I doubt a freewheel would help much, although you can run a smaller gear without spinning out.
If I was particularly concerned about my knees I'd gear right down to mid or low 60's and use two hand operated brakes.
@IR started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
I doubt a freewheel would help much, although you can run a smaller gear without spinning out.
If I was particularly concerned about my knees I'd gear right down to mid or low 60's and use two hand operated brakes.