force generation during coldEx, this link is not as apparent as in complex manual dexterity tasks that require precise coordination of force in both time and space and are also influenced by factors external to thermoregulation, such as vibrotactile sensitivity.

The present results regarding the association between manual dexterity and either {Delta}Hb or Tfing are similar to that previously reported by Brajkovic et al. (5). However, the conclusions of that study were based on observatory analysis of change over time and correlation coefficients, rather than appropriate longitudinal statistics. As the inherent dependency of within-subject observations can reduce the power of linear models (28), we adopted the generalized estimating equations approach in fitting generalized linear models to our dependently distributed response variables. The advantage of using the generalized estimating equations analysis in the present study stems from the repeated-measures nature of our research design and the need to isolate the effect of each independent variable over time. Therefore, our regression coefficients reflect the true relationship between the longitudinal changes in the parameters of hand function and the corresponding thermal variables (i.e., {Delta}Hb, Tfing, and S), excluding possible repeated-measures correlation bias.

With the addition of a brief period of submaximal exercise, we were able to incorporate in our protocol the effects of increased Hb via metabolic heat release on hand function. This paradigm also provides a more realistic real-world scenario, given that individuals exposed to cold generally go through periods of rest interspersed by periods of submaximal physical activity. Our results, therefore, suggest that absolute changes in heat content, represented primarily by body heat energy and, to a smaller extent, by local finger temperature, are better indicators of hand function compared with S.

It seems reasonable to assume that the present results may be influenced by the methodology used to calculate S and {Delta}Hb. These coefficients were calculated using thermometry and the standard Tre and Tsk weighting coefficients used for the estimation of Tb for an individual exposed to cool steady-state conditions (nude, 23°C ambient environment) (32). Thermometry calculations are generally accepted and have been previously used in a number of studies, but partitional and direct calorimetry have been shown to produce more accurate estimations in experiments that involve active heating of specific body parts during coldEx. This is attributed to the high variation in skin temperature over the heated and nonheated areas of the body, which may generate bias when the standard thermometric weighting coefficients for Tre and Tsk are applied. Estimating Tsk from 10 sites would provide more information with regard to this temperature variation across the different areas of the body, but the four-site protocol adopted in our study remains a valid approach (32). The current thermometric protocol has been adopted in the vast majority of thermoregulatory investigations to date and, despite its limitations, is considered a valid tool for monitoring the thermal state of the human body. In addition, given that the LCG used in the present study covered the entire body (including all the areas used for the calculation of Tsk), minimizing the temperature variation across the different areas of the body, the use of thermometric calculations was deemed appropriate.

In summary, we conclude that increasing Hb not only throughout but also before a coldEx is efficacious in maintaining hand function. However, it should be noted that the benefits of this prior body heat increase appear to weaken over time, and the overall advantage was less than that from maintaining Hb throughout coldEx. Additionally, the present results suggest that the best indicator of hand function during coldEx is change in absolute Hb followed by local finger temperature. In contrast, the rate of overall S does not provide efficacious indication of hand function.


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 MATERIALS AND METHODS
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The project was supported by separate Discovery Grants (S. S. Cheung; R. J. L. Murphy) from the National Sciences and Engineering Research Council (NSERC). A. D. Flouris was supported by NSERC PGS-D and Canadian Space Agency funding.


   ACKNOWLEDGMENTS
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 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
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 ACKNOWLEDGMENTS
REFERENCES
 
The authors express gratitude to the subjects who participated in the experiments, to Peter Romkey and the K. C. Irving Centre at Acadia University for providing the environmental chamber and associated technical assistance, as well as to Dr. John Kozey and David Grimshire for support with the experimental infrastructure.


   FOOTNOTES
 

Address for reprint requests and other correspondence: A. D. Flouris, Environmental Ergonomics Laboratory, School of Health and Human Performance, Dalhousie Univ., 6230 South St., Halifax, Nova Scotia, Canada B3H 3J5 (e-mail: aflouris{at}dal.ca)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


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 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 

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