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.