Torque and speed control of a DC motor
Me: You need to decide whether you want constant speed or constant torque as you can't have both at the same time. I suspect you want to set a current and voltage limit giving you a maximum torque (current) and a maximum speed (voltage).

You: I would like to set a current limit so that the motor would not exert more torque than necessary as that would take away the concept of rehabilitation. In that case I should control the voltage while setting a current limit, is that right?

If you have a DC motor and a bench power supply you can play with you can get a good intuitive feel for how this might work.



Figure 1. A typical lab / bench PSU with adjustable voltage and current
limit.

Experiment 1 - constant voltage: 


Set the voltage to 1/4 the motor's rated voltage and turn the current limit up high.
Connect up the motor and load the rotating shaft by gripping it (carefully) with your hand. Observe the voltage and current readings. You will see the current rise as the motor slows slightly. This is as close to constant speed that you can get with just a straight DC supply.


Experiment 2 - constant current:


Set the current to half the maximum observed in the previous experiment.
Connect up the motor and notice that it runs at full voltage and low current.
Gradually load the shaft by hand again. Not that at some point the power supply reaches the current limit and the voltage starts to fall while the current remains at the limit. You are now in constant torque mode.


You can play with the voltage to set the maximum speed and play with the current to set the maximum torque. The motor will run to whichever limit it hits first. In your case this is most likely to be the torque limit.

This will give you some instinct regarding the type of control you need. Then you can look for a suitable controller to meet those requirements.