The Bartlett Tendon Knee is the world`s first hybrid prosthetic built for outdoor adventures. Get Active, Get BtK! The World`s Toughest Prosthetic Leg. The Bartlett Tendon Knee utilizes the industry`s most natural feeling prosthetic, so you can spend more time enjoying the activities you love. The BtK has has years of proven performance in the most demanding and extreme conditions. The BtK is more than a prosthetic built for extreme sports. While it is built to military specifications and can handle an incredible amount of physical stress, it is also designed for use on activities that require more agile, fine-tuned movements. Whether you`re into aggressive downhill skiing, riding your bike with your kids, or just starting on your path to rehabilitation, the Bartlett Tendon Knee will help you every step of the way.