Spinal motor neuron excitability plateaus in very old age
In an observational study of 89 adults, estimates of motor neuron persistent inward currents fell by the seventh decade without further decline past age 80.
bioRxiv
In a preprint study, researchers assessed spinal motor neuron excitability across 30 young adults, 30 older adults, and 29 very old adults aged 80 and above. Participants performed isometric ankle dorsiflexion ramp contractions at 20% of maximal voluntary torque while high-density surface electromyograms recorded tibialis anterior motor units. The team used the paired motor unit technique to estimate persistent inward currents, measured as ΔF and normalized ΔF. Overall, ΔF was lower in both older groups than in young adults, with no further difference between older and very old cohorts. However, normalized ΔF showed sex-dependent patterns: it was lower in both older and very old males compared to young males, but was only lower in the very old group among females. Neuromodulatory and inhibitory indices did not differ across age groups or sexes.
Why it matters
The findings show that reductions in spinal motor neuron excitability emerge by the seventh decade and stabilize into very old age rather than progressively deteriorating. These non-linear trajectories highlight sex-specific differences in spinal cord aging and suggest intact monoaminergic neuromodulatory drive in older adults.
Caveats
This work is a preprint and has not yet completed peer review. It relies on a cross-sectional design in healthy, active participants and evaluates a single lower-limb muscle during submaximal isometric contractions.
The paper
Université de Bourgogne
bioRxiv · 29 Sep 2026 · Preprint, not peer-reviewed