EFFECT OF CAFFEINE ON FATIGUE DURING SUBMAXIMAL ISOMETRIC CONTRACTIONS AT DIFFERENT KNEE ANGLES

Introduction: In the knee extensors, the time to fatigue for intermittent isometric contractions can be increased by caffeine. Fatigue is muscle length dependent. Aim: We examined the effect of caffeine on fatigue of knee extensors at two joint angles. Methods: Ten male subjects (24±3 yr, 177±5 cm, 75±6 kg) with low caffeine intake (<200 mg·wk -1 ) volunteered. Study design was double-blind and counter-balanced. Subjects were administered caffeine (6 mg·kg -1 ) added to a non-caloric beverage or placebo, 1 hr before testing. Maximal voluntary isometric torque (MVIT) and intermittent contractions at 50%MVIT (15 s contraction, 5 s rest) were performed at knee angles of 30° and 90° (short and long length) until exhaustion. Fatigue was quantified by measurement of the MVIT 20 s post exhaustion. Surface EMG of m.vastus lateralis was analysed for root mean square (RMS). Data were analysed with 2-way ANOVA and paired t-tests with significance set at P<0.05. Results: MVIT values were similar for caffeine and placebo at both knee angles. Time to fatigue was higher at 30° for both conditions. Caffeine increased the time to fatigue, being equal by 15% (30°) and 13% (90°). The fatigue index was similar for both conditions and knee angles. Changes in RMS were similar for both conditions and knee angles [e.g. 30°: 154±14% (caffeine), 154±16% (placebo)]. Conclusions: Caffeine enhanced the time to fatigue during submaximal intermittent isometric contractions at different knee angles. However, potential mechanism(s) for the enhanced time to fatigue by caffeine do not seem to be muscle length dependent.