Jakob Škarabot

Study Design Randomised, single blind, sham-controlled parallel trial Objectives To determine the feasibility of delivering and acceptability of an upper-limb ischaemic preconditioning (IPC) protocol and to explore its acute effect on elbow extension strength and endurance in chronic spinal cord injury (SCI). Setting Outpatient spinal injuries centre, UK Methods Thirty-eight community-dwelling adults at least one-year post-SCI (m. triceps brachii Medical Research Council grade at least 4/5) were randomised (19 IPC, 19 sham) from 770 screened, of which 37 received the allocated intervention. Feasibility was assessed via adherence, adverse events and retention; acceptability via the Theoretical Framework of Acceptability questionnaire, McGill Pain Questionnaire and pain and discomfort ratings. Maximal isometric elbow extension force, intermittent isometric time to exhaustion and m. triceps brachii surface electromyography were assessed pre- and post- intervention. Results Adherence was 100% and retention rate was 97%. No adverse events were reported. IPC was rated more uncomfortable than sham (p < 0.001), but acceptability did not differ between the groups (p ≥ 0.257). Post-intervention strength and TTE did not differ between groups after adjusting for baseline (p = 0.292, p = 0.438). A significant interaction (p = 0.003) reflected lower triceps brachii sEMG at task failure following IPC. Conclusions Upper-limb IPC is feasible and acceptable in community dwelling adults with chronic SCI, but a single session protocol produced no significant changes in maximal elbow extension strength or endurance. The neuromuscular findings warrant investigation in an adequately powered efficacy trial.

Age-related skeletal muscle atrophy differs between upper and lower body muscles and may also differ between lower limb muscles, but these muscle-specific patterns remain poorly characterised. We acquired axial 3T magnetic resonance images from sex-balanced samples of thirty young (24±4 years) and thirty older adults (73±4 years). Muscle volume of seventeen lower limb muscles/compartments was quantified using manual segmentation and expressed as absolute volume, relative to body mass, and as a percentage of total muscle volume. Absolute muscle volume was lower in older adults in 13 out of 17 muscles compared to young, with substantial heterogeneity in the effect sizes ( d =0.01-3.36). Age group × muscle interaction contrasts revealed muscle-specific age-related differences in volume. Three of the quadriceps muscles demonstrated greater age-related atrophy compared to several other muscles (e.g. vastus lateralis differing from 14 of the other 16 muscles; d ≥1.54). Conversely, the anterior and deep posterior compartments of the shank and biceps femoris short head showed significantly smaller age-related atrophy than several other muscles. These interaction effects were exacerbated when expressed in relative terms. Consequently, three of four shank compartments, gluteus maximus and adductor magnus represented a greater proportion of total muscle volume in older adults, whereas the vastus lateralis, vastus intermedius, rectus femoris, semitendinosus, gracilis, and iliopsoas contributed a smaller proportion in older adults. These findings highlight the variability in age-related leg muscle atrophy in a large mixed sex cohort, resulting in a redistribution of muscle volume in the lower limb.

Age-related strength loss may reflect morphological and neural adaptations that differ between muscles. However, it is unclear how ageing affects motor unit (MU) discharge rate modulation and estimates of persistent inward currents (PICs) across contraction levels. Thirty young (24 ± 4 years) and thirty older adults (73 ± 4 years) underwent magnetic resonance imaging and performed unilateral isometric dorsiflexion and knee extension contractions. Individual MU spike trains were identified from decomposed high-density electromyography recordings of tibialis anterior (TA) and vastus lateralis (VL) during triangular contractions at 30%, 50%, and 70% of maximal voluntary force (MVF). Older adults exhibited lower isometric strength in both tasks and smaller quadriceps but greater shank anterior compartment cross-sectional area compared to young. TA MU discharge rate was lower in older adults at all contraction levels but only at ≤ 50% MVF in VL. Discharge rate hysteresis (∆F), an estimate of PIC contribution to MU discharge, was lower in older adults at all TA contraction levels, but only at 70% MVF in VL, and in both muscles older adults exhibited an attenuated increase in ∆F across contraction levels. The ascending discharge rate non-linearity did not differ between groups, except at 30% MVF in VL where it was greater in older adults. Older adults also exhibited lower acceleration and attenuation slopes in TA, but not VL. These results reveal muscle-specific differences in MU discharge rate modulation with ageing, suggesting that the age-related strength loss is likely underpinned to a greater extent by morphological and neural factors in knee extensors and dorsiflexors, respectively.

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.

Persistent inward currents (PICs) govern motoneuron output and are influenced by diffuse neuromodulation and local inhibition. When large diameter afferent feedback is lost, as in some neurological conditions, PICs might additionally amplify and prolong synaptic inputs. Here, we examined whether reducing Ia afferent transmission via ischaemic nerve block alters PIC contribution to tibialis anterior (TA) motor unit (MU) discharge. Across two experiments 12 adults (5 female) performed triangular-shaped isometric dorsiflexion to 30% (Experiments 1 and 2) and 50% (Experiment 2) maximum voluntary force (MVF) at baseline, after a 20-minute rest (control), and during occlusion after inducing an ischaemic nerve block, confirmed by abolition of the soleus H-reflex. TA myoelectrical activity measured during contractions was decomposed into MU spike trains, and from smoothed MU discharges, discharge rate hysteresis (ΔF) and ascending non-linearity (brace height) were quantified. Results from Experiment 1 involving contractions matched to absolute force levels revealed increased peak discharge rate, ΔF, and brace height post-occlusion. However, ΔF normalised to maximal theoretical hysteresis did not change across time points. In Experiment 2, where MVF was reassessed at each timepoint and contractions were matched to relative force, peak discharge rate, normalised ΔF and brace height increased post-occlusion compared to pre-, across both contraction intensities. ΔF only increased post-occlusion at 50% MVF, with no changes at 30% MVF. These results show that ischaemic block of large-diameter axons, likely reducing reciprocal inhibition, increases PIC contribution to discharge rate modulation, highlighting the role of Ia afferent input in shaping motoneuron output in humans.

Unfamiliar eccentric exercise causes muscle damage, but repeating the same exercise later substantially reduces symptoms—an adaptive phenomenon known as the repeated bout effect (RBE). We investigated the neural contributions to this response. We observed lower motor unit discharge variability and smaller increases in firing rate after the repeated bout. These findings provide evidence that motor unit-level neural adjustments are a critical component of the RBE, offering a new perspective on this protective adaptation.

Physical training alters intrinsic motoneuron properties in reduced animal preparations, especially in neurons recruited at high excitation levels. Here, we show that individuals with a history of resistance or endurance training exhibit higher discharge rates that are more linear during forceful contractions. This likely reflects a more reciprocal/push-pull excitation-inhibition coupling, leading to greater net excitation to the motor pool that may contribute to greater force production observed in trained individuals.

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.

This study explored the relationship between motoneuron output to the vastii muscles at different knee-joint angles (quadriceps lengths) and isometric contraction levels. We showed that the motor unit discharge rate was lowest at the angle of the greatest absolute torque capacity, whereas the contraction-level-induced increases in discharge rate and motoneuron excitability were the greatest in the flexed position. These findings suggest that joint-angle-dependent adjustments in sensory feedback modulate motor control of the knee-extensor muscles.