Academic journal article Physical Therapy

Perspective on Variability in the Development of Human Action

Academic journal article Physical Therapy

Perspective on Variability in the Development of Human Action

Article excerpt

Humans are designed not only with variability but for variability. Variability can be defined at many levels of analyses; for example, growth, physiology, performance, behavior, impairments, and phase relations among moving limbs can all be measured in terms of variability. Variability is inherent in human behavior, and it is necessary for exploration and skill development. However, variability also can hinder the development of functional actions and skill development when it exceeds typical limits. In this perspective article, I focus on the necessity of variability for exploration, discovery, and functional skill development and on the potential consequences of excessive or restricted variability on human action.

Built for Variability: Variability for Solutions to Action Demands

Humans live in an extraordinarily wide range of climates and circumstances. Our nervous and musculoskeletal systems afford plasticity of tissues and processes that enable us to grow, develop, heal, and adapt to a variety of human conditions throughout our lives and through the many experiences of our species. This flexibility ensures a range of responses to a variable set of challenges and offers evolutionary advantages. In detailing selection within the nervous system, Sporns and Edleman reflected that "many motor systems remain plastic throughout adult life, ready to compensate for metrics and dynamic changes in the biomechanics of motor organs, including even the loss of entire limbs." (1(p961)) Indeed, we would be functionally maladaptive if variability were not inherent in our capacities.

Variability in Coordination During Changing Body Metrics

Humans encounter dramatic changes in the metrics of body size and relational metrics among body segments during early development. These metrics are changing as the infant is exploring and attempting mastery over patterns for mobility and other functional actions. Infants are initially "top heavy," with large heads and upper bodies in comparison with their lower trunks and legs. (2) A newborn's head is 25% of the body length, (3) which poses potential challenges for dynamically controlling the head. Head control is developing while the head circumference and weight are dynamically changing.

Bartlett (3) investigated the effects of anthropometrics on gross motor skills during the first year of life. At 6 weeks of age, infants with proportionally larger heads (head size to body mass) had lower motor skills, specifically lower skills in prone head lifting. Body proportions change and the center of mass lowers as infants mature into toddlers. Infant weight, on average, more than doubles in the first year of life, and length increases, on average, approximately 50% while head circumference increases approximately 30%. (4) However, toddlers remain top heavy as they stand and begin to take steps while managing a large head at the top of the body. The width of the head is greater than the width of the pelvis (4) when an infant begins to toddle, and initially the infant has a wide base of support and makes variable steps and strides.

The coordination pattern that emerges from toddlers as they begin to walk is, in part, a function of the relation of head size to pelvis. These relational metrics between head and pelvis, however, are dynamically changing while walking is mastered. The walking patterns that emerge under these changing task demands will, of necessity, be variable--the constraints of the task are variable. Changes in movement capabilities are required to accommodate the metric changes, as well as to meet new environmental challenges.

Although these relational metrics contribute to variability in action, the relation of body metrics and motor development is not a one-to-one mapping. There is correlational evidence suggesting that top-heavy infants are later walkers, (5) but the study by Bartlett (3) and other studies (4-6) have failed to find relations between many of the tested anthropometrics, when considered independently, with crawling or walking. …

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