Isometric handgrip contraction increases tibialis anterior intrinsic motoneuron excitability in a dose-dependent manner

Persistent inward currents (PICs) contribution to motoneuron firing in the lower limb typically increase after a remote handgrip contraction, believed to result from diffuse increases of serotonergic input on the spinal cord. We investigated whether handgrip contraction intensity, duration, and/or impulse would affect tibialis anterior estimates of PICs. Multi-channel electromyograms were recorded from the tibialis anterior of 21 participants (18-40 years), during dorsiflexions at 20% of individual’s maximal torque, before and after four handgrip conditions: i) 80%15s , 80% of their maximal handgrip strength sustained for 15s; ii) 40%15s , 40% sustained for 15s; iii) 40%30s , 40% sustained for 30s; and iv) Control (no handgrip). PICs contribution to self-sustained motoneuron firing was estimated with the delta frequency (ΔF) using the paired motor unit analysis. The ‘brace height’, normalised as a percentage of a right triangle (%rTri), was used to quantify the effects of PICs on the non-linearity of firing patterns, representing the neuromodulatory drive (metabotropic regulation of motoneuron excitability) onto the motoneurons. ΔF increased by 0.33 pulses per second (pps; 95%CI 0.16–0.49, d =0.47) after 40%30s and by 0.24 pps (0.09–0.38, d =0.34) after 80%15s but remained unchanged after 40%15s and Control . Similarly, brace height increased by 2.24 %rTri (0.18–4.30, d =0.20) after 40%30s and by 2.45 %rTri (0.64–4.25, d =0.22) after 80%15s; remaining unchanged after 40%15s and Control . The increase in PICs contribution to motoneuron firing induced by a remote handgrip contraction is impulse-dependent rather than intensity or duration. The parallel increases in ΔF and brace height suggest augmented neuromodulatory input onto the spinal cord.