Transitions in a postural task: do the recruitment and suppression of degrees of freedom stabilize posture?

In this study, we examined flexibility in postural coordination by inducing transitions between postural patterns. Previous work demonstrated that the postural control system produces two task-specific postural patterns as a function of the frequency of support surface translation. For slow translat ion frequencies (< 0.5 Hz), subjects ride on the platform reminiscent of up right stance (ride pattern), and for fast frequencies (greater than or equa l to 0.75 Hz) subjects actively fixed the head and trunk in space (head fix ed pattern) during anterior-posterior platform motion. To study the adaptat ion of the postural control system, we had subjects stand on a support surf ace undergoing increases (from 0.2 to 1.0 Hz in 0.1-Hz steps) and decreases (from 1.0 to 0.2 Hz in 0.1-Hz steps) in translation frequency with the eye s open and closed. Kinematic measures of sagittal plane body motion reveale d a gradual transition between these two postural patterns as a function of frequency scaling. In both the increasing and decreasing frequency conditi ons with visual input, center of mass displacements gradually decreased and increased, respectively, whereas upper-bunk (and head) displacement decrea sed gradually within the ride pattern until a head fixed pattern was observ ed without any significant changes in displacement for translation frequenc ies at and above 0.6 Hz. Without visual input, the scaling of the ride patt ern was similar except the transition to the head fixed pattern never emerg ed with increasing frequency; instead, a less stable pattern exhibiting slo w drift in head-trunk anterior-posterior motion (drift pattern) was observe d at and above 0.5 Hz oscillations. The stability of the head fixed pattern at fast frequencies was clearly dependent on visual input suggesting that vision was more critical for trunk and head control in space at high than l ow translation frequencies. Head velocity was kept constant, and lower with vision, as translation frequency (and velocity) changed suggesting a head velocity threshold constraint across postural patterns. The gradual transit ion from the ride to the head fixed pattern was made possible by the recrui tment of available degrees of freedom in the form of ankle, then knee, and then hip joint motion. In turn, the transition from the head fixed or drift pattern was made possible by the gradual suppression of available degrees of freedom in the form of reducing hip, then knee, and then ankle motion. T he gradual change in postural kinematics without instabilities and hysteres is suggests that the ability to recruit and suppress biomechanical degrees of freedom allows the postural control system to gradually change postural strategies without suffering a loss of stability. The results are discussed in light of possible self-organizing mechanisms in the multisensory contro l of posture.