Caffeine ingestion by human athletes has been found to improve endurance performance primarily by acting via the central nervous system as an adenosine receptor antagonist (Graham, 2001). After caffeine ingestion the maximum normal concentration of caffeine in human blood plasma is 70 μM and a few studies have implied that such a concentration may directly affect skeletal muscle causing enhanced force production in both long term and short term activities (Graham, 2001). Our aim was to determine whether micromolar levels of caffeine enhance force production in isolated mammalian muscle. Eight- to ten-week-old female CD1 mice (Mus musculus) were humanely killed. An extensor digitorum longus muscle was isolated from each mouse and attached to a force transducer and a motor arm. The work loop technique was used to subject muscle preparations to cyclical sinusoidal length changes equivalent to 10% of resting muscle fibre length at a cycle frequency of 5 Hz (James et al. 2004). Electrical stimulation conditions were optimised to produce maximal work. Every 10 min for 130 min the muscle was subjected to three work loop cycles. For the first 30 min of this experiment the muscle was bathed in oxygenated (95% O₂-5% CO₂) Krebs-Henseleit solution at 35°C. The muscle was then subjected to a 60 min incubation in Krebs solution containing 70 μM caffeine, before a 40 min washout period in standard Krebs solution (Fig. 1). Each muscle was used as its own control by fitting a first order regression line through the power output data gained while in standard Krebs solution (excluding the first wash-out data point) and calculating power output during incubation as a percentage of the value indicated by the regression line. Single factor ANOVA was used to investigate the effect of caffeine on force and power using arcsine transformed data as the dependent variable, treatment as the independent variable and time as a fixed factor. Least significant difference pair-wise multiple comparisons were used as post-hoc tests. 70 μM caffeine caused a small but significant increase (2-3%; Fig. 1; P<0.05) in net power output produced between 30 and 60 min of caffeine incubation. This increase in power output was caused by an increase in force produced during the work loop. Our results indicate that micromolar levels of caffeine can directly enhance force and power output in mammalian skeletal muscle. These findings potentially confirm previous in vivo studies using humans that implied caffeine ingestion may cause acute improvements in muscle force and power (Graham, 2001).