When I play basic major scale from 8th fret, as I go up the tempo my left hand starts ache. The muscle that is tight is the base of the palm, the one that's used by Karate masters to brake bricks :-)
Neither my action is high, nor my left hand is weak. I have been practicing daily for the last 4 months and I did play guitar and bass during my teenage years (so muscles should have been adapted together with body development). I feel the pain mostly because of my pinky movements, when I stretch to fret with my index after pinky and vice-versa. I tried to change position to more classical one (bass on the left leg), but still, 60 bpm playing 16 notes and the palm is on fire. I have no other issues with my pinky when playing regular riffs. My hand is very small, glove size XS, but I don't think that this should be the limitation. Neither I feel that I fret hard. What can I do with my technique to be able to play major scales for at least 5 minutes 16th notes?