Passive-mode treadmill test effectively reveals neuromuscular modification of a lower limb muscle: sEMG-based study from experiments on ISS

The study was aimed at exploring informative value of nonlinear parameters of surface electromyogram (sEMG) to characterization of neuromuscular activity in respect with duration of space flight (SF) and stepping mode. The working hypothesis held that sEMG entropy, correlation dimension (CD), and Lyapunov's exponent, which characterize signals in terms of regularity and predictability, may inform on motor units (MUs) synchronization or content of MUs of different types in muscle activity. In addition, median frequency (MDF), mean amplitude, and kurtosis of the probability distribution of sEMG were computed. sEMG samples were collected from the soleus of 6 cosmonauts on board of the International Space Station during 5 modes of stepping on the passive-mode treadmill in the MO-3 test, and 5 study points - before and after SF on ground (preSF, postSF) and for 3 periods of time in SF (1–80, 81–120, 120–170 days of SF). Altogether, 15 individual sEMG sample sets were successfully obtained. It was found that 1) the sEMG parameters pooled for all subjects by the stepping mode, except for MDF and kurtosis, did modify in the direction of higher regularity of the signal with growing speed of stepping, 2) the conditions of SF exerted effect on MDF, Lyapunov's exponent, entropy, and kurtosis of the probability distribution of sEMG, which probably indicated on greater involvement of the faster type motor units in muscle activity. In conclusion, nonlinear sEMG parameters look promising for characterizing the neuromuscular activity of skeletal muscles under SF conditions. Nonetheless, a small number of sample sets combined with different study points in different cosmonauts allow considering these facts as promising, but not yet definitely established. • Results of passive-mode treadmill test in long duration space mission are discussed. • Characteristics of surface electromyogram of cosmonauts soleus muscle are analyzed. • Nonlinear and linear parameters of surface electromyogram are considered. • Most of studied parameters proved sensitive to the stepping speed during the test. • Space flight results in greater proportion of faster motor units in muscle activity.