To start the project, I first dismantled a hand cycle and determined the best way to digitally control the tension and assess the viability of the project.  Upon visual inspection the therapy machine was in good working condition and has enough space for components inside bicycle casing.

This is an Endorphin Hand Cycle Model EN-300 disassembled. Below shows close-up of hand cycle.

The top image shows the hand cycle without plastic cover and displays the hand crank, sprocket, fly wheel, spring and support structure.  The bottom image shows a close up of the RPM sensor attached to the flywheel.  This picture also displays a clear view of the tensioning mechanism which I will modify to allow the Arduino to control the tension via a Servo motor.