Increase in vastus lateralis aponeurosis width induced by resistance training: implications for a hypertrophic model of pennate muscle

Purpose This study aimed to ascertain whether training-induced muscle hypertrophy is accompanied by an increase in the aponeurosis width, and to infer its impact on the training-induced increase in the pennation angle. Methods Eleven young men completed a resistance training program of unilateral knee extensions for 12 weeks. Before and after training, anatomical cross-sectional area (ACSA) of the vastus lateralis and its distal aponeurosis width in the transverse plane were measured with magnetic resonance imaging. The pennation angle and fascicle length were also determined with ultrasonography at the midbelly of the muscle. The effect of change in aponeurosis width on the magnitude of training-induced increase in pennation angle was estimated by using a parallelepipedon model. Results After the training, there were significant increases in ACSA (10.7 ± 7.6 %), pennation angle (10.8 ± 7.3 %) and aponeurosis width (1.9 ± 3.1 %), whereas no significant change was found in the fascicle length. The model simulation shows that the increase in aponeurosis width by 1.9 % reduces the magnitude of increase in pennation angle by only 0.4°. Conclusions These results indicate that (1) the aponeurosis width of the vastus lateralis increases after 12 weeks of resistance training and (2) the increase in the aponeurosis width accompanying muscle hypertrophy by the amount of ~10 % does not substantially affect the increase in pennation angle.