Nina Murks

This study provides evidence that postactivation potentiation is accompanied by a reduction in soleus H-reflex amplitude and a concurrent facilitation of presynaptic spinal mechanisms. By combining global EMG and single motor unit analyses extracted from high-density surface EMG, we reveal a dissociation between spinal disinhibition and reflex depression. These findings suggest that acute postcontraction reflex suppression might be mediated by mechanisms other than presynaptic inhibition, potentially involving postactivation depression or changes in muscle spindle sensitivity.

This study investigated the spinal neural mechanisms underlying post-activation potentiation in ten healthy young males (21.9 ± 4.8 years). Participants performed a 10-second maximal isometric plantarflexion, after which we measured twitch torque and assessed spinal excitability using the soleus H-reflex, D1 presynaptic inhibition and heteronymous Ia facilitation (HF). High-density surface EMG was decomposed to track single motor unit responses. The conditioning contraction increased twitch torque by 12.2 Nm (p < 0.001) immediately and returning to baseline within nine minutes. This mechanical potentiation was accompanied by a 29% reduction in H-reflex amplitude (p < 0.001), which recovered within three minutes. 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 in higher-threshold units indicating overall spinal facilitation. These results demonstrate that post-activation potentiation involves a complex dissociation: H-reflex pathway inhibition along with facilitation of presynaptic spinal mechanisms. This paradox can be explained by either post-activation depression (caused by depletion of neurotransmitter at the Ia–motoneuron synapse) or muscle thixotropy, a contraction history-dependent decrease in muscle spindle sensitivity, which reduces the efficacy of the Ia afferent volley independently of spinal inhibitory mechanisms. Our findings highlight a dissociation between spinal presynaptic facilitation and the decreased H-reflex, underscoring the need for future studies to explicitly test the roles of post-activation depression and muscle thixotropy during post-activation potentiation. New & Noteworthy: This study provides evidence that post-activation potentiation reduces the soleus H-reflex amplitude while concurrently facilitating presynaptic spinal mechanisms. By combining global EMG and single motor unit analyses extracted from high-density surface EMG, we reveal a dissociation between spinal disinhibition and reflex depression. These findings suggest that acute post-contraction reflex suppression might be mediated by mechanisms other than presynaptic inhibition, potentially involving post-activation depression spinal mechanisms or changes in muscle spindle sensitivity.

We investigated the agreement and accuracy of manual editing of the high-density electromyogram (hdEMG) decomposition results by seven human operators with various experience levels. All operators edited the same automatically decomposed experimental hdEMG from the first dorsal interosseous (FDI), tibialis anterior (TA), vastus lateralis (VL), and biceps brachii (BB) muscles, and synthetic hdEMG from soleus (SO) and BB muscles at 10%, 30%, 50% and 70% of maximum voluntary contraction. On average, operators kept 13.7 ± 7.4 motor units (MUs) after editing and demonstrated relatively large disagreement in the calculated MU pulse trains (normalized root mean square difference) but relatively high agreement in the identified MU discharges. Inter-operator agreement positively correlated with the initial MU Pulse-to-Noise Ratio used as a quality measure of automatic MU identification, and negatively correlated with the muscle contraction level. Operators agreed more on the results of the simulated than experimental hdEMG. Among the experimental muscles tested, the greatest agreement was demonstrated for VL and the lowest for BB. We obtained similar results when comparing editing to the results of the most experienced operator and to ground truth in simulated cases: the greatest precision and sensitivity were demonstrated for VL, and the lowest for BB. The level of the operator’s experience had a significant impact on the editing of synthetic hdEMG and the detection of the first MU discharge, but not on the rate of agreement or editing time of experimental hdEMG.

Leveraging both intramuscular and high-density surface EMG recordings in the vastus lateralis, we identified alterations in motor unit (MU) properties in young adults after 21 days of bed rest. These included reduced discharge rates and changes in MU potential size and complexity, observed at both low and moderate contraction intensities. Evidence of impaired neuromuscular junction (NMJ) function and denervation was also found. Our findings indicate that medium-term disuse elicits MU-level changes not detected in shorter-duration studies.

This study provides the first evidence that orofacial and neck mobility exercises can acutely reduce spinal excitability in remote lower-limb muscles. By combining high-density surface EMG with both global and single motor unit H-reflex analyses, we demonstrate a decrease in soleus H-reflex amplitude independent of presynaptic inhibition. These findings suggest potential nonpharmacological applications for managing motoneuron hyperexcitability and promoting whole body relaxation in individuals with cervical and orofacial constraints.