Opposite actions of caffeine and creatine on muscle relaxation time in humans.
Level 2 - randomized trial
Individual randomized crossover trial in humans
PubMed 11796658 · doi:10.1152/japplphysiol.00255.2001
What was done
In a double-blind randomized crossover trial, 10 healthy male volunteers performed an electrically stimulated quadriceps exercise protocol (30 intermittent contractions: 2 seconds stimulation, 2 seconds rest). Maximal torque (Tmax), contraction time (CT, from 0.25 to 0.75 Tmax), and relaxation time (RT, from 0.75 to 0.25 Tmax) were measured before and after five conditions: creatine supplementation (Cr: 4 x 5 g/day for 4 days), short-term caffeine intake (Caf: 5 mg/kg/day for 3 days), combined creatine and caffeine (Cr+Caf), acute caffeine intake (ACaf: single dose of 5 mg/kg), and placebo.
What was found
Compared with placebo, creatine supplementation shortened muscle relaxation time by approximately 5% (P < 0.05). Conversely, 3-day caffeine intake lengthened relaxation time by approximately 10% (P < 0.05), particularly as fatigue accumulated during the protocol. Relaxation time was not significantly altered by acute caffeine alone or by combined creatine plus caffeine. Maximal torque and contraction time did not differ across experimental conditions, and initial electrically evoked torque remained at roughly 20% of voluntary maximal isometric force across all groups.
Why it matters
This study provides evidence of opposing physiological effects between creatine and multi-day caffeine supplementation on muscle contractile kinetics, showing that short-term caffeine intake can blunt creatine's capacity to accelerate muscle relaxation.
Limits
The sample size was very small (n = 10) and limited entirely to healthy young men. Muscle contractions were electrically evoked at submaximal force (~20% of voluntary maximum) rather than produced by voluntary high-intensity effort, and the study did not measure long-term athletic or functional performance outcomes.
Cited by
- partial Caffeine doses exceeding 250 mg cause an interference effect with creatine at the cellular level by opposing calcium release and uptake kinetics in the sarcoplasmic reticulum, increasing muscle relaxation time.