Balance control: looking forwards to feedforward

Cognitive demand and predictive adaptational responses in dynamic stability control

We studied the effects of a concurrent cognitive task on predictive motor control, a feedforward mechanism of dynamic stability control, during disturbed gait in young and old adults. Thirty-two young and 27 elderly male healthy subjects participated and were randomly assigned to either control or dual task groups. By means of a covered exchangeable element the surface condition on a gangway could be altered to induce gait perturbations. The experimental protocol included a baseline on hard surface and an adaptation phase with twelve trials on soft surface. After the first, sixth and last soft surface trial, the surface condition was changed to hard (H1-3), to examine after-effects and, thus, to quantify predictive motor control. Dynamic stability was assessed using the 'margin of stability (MoS)' as a criterion for the stability state of the human body (extrapolated center of mass concept). In H1-3 the young participants significantly increased the MoS at touchdown of the disturbed leg compared to baseline. The magnitude and the rate of these after-effects were unaffected by the dual task condition. The old participants presented a trend to after-effects (i.e., increase of MoS) in H3 but only under the dual task condition.In conclusion, the additional cognitive demand did not compromise predictive motor control during disturbed walking in the young and old participants. In contrast to the control group, the old dual task group featured a trend to predictive motor adjustments, which may be a result of a higher state of attention or arousal due to the dual task paradigm.

Hitting a support surface at unexpected height during walking induces loading transients

The impact phase during walking is sometimes characterised by an early loading peak, termed 'transient', followed by a brief decline in the force profile, termed 'unloading phase'. It was hypothesized that transients occur more frequently when subjects are unaware of the landing condition, and that the unloading phase represents a yield of the leg. This was tested experimentally by introducing an unexpectedly lowered or level support surface height during walking. Furthermore, associations between the unloading phase and type of foot placement, load-rate, kinesiology and centre of pressure were investigated. The transient occurred more frequently when subjects were unaware of the surface height. The amplitude of unloading was higher in flatfooted (combined), as compared to heel and toe landings. The percentage of combined landings, as well as the amplitude and duration of unloading were highest in the first unexpected level trials (UL1) and gradually decreased in the subsequent level trials, when subjects adapted to the situation. Following the UL1 unloading phase, the foot roll-off was halted, the ipsilateral knee flexed, the onset of the contralateral swing phase was delayed, and the double support phase increased. The unloading amplitude correlated significantly with the load-rate and knee flexion. It is concluded that an unexpected surface height frequently induces an early stance transient that is followed by an unloading phase, flexion response and halt in foot roll-off. These characteristics deserve further study in the context of the frequent falls induced by uneven surfaces during walking.