I would recommend using sub-maximal voluntary isometric contraction to normalize the EMG signal. (see - Burden https://doi.org/10.1016/j.jelekin.2010.07.004)
We have used the ability of the patient to maintain an anatomical segment against the force of gravity as submaximal task (see -Tabard et al. https://doi.org/10.1016/j.gaitpost.2017.10.026)
Hope this helps,
Stephane ARMAND