The extensor lag describes the lack of full knee extension with full contraction of the quadricep. Limited knee extension range of motion may also cause the lack of full extension but eliminating limited ROM as the problem, quad weakness is the major issue. The extensor lag is commonly seen after total knee replacement, ACL reconstruction or patella tendon rupture. The weakness in the quadricep extensor mechanism causes the extensor lag. Vigorous strengthening of the quadriceps is necessary to remedy an extensor lag.
In the case of a patellectomy, the client should be able to fully extend the knee with the proper stregthening regimen. Though the patella does provide a fulcrum that makes knee extension easier and more efficient, the lack of a patella doesn't mean the inability to achieve full extension.
Here is a photo of extensor lag following total knee replacement. Notice the slight lack of full knee extension or the "extensor lag". If you do not look closely you might miss it. Standing terminal knee exercise, quadriceps strengthening, leg press, short arc quads will all help restore full active knee extension.
Please share your experiences working the post rehab clients with "extensor lag".