The alpha actinin3 gene (ACTN3) encodes a protein of the Z disk of myofibrils, a polymorphism of ACTN3 results in a complete loss of the protein. ACTN3 is a fast-twitch-specific isoform, expressed only in type II skeletal muscle fibres (North and Beggs, 1996), which are very important for rapid and powerful contractions. Therefore it is possible that ACTN3 genotype may influence muscle function and performance particularly during high velocity movements. Some preliminary evidence suggests that ACTN3 RR genotype is associated with elite sprint performance (Yang et al, 2003) and higher elbow flexor strength in untrained women (Clarkson et al, 2005). The aim of this study was to conduct a thorough investigation of the influence of ACTN3 genotype on human muscle function and contractile properties. Following ethical approval a range of quadriceps muscle function measurements were taken from 79 recreationally active but non strength trained White British males on two occasions. A conventional strength testing chair was used to measure maximal isometric strength. Maximum twitches (Digitimer DS7AH, UK) were electrically evoked to assess time to peak tension (TPT) and half relaxation time (HRT). Isokinetic peak torque was measured at 3 velocities (30°.s-¹, 90°.s-¹, 240°.s-¹) using isokinetic dynamometry (Cybex Norm, USA). Relative strength at high velocities (240°.s-¹,:30°.s-¹, torque ratio) was also calculated and is recognised as a functional indicator of fibre type composition (Gur et al, 2002). Maximum angular velocity during unloaded knee extension was determined with goniometry. ACTN3 R/X genotype was determined from whole blood samples using polymerase chain reaction. ACTN3 genotype distribution was in Hardy Weinberg Equilibrium (XX 15, RX 37, RR, 27). Physical characteristics were independent of genotype. Isometric and isokinetic strength measures, and relative strength at high velocities were not influenced by genotype. There was a trend for a difference in TPT between genotypes (XX, 70±11 ms; RX, 64±8 ms; RR, 68±12 ms; ANOVA P = 0.06). No other differences were found between the ACTN3 genotypes for measures of HRT and maximum unloaded knee extension. Whilst there was a trend for a difference in TPT between genotype groups, overall there was no clear effect of ACTN3 polymorphism on muscle function and contractile properties even during high velocity movements. Either there was no influence of ACTN3 genotype on fibre type composition or the effect was too small to show an influence on the functional and contractile properties of muscle.