Title: In vivo determination of fascicle length and pennation during voluntary contraction of human muscle 722
Abstract: Fascicles in human vastus lateralis muscle (VL) were visualized as ultrasonic echoes and fascicle lengths were measured for healthy male volunteers (n=6). When the knee was fully extended from flexed position(110°) passively without muscle contraction (relaxed condition), the fascicle length decreased by 28%, from 133 to 97 mm. When VL was statically contracted with 10% of maximal voluntary contraction strength (tensed condition), shortening of fascicles (5.30%) were observed which was more pronounced when the knee was extended (30%) than when flexed (5%). Angle of pennation (fascicle angle), defined as the angle between fascicles and aponeurosis, was larger in tensed than in relaxed condition, especially when the knee was extended.Larger fascicle angles at the extended positions would cause more mechanical disadvantage in the force transmission.During relaxed condition, when the knee is fixed at above 90°, passive tension would be generated in fascicles. By contraction the force generated by muscle fibers lengthed the tendon slightly, which could result in a slim shortening of fascicles. Because a skeletal muscle consists of muscle fibers (contractile component) and tendinous tissues (elastic component), the differences in length and angle of fascicle between the relaxed and tensed conditions might be caused by the laxity of either or both of two components. The present results clearly show that the architecture of contracting muscle fibers is greatly different from that with no activation. The use of cadaver's data in the study of architecture and function of muscles would result in inaccurate, and in some cases even erroneous conclusions.