Need help...Problem with my left thumb!

[theholygideons]

This has been a problem that I've had for as long as I remember. Basically, there is an inherent difference between my left and right thumb. My right thumb is completely normal, it doesn't experience any tension at the joints when I play chords with long reach, repeated octaves, or octave tremolos. However, there seems to be a physical hindrance with my left thumb. Whenever I octave tremolo or play chords that have a large stretch between the index finger and thumb, both the joint on the thumb and the joint connecting the thumb to the hand, IP and MP joints (if i'm correct), seem to lock up.

To give a demonstration, for my right hand, if i try moving the thumb as far back as i can, just using the muscles on my right hand, the joints on the thumb don't lock up and the other fingers on my right hand start moving in order to counteract the movement of my right thumb back.

However, when i try doing the same with my left hand, the left thumb just locks in place, and the fingers on that left hand don't even need to move, since my left thumb just locks itself up.

Any anatomists around here?  please help, this has affected my piano playing in many ways, and it doesn't seem like an issue with my piano technique.