Let's start at the beginning, which is the protocol from the clinical trial. The protocol specifies a cadence of between 80 and 90 RPM, and a heart rate of between 60% and 85% of HRmax (where HRmax is simply calculated as 220 minus your age). Forty minutes per session, three sessions per week.

Some people can achieve the protocol's specification without any assistance from a motor (or from a tandem captain). Road bikes or exercise bikes can be used. Different exercise bikes come with different types of "resistance" (e.g. magnetic, fan, belt etc.). You adjust the resistance to achieve the protocol's specification. For example, you may find that when you cycle at a cadence of between 80 and 90 RPM, your heart rate is below 60% of your HRmax. If this is the case then you need to increase the resistance level to get your heart rate into the range of between 60% and 85% of your HRmax.

If you find that you cannot maintain a cadence of between 80 and 90 RPM, even when the resistance is set to the lowest level, you could consider trying a different exercise bike (i.e. one which has a lower minimum resistance setting). If you still can't maintain the specified cadence, you could consider trying an exercise bike with motor assistance.

I know that Nan has her own views on (certain?) motor-assisted exercise bikes, but I think that if such a bike enables you to achieve the protocol's specification then that should be fine.