Impact of Prolonged Exercise in the Heat and Carbohydrate Supplementation on Performance of a Virtual Environment Task

Military Medicine, Feb 2008 by Bailey, Stephen P, Holt, Craig, Pfluger, Kent C, Budde, Zina La, Afergan, Daniel, Stripling, Roy, Miller, Paul C, Hall, Eric E

ABSTRACT Purpose: The purpose of this investigation was to determine whether performance of a virtual environment (VE) task is influenced by exercise in the heat and carbohydrate supplementation. Methods: Ten males completed four exercise trials to fatigue. During each trial, subjects cycled at a submaximal workload. Subjects exercised in a normal environment (NORM) and in a hot environment on different occasions. During exercise, subjects drank 10 mL × kg^sup -1^ × hour^sup -1^ of body weight of a 6% carbohydrate beverage (CHO) or a placebo. Subjects completed a VE task before, during exercise, and after fatigue. Results: More failures occurred during placebo than CHO during exercise. The NORM CHO trial had the fewest failures at fatigue. More kills occurred during exercise in the NORM CHO. Conclusions: Performance of a VE task was negatively influenced by prolonged exercise and heat stress. CHO supplementation may have a positive impact on performance of the VE task following prolonged exercise.

INTRODUCTION

Warfighters on the battlefield are typically exposed to a myriad of physical stressors including, but not limited to, extreme environments, heavy workloads, dehydration, and impaired nutritional status. Anecdotally, these stressors are believed to have a devastating impact on the ability of these individuals to act quickly and appropriately in a combat environment. Cognitive function and mood state have been shown to deteriorate in laboratory and field1,2 experiments designed to simulate these stressful conditions. For example, Lieberman et al.1 found decrements in visual vigilance, choice reaction time, short-term memory, and mood following an 84-hour simulated military sustained operation in a group of soldiers. Lieberman et al.2 had similar finding in soldiers who were part of an elite light infantry unit following 53 hours of simulated combat activities in the heat.

A limitation of these experiments has been that the tools used to evaluate cognitive function typically do not closely approximate the actual psychomotor activities that a warfighter must perform on the batdefield. The use of a virtual environment (VE) can be useful at mimicking these activities and provide a vehicle for quantitative evaluation of performance in a simulated battlefield setting.3 Unfortunately, there is limited empirical information describing how performance of a VE task is influenced by exposure to prolonged physical and environmental stress.

Decrements in cognitive function subsequent to combat training and prolonged military exercises have been associated with symptoms of hypoglycemia.4,5 Owen et al.4 found symptoms of hypoglycemia, precipitated as a result of 48 hours of military activity, were accompanied by reductions in memory recall and vigilance. Similar results have been described following prolonged exercise in a laboratory setting.6-8 This concept is demonstrated by Grego et al.6 who found blood glucose and cognitive processing speed to be reduced in trained cyclists during 3 hours of cycling at 66% of VO^sub 2max^. In contrast, 6% carbohydrate beverage (CHO) supplementation has been shown to attenuate these impairments in cognitive function following prolonged exercise stress.8,9 For example, Welsh et al.9 were able to prevent the decrements in performance of the Stroop Color-Word test precipitated by 1 hour of intermittent high-intensity exercise by providing subjects with regular doses (5 mL × kg^sup -1^ every 30 minutes) of a CHO drink.

Cognitive function is also negatively affected by heat stress.10,11 For example, Radakovic et al.10 found that exposure to a hot environment (40°C) at rest for 90 minutes resulted in deficits in attention during a computer-based cognitive task. Interestingly, it appears that the negative effects of heat stress on cognitive function are more typically associated with more complex executive tasks12 and these deficits can be attenuated with acclimatization and fluid replacement.13 The purpose of this investigation was to examine the influence of heat and CHO supplementation on VE performance.

METHODS

Subjects

Ten moderately fit males (Table I) served as subjects in this investigation. All subjects were recent (within 5 years) graduates of the U.S. Naval Academy and signed an approved informed consent form before participation. Subjects consisted of temporary assigned duty ensigns and second lieutenants (awaiting departure to flight school, dive school, or The Basic School for the U.S. Marine Corps) and fleet lieutenants assigned to the Naval Academy as instructors. Subjects were excluded from participation if they had an orthopedic or other medical condition that prevented their participation.

Experimental Design

To participate in this investigation, subjects were required to complete a maximal exercise test and four experimental trials. Experimental trials were separated by at least 7 days and the order of experimental treatments for each subject was varied using a Latin square design. All experimental trials were initiated between 7:00 and 7:30 a.m. Before each experimental trial, subjects were required to refrain from consumption of caffeine, alcohol, over-the-counter medications, and any nutritional supplements for 24 hours and from eating for 12 hours.