when walking on a treadmill at a 4.0 pace and a 7.0 incline what would be the heaveiest hand wieghts i could use that would not compromise my healthi am 44 year old and in great shape

that's actually a fairly tough question. I think you would probably get better results by doing some light weight-training than carrying the weights ... what is the goal of carrying weight while using the treadmill? toning of the muscles or burning extra calories?

I think that carrying weight whilst running will only increase the impact to your joints which could cause problems.

Unless you have very specific goals (like the US Navy Seals specificity training - carrying gear and injured colleagues) I agree with AllCdnBoy that running and a separate weights session would be better for you, from an overall health point of view.