Resonance in the human medial gastrocnemius muscle during cyclic ankle bending exercise

1 Department of Life Sciences, University of Tokyo, Meguro-ku, Tokyo; 2 Health and Sports Science, Nippon Engineering College of Hachioji, Tokyo; 3 Consolidated Research Institute for Advanced Science and Medical Care, Waseda University, Tokorozawa, Saitama; 4 Institute of Physical Education, Keio University, Kohoku, Yokohama; 5 Computational Biomechanics Unit, Riken, Wako, Saitama; and 6 Faculty of Sport Sciences, Waseda University, Tokorozawa, Saitama, Japan Submitted 6 September 2005 ; accepted in final form 21 February 2006 We investigated the behavior of the muscle tendon unit (MTU) of the medial gastrocnemius muscle during cyclic ankle bending exercise at eight different frequencies (ranging from 1.33 to 3.67 Hz). The changes in the length of fascicle in the muscle during the exercises were determined by real-time ultrasound imaging. The coordinates of anatomical references and the ground reaction force were determined from video recording and a force plate, respectively. The length change of the MTU (the distance from the origin to insertion of the muscle) was calculated from changes in the knee and ankle joint angles. It was found that the amplitude ratio and phase difference between the fascicle and MTU lengths were both dependent on the movement frequency. At lower frequencies, the fascicle lengths varied almost in phase with the MTU length, whereas they varied out of phase at the higher frequencies. At intermediate frequency, the amplitude of the fascicle became very small compared with that of the MTU, which is considered resonance. We constructed a mechanical model of the MTU based on a notion of forced oscillation in a mass-spring system. The obtained data were well explained by the model. It was concluded that the behavior of the MTU highly depends on the movement frequency due to the viscoelasticity of the MTU. muscle mechanism; tendinous tissue; viscoelasticity; ultrasound imaging Address for reprint requests and other correspondence: D. Takeshita, Center for Neurodynamics, Dept. of Physics and Astronomy, Univ. of Missouri at St. Louis, One University Blvd., St. Louis, MO 63121 (e-mail: dtakeshita{at}aol.com )