Without modelling various muscle groups and so forth, this is going to be largely an exercise in gross approximation. I would just find an equation that appears to look nice and stick with it. (e.g. speed of blade = a.(strength - k.(weight)) )
The main problem I see with this is that there are many muscle groups in each motion, all of which act on the forces in the blade in different ways to each other. (as a martial artist I can tell you that this also includes abdominal and leg muscles)
Thus shoulder muscles would be using most of their force to counteract the gravitational force, while the back muscles will be almost exclusively pulling the sword sideways. (assuming a man in normal stance, if he is on an angle or something, stuff changes)
I think the idea here should be KISS. Spend your time on things the user will actually notice...
As an example of what I mean...
Basic speed calculation...
Basic Speed BS = a.strength - b.weight
Upward Swing = BS - c.weight
Downward Swing = BS + d.weight
Thrust = BS
Sideways = BS
a,b,c etc = costants. may be replaced with more complex formula to get exponential curves etc if need be.