Regional Heterogeneity in Vastus Lateralis Architecture Influences Fascicle Behavior During In Vivo Contractions

Skeletal muscle is heterogeneous in its architecture, with regional differences in fiber length and pennation angle that make up anatomically distinct regions. This study aimed to understand how these regional differences influence vastus lateralis (VL) behavior during isokinetic contractions in vivo. Knee extensor torque was measured in twelve healthy young adults using an isokinetic dynamometer during maximal contractions at different angular velocities (30° s^-1, 75° s^-1, 150° s^-1, 210° s^-1, 270° s^-1). The fascicle length of VL was recorded by two ultrasound devices in its distal and middle regions, and muscle-belly length was calculated as the longitudinal length change in the muscle belly. Fascicle (Vf) and muscle-belly (Vm) velocities were calculated as the first derivative of the length-time curve in the phase at constant angular velocity. Muscle-belly gearing (Gb) was calculated as Vm/Vf. At rest, greater thickness and pennation angles and lower fascicle lengths were observed in the middle vs. distal regions. During contraction, Vf and Vm increased as a function of angular velocity in both the investigated regions. The distal regions showed higher Vf and Vm values at all the investigated angular velocities. Significant differences in Gb were observed between regions but not as a function of knee angular velocity. Our data indicate that the architectural differences within a muscle affect the behavior of the active components during contraction. These results could help develop new musculoskeletal models to predict the muscle's mechanical output better.