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The mechanisms governing the increases in force production in response to short periods of strength training have yet to be fully elucidated. We examined whether muscle architectural adaptation was a contributing factor. Ultrasound imaging techniques were used to measure quadriceps muscle architecture at 17 sites in vivo in trained and untrained legs of men and women after 2.5 and 5 weeks of unilateral knee extension training, as well as in a nontraining control group. Despite increases in knee extensor strength of the trained and untrained (women only) legs, there were no changes in muscle thickness, fascicle angle, or fascicle length in any of the muscles tested. The moderate correlation between vastus lateralis thickness (middle site) and eccentric (r = 0.55; P < 0.05) and concentric (r = 0.46; P < 0.1) torque after, but not before, training is suggestive of neural rather than architectural adaptations …