The purpose of this investigation was to determine the effect of caffe
ine ingestion on work output at various levels of perceived exertion d
uring 30 min of isokinetic variable-resistance cycling exercise. Ten s
ubjects completed six trials 1 hr after consuming either 6 mg . kg(-1)
caffeine (3 trials) or a placebo (3 trials). During each trial the su
bjects cycled at what they perceived to be a rating of 9 on the Borg r
ating of perceived exertion scale for the first 10 min, a rating of 12
for the next 10 min, and a rating of 15 for the final 10 min. Total w
ork performed during the caffeine trials averaged 277.8 +/-26.1 kJ, wh
ereas the mean total work during the placebo trials was 246.7 +/- 21.5
kJ (p<.05). Blood glycerol and free fatty acid levels increased over
time to a significantly greater degree in the caffeine trials than in
the placebo trials (p<.05). However, there were no significant differe
nces between conditions in respiratory exchange ratio. These data sugg
est that caffeine may play an ergogenic role in exercise performance b
y altering both neural perception of effort and substrate availability
.