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

Abstract

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 individuals 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 ({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 (metabotropic regulation of motoneuron excitability) onto the motoneurons. {Delta}F 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 {Delta}F and brace height suggest augmented neuromodulatory input onto the spinal cord.