Academic journal article Research Quarterly for Exercise and Sport

Sensitivity to Infants with and without Down Syndrome to Intrinsic Dynamics

Academic journal article Research Quarterly for Exercise and Sport

Sensitivity to Infants with and without Down Syndrome to Intrinsic Dynamics

Article excerpt

Theorists suggest that infants who are sensitive to their own intrinsic dynamics discover movement solutions in the context of various tasks or goals. Intrinsic dynamics may be defined as the collective behavior of the system that occurs in the absence of specific task requirements; this behavior changes over time, deriving from the infants' experiences as well as their biomechanical and neurophysiological properties (Zanone, Kelso, & Jeka, 1993). Therefore, what each infant brings to the movement context, what he or she has to work with, is unique. Thelen et al. (1993), stated that "...infants' intrinsic dynamics, act as both constraints...and the physical and informational raw materials from which each infant must assemble an action to fit the specified goal..." (p. 1060).

The process of adapting one's intrinsic dynamics to the goal involves linking multiple body segments and many underlying muscle groups into a synergistic organization (Bernstein, 1967; Tuller, Turvey, & Fitch, 1982). When one reaches for an object, the trunk muscles stabilize the base from which the arms move forward. When one walks, one leg alternately provides a base of support while the opposite leg swings over ground. Components of these synergies must be mutually sensitive and responsive to the dynamic information available across the system (Kugler & Turvey, 1987; Schoner & Kelso, 1988). One way a system demonstrates its adaptability is by its responses to perturbations. For example, if an adult steps on a crack in the sidewalk, both legs rapidly but differentially modify their neuromuscular output to avoid falling and continue forward locomotion.

Little is known about how this capacity to control one's intrinsic dynamics - to organize body segments into functional reaching or walking synergies - emerges. Likely, from very early in life, limb movements, at least at some level, are not completely independent of each other. But the nature and precision of this interdependence inevitably changes over time and context.

Data from several studies suggest that, at least in some contexts, normally developing infants respond to changes in the context of one leg by adjusting the spontaneous behavior of both legs. When weights were added to one of their legs, 6-week-old infants systematically adapted the motor output of both legs (Thelen, Skala, & Kelso, 1987). Infants maintained their overall baseline (both legs unweighted) kicking frequency during weighted trials by increasing the activity of the unweighted limb and decreasing the activity of the weighted limb. A similar capacity to adapt to changes in sensory information within and between the legs was observed when 7-month-old infants produced treadmill steps. When supported on a motorized treadmill, infants matched their rate of alternating stepping to imposed changes in belt speed (Thelen, 1986). And, when the treadmill belt was "split" down the middle, causing one leg to move backward twice as fast as the other, infants maintained alternating steps (Thelen, Ulrich, & Niles, 1987). In the split-belt context, infants reduced their step rate for the leg on the fast belt and increased step rate for the leg on the slow belt. The overall step rate was intermediate between step rates used, when both belts moved either fast or slow. Nevertheless, infants' behavior on single-belt and split-belt treadmills is not uniform throughout the first year. Rather, the stability and adaptability of their motor performance improves over developmental time (Thelen & Ulrich, 1991).

Dynamic systems theory predicts that changes in infants' behavior over time in response to the same context arise from the development of one or more underlying subsystems, such as muscle strength, body size, postural control, and past experiences. While many subsystems contribute to infants' intrinsic dynamics and their emergent functional behaviors, we focused on the impact of sensory subsystems, specifically somatosensory perception. …

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