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We investigated fast pointing and fast reversal single-joint movements performed under artificial damping conditions applied with the myohaptic technology. Healthy subjects were able to scale the intensity of the agonist and antagonist electromyographic (EMG) activities when damping was added during both types of movements. Cerebellar patients were able to scale the magnitude of the agonist and antagonist EMG activities during pointing movements. For reversal movements, patients could scale the intensity of the initial agonist EMG activity but were unable to modulate the intensities of the second set of EMG discharges associated with the return to the initial position. Patients were able to adopt the appropriate motor strategy for the first phase of reversal movements, but they were unable to implement the suitable sequential strategy for the superimposition of motor plans during artificial damping. The myohaptic technology is the first to allow the detection of deficits in the implementation of sequential motor coding under the mechanical condition of artificial damping.

We report a novel form of hypometria in a patient presenting a spinocerebellar ataxia type 2 (SCA2; 32 repeats; Age: 44). We investigated fast pointing (FP) and fast reversal (FR) single-joint movements performed under artificial damping conditions using our myohaptic device called Wristalyzer, which allows an instantaneous regulation of the mechanical characteristics of a wrist manipulandum. The wristalyzer controller has a range of motion from −1 rad to +1 rad. The inertia of the motor/handgrip is 0.004 kg × m2. Movements were studied in free mode and with damping either at 0.1 Nms/rad or 0.2 Nms/rad. Subjects performed sets of FP movements and FR movements over 3 distances (targets: 0.2, 0.3, 0.4 rad). Control subjects (n = 8, mean age: 34.8 +/− 10.2 years; 3 women) were able to scale the intensity of the agonist and antagonist electromyographic (EMG) activities when damping was added during both types of movements. FR movements were accurate in the 3 mechanical conditions: mean movement amplitude was −0.0179 rad, 0.0061 rad and 0.0062 rad in the basal state, following addition of 0.1 Nms/rad and following addition of 0.2 Nms/rad, respectively. The patient scaled the magnitude of the agonist and antagonist EMG activities during FP, but FR movements were dysmetric. The second phase of reversal movements was hypometric. Movement performed in the basal condition was accurate during phase 1 but was hypometric during phase 2. The hypometria worsened with damping: mean movement amplitude (second zero-crossing of the velocity curve) was 0.088 rad in the basal state, 0.1425 rad following addition of 0.1 Nms/rad, and 0.2003 rad with addition of a damping of + 0.2 Nms/rad. This is the first demonstration in human of a deficit in the implementation of the suitable sequential strategy for the superposition of motor plans during artificial damping.

Wristalyzer is a portable robotic device combining haptic technology with electromyographic assessment. It allows to assess wrist motion in physiological and pathological conditions by applying loads and mechanical oscillations, taking into account the ergonomy and the angular positioning of the joints. The wristalyzer works in a free or loaded mode for assessment of metrics of motion and tremor, analyzes the behavior of the wrist joints and the associated muscle activities during delivery of mechanical oscillations, estimates the maximal voluntary contraction, assesses automatically the impedance of the wrist for assessment of rigidity or spasticity. Position, torques and electromyographic activities are analyzed in real time. The device characterizes the effects of damping on voluntary motion. A personal computer implements control loops and user application. This is the first standardized tool to assess wrist motion with high accuracy and reliability using the haptic technology with concomitant investigation of muscle activity.