Hyperkinetic movements are unwanted or excess movements that are frequently seen in children with neurologic disorders. They are an important clinical finding with significant implications for diagnosis and treatment. However, the lack of agreement on standard terminology and definitions interferes with clinical treatment and research. We describe definitions of dystonia, chorea, athetosis, myoclonus, tremor, tics, and stereotypies that arose from a consensus meeting in June 2008 of specialists from different clinical and basic science fields. Dystonia is a movement disorder in which involuntary sustained or intermittent muscle contractions cause twisting and repetitive movements, abnormal postures, or both. Chorea is an ongoing random-appearing sequence of one or more discrete involuntary movements or movement fragments. Athetosis is a slow, continuous, involuntary writhing movement that prevents maintenance of a stable posture. Myoclonus is a sequence of repeated, often non-rhythmic, brief shock-like jerks due to sudden involuntary contraction or relaxation of one or more muscles. Tremor is a rhythmic back-and-forth or oscillating involuntary movement about a joint axis. Tics are repeated, individually recognizable, intermittent movements or movement fragments that are almost always briefly suppressible and are usually associated with awareness of an urge to perform the movement. Stereotypies are repetitive, simple movements that can be voluntarily suppressed. We provide recommended techniques for clinical examination and suggestions for differentiating between the different types of hyperkinetic movements, noting that there may be overlap between conditions. These definitions and the diagnostic recommendations are intended to be reliable and useful for clinical practice, communication between clinicians and researchers, and for the design of quantitative tests that will guide and assess the outcome of future clinical trials.

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Muscle coordination studies repeatedly show low-dimensionality of muscle activations for a wide variety of motor tasks. The basis vectors of this low-dimensional subspace, termed muscle synergies, are hypothesized to reflect neurally-established functional muscle groupings that simplify body control. However, the muscle synergy hypothesis has been notoriously difficult to prove or falsify. We use cadaveric experiments and computational models to perform a crucial thought experiment and develop an alternative explanation of how muscle synergies could be observed without the nervous system having controlled muscles in groups. We first show that the biomechanics of the limb constrains musculotendon length changes to a low-dimensional subspace across all possible movement directions. We then show that a modest assumption—that each muscle is independently instructed to resist length change—leads to the result that electromyographic (EMG) synergies will arise without the need to conclude that they are a product of neural coupling among muscles. Finally, we show that there are dimensionality-reducing constraints in the isometric production of force in a variety of directions, but that these constraints are more easily controlled for, suggesting new experimental directions. These counter-examples to current thinking clearly show how experimenters could adequately control for the constraints described here when designing experiments to test for muscle synergies—but, to the best of our knowledge, this has not yet been done.

Numerous observations of structured motor variability indicate that the sensorimotor system preferentially controls task-relevant parameters while allowing task-irrelevant ones to fluctuate. Optimality models show that controlling a redundant musculo-skeletal system in this manner meets task demands while minimizing control effort. Although this line of inquiry has been very productive, the data are mostly behavioral with no direct physiological evidence on the level of muscle or neural activity. Furthermore, biomechanical coupling, signal-dependent noise, and alternative causes of trial-to-trial variability confound behavioral studies. Here we address those confounds and present evidence that the nervous system preferentially controls task-relevant parameters on the muscle level. We asked subjects to produce vertical fingertip force vectors of prescribed constant or time-varying magnitudes while maintaining a constant finger posture. We recorded intramuscular electromyograms (EMGs) simultaneously from all seven index finger muscles during this task. The experiment design and selective fine-wire muscle recordings allowed us to account for a median of 91% of the variance of fingertip forces given the EMG signals. By analyzing muscle coordination in the seven-dimensional EMG signal space, we find that variance-per-dimension is consistently smaller in the task-relevant subspace than in the task-irrelevant subspace. This first direct physiological evidence on the muscle level for preferential control of task-relevant parameters strongly suggest the use of a neural control strategy compatible with the principle of minimal intervention. Additionally, variance is nonnegligible in all seven dimensions, which is at odds with the view that muscle activation patterns are composed from a small number of synergies.

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Human fingers have sufficiently more muscles than joints such that every fingertip force of submaximal magnitude can be produced by an infinite number of muscle coordination patterns. Nevertheless, the nervous system seems to effortlessly select muscle coordination patterns when sequentially producing fingertip forces of low, moderate, and maximal magnitude. The hypothesis of this study is that the selection of coordination patterns to produce submaximal forces is simplified by the appropriate modulation of the magnitude of a muscle coordination pattern capable of producing the largest expected fingertip force. In each of three directions, eight subjects were asked to sequentially produce fingertip forces of low, moderate, and maximal magnitude with their dominant forefinger. Muscle activity was described by fine-wire electromyograms (EMGs) simultaneously collected from all muscles of the forefinger. A muscle coordination pattern was defined as the vector list of the EMG activity of each muscle. For all force directions, statistically significant muscle coordination patterns similar to those previously reported for 100% of maximal fingertip forces were found for 50% of maximal voluntary force. Furthermore the coordination pattern and fingertip force vector magnitudes were highly correlated (r > 0.88). Average coordination pattern vectors at 50 and 100% of maximal force were highly correlated with each other, as well as with individual coordination pattern vectors in the ramp transitions preceding them. In contrast to this consistency of EMG coordination patterns, predictions using a musculoskeletal computer model of the forefinger show that force magnitudes

The static properties of tensegrity structures have been widely appreciated in civil engineering as the basis of extremely lightweight yet strong mechanical structures. However, the dynamic properties and their potential utility in the design of robots have been relatively unexplored. This paper introduces robots based on tensegrity structures, which demonstrate that the dynamics of such structures can be utilized for locomotion. Two tensegrity robots are presented: TR3, based on a triangular tensegrity prism with three struts, and TR4, based on a quadrilateral tensegrity prism with four struts. For each of these robots, simulation models are designed, and automatic design of controllers for forward locomotion are performed in simulation using evolutionary algorithms. The evolved controllers are shown to be able to produce static and dynamic gaits in both robots. A real-world tensegrity robot is then developed based on one of the simulation models as a proof of concept. The results demonstrate that tensegrity structures can provide the basis for lightweight, strong, and fault-tolerant robots with a potential for a variety of locomotor gaits.

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