Functional assessments of the knee joint biomechanics by using pendulum test in adults with Down syndrome

Abstract

In this study, we assessed kinematics and viscoelastic features of knee joint in adults with Down syndrome (DS) by means of the Wartenberg pendulum test. This test allows the measuring of the kinematics of the knee joint during passive pendular motion of leg under the influence of gravity. In addition, by a combination of kinematic and anthropometric data, pendulum test provides estimates of joint viscoelastic properties by computing damping and stiffness coefficients. To monitor the occurrences of muscle activation, the surface electromyogram (EMG) of muscle rectus femoris was recorded. The experimental protocol was performed in a group of 10 adults with DS compared with 10 control adults without DS. Joint motion amplitude, velocity, and acceleration of the leg during the first knee flexion significantly decreased in persons with DS with respect to those without DS. This behavior was associated with the activation of rectus femoris in subjects with DS that resulted in increasing of joint resistance shortly after the onset of the first leg flexion. The EMG bursts mostly occurred between 50 and 150 ms from the leg flexion onset. During the remaining cycles of pendular motion, persons with DS exhibited passive leg oscillations with low tonic EMG activity and reduced damping coefficient compared with control subjects. These results suggest that adults with DS might perform preprogrammed contractions to increase joint resistance and compensate for inherent joint instability occurring for quick and unpredictable perturbations. The reduction of damping coefficients observed during passive oscillations could be a predictor of muscle hypotonia.

Fig. 1.

Experimental arrangement to perform pendulum test. A: the leg passively swung from an extended position to rest. An electrogoniometer recorded the angular displacements of knee joint, and a couple of surface electrodes recorded the electromyogram (EMG) activity at the rectus femoris. B: typical kinematic trajectory reporting the knee angle variations and the main landmarks for further kinematic and dynamic computations. F1–F4, angles of reversal at the end of each swing flexion; E1–E4, angles of reversal at the end of each swing extension; PA, plateau amplitude.

Fig. 2.

Examples of kinematic and EMG traces recorded in one subject without Down syndrome (DS; A) and in two subjects with DS (subjects F.O. and A.A.; B and C). Shadow areas highlight the first flexion. RI, relaxation index; B, damping coefficient; MV, mean velocity during the first flexion; MA, mean acceleration during the first flexion.

Fig. 3.

Summary of statistic comparisons between subjects with and without DS concerning kinematic [RI (A), MV (B), peak velocity (C), MA (D), peak acceleration (E)] and EMG (F) parameters. Data are expressed as means and SDs. *P < 0.05. **P < 0.01. ns, Not significant.

Fig. 4.

Relationship between changes of MA and EMG area during the first flexion. The linear regression analysis was performed separately for each group. Each data point represents the average value obtained over the 10 trials for each subject.

Fig. 5.

Principal component (PC) analysis performed on the data set of EMG responses recorded in the persons with DS during the first flexion. A: the first six components, accounting for the 86.9% of the total variance, are distributed on the basis of their occurrences over the time interval of the first swing. For each component are reported the fraction of variance accounted. The three most important components have been extracted for a better clearness and are illustrated, respectively, in B, C, and D. The axes of time are divided in percentage of the interval time between movement onset and the first flexion peak.

Fig. 6.

Distribution of the latencies of EMG bursts with respect to the onset of leg movement. In the plot are represented latencies measured in all of the data set of EMG traces recorded in persons with DS.

Fig. 7.

Damping (A) and stiffness (B) coefficients estimated for the subjects with and without DS over the three cycles following the first flexion. F1–F4, peak angles of flexions; E1–E4, peak angles of extensions. Data are expressed as means and SDs. *P < 0.05. **P < 0.01.

Fig. 8.

Relationship between the changes of mean values of damping coefficient and EMG area during the three cycles following the first flexion. The linear regression analysis was performed, collecting the data from both groups. Each data point represents the average value obtained over the 10 trials for each subject.