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Scaling of Dynamics in the Earliest Stages of Walking

KG Holt, PT, PhD, is Associate Professor, Department of Physical Therapy and Athletic Training, Sargent College of Health and Rehabilitation Sciences, Boston University, 635 Commonwealth Ave, Boston, MA 02215 (USA). Dr Holt is a fellow at the Center for the Ecological Study of Perception and Action, University of Connecticut, Storrs, Conn
E Saltzman, PhD, is Associate Professor, Department of Physical Therapy and Athletic Training, Sargent College of Health and Rehabilitation Sciences, Boston University. Dr Saltzman is a fellow at the Center for the Ecological Study of Perception and Action, University of Connecticut, and a research scientist at the Haskins Laboratories, New Haven, Conn
CL Ho, PT, ScD, Department of Physical Therapy and Athletic Training, Sargent College of Health and Rehabilitation Sciences, Boston University
BD Ulrich, PhD, is Professor and Dean, Division of Kinesiology, University of Michigan, Ann Arbor, Mich

Address all correspondence to Dr Holt at: kgholt{at}bu.edu

Background and Purpose: Although the description of mature walking is fairly well established, less is known about what is being learned in the process. Such knowledge is critical to the physical therapist who wants to teach children with developmental delays. The purpose of this experiment was to test the notion that learning to walk efficiently involves fine-tuning the body’s controllable stiffness (by co-contraction and isometric muscle contractions against gravity) to match (at a 1:1 scaling) the gravitational (pendular) stiffness of the swing leg.

Subjects: The study participants were 7 children with typical development and the newly emerged ability to walk 6 steps without falling (ages 11 months to 1 year 5 months at the onset of walking).

Methods: Pendular stiffness and spring stiffness were estimated from the equations of motion for a hybrid model with kinematic data as children walked over ground. Testing occurred once per month for the first 7 months of walking.

Results: After the first month of walking, children walked with greater spring stiffness than would be predicted by the model. The ratio began to approach the predicted value (1:1) as the months progressed.

Discussion and Conclusion: The results of this and a previous study of the pendular dynamics of gait suggest that learning to walk is a 2-stage process. The first stage involves the child’s discovery of how to conserve energy by inputting a particular muscular force at the correct moment in the cycle. The second stage involves the fine-tuning of the soft-tissue stiffness that takes advantage of the resonance characteristics of tissues. In order to address developmental delays, investigators must discover the dynamic resources used for the activity and attempt to foster their development. A number of interventions that probe this approach are discussed.