Klinische Neurophysiologie 2004; 35 - 59
DOI: 10.1055/s-2004-831971

A New Method to Evaluate Muscle Tone in the Upper Limb

A Fincziczki 1, P Krause 2, T Eggert 3, A Straube 4
  • 1München
  • 2München
  • 3München
  • 4München

Some neurological diseases can cause changes in muscle tone. These changes can occur after lesions of central motor pathways (spastic syndrome), in Parkinson syndrome (rigidity) and cerebellar hypotonia. Other syndromes with increased muscle tone are less well defined like the muscle stiffness in CRPS. The problem in clinical praxis is that there is only a limited set of evaluation methods mostly using scales, which are highly observer dependent thus limiting the possibility to do inter-rater comparisons. The pendulum test of Wartenberg is a semiautomatic method to evaluate the muscle tone of the lower limb. It is based on an estimate of muscle torque in an unvoluntary periodic movement. Lin modified the test to measure damping and stiffness of the elbow joint (ej). They applied an external sinusodial torque to the limb by a mechanical pendulum to which it was attached. Lin found that the active torques (torques not explained by inertia, gravity torques of the limb and the mechanical apparatus) in the ej could be approximated by damped sinusoial oscillations. They modelled this active torque as a sum of torques proportional to joint angle and velocity (v) [elasticity (e) and damping (d)]. However, the acceleration of a damped sinusoidal oscillation is identical to a weighted sum of joint angle and v. We asked whether the active torque is sufficiently explained by the sum of e and d, or whether a component proportional to acceleration also exists. To address this, we used a similar system with an additional possibility to modify the stable position of the limb by changing the angle between forearm and pendulum. If the model of Lin is sufficient, joint angle phi0 at which no elastic torques occurs should not depend on this change. In a control experiment we tested whether the active torque depends on visual feedback. Aligning the forearm with the pendulum, we observed d-coefficients of 0.1±0.07 N m rad/s and stiffness-coefficients of 4.5±1.0 N m/rad. No elasic torque was estimated at an angle of elbow flection (ef) of j0=91.3±5.5 deg. When the angle between forearm and pendulum was changed about 10 deg towards smaller ef, phi0 decreased on average by almost the same amount to 80.2±8.7 deg (p<0.01). With smaller ef stiffness-coefficients were slightly smaller (4.0±0.7 N m/rad; p<0.05). This indicates that the active torque might not be sufficiently described by e and d alone. Torques proportional to acceleration may be involved, too.