20.500.12556/DiRROS-29360 Spinal mechanisms in postactivation potentiation facilitation of presynaptic inhibition contrasts h-reflex amplitude reduction This study investigated the spinal neural mechanisms underlying postactivation potentiation in 10 healthy young males (21.9 ± 4.8 yr).Participants performed a 10-s maximal isometric plantarflexion, after which we measured twitch torque and assessed spinal excitabilityusing the soleus H-reflex, D1 presynaptic inhibition, and heteronymous Ia facilitation (HF). High-density surface EMG was decomposed totrack single motor unit responses. The conditioning contraction increased twitch torque by 12.2 Nm (P < 0.001) immediately and returnedto baseline within 9 min. This mechanical potentiation was accompanied by a 29% reduction in H-reflex amplitude (P < 0.001), whichrecovered within 3 min. Paradoxically, neurophysiological indices of presynaptic inhibition, D1, and HF were significantly increased (D1:P < 0.017; HF: P < 0.001), resulting in spinal facilitation. Single MU analysis revealed increased discharge probability, particularly inhigher-threshold units, indicating overall spinal facilitation. These results demonstrate that postactivation potentiation involves a complexdissociation; H-reflex pathway inhibition along with facilitation of presynaptic spinal mechanisms. This paradox can be explained by eitherpostactivation depression (caused by depletion of neurotransmitter at the Ia-motoneuron synapse) or muscle thixotropy, a contraction his-tory-dependent decrease in muscle spindle sensitivity, which reduces the efficacy of the Ia afferent volley independently of spinal inhibi-tory mechanisms. Our findings highlight a dissociation between spinal presynaptic facilitation and the decreased H-reflex, underscoringthe need for future studies to explicitly test the roles of postactivation depression and muscle thixotropy after conditioning contractions.NEW & NOTEWORTHY This study provides evidence that postactivation potentiation is accompanied by a reduction in soleusH-reflex amplitude and a concurrent facilitation of presynaptic spinal mechanisms. By combining global EMG and single motorunit analyses extracted from high-density surface EMG, we reveal a dissociation between spinal disinhibition and reflex depres-sion. These findings suggest that acute postcontraction reflex suppression might be mediated by mechanisms other than presyn-aptic inhibition, potentially involving postactivation depression or changes in muscle spindle sensitivity. HDsEMG Ia afference soleus heteroniumous Ia facilitation spinal reflex muscle contraction neural mechanisms performance enhancement true false true Angleški jezik Ni določen © 2026 The Authors. Neznano 2026-05-12 11:59:55 2026-05-12 11:59:56 2026-05-28 03:48:18 0000-00-00 00:00:00 2026 0 0 Soavtorji: Aleš Holobar, Matej Kramberger, Nina Murks, Jakob Škarabot; str. 1437-1517 iss. 6 Vol. 135 2026 0000-00-00 Zaloznikova Objavljeno NiDoloceno 0000-00-00 0000-00-00 2026-01-01 612.741 0022-3077 10.1152/jn.00031.2026 277837059 26-05-27-Kalc_277837059-1.pdf 26-05-27-Kalc_277837059-1.pdf 1 B749DD3875CFE4D26DC5DEDACBC183A2 fd54d178967a6f343d98ac40a0748c53dcde2bbcb91c6a866f482214fddd537c 51afd826-5997-11f1-8e51-001a4af901a5 https://dirros.openscience.si/Dokument.php?lang=slv&id=44350 https://journals.physiology.org/doi/abs/10.1152/jn.00031.2026 1 556ae9d2-4de9-11f1-8a5a-001a4af901a5 https://dirros.openscience.si/Dokument.php?lang=slv&id=43985 Znanstveno-raziskovalno središče Koper / Centro di Ricerche Scientifiche Capodistria 0 0 0