Total knee replacement (TKR) has some very positive outcomes. Besides pain relief, patients often report improved motion and increased function. Most of these patients started from a place of severe disability. Having come from a very inactive lifestyle, their expectations may be less than yours.
Despite good outcomes research shows there are some long-term problems after TKR. Walking speed and stair-climbing speed are as much as 50 percent slower in patients with TKR compared to those without a TKR. Quadriceps weakness and atrophy may account for these changes.
Long-term studies of patients with TKR show continued improvement of muscle activity up to three years later. Even so, the level of voluntary muscle contraction doesn't seem to come up to the level of healthy adults of the same age without a TKR. You may be having this problem.
Perhaps it's time for some formal muscle strength testing if this hasn't already been done. Ask your doctor for a more precise time frame for expected recovery and request muscle testing with a physical therapist. Getting a baseline even now can help you track your progress and find the right exercise program for you.