Frequency characteristics of heteronymous responses evoked by Achilles tendon vibration during quiet stance

Primary (Ia) sensory afferents that innervate muscle spindles provide strong synaptic input to homonymous motoneurons and are thought to play a role in balance control. In addition, Ia afferents have broad heteronymous connections; i.e., projections to motoneurons that innervate other muscles that act at the same joint as well as at different joints. The purpose of the current study was to investigate heteronymous Ia afferent connections from the triceps surae muscles to lower limb and back muscles during quiet standing in humans. We applied supra-threshold noisy vibration (10-115 Hz) to the right Achilles tendon of 12 participants maintaining quiet stance and recorded EMG activity bilaterally from homonymous (Soleus) and heteronymous muscles (Semitendinosus, Vastus Lateralis, Erector Spinae). We estimated coherence, phase, and gain between the tendon probe acceleration and rectified EMG from each muscle. We found significant coherence between the probe acceleration and EMG in ipsilateral Soleus (5-100 Hz), Semitendinosus (10-75 Hz), Vastus Lateralis (5-70 Hz), and bilateral Erector Spinae muscles (10-70 Hz). These results provide evidence that triceps surae muscle spindle afferents can influence the activity of muscles proximal to the ankle joint across a broad frequency band during quiet standing.