Effect of two doses of caffeine on muscular function during isokinetic exercise
Journal Title (Medline/Pubmed accepted abbreviation): Med Sci Sports Exerc
Year: 2010
Volume: Published ahead of print
Page numbers:
doi: 10.1249/MSS.0b013e3181e3a11d

Summary of Background and Research Design

Hypothesis:The study examined the effects of 2 doses of caffeine on peak/average torque, power output, and total work of the knee extensors and flexors during 2 bouts of high-intensity exercise. The authors hypothesized that acute caffeine intake would have an effect on muscular performance versus placebo.

Subjects:15 active men participated in the study. Mean subject characteristics included age of 26.4 ± 3.9 years, height of 177.6 ± 7.1 cm, weight of 82.7 ± 11.5 kg, physical activity of 6.8 ± 2.7 hours/week, and caffeine intake of 243.3 ± 136.3 mg/day.

Experimental design: Randomized, single-blinded, counterbalanced, crossover

Treatments and protocol:Before each exercise trial, subjects abstained from caffeine intake and intense exercise for 48 hours. Over 3 separate days, each at least 48 hours apart, subjects ingested 1 of 3 beverages (5 mg/kg anhydrous caffeine [high-dose], 2 mg/kg anhydrous caffeine [low-dose], or placebo) 1 hour before each exercise trial. Testing began with a familiarization trial to determine optimal settings for the isokinetic dynamometer for each subject. Each exercise trial consisted of a 5-minute warm-up on a cycle ergometer, followed by 2 maximal bouts of 40 repetitions of knee extension/flexion of the dominant leg at a velocity equal to 180 degrees/second on an isokinetic dynamometer. Bouts were separated by 3 minutes of passive recovery. Peak and average torque (ft/lb), power (Watt), total work (ft/lb), and work fatigue (%) were recorded for both knee extension and flexion across both bouts. The Borg 0 to 10 category ratio scale was used to assess ratings of perceived exertion (RPE). During each exercise trial, subjects reported their RPE 25 repetitions into each exercise bout. A 3 (treatment) x 2 (sets) analysis of variance (ANOVA) with repeated measures was used to examine differences in muscular performance and RPE between the caffeine and placebo treatment groups. Statistical significance as accepted at P < .05.

Summary of research findings:
  • Compared with placebo, caffeine significantly (P < .05) enhanced peak knee flexion torque, knee extension/flexion total work, and knee extension/flexion power in Bout 1 with no effect in Bout 2
  • Only the 5-mg/kg caffeine dose improved performance
    • Magnitude of improvement ranged from 5% to 8%
  • There was no difference in work fatigue across treatments (P > .05)
  • RPEs were significantly increased in the 5-mg/kg caffeine, 2-mg/kg caffeine, and placebo groups from Bout 1 (5.47 ± 1.50, 5.47 ± 1.50, and 5.27 ± 1.53, respectively) to Bout 2 (6.47 ± 1.25, 6.47 ± 1.25, and 6.27 ± 1.58, respectively; P < .05)
    • However, no effect of caffeine on RPE was exhibited versus placebo (P > .05)

Interpretation of findings/Key practice applications:

The study demonstrates that relatively high (5 mg/kg) but not low (2 mg/kg) doses of caffeine are ergogenic for maximal knee extension/flexion exercise in active men. Muscular performance was improved only during Bout 1, suggesting that caffeine may not alter performance when the muscle is already fatigued. Existing data for this exercise mode are equivocal, as some studies show no benefit of caffeine intake, whereas others reveal enhanced performance with caffeine. Despite a few methodologic discrepancies between studies, these equivocal data suggest that there is inter-individual variation in subjects' performance responses to caffeine ingestion. These inter-individual differences may be mediated by discrepancies in caffeine metabolism. Consequently, scientists should examine individual data in response to caffeine ingestion, and should be aware that aggregate data revealing no effect of caffeine intake do not necessarily demonstrate that no subjects benefited from caffeine ingestion. A simple technique to identify responders would be to determine a priori a meaningful difference in performance and to use this as a criterion to identify subjects who do and do not benefit from caffeine ingestion. There were a few limitations to the present study. Results can only be generalized to the ergogenic effects of anhydrous caffeine rather than energy drinks or other caffeine-containing products, of which less is known about their effects on exercise performance. Also, the study recruited recreationally active men, so results cannot be applied to women or trained individuals.
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