Caffeine effects on short-term performance during prolonged exercise in the heat

To determine the effect of water, carbohydrate, and caffeine ingestion on fatigue during prolonged exercise in the heat.

Seven endurance-trained cyclists (V O2max = 61 +/- 8 mL.kg.min) pedaled for 120 min at 63% V O2max in a hot-dry environment (36 degrees C; 29% humidity), ingesting either no fluid (NF), water (WAT) to replace 97% fluid losses, the same volume of a 6% carbohydrate-electrolyte solution (CES), or each of these treatments along with ingestion of 6 mg of caffeine per kilogram of body weight (NF + CAFF, WAT + CAFF, and CES + CAFF). At regular intervals during exercise, maximal cycling power (PMAX) was measured. Before and after exercise, maximal voluntary contraction (MVC), voluntary activation (VA), and electrically evoked contractile properties of the quadriceps were determined.

Without fluid replacement (NF and NF + CAFF), subjects were dehydrated by 3.8 +/- 0.3%, and rectal temperature reached 39.4 +/- 0.3 degrees C, while it was maintained at 38.7 +/- 0.3 degrees C in trials with rehydration (P < 0.05). Trials with caffeine ingestion increased PMAX by 3% above trials without caffeine (P < 0.05). MVC reductions after exercise were larger with NF (-11 +/- 5%) than for the rest of the trials (P < 0.05). MVC was reduced in WAT compared with CES + CAFF (-6 +/- 4 vs 2 +/- 4%; P < 0.05). However, NF + CAFF maintained MVC at the level of the CES trial. VA showed the same treatment response pattern as MVC. There were no differences in electrically evoked contractile properties among trials.

During prolonged exercise in the heat, caffeine ingestion (6 mg.kg body weight) maintains MVC and increases PMAX despite dehydration and hyperthermia. When combined with water and carbohydrate, caffeine ingestion increases maximal leg force by increasing VA (i.e., reducing central fatigue).