Tiring quickly, improper technique?

[Subway]

I've been practicing quite often in an attempt to get faster and faster, and I've seen improvement thus far. However, it seems that every time I really go out and slap faster and faster, my right shoulder gets tired very quickly. Like in less than 15 seconds. Is this normal or could I be doing something wrong? I think I may be pulling my hand too far away from the strings, or I could possibly have the endpin extended too far. Does this sound like anything I would want to change? And advice at all would be appreciated. Thanks.

[Bass]

Don't hurt yourself! Sounds to me you're tensing up, think about relaxing.

[kurt]

depends how high your bass is. A good guideline is to have the nut around eyebrow level. What kind of strings do you have on? normal steel strings can really burn out your shoulder, nylon or gut are great for really fast slap, takes way less effort.

Part of it is just building endurance gradually over time but you really need to concentrate on staying relaxed. Try to make the motion as fluid as possible, the faster you go the less your arm moves. You probably don't need to pull the strings as hard as you think. try to think of using speed instead of strength to achieve power like Bruce Lee. try it slow at first, relax you entire arm with your fingers just under the string. Suddenly as if out of nowhere snap the string lightly and immediately relax again. the slap down on the board should be done with the hand relaxed so it feels more like your whipping the string with your fingers. Remeber, the key is to think speed not strength.