The dexterous upper limb serves as the most important tool for astronauts to implement in-orbit experiments and operations.This study developed a simulated weightlessness experiment and invented new measuring equipment to quantitatively evaluate the muscle ability of the upper limb.Isometric maximum voluntary contractions(MVCs) and surface electromyography(sEMG) signals of right-handed pushing at the three positions were measured for eleven subjects.In order to enhance the comprehensiveness and accuracy of muscle force assessment,the study focused on signal processing techniques.We applied a combination method,which consists of time-,frequency-,and bi-frequency-domain analyses.Time-and frequency-domain analyses estimated the root mean square(RMS) and median frequency(MDF) of sEMG signals,respectively.Higher order spectra(HOS) of bi-frequency domain evaluated the maximum bispectrum amplitude(Bmax),Gaussianity level(Sg) and linearity level(Sl) of sEMG signals.Results showed that B max,S l,and RMS values all increased as force increased.MDF and S g values both declined as force increased.The research demonstrated that the combination method is superior to the conventional time-and frequency-domain analyses.The method not only described sEMG signal amplitude and power spectrum,but also deeper characterized phase coupling information and non-Gaussianity and non-linearity levels of sEMG,compared to two conventional analyses.The finding from the study can aid ergonomist to estimate astronaut muscle performance,so as to optimize in-orbit operation efficacy and minimize musculoskeletal injuries.
