According to the Center for Universal Design, headquartered in North Carolina State University, UD has seven basic principles.
Low Physical Effort
The design should bear in mind that the user needs to exert the most minimum effort for a device to function. This principle allows both the strong and the weak in stature to enjoy the benefit of a Universal Design. Comfort and ease of use is the driving force behind this principle.
In the picture a door level is presented that does not require grip strength. A close fist or an elbow can offer the door.
See 7. Size and Space