This study investigated the chronology of neural and morphological adaptations to knee extensor eccentric training and their contribution to strength gains in isometric, concentric and eccentric muscle actions. 20 male healthy subjects performed a 12-week eccentric training program on an isokinetic dynamometer, and neuromuscular evaluations of knee extensors were performed every 4 weeks. After 12 training weeks, significant increases were observed for: isometric (24%), concentric (15%) and eccentric (29%) torques; isometric (29%) and eccentric (33%) electromyographic activity; muscle thickness (10%) and anatomical cross-sectional area (19%). Eccentric and isometric torques increased progressively until the end of the program. Concentric torque and muscle mass parameters increased until the eighth training week, but did not change from this point to the twelfth training week. Eccentric and isometric activation increased at 4 and 8 training weeks, respectively, while no change was found in concentric activation. These results suggest that: 1) the relative increment in concentric strength was minor and does not relate to neural effects; 2) eccentric and isometric strength gains up to 8 training weeks are explained by the increased neural activation and muscle mass, whereas the increments in the last 4 training weeks seem to be associated with other mechanisms.

Humans preferentially recruit limbs to functionally perform a range of daily tasks, which may lead to performance asymmetries. Because initial training status plays an important role in the rate of progression during resistance training, could asymmetries between the preferred and nonpreferred limbs lead to different magnitudes of strengthening during a resistance training program? This issue motivated this study, in which 12 healthy and physically active men completed a 4-week control period followed by a 12-week isokinetic resistance training program, performed twice a week, including 3-5 sets of 10 maximal eccentric contractions for each limb. Every 4 weeks, knee extensor peak torques at concentric, isometric, and eccentric tests were measured using an isokinetic dynamometer and the sum of quadriceps muscle thickness was determined by ultrasound images. Before training, concentric peak torque was similar between limbs but isometric and eccentric peak torques were significantly smaller in the nonpreferred compared with the preferred limb (4.9 and 5.8%, respectively). Bilateral strength symmetry remained constant throughout the training period for concentric tests. For eccentric and isometric tests, symmetry was reached at the fourth and eighth training weeks, respectively. After 12 weeks, between-limb percent nonsignificant differences were -0.62% for isometric and -1.93% for eccentric tests. The sum of knee extensor muscle thickness had similar values before training and presented similar changes throughout the study for both the preferred and the nonpreferred limbs. In conclusion, the nonpreferred limb presents higher strength gain than the preferred limb at the initial phase of an isokinetic resistance training program, and this increased strength gain is not associated with muscle hypertrophy.