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

The contribution of persistent inward currents (PICs) to motoneuron firing in the lower limb typically increases after a remote handgrip contraction, believed to result from diffuse serotonergic input increases in spinal cord. We investigated whether handgrip contraction intensity, duration and/or impulse would affect PIC estimates in tibialis anterior motoneurons. Multi-channel electromyograms were recorded from the tibialis anterior of 21 participants (18-40 years), during dorsiflexions at 20% of the individuals' maximal torque, before and after four handgrip conditions: (i) 80% of their maximal handgrip strength sustained for 15 s (80%15s); (ii) 40% sustained for 15 s (40%15s); (iii) 40% sustained for 30 s (40%30s); and (iv) no handgrip (Control). The PIC contribution to self-sustained motoneuron firing was estimated with delta frequency (ΔF) using paired motor unit analysis. The 'brace height', normalised as a percentage of a right triangle (% rTri), was used to estimate the PIC effects 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 the PIC contribution to motoneuron firing induced by a remote handgrip contraction is impulse dependent rather than intensity or duration dependent. The parallel increases in ΔF and brace height suggest augmented neuromodulatory input onto the spinal cord.