title: Effect of caffeine ingestion on muscular strength and endurance: a meta-analysis.
abstract: OBJECTIVE
  Our objective was to perform a systematic review and meta-analysis of the research literature assessing the effect of caffeine ingestion on maximal voluntary contraction (MVC) strength and muscular endurance.
  
  
  METHODS
  Thirty-four relevant studies between 1939 and 2008 were included in the meta-analyses of caffeine's effects on MVC strength (n = 27 studies) and muscular endurance (n = 23 studies). Effect sizes (ES) were calculated as the standardized mean difference and meta-analyses were completed using a random-effects model.
  
  
  RESULTS
  Overall, caffeine ingestion was found to result in a small beneficial effect on MVC strength (overall ES = 0.19, P = 0.0003). However, caffeine appears to improve MVC strength primarily in the knee extensors (i.e., by approximately 7%, ES = 0.37) and not in other muscle groups such as the forearm or the knee flexors. In an attempt to offer a physiological mechanism behind caffeine's ability to improve MVC strength, a meta-analysis was run on ES from nine studies that measured percent muscle activation during MVC in trials comparing caffeine versus placebo; the overall ES (0.67) was highly significant (P = 0.00008) and of moderate to large size, thus implicating an effect of caffeine on the CNS. Caffeine ingestion was also found to exert a small beneficial effect on muscular endurance (overall ES = 0.28, P = 0.00005). However, it appears caffeine improves muscular endurance only when it is assessed using open (i.e., by approximately 18%, ES = 0.37) and not fixed end point tests.
  
  
  CONCLUSIONS
  Overall, caffeine ingestion improves MVC strength and muscular endurance. The effect on strength appears exclusively in the knee extensors, and the effect on muscular endurance appears only detectable with open end point tests.
authors.affiliation: Division of Physical Therapy, Georgia State University, Atlanta, GA 30302-4019, USA. gwarren@gsu.edu
authors.forename: Gordon L
authors.initials: GL
authors.lastname: Warren
authors.forename: Nicole D
authors.initials: ND
authors.lastname: Park
authors.forename: Robert D
authors.initials: RD
authors.lastname: Maresca
authors.forename: Kimberly I
authors.initials: KI
authors.lastname: McKibans
authors.forename: Melinda L
authors.initials: ML
authors.lastname: Millard-Stafford
mesh_terms: D002110:Caffeine / Q000494:pharmacology*; D000697:Central Nervous System Stimulants / Q000494:pharmacology*; D006801:Humans; D035002:Lower Extremity / Q000502:physiology; D053580:Muscle Strength / Q000187:drug effects*; D018482:Muscle, Skeletal / Q000187:drug effects*; D010807:Physical Endurance / Q000187:drug effects*; D034941:Upper Extremity / Q000502:physiology
chemical_list: D000697:Central Nervous System Stimulants; D002110:Caffeine