The Effect of Caffeine on Peak Torque, Muscle Fatigue and Prefrontal Cortex Blood Flow

Abstract

PURPOSE: The purpose of this study was to assess the effect of caffeine consumption on prefrontal cortex (PFC) hemodynamics and muscular fatigue during maximal isokinetic exercise testing. METHODS: Six active (exercise >3x/week, >1 hour per bout for >3 months) individuals (age 20.6+2.3yrs, body weight 72.1 +7.3kg, height 172.6 + 14.5cm) participated in a double-blind placebo controlled study. Participants attended three separate exercise sessions on the Humac Norm Isokinetic Dynamometer. Each exercise session included three sets of 30 maximal knee extensions at 180° per second using their dominant leg. Exercise sessions were separated by one week and participants were instructed to refrain from consumption of caffeine 36 hours prior to testing. One hour prior to testing, participants were administered a placebo (NC), low caffeine (LC) (3mg/kg body weight) or high caffeine (HC) (6mg/ kg body weight) dose. Time-resolved near-infrared spectroscopy monitoring (TRS0-21, Hamamatsu) was utilized throughout testing to measure hemodynamics in the PFC. Peak torque and fatigue index were analyzed. RESULTS: The fatigue index for HC (42.78%) and LC (42.82%) was lower when compared to the NC condition (47.39%; p<0.05). Peak torque per exercise session was found to increase during LC (124.17 N*m) compared to both NC (120.00 N*m) and HC (116.75 N*m). Additionally, LC showed a significant increase in total hemoglobin levels (141.21uM) compared to NC (101.05uM) and HC (97.618uM) in the PFC. CONCLUSION: The results of the study indicate that both high and low doses of caffeine were found to reduce fatigue across the bout of fatiguing exercise, but only LC resulted in increased peak torque and total hemoglobin to the PFC. Further examination of both PFC and local muscle hemodynamics should be explored to further understand the differential response between LC and HC.