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.

The influence of stimulus cue on the initiation of stepping in young and older adults.

OBJECTIVES: To investigate the influence of different reaction stimulus cues on the characteristics of ground reaction forces during the initiation of rapid forward stepping, and to determine whether age-related differences in step initiation are influenced by the type of stimulus cue used to trigger stepping. DESIGN: Case-control study. SETTING: University-based research laboratory. PARTICIPANTS: Fifteen healthy younger adults and 35 community-dwelling older adult volunteers. MAIN OUTCOME MEASURES: Subjects performed forward step initiation in response to 3 different reaction stimulus cues (light, sound, electrocutaneous) during simple reaction-time conditions. Ground reaction force data were collected and used to characterize the timing and magnitude features of the postural (weight-transfer) and step components. RESULTS: In comparison with the light and sound stimuli, the nonnoxious electrocutaneous trigger cue was associated with an increase in the magnitude of the initial displacement of the center of pressure (COP) in the mediolateral direction. Overall, older subjects were slower to initiate the postural and step components, and displayed a reduction in the initial posterior displacement of the COP. CONCLUSIONS: The postural component for lateral weight transfer was augmented by the electrocutaneous trigger cue, suggesting its potential to enhance step initiation among clinical populations. Age-related changes in stepping were unaffected by the type of stimulus cue, but the older group had a slower response initiation time and less forward propulsion. The abilities of many older persons may be compromised in situations where rapid adjustments in the base of support through stepping are triggered by environmental stimuli.

Static versus dynamic predictions of protective stepping following waist-pull perturbations in young and older adults.

The purposes of this study were: (1) to determine the frequency of protective stepping for balance recovery in subjects of different ages and fall-status, and (2) to compare predicted stepping based on a dynamic model (Pai and Patton, 1997. Journal of Biomechanics 30, 347 354) involving displacement and velocity combinations of the center of mass (COM) versus a static model based on displacement alone against experimentally induced stepping. Responses to three different magnitudes of forward waist pulls were recorded for 13 young, 18 older-non-fallers and 18 older-fallers. The COM phase plane trajectories derived from motion analysis were compared with the model-predicted threshold values for stepping. We found that the older fallers had the highest percentage of stepping trials (52%), followed by older-non-fallers (17.3%), and young (2.7%) at the lowest perturbation level. Younger subjects stepped less often than the elderly at the middle level. Everyone consistently stepped at the highest level of perturbation. Overall, the dynamic model showed better predictive capacity (65%) than the static model (5%) for estimating the initiation of stepping. Furthermore, the threshold for step initiation derived from the dynamic model could consistently predict when a step must occur. However, it was limited, especially among older fallers at the low perturbation level, in that it considered some steps 'unnecessary' that were presumably triggered by fear of falling or other factors.

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.

Stimulus parameters and inertial load: effects on the incidence of protective stepping responses in healthy human subjects.

OBJECTIVE: To test the hypotheses that the incidence of protective stepping in response to sudden translations of the support would (1) increase as a function of both the magnitude of surface displacement and velocity of platform movement, and (2) decrease in association with an increase in external loading applied to the body. DESIGN: A log-linear approach was used to analyze the incidence of stepping by testing several models incorporating different platform stimulus parameters (direction, displacement, velocity) and external loading (0% and 20% body weight). SETTING: Institutional-based research laboratory. PARTICIPANTS: Eight healthy younger adult (21 to 28 years) volunteers. MAIN OUTCOME MEASURES: The incidence and number of protective steps served as the primary planned outcome variables. RESULTS: Steps occurred more frequently for anterior (83 steps) versus posterior (45 steps) translations. Step occurrence was generally proportional to platform velocity, and increased with displacements up to 15cm, but then plateaued. External loading was associated with a reduction in the number of steps for lower magnitudes of platform motion but had little effect at higher magnitudes. CONCLUSION: The tendency to step in response to externally applied disturbances to stance appears to be a complex function of direction, velocity, displacement, and inertial load.

Reliability of measurements of body center-of-mass momentum during sit-to-stand in healthy adults.

BACKGROUND AND PURPOSE: The purpose of this study was to determine the reliability of measurements of momentum of the body's center of mass (CM) during a sit-to-stand (STS) transfer in healthy adults. SUBJECTS: Nineteen healthy adults aged 25 to 38 years (mean = 31.7, SD = 4.2) participated. METHODS: Horizontal and vertical components of CM momentum were computed for STS transfers made at three movement speeds (fast, natural, and slow) with the aid of a motion analysis system. Two force platforms detected the time when the subject lost contact with the chair and the propulsive and braking impulses in the horizontal and vertical directions. Separate intraclass correlation coefficients (ICCs) were calculated for three temporal variables (time to peak horizontal and vertical momentum and time to when the subject lost contact with the chair) and two magnitude variables (peak horizontal and vertical momentum). RESULTS: The ICCs for magnitude variables were > or = .81 for all speeds of movement. The ICCs for temporal variables ranged from .28 for fast movements to .75 for slow movements. CONCLUSION AND DISCUSSION: Measurement of peak vertical and horizontal momentum magnitudes is highly reproducible during STS transfers. Measurement of temporal variables exhibits a range of reliability estimates. Implications include consideration of the speed at which STS transfer is performed and its effect on reliability estimates and the potential differences between reliability estimates for magnitude measurements versus temporal measurements.

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.

Reliability of ground reaction force measurements during dynamic transitions from bipedal to single-limb stance in healthy adults.

The consistency of ground reaction force (GRF) variables underlying dynamic transitions from bipedal to single-limb stance during a single-leg flexion movement was examined in 18 healthy adult subjects aged 21 to 47 years (mean = 31.1, SD = 8.6). Force platforms were used to measure the GRFs during fast and natural speeds of movement. Separate intraclass correlation coefficients (ICCs) were calculated for three temporal variables (ie, onset of the propulsive and braking phases of the lateral horizontal GRF component and time to unload the flexing limb) and for two magnitude variables (ie, propulsive impulse [PROP] and braking impulse [BRAK]). The ICCs for both PROP and BRAK were > or = .73 for fast movements and > or = .88 at the natural speed. The ICCs for the temporal variables were > or = .66 at fast speeds and > or = .37 at the natural speed. We concluded that measurements of PROP and BRAK are reliable across a range of speeds during transitions in stance support and that these variables may be tightly regulated by the movement control system. Temporal variables, particularly at the natural speed, exhibited lower reliability estimates, suggesting that measurements of these events have greater variability. Reliability of measurements of GRF variables provides useful information for the clinician regarding underlying control processes governing dynamic transitions in stance support.