Effect of diet on human muscle weakness following prolonged exercise.

The effect of altering muscle glycogen on the ability of skeletal muscle to generate voluntary and electrically evoked isometric force following prolonged exercise has been investigated in five healthy male subjects. Measurements from the triceps surae were made at rest, and before and after prolonged exercise (uphill walking) at approximately 75% VO2 max in low muscle glycogen (low CHO) and high muscle glycogen (high CHO) conditions. The results showed that before exercise there was no change in maximal twitch tension (Pto), maximal tetanic tension at frequencies of 10 (Po10), 20 (Po20) and 50 HZ (Po50), and maximal voluntary contraction (MVC) in low and high CHO compared with normal. The loss of force during a 2 min electrically evoked "fatigue test" at rest was found to be higher (p less than 0.05) in low CHO and lower (p less than 0.05) in high CHO than normal. Following the prolonged exercise, muscle weakness was produced in both low and high CHO conditions, but was found to be significantly greater in the low CHO condition for the measurements of Po10 (P less than 0.01), Po20 (p less than 0.05) and MVC (p less than 0.05). It is concluded that changes in muscle glycogen alone do not alter the isometric force generating capacity of human muscle, but when combined with prolonged exercise low muscle glycogen enhances exercise-induced muscle weakness.