Biomedical subjects

E F Coyle

Publications and source records attributed to E F Coyle.

At least 19 recordsLinked to original sources

PubMed1992

Rehydration after exercise with common beverages and water.

This study assessed the effectiveness of two common rehydration beverages (a caffeinated diet cola (DC) and a 6% carbohydrate-electrolyte (CE) solution) compared with water (W) for whole body rehydration, gastric emptying and blood volume (BV) restoration during a 2 h rehydration period following exercise-induced dehydration. Subjects (mean VO2max = 4.2 +/- 0.6 l.min-1.min-1; n = 19) exercised at 60-80% VO2max in the heat (32 degrees C; 40% rh) until approximately 2.5% (1.95 +/- 0.12 kg) of their body weight (BW) was lost. After exercise, the subjects sat for 2 h in a thermoneutral environment (21 degrees C; 60% rh) and drank a volume of DC, W and CE equal to the fluid lost. Fluids were consumed in two boluses averaging 1,046 +/- 198 and 912 +/- 186 ml at 0 and 45 min of the 2 h rehydration period, respectively. At the end of the rehydration period, no fluid remained in the stomach during any of the trials as indicated by epigastric impedance. However, in all the trials the subjects were somewhat hypohydrated (range 0.6-0.9 kg BW below euhydrated BW; p less than 0.05) after the 2 h rehydration period since additional water and BW were lost as a result of urine formation, respiration, sweat and metabolism. The percentage of body weight loss that was regained (used as an index of % rehydration) during DC (54 +/- 5%) was significantly lower than that of W and CE (64 +/- 5% and 69 +/- 5%, respectively; p less than 0.05; n = 10).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1992

Load and velocity of contraction influence gross and delta mechanical efficiency.

The effects of three different cadences and five different work rates on Gross (GE) and Delta Efficiency (DE) during cycle ergometry were studied. Fifteen well-trained cyclists exercised for 30 minutes at 60, 80, or 100 RPM on three different occasions. On each occasion, the load was increased every five minutes and corresponded to approximately 50, 60, 70, 80 and 90% of VO2max. During the last three minutes of each stage, steady-state energy expenditure was calculated while work rate was recorded. In addition, the oxygen cost of unloaded cycling (CUC) was also measured. GE was calculated as the ratio of work rate to the rate of energy expenditure, whereas DE was calculated as the reciprocal of the slope of this relationship at work rates between 50 and 90% of VO2max. The CUC corresponded to 0.66 +/- 0.03 l/min, 0.77 +/- 0.04 l/min and 1.04 +/- 0.04 l/min at 60 RPM, 80 RPM and 100 RPM, respectively (p less than 0.01 for all comparisons). GE was similar at all cadences when cycling at 80 and 90% VO2max. DE increased with increasing rpm and corresponded to 20.6 +/- 0.4%, 21.8 +/- 0.6%, and 23.8 +/- 0.4% at 60 RPM, 80 RPM and 100 RPM, respectively (p less than 0.01 for all comparisons). Therefore, when trained cyclists exercise intensely (80-90% VO2max), GE is similar at cadences of 60, 80 and 100 RPM, despite the significant increase in the CUC. Thus, it is possible that delta efficiency increases with increasing cadence.

Exercise↗

PubMed1992

Carbohydrate feeding during exercise.

During strenuous exercise (i.e. 70% maximal O2 consumption) there is a progressive shift from muscle glycogen to blood glucose oxidation with increasing duration of exercise. By maintaining blood glucose concentration and the rate of carbohydrate oxidation necessary to exercise strenuously, carbohydrate consumption throughout exercise delays fatigue by 30-60 min in endurance-trained subjects. This requires exogenous glucose supplementation at rates in excess of 1 gram/min (i.e., 16 mg/kg/min) as evidenced by the observation that intravenous glucose infusion at this rate is required to maintain blood glucose at 5 mM. Exogenous glucose must be infused at a rate of 2.6 gram/min (i.e., 37 mg/kg/min), which is similar to the total rate of carbohydrate oxidation, in order to maintain blood glucose at 10 mM after 2 h of exercise. However, carbohydrate supplementation during intense exercise does not spare muscle glycogen utilization in people. This suggests that over the course of 2-4 hours of exercise at 70% VO2max, muscle glycogen and blood glucose contribute equally to total carbohydrate oxidation. Furthermore, during the latter stages of prolonged exercise, exogenous blood glucose supplementation may be capable of supplying almost all of the carbohydrate requirements of exercise at intensities up to 70% VO2max.

Blood Glucose↗

PubMed1992

Carbohydrate supplementation during exercise.

Muscle glycogen and plasma glucose are oxidized by skeletal muscle to supply the carbohydrate energy needed to exercise strenuously for several hours (i.e., 70% maximal O2 consumption). With increasing exercise duration there is a progressive shift from muscle glycogen to blood glucose. Blood glucose concentration declines to hypoglycemic levels (i.e., 2.5 mmol/L) in well-trained cyclists after approximately h of exercise and this appears to cause muscle fatigue by reducing the contribution of blood glucose to oxidative metabolism. Carbohydrate feeding throughout exercise delays fatigue by 30-60 min, apparently by maintaining blood glucose concentration and the rate of carbohydrate oxidation necessary to exercise strenuously. Carbohydrate feedings do not spare muscle glycogen utilization. Very little muscle glycogen is used for energy during the 3-4-h period of prolonged exercise when fed carbohydrate, suggesting that blood glucose is the predominant carbohydrate source. At this time, exogenous glucose disposal exceeds 1 g/min (i.e., 16 mg.kg-1.min-1) as evidenced by the observation that intravenous glucose infusion at this rate is required to maintain blood glucose at 5 mmol/L. However, at this time these cyclist cannot exercise more intensely than 74% of maximal O2 consumption, suggesting a limit to the rate at which blood glucose can be used for energy. It is important to realize that carbohydrate supplementation during exercise delays fatigue by 30-60 min, but does not prevent fatigue. In conclusion, fatigue during prolonged strenuous exercise is often due to inadequate carbohydrate oxidation. This is partly a result of hypoglycemia, which limits carbohydrate oxidation and causes muscle fatigue.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood Glucose↗

PubMed1992

Comparison of indirect calorimetry and a new breath 13C/12C ratio method during strenuous exercise.

A new stable isotope method for the determination of substrate oxidation rates in vivo is described and compared with indirect calorimetry at rest and during high-intensity exercise (30 min at 80-85% maximal O2 uptake capacity) in six well-trained cyclists. This method uses the absolute ratios of 13C/12C in expired air, endogenous glucose, fat, and protein in addition to O2 consumption and is independent of CO2 production (VCO2). Carbohydrate and fat oxidation rates at rest, calculated by both methods, were not significantly different. During exercise the breath 13C/12C ratio increased and reached a steady state after 15-20 min. Carbohydrate oxidation rates during exercise were 39.4 +/- 5.2 and 41.7 +/- 5.7 mg.kg-1.min-1 [not significant (NS)], and fat oxidation rates were 7.3 +/- 1.3 and 6.9 +/- 1.2 mg.kg-1.min-1 (NS), using indirect calorimetry, and the breath ratio method, respectively. We conclude that the breath 13C/12C ratio method can be used to calculate substrate oxidation under different conditions, such as the basal state and exercise. In addition, the results obtained by this new method support the validity of the underlying assumption that indirect calorimetry regards VCO2 as a reflection of tissue CO2 production, during exercise in trained subjects, even up to 80-85% maximal O2 uptake.

Adult↗

PubMed1992

Effect of heart failure on skeletal muscle in dogs.

To investigate the effect of heart failure (HF) on skeletal muscle, gracilis, gastrocnemius, and triceps muscle histochemistry and enzyme activity, gracilis muscle performance, and gracilis muscle 31P magnetic resonance spectroscopy (MRS) metabolic responses to exercise were compared in six sham-operated control dogs (C) and seven dogs with HF produced by 3 mo of rapid ventricular pacing. HF reduced skeletal muscle weights and decreased type II fiber area (3,315 +/- 996 to 1,750 +/- 638 microns 2; P less than 0.01). Citrate synthase and beta-hydroxyacyl CoA dehydrogenase activity also tended to decrease. Gracilis muscle inorganic phosphate-to-phosphocreatine ratios, oxygen uptake (VO2), and pH responses to exercise using supramaximal twitch stimulation, a form of exercise that produces maximal stimulation of all muscle fibers and therefore is independent of muscle mass, were similar in both groups. There was no significant difference in developed tension per gram muscle during stimulation at 0.5-200 Hz, maximal developed tension (C: 177 +/- 32 vs. HF: 173 +/- 44 g/g muscle), or muscle fatigability as assessed by percent of initial tension observed at the end of 2 min of 20 Hz stimulation (C: 52 +/- 17% vs. HF: 52 +/- 11%; all P = NS). These data suggest that HF produces skeletal muscle atrophy but that the remaining muscle exhibits normal performance and metabolism.

Animals↗

PubMed1992

Fluid ingestion during exercise increases skin blood flow independent of increases in blood volume.

The purpose of this experiment was to determine whether fluid ingestion attenuates the hyperthermia and cardiovascular drift that occurs during exercise dehydration due to increases in blood volume. In addition, forearm blood flow, which is indicative of skin blood flow, was measured to determine whether the attenuation of hyperthermia and cardiovascular drift during exercise with fluid ingestion is due to higher skin blood flow. On three different occasions, seven trained cyclists [mean age, body weight, and maximum oxygen uptake: 23 +/- 3 yr, 73.9 +/- 10.5 kg, and 4.75 +/- 0.34 (SD) l/min, respectively] cycled at a power output equal to 62-67% maximum oxygen uptake for 2 h in a warm environment (33 degrees C, 50% relative humidity, wind speed 2.5 m/s). During exercise, they randomly received no fluid (NF) or a volume of a carbohydrate-electrolyte fluid replacement solution (FR) sufficient to replace 80 +/- 2% of sweat loss or were intravenously infused with 5.3 ml/kg of a blood volume expander (BVX; 6% dextran in saline). The infusion of 398 +/- 23 ml of BVX maintained blood volume at levels similar to that when 2,404 +/- 103 ml of fluid were ingested during FR and greater than that when no fluid was ingested during the 2nd h of exercise (P less than 0.05). However, BVX and NF resulted in similar esophageal and rectal temperatures, forearm blood flow, and elevations in serum osmolality and sodium concentration during 2 h of exercise.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1992

Influence of graded dehydration on hyperthermia and cardiovascular drift during exercise.

This investigation determined the effect of different rates of dehydration, induced by ingesting different volumes of fluid during prolonged exercise, on hyperthermia, heart rate (HR), and stroke volume (SV). On four different occasions, eight endurance-trained cyclists [age 23 +/- 3 (SD) yr, body wt 71.9 +/- 11.6 kg, maximal O2 consumption 4.72 +/- 0.33 l/min] cycled at a power output equal to 62-67% maximal O2 consumption for 2 h in a warm environment (33 degrees C dry bulb, 50% relative humidity, wind speed 2.5 m/s). During exercise, they randomly received no fluid (NF) or ingested a small (SF), moderate (MF), or large (LF) volume of fluid that replaced 20 +/- 1, 48 +/- 1, and 81 +/- 2%, respectively, of the fluid lost in sweat during exercise. The protocol resulted in graded magnitudes of dehydration as body weight declined 4.2 +/- 0.1, 3.4 +/- 0.1, 2.3 +/- 0.1, and 1.1 +/- 0.1%, respectively, during NF, SF, MF, and LF. After 2 h of exercise, esophageal temperature (Tes), HR, and SV were significantly different among the four trials (P < 0.05), with the exception of NF and SF. The magnitude of dehydration accrued after 2 h of exercise in the four trials was linearly related with the increase in Tes (r = 0.98, P < 0.02), the increase in HR (r = 0.99, P < 0.01), and the decline in SV (r = 0.99, P < 0.01). LF attenuated hyperthermia, apparently because of higher skin blood flow, inasmuch as forearm blood flow was 20-22% higher than during SF and NF at 105 min (P < 0.05). There were no differences in sweat rate among the four trials. In each subject, the increase in Tes from 20 to 120 min of exercise was highly correlated to the increase in serum osmolality (r = 0.81-0.98, P < 0.02-0.19) and the increase in serum sodium concentration (r = 0.87-0.99, P < 0.01-0.13) from 5 to 120 min of exercise. In summary, the magnitude of increase in core temperature and HR and the decline in SV are graded in proportion to the amount of dehydration accrued during exercise.

Adult↗

PubMed1992

Benefits of fluid replacement with carbohydrate during exercise.

Ingestion of approximately 30-60 g of carbohydrate during each hour of exercise will generally be sufficient to maintain blood glucose oxidation late in exercise and delay fatigue. Since the average rates of gastric emptying and intestinal absorption exceed 1,250 ml.h-1 for water and solutions containing up to 8% carbohydrate, exercising people can be supplemented with both carbohydrate and fluids at relatively high rates. When cyclists exercise at competitive intensities for 2 h in the heat with a sweat rate of 1,400 ml.h-1, it is clear that the closer that fluid consumption matches sweating rate (at least up to 80% of sweating rate), the better. Increasing dehydration, due to inadequate fluid consumption, directly impairs stroke volume, cardiac output, and skin blood flow, which results in larger increases in body core temperature, heart rate, and ratings of the difficulty of exercise. This same phenomenon probably also applies to running, which argues against the notion that a certain amount of dehydration (i.e., up to 3%) is permissible and without major cardiovascular consequences. However, runners generally drink only 500 ml.h-1 of fluid and thus allow themselves to dehydrate at rates of 500-1,000 ml.h-1. The performance question boils down to "Will the time lost as a result of drinking larger volumes be compensated by the physiological benefits drinking produces and the faster running pace that might be achieved during the last half of the race?" However, if the goal is safety, which means minimizing hyperthermia, there is no question that the closer that the rate of drinking can match the rate of dehydration, the better.

Body Temperature Regulation↗

PubMed1992

Cycling efficiency is related to the percentage of type I muscle fibers.

We determined that the variability in the oxygen cost and thus the caloric expenditure of cycling at a given work rate (i.e., cycling economy) observed among highly endurance-trained cyclists (N = 19; mean +/- SE; VO2max, 4.9 +/- 0.1 l.min-1; body weight, 71 +/- 1 kg) is related to differences in their % Type I muscle fibers. The percentage of Type I and II muscle fibers was determined from biopsies of the vastus lateralis muscle that were histochemically stained for ATPase activity. When cycling a Monark ergometer at 80 RPM at work rates eliciting 52 +/- 1, 61 +/- 1, and 71 +/- 1% VO2max, efficiency was determined from the caloric expenditure responses (VO2 and RER using open circuit spirometry) to steady-state exercise. Gross efficiency (GE) was calculated as the ratio of work accomplished.min-1 to caloric expenditure.min-1, whereas delta efficiency (DE) was calculated as the slope of this relationship between approximately 50 and 70% VO2max. The % Type I fibers ranged from 32 to 76%, and DE when cycling ranged from 18.3 to 25.6% in these subjects. The % Type I fibers was positively correlated with both DE (r = 0.85; P less than 0.001; N = 19) and GE (r = 0.75; P less than 0.001; N = 19) during cycling. Additionally, % Type I fibers was positively correlated with GE (r = 0.74; P less than 0.001; N = 13) measured during the novel task of two-legged knee extension; performed at a velocity of 177 +/- 6 degrees.s-1 and intensity of 50 and 70% of peak VO2 for that activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Bicycling↗

PubMed1992

Carbohydrate and fluid ingestion during exercise: are there trade-offs?

Intense exercise (i.e.; above 60% VO2max) can be maintained for prolonged periods provided sufficient carbohydrate is available for energy and the heat generated from muscle metabolism does not cause excessive hyperthermia and/or dehydration due to sweating. It is clear that people should ingest carbohydrate during prolonged exercise (i.e.; longer than 1-2 h), which causes fatigue because of an inadequate supply of blood glucose and that fluids should also be ingested in an attempt to offset dehydration and reduce hyperthermia. Ingestion of approximately 30-60 g of carbohydrate (i.e.; glucose, sucrose, or starch) during each hour of exercise will generally be sufficient to maintain blood glucose oxidation late in exercise and delay fatigue. Since the average rates of gastric emptying and intestinal absorption can reach 1 l.h-1 for water and solutions containing up to 8% carbohydrate, exercising people can be supplemented with both carbohydrate and fluids at relatively high rates (over 60 g.h-1 of carbohydrate and 1 l.h-1 of fluid). Therefore, when sweat rate is not high (i.e.; less than 1 l.h-1), the addition of carbohydrate to fluids, and vice versa, does not prevent adequate supplementation of each, especially if large volumes are consumed to keep the stomach somewhat full and thus increase gastric emptying. Therefore, in most situations there are no trade-offs between fluid and carbohydrate.(ABSTRACT TRUNCATED AT 250 WORDS)

Body Temperature Regulation↗

PubMed1991

Timing and method of increased carbohydrate intake to cope with heavy training, competition and recovery.

Based upon the fact that fatigue during intense prolonged exercise is commonly due to depletion of muscle and liver glycogen which limits both training and competitive performance, this paper has proposed extraordinary dietary practices which generally advocate high carbohydrate intake at all times before, during and after exercise. The simple goal is to have as much carbohydrate in the body as possible during the latter stages of prolonged intense exercise when the ability for intense exercise usually becomes limiting to performance. This theory is put into practice by recommending that carbohydrate intake after exhaustive exercise should average 50 g per 2 h of mostly moderate and high glycaemic carbohydrate foods. The aim should be to ingest a total of about 600 g in 24 h. Carbohydrate intake should not be avoided during the 4 h period before exercise and in fact it is best to eat at least 200 g during this time. When possible, carbohydrate should be ingested during exercise, generally in the form of solutions containing glucose/sucrose/maltodextrins, at a rate of 30-60 g h-1. Emphasis has been placed upon eating the optimal amount and best type of carbohydrate at the proper times because these practices demand a large amount of food. When diet is not carefully planned according to these guidelines, endurance athletes tend to consume too little carbohydrate because they become satiated with high fat in their diet and they go through periods in the day when recovery of glycogen stores is suboptimal and thus precious time is wasted.

Competitive Behavior↗

PubMed1991

Carbohydrate metabolism during intense exercise when hyperglycemic.

The effects of hyperglycemia on muscle glycogen use and carbohydrate metabolism were evaluated in eight well-trained cyclists (average maximal O2 consumption 4.5 +/- 0.1 l/min) during 2 h of exercise at 73 +/- 2% of maximal O2 consumption. During the control trial (CT), plasma glucose concentration averaged 4.2 +/- 0.2 mM and plasma insulin remained between 6 and 9 microU/ml. During the hyperglycemic trial (HT), 20 g of glucose were infused intravenously after 8 min of exercise, after which a variable-rate infusion of 18% glucose was used to maintain plasma glucose at 10.8 +/- 0.4 mM throughout exercise. Plasma insulin remained low during the 1st h of HT, yet it increased significantly (to 16-24 microU/ml; P less than 0.05) during the 2nd h. The amount of muscle glycogen utilized in the vastus lateralis during exercise was similar during HT and CT (75 +/- 8 and 76 +/- 7 mmol/kg, respectively). As exercise duration increased, carbohydrate oxidation declined during CT but increased during HT. Consequently, after 2 h of exercise, carbohydrate oxidation was 40% higher during HT than during CT (P less than 0.01). The rate of glucose infusion required to maintain hyperglycemia (10 mM) remained very stable at 1.6 +/- 0.1 g/min during the 1st h. However, during the 2nd h of exercise, the rate of glucose infusion increased (P less than 0.01) to 2.6 +/- 0.1 g/min (37 mg.kg body wt-1.min-1) during the final 20 min of exercise. We conclude that hyperglycemia (i.e., 10 mM) in humans does not alter muscle glycogen use during 2 h of intense cycling.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood Glucose↗

PubMed1991

Exercise metabolism at different time intervals after a meal.

To determine how long a meal will affect the metabolic response to exercise, nine endurance-trained and nine untrained subjects cycled for 30 min at 70% of peak O2 consumption (VO2 peak) 2, 4, 6, 8, and 12 h after eating 2 g carbohydrate/kg body wt. In addition, each subject completed 30 min of cycling 4 h after the meal at an intensity that elicited a respiratory exchange ratio (RER) of 0.94-0.95. During exercise after 2 and 4 h of fasting, carbohydrate oxidation was elevated 13-15% compared with the response to exercise after an 8- and 12-h fast (P less than 0.01). The increase in blood glycerol concentration during exercise (30 to 0 min) was linearly related to the length of fasting (r = 0.99; P less than 0.01). In all subjects, plasma glucose concentration declined 17-21% during exercise after 2 h of fasting (P less than 0.01). Plasma glucose concentration also declined (15-25%) during exercise in the trained subjects after 4 and 6 h of fasting (P less than 0.05) but did not change in the untrained subjects. However, the decline in plasma glucose concentration was similar (14%) in the two groups when the exercise intensity was increased in the trained subjects (i.e., 78 +/- 1% VO2 peak) and decreased in the untrained subjects (i.e., 65 +/- 3% VO2 peak) to elicit a similar RER.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood Glucose↗

PubMed1991

Fluid replacement and glucose infusion during exercise prevent cardiovascular drift.

This study examined the influence of both hydration and blood glucose concentration on cardiovascular drift during exercise. We first determined if the prevention of dehydration during exercise by full fluid replacement prevents the decline in stroke volume (SV) and cardiac output (CO) during prolonged exercise. On two occasions, 10 endurance-trained subjects cycled an ergometer in a 22 degrees C room for 2 h, beginning at 70 +/- 1% maximal O2 uptake (VO2max) and in a euhydrated state. During one trial, no fluid (NF) replacement was provided and the subject's body weight declined 2.09 +/- 0.19 kg or 2.9%. During the fluid replacement trial (FR), water was ingested at a rate that prevented body weight from declining after 2 h of exercise (i.e., 2.34 +/- 0.17 1/2 h). SV declined 15% and CO declined 7% during the 20- to 120-min period of the NF trial while heart rate (HR) increased 10% and O2 uptake (VO2) increased 6% (all P less than 0.05). In contrast, SV was maintained during the 20- to 120-min period of FR while HR increased 5% and thus CO actually increased 7% (all P less than 0.05). Rectal temperature, SV, and HR were similar during the 1st h of exercise during NF and FR. However, after 2 h of exercise, rectal temperature was 0.6 degree C higher (P less than 0.05) and SV and CO were 11-16% lower (P less than 0.05) during NF compared with FR.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1991

Carbohydrate ingestion during prolonged exercise: effects on metabolism and performance.

It is well recognized that energy from CHO oxidation is required to perform prolonged strenuous (greater than 60% VO2 max) exercise. During the past 25 years, the concept has developed that muscle glycogen is the predominant source of CHO energy for strenuous exercise; as a result, the potential energy contribution of blood glucose has been somewhat overlooked. Although during the first hour of exercise at 70-75% VO2max, most of the CHO energy is derived from muscle glycogen, it is clear that the contribution of muscle glycogen decreases over time as muscle glycogen stores become depleted, and that blood glucose uptake and oxidation increase progressively to maintain CHO oxidation (Fig. 1.7). We theorize that over the course of several hours of strenuous exercise (i.e., 3-4 h), blood glucose and muscle glycogen contribute equal amounts of CHO energy, making blood glucose at least as important as muscle glycogen as a CHO source. During the latter stages of exercise, blood glucose can potentially provide all of the CHO energy needed to support exercise at 70-75% VO2max if blood glucose availability is maintained. During prolonged exercise in the fasted state, however, blood glucose concentration often decreases owing to depletion of liver glycogen stores. This relative hypoglycemia, although only occasionally severe enough to result in fatigue from neuroglucopenia, causes fatigue by limiting blood glucose (and therefore total CHO) oxidation. The primary purpose of CHO ingestion during continuous strenuous exercise is to maintain blood glucose concentration and thus CHO oxidation and exercise tolerance during the latter stages of prolonged exercise. CHO feeding throughout continuous exercise does not alter muscle glycogen use. It appears that blood glucose must be supplemented at a rate of approximately 1 g/min late in exercise. Feeding sufficient amounts of CHO 30 minutes before fatigue is as effective as ingesting CHO throughout exercise in maintaining blood glucose availability and CHO oxidation late in exercise. Most persons should not wait, however, until they are fatigued before ingesting CHO, because it appears that glucose entry into the blood does not occur rapidly enough at this time. It also may be advantageous to ingest CHO throughout intermittent or low-intensity exercise rather than toward the end of exercise because of the potential for glycogen synthesis in resting muscle fibers. Finally, CHO ingestion during prolonged strenuous exercise delays by approximately 45 minutes but does not prevent fatigue, suggesting that factors other than CHO availability eventually cause fatigue.

Bicycling↗

PubMed1991

Physiological and biomechanical factors associated with elite endurance cycling performance.

In this study we evaluated the physiological and biomechanical responses of "elite-national class" (i.e., group 1; N = 9) and "good-state class" (i.e., group 2; N = 6) cyclists while they simulated a 40 km time-trial in the laboratory by cycling on an ergometer for 1 h at their highest power output. Actual road racing 40 km time-trial performance was highly correlated with average absolute power during the 1 h laboratory performance test (r = -0.88; P less than 0.001). In turn, 1 h power output was related to each cyclists' VO2 at the blood lactate threshold (r = 0.93; P less than 0.001). Group 1 was not different from group 2 regarding VO2max (approximately 70 ml.kg-1.min-1 and 5.01 l.min-1) or lean body weight. However, group 1 bicycled 40 km on the road 10% faster than group 2 (P less than 0.05; 54 vs 60 min). Additionally, group 1 was able to generate 11% more power during the 1 h performance test than group 2 (P less than 0.05), and they averaged 90 +/- 1% VO2max compared with 86 +/- 2% VO2max in group 2 (P = 0.06). The higher performance power output of group 1 was produced primarily by generating higher peak torques about the center of the crank by applying larger vertical forces to the crank arm during the cycling downstroke. Compared with group 2, group 1 also produced higher peak torques and vertical forces during the downstroke even when cycling at the same absolute work rate as group 2. Factors possibly contributing to the ability of group 1 to produce higher "downstroke power" are a greater percentage of Type I muscle fibers (P less than 0.05) and a 23% greater (P less than 0.05) muscle capillary density compared with group 2. We have also observed a strong relationship between years of endurance training and percent Type I muscle fibers (r = 0.75; P less than 0.001). It appears that "elite-national class" cyclists have the ability to generate higher "downstroke power", possibly as a result of muscular adaptations stimulated by more years of endurance training.

Adolescent↗

PubMed1990

Maximal oxygen uptake relative to plasma volume expansion.

Controversy exists as to whether plasma volume (PV) expansion has the potential to increase maximal oxygen uptake (VO2max). In the present study, VO2max and exercise time to fatigue were measured in nine untrained men when plasma volume (PV) was normal and then again on the next day following two levels of PV expansion. Resting PV was expanded (via intravenous infusion of a 6% dextran solution) by 282 +/- 16 ml (i.e., PVX-1) and then by 624 +/- 26 ml (i.e., PVX-2). PVX-1 increased stroke volume (CO2 rebreathing) during submaximal exercise by 15% (P less than 0.05) above normal levels. VO2max following PVX-1 was increased 4% (P less than 0.05; 3.78 to 3.92 l/min) despite a 4% reduction in hemoglobin concentration. Exercise time to fatigue was also increased (P less than 0.05). PVX-2 resulted in an 11% (P less than 0.05) reduction in hemoglobin concentration during maximal exercise and a return of VO2max and exercise time to normal levels. In summary, we have observed in untrained men that 200-300 ml of PV expansion increases SV, measured during submaximal exercise, yet causes only a small amount of hemodilution. As a result, VO2max is increased slightly and performance is improved. Further PV expansion to levels 500-600 ml above normal results in an excessive hemodilution and a subsequent decline in VO2max and performance to normal levels. There is an optimal PV for eliciting VO2max in untrained men which appears to be approximately 200-300 ml above their normal levels.

Adult↗