Performance and muscle metabolite changes in exercise with repeated maximal dynamic contractions.

Maximal voluntary contractions (MVC) were performed by five male subjects as repeated knee extensions separated by 13-s rest periods for the purposes of studying performance capability, physiologic responses, and recruitment patterns among fast-twitch (FT) and slow-twitch (ST) extrafusal muscle fibers for this type of exercise. Exercise was performed on an isokinetic ergometer (Cybex II) as 4 MVC per min for 1 h, each contraction lasting 2 s and resulting in knee extension at a velocity of 18 degrees x s-1 from 90 forward to 57. Biopsies were taken from the m. vastus lateralis before, at the midpoint, and at the end of the 60-min period of exercise and analyzed for ATP, CP, lactate, and glycogen. Fiber type was determined by staining sections for myofibrillar ATPase and fiber glycogen content by PAS stain. Ability to produce force (torque) declined form an average of 250 Nm during the first minute to 200 Nm during the last minute of exercise. Oxygen uptake and heart rate were relatively constant for each subject and averaged 0.94 l x min-1 and 110 beat x min-1 throughout exercise. Muscle values in mmol x kg-1 at rest, at midpoint, and at end of exercise were for ATP 5.3, 5.4, and 4.1; for CP 19.4, 15.7, and 12.1; and for lactate 1.7, 3.9, and 4.5, respectively. It was estimated that approximately one third of the energy requirement for exercise was obtained from the endogenous muscle glycogen. Glycogen depletion patterns indicated the involvement of both FT and ST fibers in this type of exercise.