Cortical and spinal adaptations induced by balance training: correlation between stance stability and corticospinal activation

Aim: To determine the sites of adaptation responsible for improved stance stability after balance (=sensorimotor) training, changes in corticospinal and spinal excitability were investigated in 23 healthy subjects. Methods: Neural adaptations were assessed by means of H-reflex stimulation, transcranial magnetic stimulation (TMS) and conditioning of the H-reflex by TMS (H-cond) before and after 4 weeks of balance training. All measurements were performed during stance perturbation on a treadmill. Fast posterior translations induced short- (SLR), medium- and long-latency responses (LLR) in the soleus muscle. Motor-evoked potential- (MEP) and H-cond-amplitudes as well as H-max/M-max ratios were determined at SLR and LLR. Postural stability was measured during perturbation on the treadmill. Results: Balance training improved postural stability. H-max/M-max ratios were significantly decreased at LLR. MEPs and H-cond revealed significantly reduced facilitation at LLR following training. A negative correlation between adaptations of H-cond and changes in stance stability was observed (r = -0.87; P < 0.01) while no correlation was found between stance stability and changes in H-max/M-max ratio. No changes in any parameter occurred at the spinally organized SLR and in the control group. Conclusion: The decrease in MEP- and H-cond-facilitation implies reduced corticospinal and cortical excitability at the transcortically mediated LLR. Changes in cortical excitability were directly related to improvements in stance stability as shown by correlation of these parameters. The absence of such a correlation between H-max/M-max ratios and stance stability suggests that mainly supraspinal adaptations contributed to improved balance performance following training.