Purpose. The purpose of this study was to compare the effects of concentric with eccentric muscle actions on strength gains. Methods. 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. Results. 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). Conclusions. 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.