The sitting active and prone passive lag test: a validity study in a symptomatic knee population

[Purpose] This study aimed to determine the diagnostic utility of the sitting active and prone passive lag test in identifying terminal extension lag in unilaterally symptomatic knees. The lack of full extension at the knee leads to greater force of quadriceps activation, overloading of the weight bearing joints, abnormal gait mechanics, resulting in pain and dysfunction. [Participants and Methods] Participants were randomly assigned and evaluated by two blinded examiners, to determine the presence of extension lag at the knee. The reproducibility of test results between examiners was determined, for reliability. In addition, the ability of the test to identify the presence of extension lag in symptomatic knees and absence of extension lag in asymptomatic knees was assessed, for validity. [Results] The results revealed the test to possess an 'almost perfect' inter-rater reliability, high sensitivity, and moderate specificity. [Conclusion] The sitting active and prone passive lag test may be incorporated as a reliable and valid test to determine the presence of terminal knee extension lag in a unilaterally symptomatic knee population.