This is a problem that occurs regularly in off road motorcycle riders as well. Check the position of your wrists on the handlebars. If the bar is across the lower part of your hand with your wrists fully back, you are likely putting some pressure on your Carpal tunnel and what is known as the Palmar aponeurosis. Shift your hand forward a bit in rotation and your thumb will be more responsible for staying on the bar (wrist neutral). If your breaks are placed too far back for you to do this then adjust them a little to take this pressure off. As mentioned earlier padded gloves help as well. If there is numbness and tingling or pins and needles type feelings or it occurs at other times you may have more involved neural compression that may require treatment to relieve. Yes I am a medical professional and know just a little about these things. Good luck