The Joint VPD System Knee is a mountain biking kneepad from the brand POC. Heat it, put it on, adjust it, and it becomes one with your knee. Very flexible in use, it hardens upon impact to effectively absorb shocks. Ventilated due to its construction, you won't overheat, and the polyamide threads that make it up are highly resistant to abrasion (though that's not an excuse to scrape your knee at every turn).