Early muscular and metabolic adaptations to prolonged exercise training in humans.

A short-term training program involving 2 h of daily exercise at 59% of peak O2 uptake (VO2max) repeated for 10-12 consecutive days was employed to determine the significance of adaptations in energy metabolic potential on alterations in energy metabolism and substrate utilization in working muscle. The initial VO2max determined before training on the eight male subjects was 53.0 +/- 2.0 (SE) ml.kg-1.min-1. Analysis of samples obtained by needle biopsy from the vastus lateralis muscle before exercise (0 min) and at 15, 60, and 99 min of exercise indicated that on the average training resulted (P less than 0.05) in a 6.5% higher concentration of creatine phosphate, a 9.9% lower concentration of creatine, and a 39% lower concentration of lactate. Training had no effect on ATP concentration. These adaptations were also accompanied by a reduction in the utilization in glycogen such that by the end of exercise glycogen concentration was 47.1% higher in the trained muscle. Analysis of the maximal activities of representative enzymes of different metabolic pathways and segments indicated no change in potential in the citric acid cycle (succinate dehydrogenase, citrate synthase), beta-oxidation (3-hydroxyacyl CoA dehydrogenase), glucose phosphorylation (hexokinase), or potential for glycogenolysis (phosphorylase) and glycolysis (pyruvate kinase, phosphofructokinase, alpha-glycerophosphate dehydrogenase, lactate dehydrogenase). With the exception of increases in the capillary-to-fiber area ratio in type IIa fibers, no change was found in any fiber type (types I, IIa, and IIb) for area, number of capillaries, capillary-to-fiber area ratio, or oxidative potential with training.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscle energetics during prolonged cycling after exercise hypervolemia.

This study examined the question of whether increases in plasma volume (hypervolemia) induced through exercise affect muscle substrate utilization and muscle bioenergetics during prolonged heavy effort. Six untrained males (19-24 yr) were studied before and after 3 consecutive days of cycling (2 h/day at 65% of peak O2 consumption) performed in a cool environment (22-23 degrees C, 25-35% relative humidity). This protocol resulted in a 21.2% increase in plasma volume (P less than 0.05). During exercise no difference was found in the blood concentrations of glucose, lactate, and plasma free fatty acids at either 30, 60, 90, or 120 min of exercise before and after the hypervolemia. In contrast, blood alanine was higher (P less than 0.05) during both rest and exercise with hypervolemia. Measurement of muscle samples extracted by biopsy from the vastus lateralis muscle at rest and at 60 and 120 min of exercise indicated no effect of training on high-energy phosphate metabolism (ATP, ADP, creatine phosphate, creatine) or on selected glycolytic intermediate concentrations (glucose 1-phosphate, glucose 6-phosphate, fructose 6-phosphate, lactate). In contrast, training resulted in higher (P less than 0.05) muscle glucose and muscle glycogen concentrations. These changes were accompanied by blunting of the exercise-induced increase (P less than 0.05) in both blood epinephrine and norepinephrine concentrations. Plasma glucagon and serum insulin were not affected by the training. The results indicate that exercise-induced hypervolemia did not alter muscle energy homeostasis. The reduction in muscle glycogen utilization appears to be an early adaptive response to training mediated either by an increase in blood glucose utilization or a decrease in anaerobic glycolysis.

Training-induced hypervolemia: lack of an effect on oxygen utilization during exercise.

To investigate the effect of training-induced increases in plasma volume on maximal aerobic power, 8 male subjects (age 19 to 24 yr) underwent a 4-d training program (2 h X d-1) at an estimated 71% maximal aerobic power. Following training, plasma volume measured using 131I-human serum albumin increased by 20.3% (P less than 0.01) whereas red cell volume remained unchanged and total blood volume increased by 12.3% (P less than 0.01). During progressive sub-maximal cycle exercise, oxygen consumption, carbon dioxide production, ventilation, and blood lactate concentration remained unchanged following the training whereas heart rate was significantly elevated (P less than 0.05). Significant post-training elevations were also noted in carbon dioxide production (P less than 0.05), blood lactate (P less than 0.01), and peak power output (P less than 0.05) during maximal exercise. Maximal aerobic power and ventilation were not altered. It is concluded that hypervolemia induced by short-term exercise training does not affect oxygen consumption either during sub-maximal or maximal exercise.

Metabolic effects of two frequencies of short-term surface electrical stimulation on human muscle.

The acute effects of two different frequencies of electrical stimulation on the metabolism of the vastus lateralis muscle were studied in young male and female adults. The quadriceps muscle group of one leg was stimulated for 60 min using surface electrodes that delivered square pulses of 0.6 ms duration, either continuously at 10 Hz (n = 5) or intermittently (n = 5) at 50 Hz (12 s stimulation, 48 s recovery). Biochemical analyses revealed no significant differences in glycogen or metabolite concentrations between the two conditions. Muscle lactate and citrate concentrations were increased (p less than 0.05) for both conditions, but ATP and CP concentrations were not significantly changed from rest values after stimulation. Glycogen concentrations decreased (p less than 0.05) by 24.6 and 29.1 mmol glucose units/kg after 60 min of 50 Hz and 10 Hz stimulation, respectively. Muscle fibres were identified as slow twitch (ST) and fast twitch a and b (FTa and FTb) on the basis of myofibrillar ATPase activity. Estimates of glycogen depletion in different fibre types using histochemical techniques revealed that FTa and FTb fibres had lower glycogen contents than ST fibres after 10 Hz stimulation whereas glycogen was moderately reduced in approximately 50% of all fibre types following 50 Hz stimulation. The modest changes observed in muscle metabolism following 60 min of stimulation were less than has been noted following traditional exercise, and suggest that only some of the muscle fibres were activated in the stimulated muscles at the depth where biopsy samples were removed.