motor for continouous motion

my device helps to perform elbow continuous passive motion exercise. may i have the idea from you guys what type of motor is suitable to be used? the motor used must be low noise, can withstand continuous motion for hours without dissipated much power (hot), adjustable angle range. initially i plan to use stepper motor but i heard that stepper motor will dissipate a lot of energy if used for long period, any idea from you?

I assume low-noise has a high priority. A stepper is likely to have more noise. I suggest a brushless motor controlled through a servo loop or through a mechanical linkage for the reversal. I can see advantages to the mechanical linkage approach.

As you need variable speed, I would suggest high-frequency PWM, as in a brushless DC motor, instead of a VFD three-phase at something like 60 Hz.

John