Author(s): CHATAIN, C., ROTY, R., CATTAGNI, T., LEPERS, R., HAMARD, R., ROZAND, V., Institution: UNIVERSITÉ BOURGOGNE EUROPE, Country: FRANCE, Abstract-ID: 1355

INTRODUCTION:
Persistent inward currents (PICs) play a critical role in spinal motor neuron output by amplifying and prolonging excitatory synaptic inputs. Recent findings suggest that aging and repetitive muscle contractions reduce PICs contribution to motor neuron firing (1,2). However, it remains unknown whether very old individuals exhibit additional impairments compared with old adults, and whether fatigue-induced changes in PICs differ between young, old and very old adults.
METHODS:
Twelve young (27 ± 5 yrs), 12 old (71 ± 5 yrs) and 12 very old (83 ± 2 yrs) participants performed a fatiguing task consisting of 10 isometric triangular-shaped dorsiflexion ramps (20 s up, 20 s down) to 60% of maximal voluntary force (MVF). MVF was assessed before and immediately after the fatiguing task. Triangular contraction ramps to 20% MVF (10 s up, 10s down) were performed before and after the fatiguing task to estimate PICs. After the fatiguing task, ramps were performed up to 20% of post-fatigue MVF (relative condition) and up to 20% of initial MVF (absolute condition). Motor unit discharge rates were extracted from high-density EMG of the tibialis anterior muscle. The prolongation effect of PICs, quantified as delta F, was estimated using the paired motor unit method (3). A geometric approach was used to estimate the amplification effect of PICs (acceleration slope), the neuromodulatory drive (brace height) and the pattern of inhibition (attenuation slope) (4).
RESULTS:
MVF decreased after the fatiguing task (p<0.001) independent of age. Peak discharge rate was lower in old and very old than in young adults (p<0.01) and decreased after the fatiguing task in both relative and absolute conditions (p<0.001), with an interaction effect in relative condition only (p<0.001). Delta F was reduced in old and very old compared with young adults (p<0.01), and decreased after the fatiguing task in relative condition only (p<0.01). Brace height increased after the fatiguing task and acceleration slope decreased in relative condition (both p<0.01), whereas no effect was observed in absolute condition. Attenuation slope decreased after the fatiguing task in both conditions (p<0.01) independent of age.
CONCLUSION:
These results suggest that the similar reduction in peak discharge rate observed in old and very old adults is mainly mediated by the decrease in intrinsic motoneuron excitability, as indicated by reduced PICs prolongation effect. In addition, regardless of age, fatigue-related reduction in discharge rate appears to be explained by decreased in prolongation and amplification effects of PICs and increased synaptic inhibition. Finally, all parameters exhibited similar fatigue-induced changes across groups indicating comparable impairments in PICs prolongation and amplification effects in young, old and very old adults.

(1) Orssatto LBR et al. J Physiol (2023)
(2) Mackay K et al. J Neurophysiol (2023)
(3) Gorassini M et al. Neurosci Lett (1998)
(4) Beauchamp JA et al. J Neural Eng (2023)