Abstract
In an effort to describe the changes in water and electrolyte contents of active and
inactive muscles during prolonged effort, seven healthy men were studied during 120
min of cycling. Needle biopsy samples were taken from the deltoid (inactive) and vastus
lateralis (active) muscles before, at 10 and 120 min of exercise, and 30 min after
exercise. Water and electrolyte contents of blood and active muscle tissue showed
a significant change with the onset of exercise, while the inactive muscle was unaffected.
As a consequence of the exercise, the subjects lost an average of 2.40 liters of body
water, 93 mEq Na+, 22 mEq K+, 95 mEq Cl-, and 5 mEq Mg++. These water and electrolyte losses were not detectable in the muscle tissue sampled
at the end of exercise or after 30 min recovery. Based on the loss of Cl-, body weight and plasma volume, extracellular, intracellular, and total body water
volumes were calculated to decrease approximately 9%, 3% and 7.5%, respectively, at
the end of the exercise. These observations confirm earlier findings that exercise
and electrolyte losses in sweat and urine do not alter the calculated membrane potential
of active and inactive muscle.
Key words
muscle biopsy - extracellular volume - intracellular volume - plasma
