A non-arbitrary definition of the analyzing interval of sEMG signal measured during sustained low back extension

Recording of surface EMG signals over lower back muscles during sustained exercise protocols used for classification of different categories of low back pain subjects and healthy subjects are typically accompanied with measurement of kinematic parameters elaborating measurements per se and the interrelationship of surface EMG parameters and kinematic parameters. We propose an exercise protocol without measurement of kinematic parameters and examine the possibility to use spectral changes of the surface EMG signals during fatiguing static contractions in defining a time interval to be analyzed. The protocol was applied to fifteen subjects without low back pain and who were able to sustain long static contractions over 100 s. Variability of the slope of regression line that fits values of median frequency of the power spectral density function in a least-square sense was checked versus the duration of analyzed interval and start time selected. This study proposes that muscle contraction start time can be related to distinct spectral change of the surface EMG signal that are consequence of designed test protocol.