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

Objective

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

Methods

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).

Results

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.

Conclusions

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.

Significance

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