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Sérgio T Fonseca, Kenneth G Holt, Linda Fetters, Elliot Saltzman, Author Response, Physical Therapy, Volume 84, Issue 4, 1 April 2004, Pages 356–358, https://doi.org/10.1093/ptj/84.4.356
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We would like to thank Galloway for his commentary and the opportunity to expand on some of the ideas advanced in the article. We have organized the response to address the specific questions raised by Galloway.
Selection Theory
The problem of selection versus constraint is indeed difficult to test experimentally, and it is confounded by the fact that there are other constraints due to the task demands (eg, speed, accuracy) and by environmental factors that surely influence the movement patterns. Our view is that rather than “selection” of a stiffness or force variable, a movement pattern emerges as a function of the interplay of task demands, available dynamic resources (energy-generating and energy-conserving capabilities), and environment. In our view, the child with cerebral palsy learns about (explores) his or her dynamic resources within a given environment (eg, uneven surfaces, slippery floors, underwater) and under specific task demands (eg, walking, running, jumping). On each occasion, a number of viable patterns are available. The pattern that emerges (rather than being selected) is optimal from the perspective of stability, metabolic cost, and mechanical efficiency. Thus, although the child explores his or her mass, strength, stiffness, or body segment lengths, the “selection” occurs at the level of movement pattern. The stiffness observed in children with cerebral palsy is the product of muscle and connective tissue properties that developmentally change according to their patterns of use.1,2 In this sense, stiffness itself is a property that cannot serve as the basis for a selection process. Patterns of movements, however, provide enough diversity and, consequently, selection theory can be applied.
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