Introduction: Range of Motion (ROM) can be manipulated in resistance training programs in order to improve strength gains and sports performance. To the best of our knowledge, only two studies (Bloomquist et al. 2013; McMahon et al. 2014) have shown that different ROM training might lead to specific adaptations on muscle size, muscle force and fascicle geometry. However, in these studies, the training volume was not equalized, which might have affected the final results. Therefore, the aim of this study was to determine the effect of a 15-week partial ROM knee extension strength training on the vastus lateralis (VL) muscle architecture and mechanical properties, when the time under tension (TUT) was equalized. Methods: Nineteen male physical education students were randomly distributed to a control (CG; n = 8) or training (TG; n=11) group. In the TG the subject`s legs were also randomly selected to be trained with full ROM (FULL) or partial ROM (PAR). They performed isokinetic knee extensors concentric training on a isokinetic dynamometer (Biodex System 3 research, USA) for 15 weeks with 3 sessions per week, with either a FULL (100º of knee flexion to 0º) and PAR (60º of knee flexion to 0º) ROM. Training volume between conditions was equalized based on TUT. The VL muscle thickness (MT), fascicle length (FL) and pennation angle (PA) were assessed for both thighs by B-mode ultrasonography at rest and during maximal isometric knee extension contractions (MVCs) at fifteen knee joint angles, ranging from 30 to 100° of knee flexion (full extension=0°). VL fascicle force was calculated from MVCs with superimposed stimuli, accounting for the VL moment arm length, muscle architecture, and antagonist co-contraction. Results: MT, PA, and maximal fascicle force (MFF) increased significantly from baseline to post-training in both training groups (p<0.001). The changes ( %) were respectively for FULL and PAR, MT:8.4 ± 2.9% and 6.7 ± 3.0%; PA: 9.5±2.4% and 12.2±2.4%; MFF: 28.8±19.5% and 19.0±10.8%. Significant differences between FULL and PAR were found only on FL, which increased 4.9±1.9% (p <0,001) in FULL group, with no changes in PAR group ( : -1.1±3.4%, n.s.). The PAR group showed a specific effect on the VL force-length adaptations, presenting higher force levels at the trained angular positions. Discussion: The morphological and mechanical muscle adaptations seem to be influenced by the ROM of strength training exercises. When the ROM stimulates large muscle amplitudes, the fascicle length tends to increase. Conversely, when the amplitude of muscle action is restricted to reduced muscle lengths we are promoting identical pennation angles but specific force adaptations.
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