Author(s): TRINCHI, M., BIZET, B., MONTE, A., ZAMPARO, P., Institution: UNIVERSITY OF VERONA, Country: ITALY, Abstract-ID: 738

INTRODUCTION:
Studies performed in animals and humans (at rest) indicate that skeletal muscle is heterogeneous in its architecture, with regional differences in fibre length, pennation angle and fibre type composition that make up anatomically distinct compartments or regions (Tijs et al., 2021). To our knowledge, no studies investigated, so far, the effects of these regional differences on muscle mechanics. Thus, this study aimed to understand how regional differences in vastus lateralis (VL) muscle architecture influence dynamic muscle shape changes in vivo.
METHODS:
Twelve healthy young adults participated in the study. Knee extensor torque was measured using an isokinetic dynamometer; subjects were seated with the back supported and the hip joint flexed at 80°. After a standardised warm-up, they were requested to perform three maximal isokinetic contractions at five different angular speeds [30, 75, 150, 210, 270 °/s] over the entire range of motion. Muscle behaviour in distal and proximal regions of VL was recorded by two ultrasound devices positioned at 1/3 and 2/3 of the femur length, respectively. The maximum torque value during the isokinetic phase (at each angular velocity) was used to obtain the torque-fascicles velocity (T-V) curve for both regions. Fascicle and belly velocity were calculated as the first derivative of the length-time curve during the isokinetic phase of each trial, and the mean value was utilised to reconstruct the fascicle and belly T-V relationship for both muscle regions. Belly gearing was calculated as the ratio between muscle belly and fascicle velocity. Finally, the x-axis intercepts of the T-V curves obtained through fascicle and muscle data were obtained for both regions.
RESULTS:
At rest, greater muscle thickness (2.4±0.5 vs 1.9±0.7cm) and pennation angle (13.1±2.7 vs 11.7±2.6°) were observed in the proximal vs distal region, respectively. Maximum fascicle and muscle shortening velocities for the distal and proximal regions were 66.1±25.1 and 70.4±27.3 cm/s and 41.1±20.4 and 47.5±22.2 cm/s, respectively. Belly gearing at zero loads (intercept of the x-axis) was higher (1.14±0.05 vs 1.06±0.02) in the proximal vs distal region.
CONCLUSION:
At any given torque value, we observed differences in fascicle and muscle behaviour between muscle regions: higher muscle and fascicle velocity values were observed in the distal region, and the proximal one exhibited higher values of belly gearing compared to the distal one. These data suggest that the architectural differences within a muscle could affect the behaviour of the active components during contraction. Moreover, the differences in uncoupling behaviour between regions allow the VL to operate with similar Force-Velocity potential across its length.