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. High-density electromyograms were recorded from the tibialis anterior of 21 participants (18-40 years) during 20% dorsiflexion 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 motoneuron firing was estimated with the delta frequency ({Delta}F) 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 (regulation of neural activity via neurotransmitter actions) onto the motoneurons. {Delta}F increased by 0.30 peaks per second (pps; 95%CI 0.11-0.49, d=0.37) after 40%30s and by 0.20 pps (0.04-0.36, d=0.24) after 80%15s but remained unchanged after 40%15s and Control. Similarly, brace height increased by 2.39 %rTri (0.55-4.23, d=0.25) after 40%30s and by 2.74 %rTri (1.14-4.34, d=0.28) 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 {Delta}F and brace height suggest augmented neuromodulatory input onto the spinal cord.