Purpose: A common nonsense polymorphism (577XX) within the ACTN3 gene results in a total deficiency of the Alpha-actinin 3 (AA3) protein (North et al. 1999: Natural Genetics :21: 353-354) and has been purported to influence muscle performance capabilities during activities involving rapid production of high force (MacArthur et al., 2004: Bioessays :26: 786-795). Studies have shown the detrimental consequences of AA3 absence on post fatigue neuromuscular performance (MacArthur et al. 2008: HumMolGenetics 17:1076-1086) in animal models. There are no studies to date that have investigated fatigue-related neuromuscular performance in human populations in relation to ACTN3 polymorphisms. Participants: 36 recreationally active European Caucasian males participated in the study. Methods: Retrospective genotyping of whole blood via PCR and electrophoresis techniques confirmed that 27 participants possessed the R allele (expression of AA3 protein) and 9 participants were homzyogtic for the X allele (AA3 deficient). Estimates of knee extensor volitional static peak force (PF), peak twitch force (PTF), rate of force development and twitch half relaxation time (THalf) were obtained prior to and following i) a fatigue intervention consisting of 4 bouts of 3 x 10 second maximal isometric contractions (45o knee flexion) separated by 5 seconds and ii) a control condition of equivalent duration consisting of no exercise. The control condition was performed first to minimise any carry over effects. Results: Analysis of baseline performance revealed significant slower twitch half relaxation time in the AA3 deficient group (P<0.05) vs. AA3 expression group. Mixed-model repeated measures ANOVA revealed a significant exercise-related impairment in PF of a similar amount in both genotype groups (~18% vs. baseline, F[1, 34] = 33.9, p<0.001). No impairments to other indices of performance were observed following the fatiguing exercise. Conclusions: THalf appears to be lengthened in AA3 deficient individuals, suggesting that AA3 deficiency is associated with the contractile properties of fast twitch muscle fibres (Vincent et al. 2007: Physiological Genomics: 32: 58-63). However, the lack of a differential change in PF and PTF throughout the fatigue protocol between the two groups suggests that more research is needed with larger cohorts to explore further the issues of fatigue-related muscular performance and ACTN3 genotype.