Fig. 2.

The axial hip rotational stiffness versus angular rotation relationship used to simulate femoracetabular impingement (FAI), where θ is the hip internal rotation angle; θ_FAI is the hip internal rotation angle at the end of the range of motion secondary to impingement; θ₁ is the hip internal rotation angle where the impingement begins and is set to 5°; k₁ is the stiffness coefficient when the impingement does not occur and is set to 0.5 Nm/deg; k₂ is the stiffness coefficient when the hip internal rotation angle exceeds θ_FAI and is set to 5 Nm/deg