Effects of Pre-exercise Muscle Glycogen Status on Muscle Phosphagens, Sarcoplasmic Reticulum Function, and Performance During Intermittent High Intensity Exercise

Eight competitive cyclists performed two cycling trials, one following a high carbohydrate diet (H-CHO) and the other following a low carbohydrate diet (L-CHO). Trials consisted of repeated 60s maximal effort sprints to fatigue at a workload designed to elicit 125-135% VO_2peak at 90rpm. Three min of recovery separated sprints. Muscle biopsies taken at rest (biopsy 1), 85% max interval rpm (biopsy 2), and 70% max interval rpm (biopsy 3) revealed a main effect of diet on muscle glycogen levels: 609 ± 38 HCHO vs. 390 ± 42 mmol/kgdw L-CHO at biopsy 1, 383 ± 29 vs. 252 ± 28 mmol/kgdw at biopsy 2, and 346 ± 29 vs. 196 ± 18 mmol/kgdw at biopsy 3 (p<0.01). Similar decreases in muscle glycogen (45%), creatine phosphate (CP) (35%), and sarcoplasmic reticulum (SR) Ca²⁺-uptake (56%) were shown in both trials from biopsy 1 to 3. SR Ca²⁺-release decreased by 53% in H-CHO subjects and 36% in L-CHO subjects. Total exercise time tended to be longer in H-CHO than L-CHO subjects (57.5 ± 10 vs. 42.0 ± .89min) (p=0.09). H-CHO subjects exercised significantly longer than L-CHO subjects from biopsy 2 to 3 (33.6 ± 10 vs. 18 ± 3.6min) (p< 0.05). Results suggest that fatigue from 40- 60min of intermittent 60s high intensity cycling intervals is associated with reductions in muscle glycogen, CP, and SR function, and that the latter part of performance is impaired by low muscle glycogen. These data do not support a relationship between muscle glycogen status and SR function in intermittent high intensity exercise.