So this weekend was our fourth mountain biking escapade. We arrived at Markham Park promptly at 8am. We've done these trails before, but this time was different. Going down one of the larger slopes, my chain gave out and I must have over-pedaled. 
At the same time, I must have braked because my rear tire came out. That was it. Off the bike, I slid for about 20 feet, and got up immediately.
I walked my bike off the trail and inspected the damage - nothing big, just a bent water bottle cage. Then it hit me like a ton of bricks, I couldn't breathe and became incredibly nauseous. Luckily that only lasted for a few minutes. I fixed the chain and got back on the bike to finish the trail. My arms, rib cage and wrist are pretty banged up, but I should be back out there next weekend!
