US Patent 9339678 Modular resistance force system

A modular resistance force system includes an axle configured to be rotatable around a rotational axis and one or more resistance mechanisms. Each of the one or more resistance mechanisms includes a resistance element disposed about a portion of the axle, a resistance element housing configured to house the resistance element and a resistance substance disposed between the resistance element and the resistance element housing. Either the resistance element or the resistance element housing is selectively engaged to rotate with the axle. A resistance between the resistance element and the resistance substance causes a force to be applied to the axle when the resistance element and the resistance element housing move relative to each other.