Metabolic and performance responses to constant-load vs. variable-intensity exercise in trained cyclists

We studied glucose oxidation (Glu(ox)) and glycogen degradation during 140 min of constant-load [steady-state (SS)] and variable-intensity (VI) cyclin g of the same average power output, immediately followed by a 20-km perform ance ride [time trial (TT)]. Six trained cyclists each performed four trial s: two experimental bouts (SS and VI) in which muscle biopsies were taken b efore and after 140 min of exercise for determination of glycogen and perio dic acid-Schiff's staining; and two similar trials without biopsies but inc orporating the TT. During two of the experimental rides, subjects ingested a 5 g/100 ml [U-C-14]glucose solution to determine rates of Glu(ox). Values were similar between SS and VI trials: O-2 consumption (3.08 +/- 0.02 vs. 3.15 +/- 0.03 l/min), energy expenditure (901 +/- 40 vs. 904 +/- 58 J.kg(-1 ).min(-1)), heart rate (156 +/- 1 vs. 160 +/- 1 beats/min), and rating of p erceived exertion (12.6 +/- 0.6 vs. 12.7 +/- 0.7). However, the area under the curve for plasma lactate concentration vs. time was significantly great er during VI than SS (29.1 +/- 3.9 vs. 24.6 +/- 3.7 mM/140 min; P = 0.03). VI resulted in a 49% reduction in total muscle glycogen utilization vs. 65% for SS, while total Glu(ox) was higher (99.2 +/- 5.3 vs. 83.9 +/- 5.2 g/14 0 min; P < 0.05). The number of glycogen-depleted type I muscle fibers at t he end of 140 min was 98% after SS but only 59% after VI. Conversely, the n umber of type II fibers that showed reduced periodic acid-Schiff's staining was 1% after SS vs. 10% after VI. Despite these metabolic differences, sub sequent TT performance was similar (29.14 +/- 0.9 vs. 30.5 +/- 0.9 min for SS vs. VI). These results indicate that whole body metabolic and cardiovasc ular responses to 140 min of either SS or VI exercise at the same average i ntensity are similar, despite differences in skeletal muscle carbohydrate m etabolism and recruitment.