Vastus intermedius muscle architecture predicts the late phase of the knee extension rate of force development in recreationally resistance-trained men

Objectives: The current study investigated the correlation between quadriceps muscle architecture and the knee extension rate of force development (RFD). Design: cross-sectional study. Methods: Muscle thickness, pennation angle and fascicle length normalized per the thigh-length were measured via ultrasound in vastus lateralis, rectus femoris, vastus intermedius and vastus medialis. The knee extension rate of force was assessed isometrically at 90° knee angle and calculated in different 50-ms epochs (0-50, 50-100, 100-150, 150-200 and 200-250ms). The maximum voluntary contraction was also recorded. Results: Large correlations were observed between vastus intermedius muscle thickness and the 100-150ms (r=0.694, p=0.004), 150-200ms (r=0.597, p=0.019) and 200-250ms (r=0.546, p=0.045) epochs. Large correlation was observed between vastus intermedius normalized fascicle length and 100-150ms (r=0.570, p=0.043) and large correlations with 150-200ms (r=0.643, p=0.010) and 200-250ms (r=0.629, p=0.012) epochs. Additionally, large correlations were observed between vastus lateralis normalized fascicle length and the 100-150ms (r=0.535, p=0.049), 150-200ms (r=0.629, p=0.016) and 200-250ms (r=0.563, p=0.046) epochs. Vastus intermedius muscle thickness predicted 59% (R2=0.581, p=0.002) of the RFD of the 100-150ms epoch; vastus intermedius muscle thickness and fascicle length predicted 51% (R2=0.506, p=0.029) of the 150-200ms epoch; vastus intermedius and vastus lateralis fascicle length predicted 48% (R2=0.483, p=0.037) of the 200-250ms epoch. No further correlation was observed. Conclusions: Fascicle length and muscle thickness were observed as predictive of the late phase of the rate of force development. Vastus intermedius muscle architecture has a primary role in the knee extension RFD.