Effect of carbohydrate beverage ingestion on central versus peripheral fatigue: a placebo-controlled, randomized trial in cyclists.

We investigated whether carbohydrate ingestion delays fatigue in endurance-trained cyclists via peripheral or central mechanisms. Ten men (35 ± 9 years of age) and 10 women (42 ± 7 years of age) were assigned, in a double-blind, crossover design, to a sports drink (CHO) or to a placebo (PL). The following strength measures were made 3 times (before exercise, after a time trial (TT), and after a ride to exhaustion): (i) maximal voluntary contraction (MVC); (ii) MVC with superimposed femoral nerve magnetic stimulation to measure central activation ratio (CAR); and (iii) femoral nerve stimulation in a 3-s pulse train on relaxed muscle. The subjects cycled for 2 h at approximately 65% of peak oxygen consumption, with five 1-min sprints interspersed, followed by a 3-km TT. After strength testing, the cyclists remounted their bikes, performed a brief warm-up, and pedaled at approximately 85% peak oxygen consumption until unable to maintain workload. Changes in metabolic and strength measurements were analyzed with repeated-measures ANOVA. From before exercise to after the TT, MVC declined in men (17%) and women (18%) (p = 0.004), with no effect of beverage (p > 0.193); CAR decreased in both sexes with PL (p = 0.009), and the decline was attenuated by CHO in men only (time × treatment, p = 0.022); and there was no evidence of peripheral fatigue in either sex with either beverage (p > 0.122). Men rode faster in the TT with CHO (p = 0.005) but did not improve performance in the ride to exhaustion (p = 0.080). In women, CHO did not improve performance in the TT (p = 0.173) or in the ride to exhaustion (p = 0.930). We concluded that carbohydrate ingestion preserved central activation and performance in men, but not in women, during long-duration cycling.