The present study aimed to determine the influence of different acute simulated altitude exposures (normobaric hypoxia; Fraction of inhaled oxygen (FiO2) = 0.18 / 0.16 / 0.13, i.e.: 1500 m, 2500 m, 4400 m a.s.l.) on the H-wave during submaximal isometric contraction at 30% MVC.
Thirteen healthy volunteers performed the following four tests in a randomised order: Normobaric normoxic test (placebo) and three normobaric hypoxic exposures as detailed above. The wash-out period between the tests was at least one day. The acute changes in spinal properties were assessed during each experimental condition, via tracking the H-wave in the soleus muscle. In addition, the capillary oxygen saturation (SpO2) and heart rate were measured continuously.
SpO2 significantly decreased in all three hypoxic environments (FiO2 = 0,18 / 0,16 / 0,13) (all; p<0,05), in comparison to the normoxic values. A comparison between different hypoxic levels at the same time intervals did not show statistically significant differences. A comparison of the H/M ratio at rest in normoxia and during air mixture breathing or placebo, showed a statistical increase only at a simulated height of 2500 m a.s.l. Meanwhile, a comparison of the H/M relationship, showed an increase of the H-reflex during rest in all three hypoxic mixtures at rest (0.18 = + 8%, 0.16 = + 16%, 0.13 = + 13%). The H/M ratio did not change significantly during the submaximal isometric contractions in hypoxia compared to the values during hypoxic rest, while the H/M relationship showed statistically significant increased excitation in all three simulated hypoxic conditions. The maximum amplitude of the M-wave at rest and during submaximal contractions remained unchanged. In addition, the positive correlation of SA and spinal excitability in certain simulated conditions (FiO2 = 0.16 / 0.13, i.e., 2500 m, 4400 m a.s.l.) suggests, that the SpO2 plays an important role in soleus excitability under hypoxic conditions.
We note that the net inflow of alpha motor neurons in acute hypoxia seems to increase, during both rest and submaximal isometric contractions. However, as there were no statistically significant differences between the simulated hypoxic conditions, it seems that the limitation in the neural transfer occur at the supraspinal level due to the limited systemic oxygen availability. Meanwhile, the factors at the peripheral level seem to remain unchanged. If the excitation of the sarcolemma is preserved, the translation of the action potentials is uninterrupted and the force generating ability therefore remains preserved.