Cam-type femoroacetabular impingement (FAI) is characterized by an abnormal femoral head extension that disrupts normal joint biomechanics and leads to early hip osteoarthritis. Comparing the symptomatic hip with the contralateral, asymptomatic side is essential to identify potential compensatory mechanisms that may also influence the asymptomatic hip. The purpose was to examine the hip joint kinematics and kinetics in patients with unilateral FAI and compare them with their contralateral, unaffected hip and healthy controls during level walking and maximal squatting.

Fifteen patients with cam FAI (n = 15, m:f = 8:7; age = 27 ± 8; BMI = 24 ± 5) and fifteen healthy control participants (n = 15, m:f = 6:9; age = 25 ± 7; BMI = 23 ± 4), as confirmed by MRI, were recruited. Each participant underwent biomechanical assessments at our motion capture laboratory, capturing their hip functional kinematics using a motion capture system; and kinetics using instrumented force plates. Each participant completed five level walking trials and then five maximal squatting trials at a self-selected pace. The kinematics and kinetics waveforms were compared using statistical non-parametric mapping between the affected and unaffected hips of FAI patients (Wilcoxon signed-rank test) and between the control group and each FAI group (Mann-Whitney U test).

During walking, the FAI patients exhibited decreased range of motion across in all three planes, with the most notable findings being increased abduction and internal rotation of the FAI patients’ unaffected side compared to their contralateral affected side and to the controls. The FAI group also showed decreased hip extension and abduction moments bilaterally. During squatting, although the FAI group demonstrated increased hip abduction and internal rotation in the unaffected hip; and were unable to squat as deep (46 ± 14%) compared to the control group (51 ± 15%). The FAI group also reduced their extension moments on their affected side compared to their contralateral unaffected hip and to controls. The FAI group showed decreased abduction moments on both sides compared to controls near maximal depth (40% squat) and during squat ascent (80% squat, p < 0.05).

The most important finding was the compensatory increase in hip abduction and internal rotation in the FAI group's unaffected hip during both walking and squatting. This indicates that the contralateral hip adapts to the affected side's symptoms and functional limitations. These compensatory mechanisms, including reduced extension and abduction moments, likely minimize pain to maintain function. As the deep squatting task involves significant hip flexion and internal rotation, often triggering FAI symptoms, patients increased abduction and leaned their trunk to compensate and maintain similar hip flexion angles. Furthermore, the reduced extension and abduction moments further suggest altered abductor synergies in the FAI group. The observed compensatory increases in hip abduction and internal rotation on the unaffected side during both walking and squatting tasks suggest that the contralateral hip undergoes significant biomechanical adaptations.