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J Appl Physiol 92: 179-187, 2002;
8750-7587/02 $5.00
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Vol. 92, Issue 1, 179-187, January 2002

Physiological and metabolic responses to a hill walk

P. N. Ainslie1, I. T. Campbell2, K. N. Frayn3, S. M. Humphreys3, D. P. M. Maclaren1, and T. Reilly1

1 Research Institute for Sport and Exercise Sciences, Liverpool John Moores University, Liverpool L3 2ET; 2 University Department of Anaesthesia, University Hospitals of South Manchester, Withington Hospital, Manchester M20 2LR; and 3 Oxford Lipid Metabolism Group, Radcliffe Infirmary, Oxford OX2 6HE, United Kingdom


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The physiological and metabolic demands of hill walking have not been studied systematically in the field despite the potentially deleterious physiological consequences of activity sustained over an entire day. On separate occasions, 13 subjects completed a self-paced hill walk over 12 km, consisting of a range of gradients and terrain typical of a mountainous walk. During the hill walk, continuous measurements of rectal (Tre) and skin (Tsk) temperatures and of respiratory gas exchange were made to calculate the total energy expenditure. Blood samples, for the analysis of metabolites and hormones, were taken before breakfast and lunch and immediately after the hill walk. During the first 5 km of the walk (100- to 902-m elevation), Tre increased (36.9 ± 0.2 to 38.5 ± 0.4°C) with a subsequent decrease in mean Tsk from this time point. Tre decreased by ~1.0°C during a 30-min stop for lunch, and it continued to decrease a further 0.5°C after walking recommenced. The total energy intake from both breakfast and lunch [5.6 ± 0.7 (SE) MJ] was lower than the energy expended [14.5 ± 0.5 (SE) MJ; P < 0.001] during the 12-km hill walk. Despite the difference in energy intake and expenditure, blood glucose concentration was maintained. The major source of energy was an enhanced fat oxidation, probably from adipose tissue lipolysis reflected in high plasma nonesterified fatty acid concentrations. The major observations were the varying thermoregulatory responses and the negative energy balance incurred during the hill walk. It is concluded that recreational hill walking can constitute a significant metabolic and thermoregulatory strain on participants.

energy balance; thermal regulation; field study


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

THE PROLONGED DURATION OF a typical hill walk places exceptional demands on the participants. The specific demands of hill walking incorporate activity of varying intensity and duration, both of which are influenced by factors such as the physical fitness of the participant, dietary intake, backpack weight, environmental weather conditions, and severity of the terrain. Besides, hill walkers can be caught unexpectedly and unprepared when rain and wind accompany outdoor activities in cool weather. Decreased thermal insulation of wet clothing presents a serious challenge to body temperature regulation, which can be compounded by fatigue associated with prolonged exercise such as hill walking (33, 36, 38, 42).

The problem of wet-cold hypothermia is recognized by search-and-rescue organizations worldwide. Nevertheless, their ability to design educational material concerning this hazard is hindered by lack of knowledge of the physiological and psychological responses (42). The information that is available derives from the pioneering work of Pugh (36-38), supplemented by anecdotal descriptions of exposure incidents (23, 25, 36, 41).

Pugh (37) proposed that maintaining an oxygen uptake (VO2) of 2-2.5 l/min or 50-60% maximum oxygen uptake (VO2 max) would offset heat loss and combat the debilitating effects of the cold, wet, and windy environment. Although these observations were based on only three subjects, recent work by Weller et al. (45) supported Pugh's postulate. Both experiments were based in an environmental chamber, and subjects exercised on a cycle ergometer and treadmill, respectively. Pugh (37) and Weller et al. (45) showed that, when exercise metabolism is reduced, the increase in shivering may be insufficient to prevent a decrease in deep body temperature. Weller et al. (45) reported that rectal temperature (Tre) and the metabolic responses to an initial 120-min phase of exercise at ~60% of peak VO2 (VO2 peak) were not influenced by the cold stress of a wet and windy environment. However, during a subsequent 240-min phase of exercise at 30% VO2 peak, Tre was lowered by 0.6°C, whereas the following were elevated: VO2 (25%), the proportion of carbohydrate (CHO) oxidized, and the venous concentrations of lactate, glucose, norepinephrine, and epinephrine.

The studies reviewed have been limited to simulated conditions. Consequently the influence of a "typical" day's hill walk on a range of physiological and metabolic variables, in the field, has not been established. Given such limitations, the aim of the present study was to investigate selected responses to a typical hill walking event to gauge the overall physiological and metabolic strain. The field conditions would likely impose additional stresses not encountered during simulated conditions, such as stoppage of activity for fluid and food intake (transiently altering the balance between heat production and heat dissipation), in conjunction with varying terrain and weather conditions. Furthermore, we aimed to quantify both the energy cost of such activities and relevant responses that are important in the safety of hill walkers, such as the potential thermal stress, impaired psychomotor performance, and the ability to maintain glycemia. This type of study may be important in adding to the mostly anecdotal information regarding exposure and recreational activities. Because of the continuously changing intensity of activity, it is unlikely that individuals will be able to operate at or above 50-60% VO2 max, the VO2 "cutoff" point described by Pugh (37) for combating heat loss during a hill walk. Although Pugh's postulate may have an element of truth, it would depend on factors such as favorable ambient temperature, the clothing worn, terrain, and physiological capabilities of the participants. The first hypothesis, therefore, is that the VO2 cutoff is not a realistic component of hill walking. Second, it was hypothesized that a seemingly "normal" hill walk in possible adverse, but not uncommon, conditions leads to a significant physiological, psychomotor, and metabolic stress on the body.


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Subjects

Thirteen subjects (11 men and 2 women) participated in this study, which was reviewed and approved by the Human Ethics Committee of Liverpool John Moores University. The subjects gave written consent to participate in the study after they had been fully informed of the nature, purpose, and possible risks associated with the study. The physical characteristics of the subjects are shown in Table 1. The majority of the subjects were active and experienced hill walkers. Experiments were conducted from January through March. Body density and percentage of body fat (%fat) were estimated from skinfold thicknesses over the biceps, triceps, and subscapular and suprailiac areas (14). Fitness level was established by using a continuous incremental treadmill running test to exhaustion (3). A plateau in the VO2-to-work relationship was reached in only four subjects; therefore, the highest aerobic power was expressed as VO2 peak and not as VO2 max.

                              
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  		Table 1.   Physical characteristics of the subjects

Protocol and Procedures

On separate occasions, subjects completed a 12-km (8 mile) hill walk. The course varied in elevation from 100 to 902 m above sea level and consisted of a range of gradients and terrain typical of a mountainous hill walk. A caravan was used as a temporary field laboratory and for living accommodation, and was located at the start and end of the hill walk. Subjects woke each morning between 0500 and 0530 and completed the preliminary experiments before the hill walk (Fig. 1). Self-paced walking began each day between 0700 and 0800. Before the walk and on its completion, subjects weighed themselves nude. Subjects were permitted fluid and food ad libitum. They selected their own food and fluids for the walk, which were preweighed before the walk. The energy gained from CHO, fat, and protein was subsequently determined by using standardized food tables (30). After initial weighing, the participants inserted a rectal temperature probe to a depth of 10 cm beyond the anal sphincter. Skin temperature (Tsk) was assessed by the placement of temperature thermistors on the chest, forearm, thigh, and shin. Thermistors and the rectal probe were connected to a data logger (Squirrel meter 1000, Grant Instruments, Cambridge, UK) that recorded data every 6 min. On the walk, a rest period of ~1-3 min was allowed every time thermal measurements were made, and 30 min were allowed for lunch (Fig. 1). During the hill walk, respiratory gas-exchange measures were obtained with a portable telemetry system (Metamax, Cortex Biophsik, Borsdorf, Germany). All subjects carried a lightweight waterproof backpack that contained the Metamax system and thermal logger to allow continuous recording of respiratory gas exchange and Tre and Tsk, respectively. The loaded pack weighed 9.5 kg, which is consistent with a hill-walking scenario.


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  		Fig. 1.   Illustrated profile of the hill walk and associated measurements. HR, heart rate; BS, blood sample; BM, nude body mass; US, urine osmolality; Psychomotor, profile of mood state, rating of perceived exertion, reaction time, and grip strength measurements. *Windchill index measurement.

Measurements and Analysis

Temperature and heart rate. Tre and Tsk were monitored continuously and recorded every 6-min with the data logging system. Heart rate (HR) was recorded by means of short-range radiotelemetry (PE3000 Sports Tester, Polar Electro, Kempele, Finland) every minute and were subsequently averaged over 5-min blocks. Mean Tsk (<A><AC>T</AC><AC>&cjs1171;</AC></A>sk) was estimated by the formula of Ramanathan (39): <A><AC>T</AC><AC>&cjs1171;</AC></A>sk (°C) = 0.3(chest) + 0.3(arm) + 0.2(thigh) + 0.2(shin). Mean body temperature (<A><AC>T</AC><AC>&cjs1171;</AC></A>b) was estimated by the method of calculation used by Bittel (6): <A><AC>T</AC><AC>&cjs1171;</AC></A>b = x · Tre + (1 - x) · <A><AC>T</AC><AC>&cjs1171;</AC></A>sk, where x represents the cold weighting coefficient of 0.67. During one of the experiments, the data-logging system malfunctioned. Furthermore, one subject was not willing to use a rectal probe, and therefore the Tre data were based on 11 subjects and the <A><AC>T</AC><AC>&cjs1171;</AC></A>sk data were based on 10 subjects.

Environmental measurements. Environmental air, dry, and wet bulb temperatures and velocity were recorded with a digital sling psychrometer thermohygrometer and a kestrel vane anemometer, respectively. Wind chill index was calculated from the air temperature and velocity by the equation of Nishi and Gagge (32): Ko = (33 - Ta )(10V0.5 - V + 10.45), where Ko is the cooling power of the environment (kcal · m-2 · h-1), Ta is the ambient temperature (°C), and V represents the air velocity (m/s).

Thermal balance. All thermal balance data (rates) are expressed in units of watts per square meter. The rate of heat gained or lost (heat debt) from the body mass (±S) was computed from the equation of Burton (9): ±S = Delta <A><AC>T</AC><AC>&cjs1171;</AC></A>b · mass · cti · SA-1, where Delta <A><AC>T</AC><AC>&cjs1171;</AC></A>b is the change in <A><AC>T</AC><AC>&cjs1171;</AC></A>b during each exercise period (from the preceding resting value to the end of the exercise period), cti is the average specific heat of the body tissues (1.29 W · kg-1 · °C-1), and SA is the body surface area (m2). The thermal balance was calculated and averaged every 2 km of the walk.

Indirect calorimetry. Continuous assessment of respiratory gas exchange was performed by the use of a portable telemetry system. Signals from the Metamax system were logged and subsequently retrieved at the end of the hill walk. This procedure allowed for continuous monitoring of VO2, ventilation, and respiratory exchange ratio. The percent contributions of the CHO and fat oxidation were estimated from nonprotein (NP) VO2 and respiratory exchange ratio (RER) data with the use of the following formulas: %CHO = [(RERNP - 0.707)/(1 - 0.707)] and %fat = 100 - %CHO. It was assumed that protein oxidation contributed 12.5% of energy expenditure at rest and that exercise did not alter this relative rate of protein utilization (45). The respiratory quotients of CHO, fat, and protein were taken as 1.00, 0.707, and 0.81, respectively. Oxidation rates (g/min) were estimated for CHO, fat, and protein, assuming 0.829, 2.019, and 0.966 liters of oxygen were consumed per gram of substrate oxidized (28), respectively. During the hill walk, CHO and fat oxidization rates were calculated and averaged over 10-min blocks at predetermined points in accordance with the other measurements. Total oxidation rates were then averaged for the whole hill walk. Energy expenditure was calculated from the averaged VO2 and CO2 production from the whole walk by using the formulas of Elia and Livesey (15). Before use, the Metamax system was calibrated using both calibrated gas and ambient air. The volume transducer was calibrated using a 3-liter syringe. To decrease any error, the system was recalibrated during the hill walk when the subjects stopped for lunch. Because of technical problems during three of the walks, the respiratory gas exchange data were based on 10 subjects.

Psychomotor measurements. The Profile of Mood State (POMS) was measured using 65 ratings each on a 5-point rating scale. The scales are factored into six mood scores: depression/dejection, tension/anger, anger/hostility, confusion/bewilderment, fatigue/inertia, and vigor/activity (31). In addition, the subjects were asked to rate their overall ratings of perceived exertion (RPE) on a 6-20 scale (8) from the start of the walk until the lunch stop, and from the lunch stop until the completion of the walk. Reaction time (cognitive function) tests (Hick's law) (1-, 2-, 4-, and 8-choice reaction time for a finger response) were assessed on a laptop computer before, during, and after the walk. Subjects were fully familiarized with the use of the equipment. Grip strength (motor function) was assessed by means of a handgrip dynamometer (Takei, Narragansett, Japan).

Blood and urine sampling and analysis. Blood samples were obtained from subjects in a semireclined position before, during, and immediately on completion of the walk as illustrated in Fig. 1. The venous blood samples (5 ml) were drawn from a superficial forearm vein with minimum stasis, then immediately placed in a vacuum flask containing ice. From the blood samples, serum was separated rapidly at ~4°C and frozen for later determination of serum nonesterified fatty acids (NEFA) and triacylglycerol (TAG) concentrations by enzymatic methods. In addition, a portion of the whole blood was immediately deproteinized with perchloric acid (7% wt/vol) in preparation for whole blood glycerol, lactate, 3-hydroxybutyrate (3-OHB), and glucose determination by enzymatic methods. All enzymatic methods were adapted to a Monarch centrifugal analyzer (Instrumentation Laboratory, Warrington, UK). Serum insulin concentrations were determined with a double-antibody radioimmunoassay (Pharmacia and Upjohn, Milton Keynes, UK). Serum cortisol concentrations were determined by using a solid-phase radioimmunoassay (Diagnostic Products, Llanberis, Wales, UK). Some of the uncoagulated blood was also used for the measurement of hemoglobin and packed cell volume (conventional microhematocrit method). Plasma volume changes were calculated from changes in hemoglobin and packed cell volume relative to initial resting values as described by Dill and Costill (12).

Urine was collected during the rest day before the hill walk, in which subjects performed no exercise, and during the hill walk at the following times: 0800-13:00, 1301-1800, and 1801-2000. From these collections, a 5-ml mixed sample was removed. Urine epinephrine, norepinephrine, creatinine, and dopamine concentrations were then analyzed by using high-performance liquid chromatography with electrochemical detection (in-house method, Dept. of Clinical Chemistry, The Royal University Hospital, Liverpool, UK). Index of dehydration was determined in triplicate using urine osmolality determined by freezing-point depression (model 3300 Advanced Microosmometer, Vitech Scientific, West Sussex, UK). For the urine osmolality, a 5-ml sample was produced after the first void of the day, and then from the first sample after the walk.

Statistical analysis. Variables are presented as means ± SE. Data were initially tested for normality, before being analyzed by repeated-measures ANOVA. The ANOVA results were corrected by the Huynh-Feldt epsilon -adjusted degrees of freedom when the violation to sphericity was minimal (>0.75), and the Greenhouse-Geisser correction was used when sphericity was violated (<0.75) and significant condition and condition-time interactions were identified (16). Post hoc tests (honestly significantly different) were performed to isolate any significant differences. Student's paired t-tests ascertained between-condition differences when a variable was measured once. Statistical significance was set at P <=  0.05 for all statistical tests.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Exercise Duration

All subjects completed the 12-km hill walk. The mean (range) duration for the hill walk was 348 (245) min. The differences in the time to complete the walk were due mainly to variations in weather conditions and terrain. Both cold, wet, and windy weather and deep snow underfoot led to an increased time to complete the hill walk.

Energy Balance

Energy intake and energy expenditure during the hill walk are given in Table 2. Total energy intake was lower than total energy expended during the hill walk in all subjects. CHO, fat, and protein comprised 65, 25, and 10%, respectively of all food consumed, in contrast to 47, 42, and 11%, respectively, of fuels oxidized. The energy intake from both CHO and fat was lower than the amount oxidized (P < 0.001), leading to the lower energy intake relative to expenditure (P < 0.001). The relatively high energy expenditure of 14.5 ± 0.5 MJ reflects the high energetic cost of hill walking, even when pursued over a relatively short duration. The negative energy balance is also shown by a decrease (72 ± 2 to 70 ± 2 kg; P <0.05) in nude body mass as a consequence of the walk.

                              
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  		Table 2.   Total energy, CHO, fat, and protein intake vs. total energy expenditure, CHO, fat, and protein oxidation during the hill walk

Thermoregulatory and Environmental Data

The mean (range) of the recorded environmental data for air velocity, air temperature, and wind chill index was 2.8 (0.1-10.4) m/s, 6.4 (-1.3-13.2)°C, and 520 (176-1,239) kcal · m · -2 · h-1, respectively. These figures highlight the variability in the weather conditions over the period of testing. Five of the walks were completed in cold, wet, and windy weather. The surface conditions on the walks tended to vary with the weather. Snow and ice were regularly encountered, along with high winds, as reflected by a high windchill index; these factors represent walking in very demanding climatic conditions.

The rise in Tre, illustrated in Fig. 2C, is a typical response during both exercise and the initial stages of cooling because of peripheral vasoconstriction decreasing the return of cooled blood from the periphery. Also reflected at this point were the apparent decreases in <A><AC>T</AC><AC>&cjs1171;</AC></A>sk (Fig. 3A). <A><AC>T</AC><AC>&cjs1171;</AC></A>b was maintained relatively well during the hill walk, until the stop for lunch at midwalk (Fig. 2A). A loss in <A><AC>T</AC><AC>&cjs1171;</AC></A>b was evident at this point, before a subsequent rise when walking commenced again. Figure 3 illustrates an apparent temperature "afterdrop." This drop was reflected in a further decrease in Tre after walking recommenced. <A><AC>T</AC><AC>&cjs1171;</AC></A>sk showed a typical rise during the 30-min rest at the midpoint of the walk, followed by a subsequent decrease when walking commenced (Fig. 3). Thermal balance demonstrated an initial decrease during the first 2-km walk, before gaining a positive balance (Fig. 2B). A marked negative balance (-356 ± 23 W · m2) was observed during the lunch stop and for the subsequent 2 km after this, before gaining a positive balance again.


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  		Fig. 2.   Thermoregulatoy responses during the hill walk. Values are means ± SE for 11 subjects. A: mean skin temperature. B: thermal balance. C: rectal temperature. D: mean body temperature. *Point at which a 30-min rest was taken for lunch.



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  		Fig. 3.   Apparent temperature "afterdrop" that was evident in 3 subjects after the 30-min stop for lunch. Each line represents an individual subject.

During the ~30-min stop at midwalk, three subjects showed a pronounced shivering response after ~15 min of rest. Tre fell ~1.0°C from the level observed during the prelunch walking. Before the lunch stop, two of these subjects showed early signs of exposure; symptoms included stumbling, withdrawal from voluntary conversation, and slowing down in pace. Decreases in thermal balance were also observed at this time. The majority of the subjects complained of feeling "very cold" and wanting "to speed up" during the period after the lunch stop: these subjective impressions usually lasted ~15-45 min, depending on the weather conditions.

Psychomotor performance. The POMS profile showed an expected increase in tension and confusion before the walk (P < 0.05) and an increase in fatigue (P < 0.05) postwalk relative to both before and at midwalk. Overall RPE from the start of the walk to the lunch stop and to the final part of the walk was 15 ± 2 and 13 ± 3, respectively. There was a small decrease in grip strength (45.4 ± 2.7 to 43.5 ± 2.8 kg/m2) from prewalk to postwalk (P < 0.01). Any changes in reaction time were less evident. The only significant changes in reaction time were evident in "one-finger" reaction time (P <0.01) and in the recorded errors (4 finger) (P < 0.05, not shown), both after completion of the walk. However, the normal circadian variation of an accelerated reaction time from morning to afternoon was clearly not evident (not shown).

Respiratory gas exchange and HR responses. During the first ~5 km of ascent, RER increased from 0.82 ± 0.03 at base to 0.89 ± 0.02. During the descent, RER gradually fell to <0.84 ± 0.02 for the final 5 km of the walk (Fig. 4A). RER changes are reflected in the oxidation rates of CHO and fat shown in Fig. 4B. During ascent, both CHO and fat oxidation increased. After the first 4 km, CHO oxidation decreased for the duration of the walk, with fat oxidation remaining elevated (Fig. 5). Increases in the VO2 and HR were evident during the rise in altitude over the first 4.5 km of the hill walk (Fig. 5, D and C). From ~3 km until the descent of the hill walk (~6 km), subjects were operating at ~50% VO2 peak with an average HR of 148 ± 8 beats/min; during the descent, this value fell to ~25-40% of VO2 peak with HR averaging 126 ± 5 beats/min (Fig. 5, D and C).


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  		Fig. 4.   Respiratory exchange ratio (A) and carbohydrate oxidation (CHOox) and fat oxidation (Fatox) rates (B) during the hill walk. Values are means ± SE for 11 subjects.



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  		Fig. 5.   Respiratory gas exchange and heart rate results from the hill walk. A: ventilation. B: maximal oxygen uptake (VO2 peak). C: heart rate. D: oxygen uptake (VO2).

Blood and urine constituents

There were no significant changes in plasma volume during the walk. Consequently, circulating concentrations have not been corrected for hemoconcentration. Table 3 gives concentrations of the blood constituents. The energy metabolites 3-OHB, lactate, glycerol, and NEFA increased from prewalk to midwalk (P < 0.001). In contrast, there was no change in TAG concentrations. Insulin increased significantly, whereas cortisol decreased significantly (P < 0.01) postwalk, relative to both prewalk and midwalk. Table 4 gives the concentrations of the urine catecholamine collections. Generally, the hill walk led to a marked elevation in urinary epinephrine and norepinephrine compared with the rest day. In addition, urine osmolality increased prewalk to postwalk (from 603 ± 86 to 744 ± 71 mosmol/kgH2O; P < 0.05).

                              
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  		Table 3.   Whole blood and serum constituents before, during, and after the hill walk


                              
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  		Table 4.   Urinary catecholamine concentration during a rest day and monitored hill walk


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The main finding of this study was that a seemingly normal outdoor hill walk in adverse, but not uncommon, conditions led to a significant physiological stress on the body. Despite these stresses, which included dehydration, high thermal stress, and marked negative energy balance, subjects demonstrated only slight impairment in some of the measured psychomotor tests throughout the walk. Furthermore, the hill walk significantly altered the hormonal and metabolic milieu. Despite the large difference in energy intake and expenditure, a normal blood glucose level was maintained. The major source of energy, in the monitored walk, was an enhanced fat oxidation probably from adipose tissue lipolysis.

Physiological Responses

Pugh (37) described a VO2 cutoff point, above which individuals exercising in a cold, wet, and windy environment would not experience any influence on the physiological responses to exercise, i.e., drop in core temperature, mental impairment, extreme fatigue, and exhaustion. Below this point, there would be an obligatory increase in energy expenditure and subnormal Tre and muscle temperatures. Weller et al. (45) suggested that the cutoff point is likely to depend on factors, such as clothing insulation, body morphology, mass, and body fatness (43), and may account for the random nature of the hypothermic casualties described by Pugh (35, 37). The present observations highlight the variability in VO2 in response to the hill walk, which is likely to depend on such factors as terrain, gradient, weather condition, backpack weight, exercise intensity, preceding diet, and thermal stress. It was only during the high-intensity part of the walk that subjects reached this cutoff point. This cutoff point was clearly variable. Because the hill walkers in this study walked at their own pace, it could be cautiously concluded that hill walkers do not consistently operate at, or above, this cutoff level.

Weller et al. (45, 46) demonstrated heat loss was greater in low-intensity (~30% VO2 peak) than high-intensity (~60% VO2 peak) walking, illustrating how the rate of heat production during high-intensity exercise will offset heat loss to the environment more effectively than low-intensity exercise. This observation has important implications for hill walking in that, during the low-intensity phase of a hill walk (e.g., walking downhill, navigation, and so on), an increased heat loss relative to heat production may be experienced. The factors already mentioned, along with individual variation in effective cold stress under given environmental conditions, may lead to a compromise in the ability to operate safely in the mountainous environment.

The observations of the present study regarding the thermal stresses involved in a hill walking event have provided some novel results. The results show generally higher values for the Tre profile than described in previous studies (42, 45, 46); this increase is most likely due to the elevated thermal insulation from the protective clothes worn by the hill walkers, causing a decrease in heat loss, which will subsequently increase Tre. The maintenance of normal core temperature during cold stress depends on the subject's ability to generate enough heat to offset heat loss to the environment (33, 46). This clearly was not a problem for the hill walkers because of high levels of prolonged exercise intensity.

When the subjects stopped for lunch and measurements midwalk for ~30 min, the exercise hyperthermia was canceled out by the decreased heat production and increased heat loss through conduction and radiation. The initial physiological responses to cold exposure to maintain core temperature in the cold are peripheral vasoconstriction to reduce heat loss and shivering to generate heat. Although shivering was not quantified directly, pronounced shivering was noted in four of the subjects. Once peripheral vasoconstriction is maximized, core temperature can only be maintained by an increased heat production, i.e., shivering, which is thought to be the major contributor to the cold-induced increase in heat production (13). The core temperature continued to fall after subjects began walking after lunch. This temperature afterdrop has been reported in a number of cold-water-immersion studies (17, 33) but, to our knowledge, has not been reported in circumstances such as those of the present study. Even though Tre did not drop below 36°C, this afterdrop may describe the reason for hill walkers slipping into the first stages of hypothermia after stopping for a rest. The suggested mechanisms for the afterdrop are at present subject to controversy (10, 17, 27, 44). The extent of this afterdrop was also reflected by the high negative thermal balance (-356 ± 23 W · m2). Weller et al. (45) reported similar values for thermal balance after 240-360 min of walking in a simulated cold, wet, and windy environment with subjects wearing a minimum of clothing. The high negative thermal balance highlights the thermal stress placed on the human body, even when apparently prepared for the prevailing weather conditions

One possible explanation for this continued drop in Tre may be the decrease in exercise intensity (and hence metabolic heat production) after lunch when the subjects were walking downhill. Studies of cold-water immersion have shown Tre to continue to fall after immediate removal from cold water, even when subjects are actively rewarmed (27). The extent to which this afterdrop can be limited by an increase in exercise intensity in this scenario merits further research.

Psychomotor Response

The unremarkable changes in the psychomotor tests demonstrate that, despite serious physiological stress, the subjects demonstrated normal motor control during the walk. The small decrease in grip strength suggests that, over the monitored walk, motor function was near normal, despite potential cooling of the peripheral tissues.

Energy Balance

The recorded energy expenditure of 14.5 MJ highlights the high energetic cost of the 12-km hill walk and is comparable to the 12-MJ energy expenditure in the studies of Greenhaff et al. (18) and Maughan et al. (29). These latter studies were based on a flat 37-km walk, which corresponded to operating at 17% of VO2 max, with unrestricted access to either a mixed or CHO diet, respectively. In contrast, subjects in the present study operated at ~50% of VO2 max during the first 5-km on the predominately uphill section and at ~30% during the final 7-km, which was on the downhill section of the walk. This suggests that the main determinant of energy expenditure, in a hill walk is the relative difficulty of the walk, in terms of the gradient and terrain, both of which will contribute to a greater level of exercise intensity. Additionally, Maughan et al. demonstrated that a weight loss of 2 kg over their 4-day walk was only apparent in the group who had a low-CHO intake but was not observed in the group ingesting a high-CHO diet. They concluded that the loss of body weight was a consequence of the gradual use of the hepatic and muscle glycogen stores and loss of associated water. In the present study over 1 day, the weight loss also averaged 2 kg, with subjects consuming a mixed diet. The normal whole body muscle glycogen pool amounts to some 400 g (21). Water is stored in association with this glycogen in a ratio of ~3-4 g water/g glycogen (5, 34), suggesting that the observed body weight loss of 2 kg in the present study may reflect a small decrease in whole body glycogen level. Supporting this postulated decrease in whole body glycogen was an observed negative CHO balance of 200 g. In addition, this negative CHO balance of 200 g would equate to an ~800-g loss of body water, suggesting a high element of dehydration, which was also reflected by the increased urine osmolality concentrations. The fundamental difference in the relative intensities of the walk, compared with that of Maughan et al., may largely explain the magnitude of the weight loss in the present study. However, subjects in the studies described (18, 19) had unrestricted access to food, whereas, in the present study, they supplied their own food that they considered appropriate for the conditions. The unrestricted food may have attenuated any potential negative energy balance in the aforementioned studies. In the present study, the negative energy balance, the failure to provide enough fuel for the exercise duration and intensity, could well have led to compromises in both physiological and psychological functioning if the duration were more prolonged. This suggestion generates important considerations for the hill walker with regard to nutrient intake both before and during the walk. For example, because the subjects were clearly in negative energy balance, a suggestion is that they should take more food and/or foods with higher energy content to help prevent this from occurring and provide a measure of protection if the walk becomes unexpectedly prolonged.

Metabolic Responses

The measurements made at the midpoint of the walk showed an enhanced lipolysis, demonstrated by an almost fourfold increase in NEFA concentrations accompanied by high glycerol and 3-OHB concentrations. Fatty acids delivered from adipose tissue are the predominant fuel for sustained exercise at moderate intensity (2, 11, 24). There is usually a surge in plasma NEFA concentrations shortly after exercise, presumed to reflect a continued high rate of lipolysis when muscle NEFA uptake has suddenly diminished (22), and this may have been responsible for some of the elevation in NEFA concentration observed. The stimulus for lipolysis during exercise is mainly adrenergic (4), reinforced by decreased insulin concentrations. It is likely that the former stimulus was greater in our subjects than in many exercise studies because of the adverse conditions. Even though large variations were present in urine catecholamine concentrations, the results clearly indicate a general stress response to the hill walk compared with the prior day.

In contrast, NEFA, glycerol, and 3-OHB concentrations were considerably lower at the end of the walk than midway. The food intake midwalk is likely to have influenced the pattern of lipid mobilization during the final part of the walk. Ahlborg and Felig (1) and Krzentowski et al. (26) showed that CHO feeding before or during mild-intensity, prolonged exercise decreased the amount of energy derived from fat oxidation and increased proportionally the amount of energy-derived blood glucose. However, in the present study, the decreased NEFA, glycerol, and 3-OHB concentrations postwalk coincided with no evident decreases in fat oxidation assessed by indirect calorimetry. This maintained fat oxidation might be accounted for by fatty acids derived from the body's intramuscular stores (40). The relatively high circulating plasma insulin levels recorded at the end of the walk (~12 mU/l) would be expected to lead to a decrease in adipose tissue lipolysis, whereas intramuscular lipoysis is not so readily inhibited by insulin (7, 19).

Hypoglycemia, which would affect both fatigue and the shivering response (20), was not observed in this study at any time. The data indicate that the liver was able to meet glucose requirements by a combination of glycogenolysis and gluconeogenesis, supplemented by the midwalk carbohydrate intake.

In summary, the pattern of substrate mobilization and utilization is likely to vary according to the intensity and duration of the walk, level of fitness, environmental conditions and preceding diet of the participant. It is apparent that the energy expenditure during the hill walk exceeded energy intake, apparent in both total CHO and fat oxidation rates for the walk. The large energy expenditure observed served to highlight the high energetic cost of such hill-walking events, even when completed over a relatively short duration. The observations generate important implications for hill walkers with regard to nutritional strategies for preventing some of the potential detrimental effects of operating at a marked negative energy balance. The negative energy balance may lead to a compromise in physiological function and safety if activity is performed over a prolonged period. Nevertheless, despite the physiological stress and the difference between energy intake and expenditure, blood glucose was maintained. The major source of energy, in the monitored hill walk, was an enhanced fat oxidation probably from adipose tissue lipolysis.

It is evident that more research on this topic is desirable. Future research should extend the findings of this study into hill-walking events over a longer duration. Furthermore, there is little in the literature with respect to repetitive high-intensity hill walks completed over 1-2 wk. Finally, the optimal fluid and nutritional strategies need to be quantified for activity of this form.


    ACKNOWLEDGEMENTS

The subjects in this study deserve our special thanks. We admire their bravery to volunteer and the enthusiasm and persistence they maintained, despite the arduous testing and climatic conditions. We acknowledge the help of Dr. N. Roberts for the analysis of the urine catecholamines.


    FOOTNOTES

The study was supported by Mars Incorporated.

Address for reprint requests and other correspondence: P. N. Ainslie, Research Institute for Sport and Exercise Sciences, Liverpool John Moores Univ., Liverpool L3 2ET, UK (E-mail: humpains{at}livjm.ac.uk).

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.

Received 15 June 2001; accepted in final form 29 August 2001.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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J APPL PHYSIOL 92(1):179-187
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