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The context of voluntary movement during facial assessment has significant effects on the activity of facial muscles. Using automated facial analysis, we found that healthy subjects instructed to blow produced lip movements that were longer in duration and larger in amplitude than when subjects were instructed to pucker. We also determined that lip movement for puckering expressions was more asymmetric than lip movement in blowing. Differences in characteristics of lip movement were noted using facial movement analysis and were associated with the context of the movement. The impact of the instructions given for voluntary movement on the characteristics of facial movement might have important implications for assessing the capabilities and deficits of movement control in individuals with facial movement disorders. If results generalize to the clinical context, assessment of generally focused voluntary facial expressions might inadequately demonstrate the full range of facial movement capability of an individual patient.

The target article by Dr. Prilutsky is based on three incorrectly derived mathematical rules concerning force-sharing among synergistic muscles associated with a cost function that minimizes the sum of the cubed muscle stresses. Since these derived rules govern all aspects of Dr. Prilutsky's discussion and conclusion and form the basis for his proposed theory of coordination between one-and two-joint muscles, most of what is said in the target article is confusing or misleading at best or factually wrong at worst. The aim of our commentary is to sort right from wrong in Dr. Prilutsky's article within space limitations that do not allow for detailed descriptions of mathematical proofs and explicit discussions of the relevant experimental literature.

Muscle stress is plainly one of the physical variables that the central nervous system probably wishes to minimize. This criterion does not uniquely define the patterns of muscle activation. It fails to explain the degree of coactivation of muscle antagonists that is widely found, and it cannot explain why two movements or movement segments that follow an identical trajectory driven by identical joint torques can be driven by different patterns of muscle activation. Muscle contraction provides for both net joint torque and limb stability. The minimization of the sum of muscle stresses, raised to any power, is an insufficient rule.

Apraxia is a complex movement disorder that frequently occurs following left hemisphere stroke. Studies on patients with apraxia constitute an especially interesting body of literature for motor control researchers who seek to understand the cognitive mechanisms involved in the voluntary control of movement. Reciprocally, among apraxia researchers, great interest exists concerning the ways in which methods and theory from the field of motor control can be brought to bear in the clinical and empirical evaluation of this disorder. Here we will review representative evidence on the etiology, frequency, and assessment of apraxia, and suggest how research methods and theories from the field of motor control can be applied to, and also benefit from, a deeper understanding of apraxia. Parallels are proposed between the major cognitive models of apraxia and motor control to facilitate translation of terminology and concepts, and to enrich the emerging dialogue between these two complementary research domains.

An adequate language is a prerequisite for progress in any area of science, including movement science. Notions of structural units and synergies and the principle of minimal interaction are revisited, discussed, and illustrated with a few examples from recent studies. Equilibrium-point hypothesis is considered an example of identifying significant variables in the control of a voluntary movement.

Conversion Disorder affects voluntary motor and sensory function and involves unexplained neurological symptoms without an organic cause. Many researchers have attempted to explain how these symptoms arise but the neural correlates associated with Conversion Disorder remain largely unknown to clinicians and neuroscientists alike. This review focuses on investigations of Conversion Disorder (with motor symptoms) when deficits in voluntary movement occur. No single consistent hypothesis has emerged regarding the underlying cortical mechanisms associated with motor Conversion Disorder. However, findings from electrophysiology, neuroimaging, and behavioral research implicate the involvement of prefrontal networks. With further research using measurement techniques precise in spatial as well as temporal resolution, the conflict associated with two views of the neural correlates of motor Conversion Disorder may be resolved. This will provide a better understanding of the impairment associated with the preparation, generation, and execution of intentional movement in Conversion Disorder.

Bradykinesia is a cardinal symptom of Parkinson’s disease (PD). Both aging and disease are shown to be associated with decreased adaptability to environmental stresses characterized by reduced complexity (or increased regularity) of biorhythms. The purpose of this study was to investigate the regularity of movement in individuals with PD, and the effect of dopaminergic medication. Nine subjects with PD and eight controls performed wrist flexion/extension movements at maximal velocity and range of motion. Subjects with PD were tested under two medication conditions. Approximate entropy (ApEn) was calculated to assess the regularity of the movement, with the smaller value associated with the greater regularity. Data revealed that subjects with PD had lower ApEn values than controls. Medication did not alter the ApEn values. These findings demonstrate that impaired voluntary movement in individuals with PD is associated with increased regularity of movement and this exaggerated regularity appears less sensitive to anti-PD medication.

Transcranial magnetic stimulation (TMS) of the motor cortex was used to produce compound motor evoked potentials (cMEPs) in the first dorsal interosseus (FDI) muscle. The size of cMEPs was measured as an index of corticospinal excitability before and after initiation of a simple reaction task (SRT). The SRT, consisting of an abduction of the right index finger against a vertical support in response to a 1 kHz cueing tone, was performed in 6 healthy male subjects. cMEPs were facilitated when timed to occur just before the fastest simple reaction time (fSRT). When the cMEP was placed 15.5 ± 1.5 ms before the fSRT, its amplitude increased to 176 ± 36% of the control response seen in the relaxed state (no SRTs). Facilitation of the cMEP increased to 382 ± 43% of the control when it was placed 11.9 ± 1.5 ms after the fSRT. The facilitation of cMEP responses prior to the SRT is discussed with particular reference to the premovement potential that may be recorded over the cortex prior to a voluntary movement.