Amuscle's ability to produce force during a specific movement is determined by the type of muscle action (concentric, eccentric, or isometric). For example, based on the forcevelocity relationship of muscle, the muscle force decreases with increasing speed of movement during a concentric action, and the force increases during an eccentric action. Superimposing the force-velocity relationship on the length-tension relationship, the force will be optimal at a certain muscle (sarcomere) length during isometric and slow concentric actions. 22The use of kinematics and electromyography (EMG) to determine the type of muscle action assumes that the change in length of the muscle-tendon complex will reflect the change in length of the muscle t Study deSign: A single-group descriptive experimental design.t ObjectiveS: To determine the fascicle length change in the tibialis anterior (TA) and the vastus lateralis (VL) muscles during walking.t bAckgrOund: The length of the muscle fibers during isometric actions and during dynamic functional activities is affected by the compliance of the tendon and aponeurosis. The TA and VL muscles have important functions both in stance and swing phases of gait. Therefore, it is important to understand the dynamics of the muscle length change as it relates to the type of muscle actions in walking.t MethOdS And MeASureS: Nine healthy subjects performed treadmill walking while fascicle length, muscle activity (electromyographic signal), and joint angle (knee and ankle) were recorded. Fascicle length was measured using real-time ultrasound imaging. Fascicle length and joint angle during the gait cycle were analyzed using a repeated-measures analysis of variance.t reSultS: During the initial portion of stance, when the TA and VL muscles were active, the ankle plantar flexed and the knee joint flexed, suggesting muscle-tendon complex lengthening, but the fascicle length of both muscles remained constant (TA, P = .93; VL, P = .22). The TA muscle was again active during the initial portion of swing phase, while the ankle dorsiflexed, and the fascicle length decreased (P,.05). The VL muscle became active again at the end of swing as the knee extended, and the fascicle length decreased (P,.05).t cOncluSiOnS: The lack of change in fascicle length during the initial portions of stance phase suggests a nearly isometric muscle action of the TA and VL. There is a possible interaction occurring between the fascicle and tendon in the TA and VL such that the tendon lengthens to allow joint motion and potentially to store elastic energy.