When I touch the lowest string string to make palm mute or just muting it (basically all the time) I always feel a slight tense on my shoulder of the picking hand.
I could play like that for hours, but once in a while I have to stop for half a minute, because it grows gradually.
i totally fucked my shoulder. i have no idea how but it's sore all the time. i can barely lift it. depending on the way i let it rest it may or may not hurt and it get worse throughout the day. kill me.