Caffeine improves endurance exercise performance, but its ergogenic mechanism(s) remain unclear.
This investigation sought to examine the effects of caffeine on perceptual and physiological responses to endurance exercise.
Two experiments were performed. In study A, 14 participants were tested. Maximal voluntary strength (MVC) and motor-unit recruitment (%ACT) of the knee extensors and elbow flexors were tested before and 60 min after ingestion of a 5-mg·kg⁻¹ dose of caffeine or placebo and after completion of 40 min of exercise (30 min of submaximal leg or arm cycling followed by a 10-min time-trial performance). Muscle pain, RPE, and cardiorespiratory variables were assessed throughout. To determine the effects of caffeine on muscle pain and RPE during high-intensity exercise, a second study (study B) was performed. Twelve participants exercised at 95% of their gas exchange threshold (GET) and at 70% of the difference between their GET and VO(2peak) (70%Δ) after caffeine and placebo ingestion.
Compared to placebo, caffeine improved MVC (6.3%, P = 0.014) and %ACT (5.5%, P = 0.013) in the knee extensors, but not the elbow flexors, and reduced muscle pain (P < 0.05) and RPE (P < 0.05) during both submaximal cycling modalities. Caffeine ingestion improved time-trial performance during leg cycling (4.9% ± 6.5%, P = 0.03), but not arm crank cycling (2.1% ± 8.2%, P = 0.28), but the effect on pain and RPE was eliminated. Caffeine ingestion had no effect on pain or RPE during cycling at 95% GET and 70%Δ.
Our results suggest that augmented strength and motor-unit recruitment, rather than reductions in pain and effort, may underlie caffeine's ergogenic effect on endurance exercise.