Hello guys! I wouldn't say that I am "new" to playing the guitar, yet I think I have a few bad habits.

When I play basic chords, especcially shifting fast back and fourth, my thumb tends to hurt a lot after a short while.
Now, I watched a left hand tutorial on youtube, but I still haven't figured out a few things.

Do you put pressure on the thumb, or is it basically just resting there ?
Also, I saw that he (link, don't know how to upload pics, also go to 04:31) - 6GnAdRVIaDE has the thumb bent/curved upward, further away from the neck. Is this normal when you play basic chords, and/or is it okay to have it sort of relaxed, not curved that much and the hand closer to the neck?
I'm confused.

Anyway, I hope that you guys can shed some light on this topic

- Tommy
My thumb rests. It barely touches the neck. There should be no pressure on your fretting hand thumb. It sounds like you're more or less gripping the neck with your hand, and that's not good.
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Excessive tension is never a good idea, no matter what your hands are doing. Barre chords typically need more help from the thumb, to sound cleanly.

Unnatural bending of the thumb joint is a terrible idea.

Are you playing acoustic or electric? What gauage strings? How high is your action? Are you tuned in concert pitch?
As the others suggested, it sounds like you are experiencing unwanted tension in your fretting hand. Your fretting hand should rest on the fretboard, and there should be absolutely little to no force (pushing) coming from your thumb.

Maybe it's because of the tension on the strings? I know that on most acoustics, especially ones that beginners play on, the action is really high. Add that into the typically thicker string gauge, you're probably exerting more pressure and tension from your fretting hand.
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You do need to keep your thumb as relaxed as possible, but it is still used to help your other fingers play. I've been playing guitar about 5 years now and I would say it's 90% technique and 10% strength (in terms of fretting). I've noticed that the tendon (or whatever it is) on my left thumb is noticeably bigger than my right from "working out" playing guitar.
Thank you all for replying! I think i've figured out where my problem was. Action and gauge are all in the green area. I did tend to curl my thumb around the neck, also with a tight grip around it. I now have to practise doing it right.Thumb more or less resting, not as close and playing only with the tip of my fingers.

Thanks again guys! You really helped me out!