Difference in fascicle behaviors between superficial and deep quadriceps muscles during isometric contractions

Introduction: In this study we examined changes in fascicle length and pennation of vastus intermedius (VI) and vastus lateralis (VL) from rest to isometric contraction. Methods: Fourteen healthy men performed submaximal (30% and 50% of maximal voluntary contraction; MVC) isometric knee extensions at 60° and 110° knee flexion (0°=full extension). Panoramic images of VL and VI were obtained at rest and during contractions to measure fascicle length and pennation using extended field-of-view ultrasonography, after confirmation that the technique could be used reliably during sustained submaximal isometric contractions. Results: The relative change in fascicle length from rest to 50% MVC was greater in VI than VL at 110° (P<0.05). Pennation increased during contraction at 60° for VL (P<0.05), but not for VI. Conclusion: Muscle-dependent fascicle behaviors were observed during contraction. These findings may provide a mechanistic underpinning for differences in sarcomere length changes, relative force contribution, and blood flow during contraction.