Effects of caffeine on session ratings of perceived exertion

Journal Title (Medline/Pubmed accepted abbreviation):Eur. J. Appl. Physiol.
Year: 2013
Volume: 113
Page numbers: 721-727
doi: 10.1007/s00421-012-2480-z

Summary of background and research design:
Background: Caffeine is a popular stimulant among athletes and non-athletes alike. It has been shown to enhance mental capacities (ex. focus) and physical performance in both aerobic and anaerobic exercises. It has also been shown to reduce ratings of perceived exertion, meaning that it makes exercise feel easier.

Hypothesis: A pre-workout dose of 6 mg caffeine/kg body weight will decrease the ratings of perceived exertion during a 30 min, fixed-load cycling bout.

Experimental Design: randomized, double-blind, placebo-controlled, cross-over

Subjects: A total of 15 participants, 10 males and 5 females, age 24.5 ± 4.6 y. Participants were not selected based on typical caffeine consumption.

Caffeine: 6 mg/kg body weight (510 mg for a 187 lb. person, this is equal to about 5 cups of moderately strong coffee) fed in capsule form
Placebo: maltodextrin, delivered in an equivalent capsule

Protocol: The participants were first evaluated for height, weight, and body composition using skin fold measurements. They were also evaluated for maximum oxygen capacity (VO2max) on a stationary bicycle. They then reported to the laboratory two more times to complete the exercise protocol with one of each of the treatments. They consumed the treatment one hour before reporting to the laboratory. After a 5 min warm up on the bicycle, they cycled for 30 min at 60 revolutions per minute and a workload of 75% of their previously determined VO2max. Heart rate was monitored and ratings of perceived exertion were inquired about every 5 min. They rested for 20 min after the cycling bout and then completed a questionnaire regarding symptoms that were experienced during the sessions (ex. fatigue, nervousness, stomach distress).

Summary of research findings:
  • Ratings of perceived exertion were significantly lower with caffeine compared to the placebo, suggesting that the workout felt easier (p < 0.05). This is true for the session overall and for individual time points during the session at 15, 20, 25, and 30 min.
  • Specifically, 53% of the participants exhibited at least a 1-point decrease in their rating of perceived exertion for the session (the total scale ranges from 6-20). These participants were labeled "responders" to caffeine. Interestingly, the authors did not comment on these participants? typical caffeine consumption, which may have provided insight into how much tolerance some of these people have to caffeine.
  • Similarly, ratings of perceived exertion for legs and breathing were also lower during the last half of the cycling bout.
  • Heart rate was lower at the 5 and 10 min time points during the trial with caffeine. At 15, 20, 25, and 30 min, there were no statistical differences in heart rate between trials.
  • The participants felt more fatigued during the placebo trial compared to the caffeine trial.
  • With caffeine, the participants felt more nervous and restless and felt more stomach distress and tremors.
  • There were no differences in mood between the two trials.

Key practice applications: Caffeine consumption made the exercise session feel easier to complete. However, the participants felt negative side effects including nervousness and gastrointestinal distress. This is important because ratings of perceived exertion are used to help identify athletes that are overtraining. Caffeine usage should be taken into account when interpreting athletes? feelings about an exercise session, as their actual workload may be underestimated.
    The amount of caffeine consumed here was fairly large (about 500 mg) whereas 100-200 mg is a more typical dose of caffeine in commercially available pre-workout supplements.

Limitations: People can quickly build up a tolerance to caffeine. The authors did not report the current caffeine consumption of their participants or ask their participants if they were experiencing any withdraw symptoms when they had the placebo trial.
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