CHANGES IN PLASMA ELECTROLYTES AND MUSCLE SUBSTRATES DURING SHORT-TERM MAXIMAL EXERCISE IN HUMANS

This study compared the effects of 3 short-term maximal exercise tests lasting 10, 30, and 90 sec upon blood volume, plasma electrolytes, gl ucose, glycerol, lactate and skeletal muscle ATP, PC, glycogen, and la ctate concentrations. Seven sedentary male subjects were recruited and 5 of them were randomly assigned to each of the 3 protocols. The test were performed on a modified ergocycle at workloads of 0.9, 0.075, an d 0.05 kp . kg(-1) body mass, respectively. Muscle biopsies were taken from the vastus lateralis before and immediately after each exercise. Venous blood samples were collected before, immediately after, and du ring the recovery (5, 20, 60, and 120 min). Plasma volume decreased du ring the 30- and 90-sec tests and was increased in all tests after 60 min of recovery. Plasma K+ increased during all tests and returned to normal values 5 min postexercise, except after the 90-sec test where i t fell below resting values. Plasma Na+ and Cl- were unaffected. Blood lactate increased in all tests, glucose increased after the 90-sec te st, and glycerol increased after the 30- and 90-sec tests. All 3 tests brought relatively similar changes in muscle ATP, PC, and glycogen wh ile muscle lactate changes were related to exercise duration. These re sults suggest that a normal hydration status is important for a subjec t undergoing short training bouts, and that high-intensity tests of sh ort duration do not require a special nutritional regimen to enhance g lycogen reserves.