The seat height of a properly fitting balancing bike can be adjusted 1" to 1.5" lower than a child's inseam (measured from crotch to floor without shoes on) to allow for a "athletic stance" with a small knee bend. The bottom bracket should be no higher than the child's hip joint for proper fit and maintenance. A standard bicycle has top tubes that are about 42 inches high, so a 32-inch waist is about the maximum size that can ride one safely. The rider's age also affects the height that should be allowed between the saddle and the ground. Typically, children start out by sitting on the seat with their legs splayed outward, learning how to sit up straight. As they get older, they can learn how to sit up straighter even though they're still not fully grown. For most adults, the seat should be around the level of the heart or just below it. The top tube should be long enough to allow for a relaxed posture with the head held high. Adults who are very tall or short may need a bike with a taller or shorter top tube, respectively. Children's bikes usually have seats that are about 30 inches high, which is low enough to protect them if they fall. Some kids like to squat down close to the ground when they ride, so they can see what's going by.