Thresholds for inducing protective stepping responses to external perturbations of human standing.

Standing subjects were unexpectedly pulled forward to identify a threshold boundary that evokes stepping in terms of the size of the pull relative to the base of support (BoS). Performances in a range of sensorimotor tests were correlated with the threshold boundary parameters. Younger and older subjects were studied to identify age-related changes in stepping and the threshold boundaries. The threshold boundary had a forward limit (T(L)) that, when crossed, always made subjects step no matter how slowly they were pulled. As velocity increased, the threshold position that produced a step shifted nearer to the ankles. Eventually a pull velocity was reached above which velocity had no further effect and a position threshold (T(H)) was identified behind which subjects never stepped. Thus the position threshold boundary for stepping is a posterior-going sigmoidal function of perturbation velocity. Older subjects stepped more than the young (69% vs. 40% of trials). For the older subjects, T(L) (91% vs. 107% BoS) and T(H) (59% vs. 72% BoS) were closer to the ankles, and the transition between T(L) and T(H) occurred at lower velocities (96% vs. 121% BoS.s(-1)). Across the entire study population many sensorimotor factors were associated with T(L) and T(H). However, these associations were not present when age was removed as a factor. Thus, although the older subjects use protective stepping more often, this cannot be attributed directly to the sensorimotor factors tested here. It can be explained by stepping as a triggered response to the perturbation event rather than later sensory input about body movement.

Lateral stability during forward-induced stepping for dynamic balance recovery in young and older adults.

BACKGROUND: Balance dysfunction related to lateral instability has been associated with falls and fall-related injuries among older individuals. Protective stepping for dynamic balance recovery requires the effective control of lateral body motion. This study investigated the relationship between aging, falls, and lateral stability during forward-induced stepping for dynamic balance recovery. METHODS: Forward steps were induced by a motor-driven waist-pull system in 12 younger adults, 20 healthy community-dwelling older adult nonfallers, and 18 older adults who had reported falls. Group differences in kinetic and kinematic stepping characteristics for a range of postural disturbance magnitudes were evaluated. RESULTS: Despite group similarities in anticipatory postural adjustments for minimizing lateral instability, the older fallers demonstrated significantly greater sideways body motion toward the stepping side at first-step foot contact and a more laterally directed foot placement. During the first step, forward-stepping characteristics were generally comparable between the groups, but the older fallers had an earlier liftoff time and longer step duration. CONCLUSIONS: During forward-induced protective stepping, otherwise healthy older adults who had experienced falls showed particular differences in their control of lateral body motion that were not attributable to changes in anticipatory postural mechanisms. Aging changes in controlling lateral body motion during protective stepping appear to involve factors that intervene between the first-step liftoff and foot contact and/or adaptations in stepping patterns related to prior planning.

Electromyographic analysis of postural responses during standing leg flexion in adults with hemiparesis.

The purpose of this study was to examine muscle activation patterns during standing leg single leg flexion in adults with hemiparesis. Specifically, the electromyographic activation patterns of the flexing limb biceps femoris and gluteus medius, and the stance limb gluteus medius muscles were analyzed as a function of whether the muscles were paretic or not. Delayed activation of the affected flexing side gluteus medius, as compared with unaffected flexing side gluteus medius, resulted in it being activated simultaneous with the flexing biceps femoris rather than preceding it as was previously found in healthy subjects. This suggests a temporal change in the sequential mode of coordination of the postural and intended components of the task. In addition, the magnitude of the electromyographic integrals of both the affected and unaffected flexing side gluteus medius in the early propulsive phase of the task was significantly reduced in comparison with healthy subjects. These alterations can be attributed to spatial alterations in the sequential form of organization or to a shift to a different mode of neural control in order to perform a relatively novel task. These results suggest a potential adaptive capacity in these individuals.

Alterations in weight-transfer capabilities in adults with hemiparesis.

BACKGROUND AND PURPOSE: The purposes of this study were (1) to examine the position and displacement in the frontal plane of the body's center of mass (CM) with respect to the base of support during single-leg flexion movements in adults with hemiplegia and (2) to examine their relationship with other clinical scores. SUBJECTS: Fourteen ambulatory adult volunteers with hemiparesis of the right side of the body due to cerebrovascular accident participated in the study. METHODS: Subjects performed single-leg flexion movements with the paretic and nonparetic limbs while standing on two separate force platforms. Motion analysis and force platform data were used to determine the displacement of the CM. RESULTS: Successful performance of the transfer and holding single-limb stance occurred for 48% (to the nonparetic side) and 20% (to the paretic side) of the trials. Lack of success was due to insufficient displacement of the CM (26% of the trials to the nonparetic side and 17% of the trials to the paretic side) or a failure to maintain single-limb stance (26% of all trials to the nonparetic side and 63% of the trials to the paretic side). Overall, the final position of the CM with respect to the single-limb support region did not differ between sides. Successful performance was highly to moderately associated with clinical assessment scores for motor function and balance. Its association with gait velocity, however, was poor. CONCLUSION AND DISCUSSION: A classification scheme that can distinguish between four categories of bipedal to single-limb stance transitions has been established. Issues concerning clinical assumptions pertaining to the relationship between static and dynamic motor dysfunction in adults with hemiparesis are discussed.

Kinetic analysis of dynamic transitions in stance support accompanying voluntary leg flexion movements in hemiparetic adults.

The purpose of this study was to examine the extent to which adult hemiparesis due to cerebrovascular accident may alter the dynamic transitions from bipedal to single limb stance that accompany rapid voluntary single leg flexion movements while standing. Eight postacute hemiparetic adults performed rapid single-leg-flexion movements with the paretic (PL) and nonparetic (NL) limbs while standing on two separate force platforms that recorded the individual and resultant lateral horizontal (FY) ground reaction force (GRF) components acting on the body in the frontal plane. The results showed that for NL movements, the ipsilateral FY force-time integral contributed a significantly greater proportion to the resultant FY than that recorded beneath the upcoming stance PL, whereas PL movements showed a reverse trend indicating differences in the spatial distribution of GRFs. For some subjects, delays in the initial FY onset times between limbs and reversals in the normal direction of initial force application beneath the PL were observed. Such changes in the spatial and temporal aspects of GRF production may affect dynamic lateral weight transfer function regardless of the direction of total body motion. Implications for clinical practice pertaining to interventions that emphasize speed as well as magnitude of paretic muscle torque production, and factors related to the selection of movement activities for the retraining of dynamic weight transfer function are discussed.