Tendon vibration during submaximal isometric strength and postural tasks.

To establish the effect of tendon vibration (TV) on steady submaximal strength and static balance control, 29 women performed isometric plantarflexions at 10, 20, 30 and 50% of MVC and postural tasks of increasing difficulty (Normal Quiet Stance, NQS, Sharpened Tandem Stance and One Leg Stance) with vibrators (80 Hz) applied to the Achilles tendon. Both tasks were performed under four conditions (10 s each): eyes open, eyes closed, eyes closed with TV, eyes open with TV. During the isometric tasks, the application of TV increased the plantarflexion torque at 20 and 30% of MVC, accompanied by a greater (p < 0.001) activation of agonists (Medial Gastrocnemius, MGAS and Soleus, SOL), as well as a greater (p < 0.05) coactivation of the antagonist (tibialis anterior, TA). Non-significant differences were observed at 10 and 50% of MVC. During posture, TV resulted in a backward displacement of the Centre of Pressure only for NQS and an increase (p < 0.01) in EMG activity of the ankle muscles in all balance tasks. The MGAS contraction during TV application was confirmed (by ultrasonography) at ten participants by an increase of the pennation angle and decrease of its fascicle length (p < 0.05). The vibration-induced muscle excitation and accompanying increase in motor output, probably due to excitatory Ia afferent input, was confirmed during strength and postural tasks. However, motor output attenuates when the magnitude of central drive to the ankle muscles increases.