Corticospinal excitability of the soleus muscle during water immersion

Water-based exercising is a common form of physical activity used in both rehabilitation and athletic training. However, despite being widely used its’ effects on neuromuscular control is poorly understood. Therefore, the main aim of this study was to investigate how a brief (15 min) immersion to thermoneutral (34°C) water would affect corticospinal excitability and/or intracortical circuitry when soleus muscle is targeted, compared to dry land measures. The experiments were conducted with the subject either resting (passive experiment) and separately during a low-level submaximal (20% of maximal voluntary contraction) isometric voluntary contraction (active experiment) while seated. The depth of immersion was at sternum level. In total nine (9) subjects underwent the measurement protocols, results from six (6) of the total were analysed. Corticospinal excitability was assessed with transcranial magnetic stimulation via input-output -curve. Intracortical circuitry was measured with short-interval intracortical inhibition and intracortical facilitation for inhibition and facilitation, respectively. In addition, in both experiments force (Newtons) and electromyography (root mean square) values were measured and analysed. The main findings of the study were that at rest, the corticospinal excitability was significantly higher on dry land at mid (120% of the resting motor threshold, p = 0,001) and high (140% of the resting motor threshold, p = 0,007) end of input-output -curve. However, these changes were not reflected at 130% of the resting motor threshold level (p > 0,05). In the active experiment the input-output -curve had no statistically significant differences (p > 0,05 for all the measured levels). Intracortical inhibition was unaltered (p > 0,05) in both experiments, while intracortical facilitation measurement protocol was deemed unsuccessful. Also, in both experiments both the force values and electromyography values during maximal voluntary contraction had no statistically significant differences (p > 0,05). In the active experiment the level of background electromyography activity was also similar in water than in dry land (p > 0,05). Furthermore, the motor thresholds (measured as percentage maximal of stimulator output) and all stimulation intensities were similar in both conditions in both experiments (p > 0,05). Taken together, the results of this study suggest that water immersion alone is not enough to alter corticospinal excitability of the soleus muscle at either rest or during a low-level submaximal isometric contraction. Similar results have been obtained when upper extremity muscles are targeted in either partial or complete water immersion (Sato et al. 2014; Sato et al. 2015). Further, the intracortical circuitry seemed to remain unaltered by the environmental change. While hypothesized, spinal excitability may have an effect in the observed increase in corticospinal excitability in the resting experiment. Further studies are needed to clarify the contribution of spinal and cortical mechanisms in the observed changes during thermoneutral water immersion