SPRING TECH SUSPENSION technology, a patented specification, makes it possible to adjust the surface hardness according to the user with the five different color choices placed at the bottom part of the treadmill. Each color resembles a different surface hardness. You can choose 10 % hardness with the red one; 20% with orange, 30% with the blue, 40% with the purple and 50 % with the green one. Thanks to them, walking on the treadmill will be no different from walking in the heart of the nature.