Vastus Intermedius And Vastus Lateralis Fascicle Length Changes During Maximal Eccentric Knee Extensor Contractions

Abstract

Understanding fascicle behaviors during muscle contraction is essential for our understanding of muscle function. Several studies have examined fascicle behaviors during contractions of different muscles, however little is known about vastus intermedius (VI) fascicle behavior during eccentric contractions. The VI fascicles originate directly on the femur, but those of vastus lateralis (VL) originate on a compliant aponeurosis. Thus, it is assumed that fascicle behavior is different between VI and VL during eccentric knee extensor contractions. PURPOSE: This study compared VI and VL fascicle length changes during maximal eccentric knee extensor contractions using ultrasonography. METHODS: Thirteen healthy men (age: 27.1 ± 3.5 y, height: 176.1 ± 6.1 cm, body mass: 76.0 ± 7.9 kg) performed passive knee flexions and maximal eccentric knee extensor contractions through the same knee joint range of motion from 35° to 105° (0° = full extension) at an angular velocity of 30°/s. B-mode ultrasound images of VI and VL were simultaneously recorded at 30 Hz using two serially-arranged ultrasound probes (11 cm imaging window) placed on the skin surface over VL. VI and VL fascicle lengths at knee joint angles of 40° and 100° were measured from the images, and the magnitude of fascicle lengthening from 40° to 100° was calculated. RESULTS: VI and VL fascicle lengths at 40° were 92 ± 13 mm and 99 ± 8 mm during passive knee flexion, and 72 ± 7 mm and 75 ± 8 mm during eccentric contraction, respectively. VI and VL fascicles were lengthened similarly during passive knee flexion (VI: 36 ± 5 mm, VL: 33 ± 9 mm), but VI fascicles were lengthened more (35 ± 9 mm) than VL fascicles (24 ± 5 mm) during eccentric contraction (P<0.05). The magnitude of fascicle lengthening was similar between passive knee flexion and eccentric contraction for VI, but was smaller during eccentric contraction than passive knee flexion for VL (P<0.05). CONCLUSIONS: These results show that VI fascicles are lengthened more than VL fascicles during eccentric contractions. This may suggest that either VI is less activated or VI receives greater mechanical stress during eccentric contraction when compared with VL. Further study is necessary to investigate how the different fascicle behaviors affect the magnitude of muscle damage induced by eccentric contractions between the muscles.