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In the target article Mark Latash has argued that there is but a single bona-fide theory for hand motor control (referent configuration theory). If this is true, and research is often phenomenological, then we must admit that the science of hand motor control is immature. While describing observations under varying conditions is a crucial (but early) stage of the science of any field, it is also true that the key to maturing any science is to vigorously subject extant theories and budding laws to critical experimentation. If competing theories are absent at the present time is it time for scientists to focus their efforts on maturing the science of hand motor control through critical testing of this long-standing theory (and related collections of knowledge such as the uncontrolled manifold)?

Most of our daily actions involve movements of the hand. The neuronal pathway contributing to the control of hand movements are complex and not yet completely understood. Recent studies highlight how task-dependent changes in cortical and subcortical pathways driven by contralateral and ipsilateral influences may open avenues to further understand the complexity of hand actions in healthy and disease. In the following section studies using transcranial magnetic and electrical stimulation in healthy subjects and in individuals with chronic incomplete spinal cord injury will be highlighted to further understand neuronal pathways involved in the control of voluntary activity by hand muscles.

The purpose of this commentary is to discuss factors that limit consideration of the equilibrium point hypothesis as a scientific theory. The EPH describes control of motor neuron threshold through the variable lambda, which corresponds to a unique referent configuration for a muscle, joint, or combination of joints. One of the most compelling features of the equilibrium point hypothesis is the integration of posture and movement control into a single mechanism. While the essential core of the hypothesis is based upon spinal circuitry interacting with peripheral mechanics, the proponents have extended the theory to include the higher-level processes that generate lambda, and in doing so, imposed an injunction against the supraspinal nervous system modeling, computing, or predicting dynamics. This limitation contradicts evidence that humans take account of body and environmental dynamics in motor selection, motor control, and motor adaptation processes. A number of unresolved limitations to the EPH have been debated in the literature for many years, including whether muscle resistance to displacement, measured during movement, is adequate to support this form of control, violations in equifinality predictions, spinal circuits that alter the proposed invariant characteristic for muscles, and limitations in the description of how the complexity of spinal circuitry might be integrated to yield a unique and stable equilibrium position for a given motor neuron threshold. In addition, an important empirical limitation of EPH is the measurement of the invariant characteristic, which needs to be done under a constant central state. While there is no question that the EPH is an elegant and generative hypothesis for motor control research, the claim that this hypothesis has reached the status of a scientific theory is premature.

In the article by Krüger M, Eggert T, and Andreas S, “Age-Related Differences in the Stabilization of Important Task Variables in Reaching Movements,” in Motor Control 17(3), Figure 2 was not included due to an error in the production process. We apologize for the error. The online version of the article has now been corrected to include the figure: http://journals.humankinetics.com/mc-back-issues/mcvolume-17-issue-3-july/age-related-differences-in-the-stabilizationof-important-task-variables-in-reaching-movements