How does caffeine increase exercise capacity but decrease myocardial flow reserve?
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Caffeine↗
Biomedical subjects
Tom M McLellan
Publications and source records attributed to Tom M McLellan.
The management of heat stress for the firefighter: a review of work conducted on behalf of the Toronto Fire Service.
This report provides a summary of research conducted through a grant provided by the Workplace Safety Insurance Board of Ontario. The research was divided into two phases; first, to define safe work limits for firefighters wearing their protective clothing and working in warm environments; and, the second, to examine strategies to reduce the thermal burden and extend the operational effectiveness of the firefighter. For the first phase, subjects wore their protective ensemble and carried their self-contained breathing apparatus (SCBA) and performed very light, light, moderate or heavy work at 25 degrees C, 30 degrees C or 35 degrees C. Thermal and evaporative resistance coefficients were obtained from thermal manikin testing that allowed the human physiological responses to be compared with modeled data. Predicted continuous work times were then generated using a heat strain model that established limits for increases in body temperature to 38.0 degrees C, 38.5 degrees C and 39.0 degrees C. Three experiments were conducted for the second phase of the project. The first study revealed that replacing the duty uniform pants that are worn under the bunker pants with shorts reduced the thermal strain for activities that lasted longer than 60 min. The second study examined the importance of fluid replacement. The data revealed that fluid replacement equivalent to at least 65% of the sweat lost increased exposure time by 15% compared with no fluid replacement. The last experiment compared active and passive cooling. Both the use of a mister or forearm and hand submersion in cool water significantly increased exposure time compared with passive cooling that involved only removing most of the protective clothing. Forearm and hand submersion proved to be most effective and produced dramatic increases in exposure time that approximated 65% compared with the passive cooling procedure. When the condition of no fluid replacement and passive cooling was compared with fluid replacement and forearm and hand submersion, exposure times were effectively doubled with the latter condition. The heat stress wheel that was generated can be used by Commanders to determine safe work limits for their firefighters during activities that involve wearing their protective clothing and carrying their SCBA.
Female↗
Caffeine maintains vigilance and marksmanship in simulated urban operations with sleep deprivation.
PURPOSE: The purpose of this study was to examine the effects of caffeine (CAF) on physical, vigilance, and marksmanship tasks in soldiers during a sustained 55-h field exercise. METHODS: There were 30 soldiers (23.6 +/- 4.5 yr, 81.8 +/- 10.3 kg) who were divided into a placebo (PLAC) and a CAF group. After a period of restricted sleep of 3 h during the first night, a period of sustained wakefulness began that ended at 11:00 of the third day. PLAC or CAF doses of 100 mg, 200 mg, 100 mg, and 200 mg were administered at 21:45, 23:45, 01:45, and 03:45, respectively. At 22:00 of day 2, subjects began two cycles of marksmanship, urban operations vigilance, and psychomotor vigilance (PVT) testing which ended at 06:00 of day 3. RESULTS: CAF maintained marksmanship vigilance at 85% throughout the second night as compared with PLAC, who significantly declined to 61.4 +/- 28.2% overnight. Marksmanship accuracy also decreased significantly in PLAC from 95.1 +/- 8.3% to 83.3 +/- 19.2%, but no change was observed in CAF. Urban operations vigilance decreased for both groups over the night, but the decrease was less for CAF (81.2 +/- 14.4% to 63.4 +/- 24.1%) compared with PLAC (77.6 +/- 19.2% to 44.0 +/- 30.2%). Reaction time and the number of major and minor lapses with the PVT significantly increased in PLAC but were unaffected in CAF. CONCLUSIONS: It was concluded that CAF was an effective strategy to sustain vigilance and psychomotor performance during military operations involving sleep deprivation.
Adult↗
Caffeine maintains vigilance and improves run times during night operations for Special Forces.
PURPOSE: This study examined the effects of caffeine (CAF) on vigilance, marksmanship, and run performance during 27 h of sustained wakefulness in Special Forces personnel. METHODS: There were 31 soldiers (29.8 +/- 5.4 yr, 86.4 +/- 8.6 kg) who were divided into placebo (PLAC, n = 15) and CAF (n = 16) groups. A 6.3-km control run was completed on the morning of Day 1. In the evening of Day 2, soldiers performed a control observation and reconnaissance vigilance task (ORVT) in the field. This 90-min task was repeated twice more between 02:00 and 06:00 on Day 3 during an overnight period of sleep deprivation. Marksmanship was assessed before and after the ORVT. PLAC or 200 mg of CAF gum was administered at 01:45, 03:45, and approximately 06:30 on Day 3. A final 6.3-km run commenced within 30 min of receiving the final dose. RESULTS: ORVT was maintained in CAF at control levels of 77 +/- 13% during the overnight testing. However, values decreased significantly for PLAC from 77 +/- 15% to 54 +/- 29% and 51 +/- 31% during the first and second overnight testing periods, respectively. CAF had no effect on marksmanship but improved 6.3-km run times by 1.2 +/- 1.8 min. Run times slowed for PLAC by 0.9 +/- 0.8 min from approximately 35 min during the control run; the changes in performance were significant between groups. CONCLUSIONS: It was concluded that CAF maintained vigilance and improved running performance during an overnight field operation for Special Forces personnel.
Adult↗
The impact of prior coffee consumption on the subsequent ergogenic effect of anhydrous caffeine.
This study examined whether the prior consumption of coffee (COF) decreased the ergogenic effect of the subsequent ingestion of anhydrous caffeine (CAF). Thirteen subjects performed 6 rides to exhaustion at 80 % VO2max 1.5 h after ingesting combinations of COF, decaffeinated coffee (DECOF), CAF, or placebo. The conditions were DECOF + placebo (A), DECOF + CAF (5 mg/kg) (B), COF (1.1 mg/kg caffeine) + CAF (5 mg/kg) (C), COF + CAF (3 mg/kg) (D), COF + CAF (7 mg/kg) (E), and colored water + CAF (5 mg/kg) (F). Times to exhaustion were significantly greater for all trials with CAF versus placebo (trial A). Exercise times (in minutes) were: 21.7 +/- 8.1, 29.0 +/- 7.4, 27.8 +/- 10.8, 25.1 +/-7.9, 26.4 +/- 8.0 and 26.8 +/- 8.1 for trials A through F, respectively. In conclusion, the prior consumption of COF did not decrease the ergogenic effect of the subsequent ingestion of anhydrous CAF.
Administration, Oral↗
Caffeine improves physical performance during 24 h of active wakefulness.
BACKGROUND: Reductions in both cognitive and physical performance occur during periods of sleep loss with sustained operations. It was the purpose of this study to examine the effects of caffeine on activities chosen to simulate the physical challenges that might occur during a military scenario involving a period of sleep loss. METHODS: There were 16 subjects (26.7 +/- 7.8 yr, 83.8 +/- 11.0 kg) who completed a double-blind caffeine and placebo trial involving a control day and sleep period followed by 28 h of sleep deprivation. A 400-mg dose of caffeine was administered at 21:30 followed by subsequent 100-mg doses at 03:00 and 05:00. At 22:00, subjects began a 2-h forced march followed by a sandbag piling task. A treadmill run to exhaustion at 85% of maximal aerobic power was performed at 07:00 of the second day of sleep deprivation. RESULTS: Caffeine had no effect on the heart rate or oxygen consumption, but rating of perceived exertion (RPE) was reduced with caffeine during the forced march. Time to complete the sandbag piling task during set 1 was significantly reduced with caffeine (12.9 +/- 1.0 min) compared with placebo (13.8 +/- 1.0 min) but there was no difference during set 2 and RPE was increased. Time to exhaustion was significantly increased 25% during the run with caffeine (17.0 +/- 4.4 min) compared with placebo (13.5 +/- 3.3 min), and caffeine maintained performance at control levels (16.9 +/- 4.6 min). CONCLUSIONS: It was concluded that caffeine is an effective strategy to maintain physical performance during an overnight period of sleep loss at levels comparable to the rested state.
Adult↗
Caffeine restores engagement speed but not shooting precision following 22 h of active wakefulness.
BACKGROUND: Current military missions occasionally require combat readiness of soldiers who might be experiencing a sustained period of activity without sleep. Strategies to overcome the debilitating effects of sleep deprivation include the ingestion of caffeine. Unknown is the efficacy of caffeine use on specific elements of target detection and marksmanship following a modest period of sustained wakefulness. METHODS: There were 20 subjects (mean +/- SD of 26.7 +/- 7.2 yr of age, 179 +/- 6 cm in height, and 84.5 +/- 10.8 kg in weight) who participated in double-blind caffeine and placebo trials where each trial involved a 24-h control period (with sleep) followed by 22 h of mixed mental and physical activity with no sleep. At the end of this period, subjects engaged in a 1-h rifle-shooting task. Subjects ingested 400, 100, and 100 mg of caffeine or placebo at 7.5, 3, and 0 h, respectively, prior to shooting. Measures of shooting performance included target engagement time (between target appearance and firing), friend-foe discrimination, accuracy, and precision. RESULTS: Most measures of performance were degraded in the placebo sleep-deprived condition, but only the target engagement time and the number of shots fired were restored by caffeine ingestion. CONCLUSIONS: These findings concur with other research involving different periods of sleep deprivation, and indicate that the cognitive component of the shooting task (i.e., target detection) can benefit from caffeine whereas the psychomotor component (marksmanship) does not. It appears that once the target is detected, the subject is sufficiently aroused to engage the target regardless of the subject's level of alertness prior to detection.
Adult↗
Effect of repeated caffeine ingestion on repeated exhaustive exercise endurance.
PURPOSE: The purpose of this study was to examine the effect of repeated doses of caffeine on repeated exercise endurance. METHODS Nine male caffeine users performed exercise rides (ER) to exhaustion at 80% VO(2max) after ingesting a placebo, 5 mg x kg-1 of caffeine, or 2.5 mg x kg-1 of caffeine 1 h before the ER. Two ER were performed weekly on the same day once in the morning (AM) and 5 h later in the afternoon (PM). There were four treatments containing either caffeine or placebo, i.e., trial A representing 5-mg x kg-1 caffeine in the AM and 2.5-mg x kg-1 caffeine in the PM; trial B, which was placebo in both AM and PM; trial C representing 5-mg x kg-1 caffeine in the AM and placebo in the PM; and trial D representing a placebo in the AM and 5-mg x kg-1 caffeine in the PM. The order of the treatment trials was double blind and randomized. RESULTS: Caffeine ingestion significantly increased exercise time to exhaustion in the AM (trial A 24.9 +/- 10.2 min and trial C 21.8 +/- 4.9 vs trial B 18.0 +/- 6.4 min and D 17.7 +/- 4.3 min). This effect was maintained in the PM and greater than placebo (B 18.3 +/- 4.8 min) regardless of whether redosing (trial A 21.5 +/- 8.6 min) or placebo (trial C 21.0 +/- 6.8) followed the initial morning dose. Caffeine dosing in the PM (trial D 22.4 +/- 7.2 min) also increased ER after placebo trial D in the AM. CONCLUSIONS: It was concluded that redosing with caffeine after exhaustive exercise in the AM was not necessary to maintain the ergogenic effect of the drug during subsequent exercise 6 h later.
Administration, Oral↗
Effect of ingesting caffeine and ephedrine on 10-km run performance.
BACKGROUND: The ingestion of either caffeine (C) or ephedrine (E) has been shown to improve performance during high-intensity aerobic activity lasting 10-20 min, with an additive effect being found when the combination (C + E) was ingested. It was the purpose of this study to determine if the addition of E to C would improve performance in activity lasting longer than 20 min. METHODS: One and one half hours after ingesting a placebo (P), C (4 mg/kg), E (0.8 mg/kg), or C + E, 12 subjects performed a 10-km run while wearing a helmet and backpack weighing 11 kg. The trials were performed in a climatic suite at 12-13 degrees C, on a treadmill where the speed was regulated by the subject. VO(2), VCO(2), V(E), heart rate (HR), and rating of perceived exertion (RPE) were measured during the run at 15 and 30 min, and again when the individual reached 9 km. Blood was sampled at 15 and 30 min and again at the end of the run and assayed for lactate, glucose, and catecholamines. RESULTS: Run times (mean +/- SD), in minutes, were for C (46.0 +/- 2.8), E (45.5 +/- 2.9), C + E (45.7 +/- 3.3), and P (46.8 +/- 3.2). The run times for the E trials (E and C + E) were significantly reduced compared with the non-E trials (C and P). Pace was increased for the E trials compared with the non-E trials over the last 5 km of the run. VO(2) was not affected by drug ingestion. HR was elevated for the ephedrine trials (E and C + E). RPE remained similar for all trails. Caffeine increased the epinephrine and norepinephrine response associated with exercise and also increased blood lactate, glucose, and glycerol levels. Ephedrine reduced the epinephrine response but increased dopamine and FFA levels. CONCLUSION: The previously seen additive nature of E and C was not evident in this study, with the primary ergogenic effect being attributed to E.
Adult↗
Perceptual versus physiological heat strain during exercise-heat stress.
PURPOSE: The physiological strain index (PSI) has been proposed as a universally applicable measure of exercise-heat strain. Unknown is whether this index, based on normalized increases in core temperature and heart rate, is matched by its perceptual analog. METHODS: By using a similar mathematical construct to the PSI, the perceptions of thermal sensation and perceived exertion were combined, and the resultant index, PeSI, was compared with its physiological counterpart, denoted as PhSI, for the exercise-heat stress specific to this study. Twenty-six young and healthy subjects wore semi-impermeable clothing and walked (3.5 km.h(-1)) under hot conditions (40 degrees C and 30% RH) until exhaustion or when their core temperature reached 39.5 degrees C. Subjects were divided into two fitness groups [endurance trained (T) and untrained (U)] comprised of 10 men and 3 women each. U subjects had a higher level of body fatness (mean +/- SD 18.1 +/- 5.3 vs 12.6 +/- 4.5%; P=0.010) and a lower level of aerobic fitness ((.)VO(2max)= 43.6 +/- 3.8 +/- vs 59.0 +/- 6.2 mL.min(-1).kg(-1); P<0.001). RESULTS: During the first hour of exposure, there was no group difference in PhSI, yet T perceived their physiological strain (PeSI) lower than U (P=0.002). Further, the indices were not different for U whereas PhSI was higher than PeSI for T (P=0.008). At the end of the exposure, T had a higher value of PhSI than U (8.23 +/- 0.72 vs 6.74 +/- 1.47; = 0.002), but there was no group difference in PeSI. Although the indices were again not different for U, PhSI at the end was higher than PeSI for T (6.14 +/- 1.68; P<0.001). CONCLUSION: T underestimated and U consistently perceived their physiological strain, as defined by PhSI, in accordance with the measured increases in core temperature and heart rate throughout an exposure to uncompensable exercise-heat stress.
Adult↗
Exercise endurance 1, 3, and 6 h after caffeine ingestion in caffeine users and nonusers.
The purpose of the present study was to examine the duration of caffeine's ergogenic effect and whether it differs between users and nonusers of the drug. Twenty-one subjects (13 caffeine users and 8 nonusers) completed six randomized exercise rides to exhaustion at 80% of maximal oxygen consumption after ingesting either a placebo or 5 mg/kg of caffeine. Exercise to exhaustion was completed once per week at either 1, 3, or 6 h after placebo or drug ingestion. Exercise time to exhaustion differed between users and nonusers with the ergogenic effect being greater and lasting longer in nonusers. For the nonusers, exercise times 1, 3, and 6 h after caffeine ingestion were 32.7 +/- 8.4, 32.1 +/- 8.6, and 31.7 +/- 12.0 min, respectively, and these values were each significantly greater than the corresponding placebo values of 24.2 +/- 6.4, 25.8 +/- 9.0, and 23.2 +/- 7.1 min. For caffeine users, exercise times 1, 3, and 6 h after caffeine ingestion were 27.4 +/- 7.2, 28.1 +/- 7.8, and 24.5 +/- 7.6 min, respectively. Only exercise times 1 and 3 h after drug ingestion were significantly greater than the respective placebo trials of 23.3 +/- 6.5, 23.2 +/- 7.1, and 23.5 +/- 5.7 min. In conclusion, both the duration and magnitude of the ergogenic effect that followed a 5 mg/kg dose of caffeine were greater in the nonusers compared with the users.
Administration, Oral↗
Commercial sport drinks versus light meal combat rations: effect on simulated combat maneuvers.
This study compared a light meal combat ration (LMCR) to specific commercial sport drinks (CSD) and the effect of their ingestion on time to exhaustion during simulated combat maneuvers (SCM). The SCM consisted of three activities: a 2-hour march at 50% of maximal aerobic capacity (VO2max); a subsequent 1-hour run at 70% VO2max; and a run to exhaustion at 80% VO2max. During SCM, the subjects consumed one of four different meals: three CSD (Ergo, Go Sports, and Gatorlode), and the LMCR. In addition, one SCM was conducted with half-rations. Oxygen consumption, heart rate, and rating of perceived exertion were evaluated during each phase of the SCM. Time in minutes (mean +/- SD) to exhaustion at 80% VO2max for Ergo (42.3 +/- 8.9), Go Sports (39.4 +/- 13.3), and Gatorlode (37.7 +/- 8.6) was not significantly different from that for LMCR (36.4 +/- 13.0) but was greater than that for half-LMCR (30.3 +/- 9.3). O2 consumption, heart rate, and rating of perceived exertion were not affected by meal type but did increase over time for each stage of the SCM. We conclude that the amount of calories ingested was responsible for the differences noted in time to exhaustion. We further conclude that the CSD represent a readily available source of energy and fluid that could be used to replace and/or supplement the current LMCR.
Adult↗
Effect of modafinil on core temperature during sustained wakefulness and exercise in a warm environment.
BACKGROUND: Previous studies have revealed that modafinil elevates resting core temperature during periods of sustained wakefulness. The purpose of this study was to examine the effects of modafinil on core temperature during rest and exercise throughout 40 h of sustained wakefulness in a warm environment. METHODS: Ten males performed a drug session (three 100 mg doses per day) and a placebo session that involved a control day, 40 h of sustained wakefulness, and a recovery sleep. For 38 h of the sustained wakefulness, subjects were exposed to 30 degrees C with 50% relative humidity. During the afternoon of both days of wakefulness and during the early morning of the second day, subjects performed 2 h of exercise at 60% VO2max while exposed to the warm environment. RESULTS: The data revealed that rectal temperature (Tre) was elevated at rest 0.15-0.2 degrees C following modafinil ingestion throughout the period of sustained wakefulness. This increase in body temperature at rest was due to an increase in heat production during the first day of wakefulness followed by a lower evaporative heat loss during the second day. During exercise, an inconsistent effect of the drug on Tre was observed throughout the 38-h period. On the first afternoon, the impact of modafinil on Tre was no longer evident after 20 min of exercise. In contrast, during the early morning and afternoon of the second day, the effects of the drug on Tre at rest remained during exercise. For seven subjects who had Tre data for 80 min during all exercise periods, Tre during the placebo session was 38.9, 38.4, and 38.7 degrees C after 80 min of exercise for periods one, two and three, respectively, whereas the corresponding values during the modafinil session were 38.8, 38.7, and 38.9 degrees C. CONCLUSION: With a greater cumulative dose of the drug, Tre remained elevated throughout the exercise period to an extent similar to the increase observed under resting conditions when compared with the placebo condition.
Adult↗