Unraveling of an original mechanism of hypometria in human using a new myohaptic device — The Wristalyzer

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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.