Hmmmm well myometer measures the electric activity in which in the population you are interested may not the best solution. In the other hand requires to pefrom an isometric action same with simple MMTs and using an handheld dynamometer. Perhaps a simple pulley weight lift can be more objective and countable. Whatever approach you decide make sure that your arse is going to be on the clear due to the age and population you are interested in.
What kind of surgery we talking about and which part of the body that requires strength mesurements. I would be more concern regarding movement dexterity rather than strength that can be easily build up.
You may also need to ask the Drs or the hospital for online library codes so you can further expand your search if possible.
I will have a look and if i find anything safe will let you know ASAP