Training overload : carbohydrate balance and muscular fatigue

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Kirwan, John P.
Costill, David L.
Issue Date
Thesis (Ph. D.)
Human Performance Laboratory
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This study was designed to investigate the relationship between dietary carbohydrate consumption and muscle glycogen storage during five days of intense training. Ten highly trained distance runners performed two diet-training regimens. Training consisted of running 1.5 times each runner's average daily training distance (~20 km) at ~80% Vo2 max. During one of these intense training periods the runners ate slightly more carbohydrate (8.0 g•kg•d-1) than was required to meet the energy requirements of exercise and normal activity (EQ-CHO). During the second regimen (LO-CHO) the runners ate approximately half (3.9 g•kg•d-1) as much carbohydrate as in the previous regimen. Each regimen was preceded by a three day control period during which carbohydrate intake was maintained at 6.2 g•kg•d-1 and training was reduced to 80% of the runners normal training distance. At the end of each regimen the runners rested for three days and carbohydrate consumption was maintained at 3.8 g•kg•d-1.Compared to the EQ-CHO regimen oxygen consumption measured during standard exercise tests performed at 65% (SET 80) and 80% (SET80) of Vo2 max was greater during the LO-CHO regimen. Corresponding respiratory exchange ratios were lower during these tests. Overall (12.9 + 0.4 vs 13.7+ 0.5 units) and leg (13.3 + 0.3 vs 14.4 + 0.5 units) ratings of perceived exertion were higher during the SET80 at the end of the LO-CHO regimen. Total muscle glycogen levels were lower following the LO-CHO regimen (90.6 + 8.8 vs 66.4 + 7.8 mmol•kg-1 w.w. for the EQ-CHO vs LO-CHO regimens). A linear relationship was observed between histochemical and direct chemical analysis of muscle glycogen content (r=0.93). Resting muscle glycogen content was the same in type I, IIA and IIB fibers before the intense training period of both regimens. The glycogen content of type I, IIA and IIB fibers was lower after the LO-CHO regimen. Frequency distribution analysis of the glycogen content in individual fibers revealed that ~27% of type I fibers, 17% of type IIA fibers and 0% type IIB fibers had optical densities below 0.2 units (54 g•kg•d-1) following the intense training period of the LO-CHO regimen. Glycogen repletion during the three days of rest was greater after the LO-CHO regimen (22.2 vs 56.5 mmol•kg-1 w.w. for the EQ-CHO vs LO-CHO regimens).These data indicate that inadequate dietary carbohydrate consumption during successive days of intense training leads to incomplete glycogen repletion and selective glycogen depletion from type I muscle fibers. These events produce a greater perception of effort and decreased running economy during exercise.