A study followed 48 men and women for 10 months after they had a unilateral knee replacement.  The researchers found that overall pain levels were significantly lower, but that the effected leg still demonstrated 14% weakness in muscle strength as compared to the non-surgical leg.  What this means is that those individuals still had trouble with activities that required higher levels of strength, including walking up and down stairs and walking up hill.  Bottom line:  keep up with your strengthening exercises long after your range of motion has increased and your pain levels have improved.

A.Valtonen, T. Poyhonen, A. Heinonen, S. Sipila
Physical Therapy Oct 2009;89:10:1072-1079