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J Appl Physiol 96: 1968, 2004; doi:10.1152/japplphysiol.00211.2004
8750-7587/04 $5.00
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COMMENTARY

HIGHLIGHTED TOPICS
Neural Control of Movement

Commentary

Studies over the past 20 years have shown that, far from being immutable, spinal reflexes vary at different phases of movement and differ by task. The first featured article in this issue of the Journal, "Spinal reciprocal inhibition in human locomotion," by Dr. A. Kido and colleagues (1), examines spinal inhibition during different locomotor tasks. This study represents the first of its kind to examine reciprocal inhibition in running as well as in standing and walking. When standing still, stretch reflexes help stabilize the body, and there is relatively high gain to correct small perturbations. During walking, stretch reflexes turn on and off at different phases of the step cycle but are generally lower than when standing still. Although reflexes likely provide only a fraction of the force needed to push the body off the ground, they may play important roles in modulating the amount of force needed for locomotion across different ground surfaces. Whereas previous studies have addressed the important roles of these reflexes in the control of movement, reciprocal inhibition has been less studied, and some controversy exists as to whether it is modulated at all during the step cycle. To clarify this issue, these investigators studied reciprocal inhibition in an antagonistic pair of human calf muscles, soleus and tibialis anterior, at speeds ranging from 0 (standing) to 9 km/h (running). They found that reciprocal inhibition decreased substantially with speed and to a lesser extent with task (e.g., running vs. walking at the same speed of 6 km/h). The functional significance of this modulation remains uncertain, but, clearly, as this study illustrates, different spinal reflexes are individually modulated, depending on the particular requirements of the reflex involved in a task such as locomotion.

In the second featured article, entitled "Simultaneous control of hand displacements and rotations in orientation-matching experiments," Drs. E. Torres and D. Zipser (2) address the independence of the spatial and temporal components of voluntary motion. The basis for this study was a recently proposed theoretical framework that suggests a geometric stage between sensory perception and motor action that functions as a brain mechanism to simulate goal-directed behavior without actual implementation. These investigators employed a simple but unconstrained orientation-matching experiment that revealed that simultaneous control of hand transport and rotation is robust to changes in speed, initial arm configuration, and target orientation. This finding is consistent with this theoretical framework and suggests that learning to produce a new geometric, time-independent strategy and learning to become proficient in the temporal domain of motion are distinct processes. Consequently, the smoothness and gracefulness of automatic movements can develop independently of geometric path. Such independence implies that an area of the brain is responsible for encoding the time-free directional signal this method proposes. An ideal candidate appears to be the posterior parietal cortex, a highly cognitive area where neural correlates of several of the key ingredients necessary for simulating this type of geometric motion in the brain are known to coexist. Such signals include those related to sensory goals (for defining the target of action), arm geometry (for enabling task-dependent metric identification), and coordinate transformations (for building smooth-differentiable maps). In addition, movement intentions in the absence of motion and a dynamics-free representation of movement previously described in this region are consistent with time-free representation of movement. This theoretical framework will not only serve to pose new questions about the formation of cognitive strategies in complex motions at both the behavioral and neurophysiological levels but will also provide the tools suitable for analysis of such high-dimensional spaces.

Gary C. Sieck

REFERENCES

  1. Kido A, Naofumi T, and Stein RB. Spinal reciprocal inhibition in human locomotion. J Appl Physiol 96: 1969-1977, 2004.[Abstract/Free Full Text]
  2. Torres EB and Zipser D. Simultaneous control of hand displacements and rotations in orientation-matching experiments. J Appl Physiol 96: 1978-1987, 2004.[Abstract/Free Full Text]




This Article
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