Every person I know who describes an injury from skateboarding describes it as a freak accident or explains in detail the odd things that went wrong--me included. My 4WD skateboard is a lot of fun and I've optimized the acceleration and braking carefully. So, of course, it was a freak happenstance when it slammed on the brakes at 20 MPH and disappeared from under me. Half an hour before I was riding nekkid--shorts, a T-shirt and slippahs. I came to my senses and armored up. Shoes, long pants, shin guards, elbow guards, wrist guards, helmet. I hit the ground, bounced and skidded to a stop. No abrasions, no breaks, no blood. Just three weeks of hobbling around feeling like I was hit my baseball bats. I still don't know what caused it, and I've decided it will make a nice base for an autonomous robot.