Relative motion between the paradoxical muscle fiber movement and the human body center of mass while voluntary bodily swaying and walking

Recent study had demonstrated that the medial gastrocnemius (MG) muscle fiber during upright standing moved as paradoxical relative to the anatomical demands, and suggested that the predictive neural activities were important for the postural sway. In this respect, we estimated the changes in the length of the muscle fiber (MFL) of MG while upright standing tasks in which the changes of center of mass (COM) and the length of moment arm were different. The aim of this study was to estimate the relationship of the behavior of MFL and the length of moment arm under three muscle fiber movements in vivo revealed by time series of the ultrasound images;1) subjects swayed their body voluntarily in an anterior-posterior direction, 2) standing on their tiptoe or heels which could induce the same angle of the ankle plantar-and/or dorsi-flexion compared with those of bodily swaying in anterior-posterior direction, and 3) treadmill walking at 3.0 and 4.0 km/h. The hip, knee and ankle angular displacements were recorded by goniometers. The center of pressure and the angle of ankle had used as a visual feedback signal in two postural tasks. The MFL of MG shortened when the position of the COM located in front of the ankle joint, and lengthened when the position of the COM located behind the ankle joint. These results suggested that the changes in the MFL of MG were determined by the COM position relative to the ankle joint.