Competing Effects of Agonist and Antagonist Vibration on the Proprioceptive Sense of Force

ABSTRACT Motor unit (MU) firing is affected by motoneuronal persistent inward currents (PICs), which heavily contribute to gain control of motor output. PICs are highly sensitive to inhibition; for instance, Ia reciprocal inhibition via antagonist muscle vibration drastically reduces discharge rate hysteresis (ΔF), an estimate of PIC magnitude. A direct link between sensitivity of PICs to inhibition and voluntary force control, however, has not been established. To determine whether force control is altered with inhibition of PICs, we recorded high-density surface EMG from the tibialis anterior, while 11 participants (5F; 6M) completed and isometric force reproduction task. Tendon vibration was applied to the agonist or antagonist muscle during the first (with visual feedback) or second contraction (without visual feedback) and participants were asked to match percieved effort across contractions, in an attempt to match neural drive to the motor pool. In support of our hypothesis, torque and MU firing rates were reduced when vibration was applied to the antagonist (torque: p < .0001; MU firing rate: p < .0001), but not agonist (torque: p = .9980; MU firing rate: p = .312) muscle tendon in the second contraction, compared to control. Conversely, when vibration was applied during the first contraction, opposite effects were observed. These results suggest that PICs play a role in the proprioceptive sense of force, offering a potential link between PICs and voluntary force control, which may be important for understanding and treatment of motor impairments. KEY POINTS Motoneuronal persistent inward currents amplify synaptic currents and therefore heavily contribute to motor output, however they are extremely sensitive to Ia reciprocal inhibition induced by muscle tendon vibration. We show that modulation of PICs severely impacts human force sense using an effort-based force reproduction paradigm which enabled us to manipulate combinations of tendon vibration and visual feedback. These findings provide a link between PICs and functional motor output, which may be important for understanding neurological impairments and informing rehabilitation strategies.