<bold>Purpose.</bold> The purpose of this study was to compare the effects of concentric with eccentric muscle actions on strength gains. <bold>Methods.</bold> Forty-two untrained men were randomly divided into three groups: the concentric experimental (CE), the eccentric experimental (EE) and a control (C). The CE group performed only concentric muscle actions at 80% of one repetition maximum (1 RM) and the EE group performed only eccentric muscle actions at 120% of 1 RM. Both groups trained by performing three sets of 10-12 repetitions for eight weeks of biceps curl (BC) and bench press (BP) exercises. The C group did not engage in any type of training. <bold>Results.</bold> Analyses performed within the CE group found that there were significant improvements in muscle strength in the eighth week of BP (Δ % = 26.9%, p = 0.01) and in the fourth and eighth week of BC (Δ % = 22.1%, p = 0.00 and Δ% = 32.1%, p = 0.00, respectively). Analyses of the EE group found that there were significant improvements in muscle strength in the fourth and eighth week of BP (Δ% = 13.7%, p = 0.00 and Δ% = 28.4%, p = 0.00, respectively). Between the two groups (CE versus EE), comparisons showed that the CE group performed significantly better than the EE group in the fourth and eighth week of BC (p = 0.00 and p = 0.00, respectively). <bold>Conclusions.</bold> These findings indicate that those who do not train should perform concentric muscle actions in the first 8 weeks of training in order to generate accelerated strength improvement.
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