Fascicle shortening upon activation in voluntary human muscle contractions

Abstract

BACKGROUND: The dependence of fascicle length on complex interactions with joint angle and force challenges the interpretation of in vivo joint mechanics, muscle mechanical properties, contractile behavior, and muscle function. AIM: The purpose of this study was to determine the complex interaction between muscle activation, joint angle, and fascicle length for isometric contractions of the human vastus lateralis muscle (VL). METHOD: Knee extensor torques, joint angles, EMG activation, and fascicle lengths were determined in nine healthy subjects during maximal and submaximal isometric contractions. RESULTS: Fascicle shortening during isometric contractions depended on muscle-tendon unit length/joint angle and activation, reaching a maximum between angles where VL had its maximum force potential and minimum resistance to fascicle shortening. Maximal fascicle shortening shifted to shorter muscle-tendon unit lengths with decreasing activation. CONCLUSION: Fascicle shortening upon activation depends crucially on the force generating potential and stiffness of the muscle and can reach 30% of the resting fascicle length. Not accounting for the complex interactions between muscle length, force potential, muscle structure, and muscle stiffness has led to erroneous interpretations of the function and properties of healthy and diseased muscles.