Quantitative analysis of knee joint mobility in adults with Down Syndrome

INTRODUCTION Down syndrome (DS) is the most frequently occurring chromosomal abnormality. Typically, individuals with DS exhibit ligament laxity and reduced muscle strength and tone. Motor disability includes longer reaction times, balance and postural deficits and increasing of co-contraction of agonist and antagonist muscles (1, 2). In this study, we assessed knee joint mobility in adult persons with DS by means of the Wartenberg test. This test allows performing a quantitative analysis of the kinematics of knee joint during passive pendular motion of leg under the influence of gravity (3). Two possible outcomes can be envisioned depending on the ability of DS persons to allow a passive leg motion. Given ligament laxity and hypotonia, we hypothesized that knee joint excursion should be larger in DS subjects than in controls. An alternative outcome can be exhibited if the DS persons would respond to leg dropping by performing a co-contraction action. In this case the range of leg motion would be reduced with respect to the control group. MATERIAL AND METHODS This study involved ten persons with DS, ranging in age from 18 to 32 years, and ten age and gender matched control subjects. According to the testing protocol (3), each subject was investigated in sitting up position, lying comfortably on an examination couch, so the calf and heel did not contact the couch when the knee was a maximum flexion. The DS participants received extensive explanation and demonstration of the procedure until they showed a good level of understanding. For each trial, the leg fell down from horizontal position and swung liberally between flexions and extensions until it stopped close to vertical position. The pendulum test was repeated at least ten times, with approximately a minimum rest of one minute between the trials. The angular displacements, recorded at the knee joint, were captured with electro-goniometer to measure angular displacement. The surface EMG of rectus femoral was obtained from a bioelectric amplifier connected to a pair of surface electrodes. Measurements from each kinematic trajectory were elaborated to obtain the following parameters: relaxation index: RI = F1Amp/PA, where F1Amp is the first peak flexion angle with respect to the onset angle and PA is the plateau amplitude measured between onset angle and resting angle; mean values of angular velocity (VF1) and angular acceleration (AF1) of knee movements during the first flexion. In addition, using kinematic and anthropometric data, we computed damping (B) and stiffness (K) coefficients in order to estimate joint visco-elastic properties. RESULTS The values of angular position, mean velocity and acceleration of the first leg flexion were lower in DS persons than in control group and the differences were statistically significant (RI, p < 0.01; VF1, p < 0.05; AF1, p < 0.01). Surface EMGs of muscle rectus femoris recorded during the first flexion exhibited large phasic activity in persons with DS, whereas the EMGs of controls were characterized by low tonic activity. Thus, the values of total EMG area measured in DS subject were higher with respect the values observed in control group (p < 0.05). We compared the total area of EMG with the mean values of acceleration during the first swing and we found that the reduction of acceleration detected in DS persons contributed for a large fraction of the variance of EMG area (R2 = 0.65); on the contrary, control group showed a significant smaller correlation (R2 = 0.12). With respect to the first three oscillations, DS persons showed values of damping coefficient lower than control subjects (p < 0.05). The values of stiffness coefficients in DS persons were higher than in controls across the first three swings but only for the first flexion it was statistical different between the two groups (p < 0.05). DISCUSSION The limited range of motion during the first knee flexion and the parallel increase of muscular activation observed in DS persons, indicate a possible neuronal compensatory mechanism performed by these subjects in order to increase stiffness and to improve joint stability (2). BIBLIOGRAPHY (1) Shumway-Cook A. and Woollacott MH. Physical therapy 1985; 65: 1315-1322; (2) Aruin AS et al., Am J Ment Retard. 1996; 101: 256-68; (3) Wartenberg R. Neurology 1951; 1:18-24