Knee joint kinematics of the pendulum test in children with and without Down syndrome

•We studied the biomechanics of the pendulum test in children with and without DS.•Children with DS had a lower first flexion excursion and a lower relaxation index.•Children with DS had a lower mean and peak velocity and acceleration of leg swing.•External ankle load reduced the medial-lateral knee shift in children with DS.•Children with DS had less passive knee motion and increased joint stiffness. The Wartenberg pendulum test is a common clinical test that is used to measure stiffness about the knee in persons with and without disabilities such as cerebral palsy and Down syndrome (DS). Adults and adolescents with DS show fewer number of swing cycles and a lower relaxation index than healthy controls. However, it is not clear if children with DS show a similar trend compared to typically developing (TD) children. Was the knee joint kinematics different between children with and without DS during the pendulum test? Thirteen children with DS and 13 TD children participated in this study. There were two load conditions: no load (NL) and with ankle load (AL) equal to 2 % of the subject’s body weight. Five trials of a pendulum test were collected for each condition. The DS group showed a smaller first flexion excursion, a lower relaxation index, lower mean and peak velocities and accelerations during the first and second flexion and extension, and greater variability of acceleration during the first flexion than the TD group across both load conditions. This suggests that the DS group may have greater stiffness of the knee than the TD group to compensate for joint instability. The pendulum test appears to be a valid test to evaluate the passive stiffness of the knee in children with DS. The lower relaxation index in children with DS suggests that larger bursts of quadriceps may be activated during a pendulum test, particularly in the first flexion excursion, to assure the knee joint stability.