Validation of the filling factor index to study the filling process of the sEMG signal in the quadriceps

The filling factor of the sEMG signal at low contraction forces in the quadriceps muscles is influenced by the thickness of the subcutaneous layer

“…However, this initial "pulsatile condition" is not fulfilled in all muscles. In fact, in our previous study, we found that the initial sEMG activity was pulsatile for most of the vastus lateralis and medialis examined (75% and 87%, respectively), but not for the rectus femoris (3%) (Rodriguez-Falces et al, 2023). Interestingly, the rectus femoris sEMG signal contained many MUP discharges of small amplitude, which overlapped extensively, i.e., a signal which looks "continuous" (see Figure 1b3).…”

“…If the presence of pulsatile sEMG activity at low contraction forces is more easily found in muscles with thin subcutaneous layers, then one would expect to find a higher percentage of initiallypulsatile sEMG activity in males than in females, given the thicker subcutaneous layers in the latter (Caresio et al, 2015). In our previous study, the filling process of the sEMG signal was investigated in the quadriceps muscles only for healthy young males (Rodriguez-Falces et al, 2023). Therefore, it is yet to be determined how the filling process of the sEMG signal would be in these same muscles for healthy young females.…”

“…This is partly due to the fact that the sEMG characteristics during the filling process depend not only on neural factors, but also on the anatomical specificities of the muscle (such as thickness of the subcutaneous layers, arrangement of muscle fibers, etc.) and the spatial organization of motor units within the muscle (Rodriguez-Falces et al, 2023).…”

“…A condition necessary to study the filling process of the sEMG signal is that, at the beginning of this filling process, the sEMG activity must be "pulsatile" (see Figure 1A3), i.e., composed of a few largeamplitude MUPs, clearly standing out from background noise (Rodriguez-Falces et al, 2023). Only if the sEMG activity at low force levels is pulsatile, the successive activation of additional motor units with increasing contraction force will progressively fill up the baseline with MUPs, until this baseline is completely obscured, thus giving the opportunity to examine the filling process in a comprehensive manner.…”

“…The present objectives will be addressed using a new analysis tool called the filling factor, which is derived from the probability density function (PDF) of the sEMG amplitudes factor (Navallas et al, 2023). The filling factor allows to quantify the degree to which an EMG signal has been filled, and thus has the potential to characterize the EMG filling process, and also to differentiate between a "pulsatile" and a "continuous" sEMG activity (Rodriguez-Falces et al, 2023).…”