We are studying the use of programmable constraint machines for rehabilitation and as assistive devices in materials handling. In this paper we describe initial experiments in human interaction with a two-joint passive programmable constraint device, or cobot. The user grasps the manipulandum at a handle, and the manipulandum implements a smooth, hard, low friction constraint curve. Initial experiments in reaching tasks subject to such a constraint show that subjects apply significant forces against the constraint, in a manner dependent on the movement speed and constraint shape. These forces can be broken into passive forces (due simply to the dynamics of the human arm) and forces actively generated by the muscles. Some motor adaptation is also evident.