# Spinal excitability is enhanced by transcranial magnetic stimulation of the motor cortex in children and adolescents

## TL;DR

Transcranial magnetic stimulation (TMS) increases spinal excitability in children and adolescents, similar to adults, but with differences in how muscle activity affects this response.

## Contribution

The study reveals age-specific differences in spinal excitability modulation by TMS during muscle activation in children and adolescents.

## Key findings

- TMS enhanced spinal excitability at rest and during agonist muscle activation in children and adolescents.
- Unlike in adults, agonist muscle activity did not further increase TMS-induced facilitation in children.
- Spinal excitability modulation in children showed a later facilitation at specific timing intervals during rest.

## Abstract

•TMS induced supraspinal input changes the spinal excitability in children and adolescents.•TMS enhanced spinal excitability at rest and during agonist, but not antagonist muscle activation, similarly as in adults.•Unlike in adults, agonist activity did not potentiate the faciliatory effect of TMS.

TMS induced supraspinal input changes the spinal excitability in children and adolescents.

TMS enhanced spinal excitability at rest and during agonist, but not antagonist muscle activation, similarly as in adults.

Unlike in adults, agonist activity did not potentiate the faciliatory effect of TMS.

To investigate the modulation of spinal excitability to sub-motor threshold transcranial magnetic stimulation (TMS) in healthy children and adolescents.

We paired sub-motor threshold TMS (conditioning) with electrical tibial nerve stimulation (test) to assess changes in the soleus H-reflex at eleven conditioning-test (C-T) intervals. Eleven participants (7.5–16.2 years) were studied under three conditions: rest, voluntary agonist activation (ankle plantar flexion), and voluntary antagonist activation (ankle dorsiflexion).

TMS-induced supraspinal volley facilitated the soleus H-reflex amplitude significantly during rest and agonist activation, but not during antagonist activation. The facilitation of H-reflex was similar during rest and agonist activation between C-T intervals from –2 ms (i.e. peripheral stimulation before cortical) to +18 ms (i.e. cortical stimulation before peripheral). Specific to rest condition, a later facilitation occurred at C-T intervals of approximately +60 ms.

Spinal excitability modulation in children and adolescents showed similarities to earlier evidence in adults, with the distinction that, unlike in the adults, agonist activity did not further enhance the supraspinal facilitation of the H-reflex compared to the rest condition in children.

These results provide insight into the maturation of motor control and regulation of spinal excitability, offering possibilities to identify typical and atypical developmental trajectories.