The measure that matters is power, not heart rate. Until you can measure it in both scenarios, all you know is your heart is working harder with shorter cranks. But efficiency is providing the most power for the least cost in resources. Also, are you tracking your pedal RPM? It has been known forever that riding a given speed and therefor power in a lower gear lowers the load on the legs but raises heart rate and oxygen consumption. This may be what's happening on your trainer with the shorter cranks helping you spin faster.
Remember power, what you are after ultimately, is the force you apply to the pedals times the pedal RPM times the crank length. If you keep the same RPM and pedaling force, you will only be doing 145/170 = 85% as much work on the trainer. (Can you change the trainer cranks to 170? I would if I could. Edit: I see you answered this. I'd consider this trainer a temporary and not ideal solution unless you are training for BMX short crank riding.)
Last edited by 79pmooney; 04-29-22 at 11:01 AM.