Straight leg raise versus knee extension angle: which structure limits the test in asymptomatic subjects?

This study aimed to determine if the first onset of symptoms (discomfort) during the straight leg raise (SLR) (hip flexion with an extended knee) and the Knee Extension Angle (KEA) tests (knee extension with 90°of hip flexion) results from nervous or muscular structures in asymptomatic individuals. The secondary objective was to investigate if the gender influences the structure related to the discomfort.

This cross-sectional study consisted of a single assessment session during which the structure related to participants' discomfort during the KEA and SLR was identified. For this identification, a structural differentiation (SD) was conducted during both tests using passive mobilization of the cervicothoracic spine in flexion and extension. Changes in participants' discomfort were monitored during the SD to determine whether a change or lack of change was consistent with variations in the load applied to the suspected structures either muscular or neural. If the structure related to the participants' discomfort could not be identified, two additional tests were conducted: the lateral SLR and the Slump test.

One hundred and seventy-eight individuals were included. Median [IQR] age was 21 years [20;23], and 57.3% were female. The structure related to participants' discomfort was similar for the SLR and the KEA (p = 0.451): neural for 72.5% of participants in the SLR and 75.8% in the KEA. Gender only influenced the structure identified in the KEA test, with a significantly higher rate of nerve-related discomfort in females than males and a significantly higher rate of muscle-related discomfort in males (p = 0.002).

In asymptomatic individuals, the discomfort induced by the SLR and the KEA tests could be related to either muscular or neural structures. Therefore, structural differentiation is necessary to identify the structure causing the discomfort in both research and clinical practice.