Spinal mechanisms in postactivation potentiationKalc, Miloš (Avtor) Holobar, Aleš (Avtor) Kramberger, Matej (Avtor) Murks, Nina (Avtor) Škarabot, Jakob (Avtor) HDsEMGIa afferencesoleusheteroniumous Ia facilitationspinal reflexmuscle contractionneural mechanismsperformance enhancementThis 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.20262026-05-12 11:59:55Neznano29360UDK: 612.741ISSN pri članku: 0022-3077DOI: 10.1152/jn.00031.2026COBISS_ID: 277837059sl© 2026 The Authors.