Elucidation of Architectural and Compositional Factors Associated With Inter-Individual Variability in Passive Shear Modulus of the Human Vastus Lateralis in Young Healthy Males.

Human skeletal muscle exhibits substantial inter-individual heterogeneity in passive mechanical properties; however, the mechanistic basis underlying this variability remains poorly understood. We aimed to comprehensively examine how compositional and architectural factors are associated with individual variability in passive shear modulus of the human vastus lateralis (VL). In 46 healthy young males, the shear modulus of VL was measured from 0° to 110° of knee flexion in 10° increments using ultrasound shear wave elastography to determine the slack angle (angle at which the shear modulus first began to increase), shear modulus in the slack region (values before the slack angle), and Δshear modulus in the lengthened position (+35° from the slack region). Muscle carnosine concentration (a proxy for muscle fiber type composition), intramuscular fat fraction, 3D muscle architecture (fascicle length and pennation angle), and knee extensor moment arm were quantified using a magnetic resonance scanner. The inter-individual variability was greater for the Δshear modulus in the lengthened position (54.5% [coefficient of variation]) than for the shear modulus in the slack region (16.4%). The shear modulus in the slack region was not associated with any of the measured variables, whereas Δshear modulus in the lengthened position was only associated with pennation angle (corrected p = 0.004, r = -0.488), explaining approximately 20% of the variance. These findings indicate that pennation angle is associated when the muscle is lengthened for a given joint angle change; yet even this factor explains only a limited portion of the inter-individual variability in VL passive shear modulus.