Effects of repeated Achilles tendon vibration on triceps surae stiffness and reflex excitability

Clinical studies frequently report an increase in stiffness and a loss of range of motion at joints placed in disuse or immobilization. This is notably the case for subjects maintained in bed for a long period, whilst their joints are not affected. Recently we documented on healthy subjects the benefit in terms of force and activation capacities of the triceps surae offered by vibrations applied to the Achilles tendon. Knowing that stiffness changes may contribute to force changes, the aim of the present study was to investigate the effects of tendon vibration on the triceps surae stiffness of healthy subjects. The vibration program consisted in 14 days of 1h daily Achilles tendon vibration applied at rest. Nineteen healthy students were involved in this study. Before and at the end of the vibration program, musculo-tendinous stiffness in active conditions was determined by use of a quick-release test. Passive stiffness was also analyzed by a flexibility test: passive torque-angle relationships were established from maximal plantar-flexion to maximal dorsiflexion. Passive stiffness indexes at 10°, 15° and 20° dorsiflexion were defined as the slope of the relationships at the corresponding angle. Tendinous reflex, influenced by stiffness values, was also investigated as well as the H reflex to obtain an index of the central reflex excitability. After the program, musculo-tendinous stiffness was significantly decreased (p=.01). At the same time, maximal passive dorsiflexion was increased (p=.005) and passive stiffness indexes at 10°, 15° and 20° dorsiflexion decreased (p<.001; p<.001 and p=.011, respectively). Tendinous reflex also significantly decreased. As the triceps surae parameters are diminished after the vibration program, it could be beneficial to immobilized persons as hypo-activity is known to increase muscular stiffness.