Postural preparation to making a step: is there a 'motor program' for postural preparation?

Expectation of volitional arm movement has prolonged effects on the grip force exerted on a pinched object

Humans closely coordinate the grip force exerted on a hand-held object with changes in the load arising from the object's dynamics. Recent work suggests the grip force is responsive to the predictability of the load forces as well. The well-known grip-force-load-force coupling is intermittent when the load arising from volitional movements fluctuates predictably, whereas grip force increases when loads are unpredictable. Here, we studied the influence of expected but uncertain volitional movements on the digit forces during a static grasp. Young, healthy participants used a pinch grasp to hold an instrumented object and track visual targets by moving the object. We quantified the mean grip force, the temporal decline in grip force (slacking), and the coupling between the pressing digit forces that yield the grip force during static prehension with no expectation of movement, and during the static phase of a choice reaction time task, when the participant expected to move the object after a variable duration. Simply expecting to move the object led to sustained (for at least 5 s) higher magnitude and lower slacking in the grip force, and weaker coupling between the pressing digit forces. These effects were modulated by the direction of the expected movement and the object's mass. The changes helped to maintain the safety margin for the current grasp and likely facilitated the transition from static to dynamic object manipulation. Influence of expected actions on the current grasp may have implications for manual dexterity and its well-known loss with age.

Postural balance effects from exposure to multi-axial whole-body vibration in mining vehicle operation

Twenty participants (18 males and 2 females) completed postural stability assessments before and after 4-h exposure to whole body vibration (WBV) in four experimental conditions: (a) vertical-dominant WBV with vertical passive air suspension, (b) multi-axial WBV with vertical passive air suspension, (c) multi-axial WBV with multi-axial active suspension, and (d) no WBV condition. Center of pressure (COP)-based postural sway measures significantly increased following multi-axial WBV exposure. Increase in COP velocity and displacement following multi-axial WBV was significantly higher than the increase in all the other exposure conditions. However, no significant differences between the WBV conditions were observed in functional limits of stability and anticipatory postural adjustments. While our results show standing balance to be impaired following the multi-axial WBV exposure of off-road mining vehicles, dynamic stability across a broader range of conditions needs to be understood to causally link postural stability decrements to increased fall-risks associated with off-road vehicle operators.

The relative and absolute reliability of center of pressure trajectory during gait initiation in older adults

It has been thought that for scientific acceptance of a parameter, its psychometric properties such as reliability, validity and responsiveness have critical roles. Therefore, this study was conducted to estimate how many trials are required to obtain a reliable center of pressure (COP) parameter during gait initiation (GI) and to investigate the effect of number of trials on the relative and absolute reliability. Twenty older adults participated in the study. Subjects began stepping over the force platform in response to an auditory stimulus. Ten trials were collected in one session. The displacement, velocity, mean and median frequency of the COP in the mediolateral (ML) and anteroposterior (AP) directions were evaluated. Relative reliability was determined using the intraclass correlation coefficient (ICC), and absolute reliability was evaluated using the standard error of measurement (SEM) and minimal detectable change (MDC₉₅). The results revealed with respect to parameter, one to five trials should be averaged to ensure excellent reliability. Moreover, ICC, SEM% and MDC₉₅% values were between 0.39-0.89, 4.84-41.5% and 13.4-115% for single trial and 0.86-0.99, 1.74-19.7% and 4.83-54.7% for ten trials averaged, respectively. Moreover, the ML and AP COP displacement in locomotor phase had the most relative reliability as well as the ML and AP median frequency in locomotor phase had the most absolute reliability. In general, the results showed that the COP-related parameters in time and frequency domains, based on average of five trials, provide reliable outcome measures for evaluation of dynamic postural control in older adults.

Gait initiation is impaired in subjects with Parkinson's disease in the OFF state: Evidence from the analysis of the anticipatory postural adjustments through wearable inertial sensors

People with Parkinson's disease (PD) typically demonstrate impaired anticipatory postural adjustments (APAs) that shift the body center of mass forward (imbalance) and over the stance leg (unloading) prior to gait initiation. APAs are known to be smallest when people with PD are in their OFF-medication state compared to ON-medication or healthy controls. The aim of this pilot study is to validate a previously developed method for the assessment of gait initiation on PD patients in OFF state with body-worn, inertial sensors. Ten subjects with mild-to-moderate idiopathic PD and twelve healthy controls of similar age performed three gait initiation trials. The spatio-temporal parameters of APAs were extracted from three wearable sensors, placed on the shins and on the lower back, and validated with two force plates. Temporal parameters extracted from sensors and force plates, as well as the trunk medio-lateral acceleration and the correspondent displacement of the center of pressure, were significantly correlated. Subjects with PD showed hypometric adjustments in the medio-lateral direction (p-value<0.003) and increased duration of the unloading phase (p-value=0.04). The unloading phase was significantly longer than the imbalance (p-value=0.003) only in subjects with PD. The validity of the method of quantifying APAs from inertial sensors was confirmed in PD subjects by comparison with force plates. Sensitivity in discriminating PD patients from healthy controls was proven by both spatial and temporal parameters. Objective measures of gait initiation deficits with wearable technology provides valuable instrument for the assessment of gait initiation in clinical environments.

Postural Preparation to Stepping: Coupled Center of Pressure Shifts in the Anterior-Posterior and Medio-Lateral Directions

We explored changes in the postural preparation to stepping introduced by modifications of the initial coordinates of the center of pressure (COP). We hypothesized that the postural adjustments in the anterior-posterior direction would persist across all initial COP manipulations while the adjustments in the medio-lateral direction would be highly sensitive to the initial COP coordinate. Healthy subjects stood on a force plate, shifted the body weight to one of the initial conditions that spanned the range of COP coordinates in both directions, and initiated a single step or started to walk. No major changes were observed between the stepping and walking conditions. Changes in the initial COP coordinate in the medio-lateral direction led to scaling of the magnitude of the COP shift in that direction prior to stepping accompanied by a nearly proportional change in the COP shift in the anterior-posterior direction. Changes in the initial COP coordinate in the anterior-posterior direction led to scaling of the magnitude of the COP shift in that direction prior to stepping without consistent changes in the COP shift in the medio-lateral direction. We interpret the results as reflecting a neural organization using a small set of referent body configurations for the postural adjustments.

The effects of mild traumatic brain injury on postural control

PRIMARY OBJECTIVE

The purpose of this study was to investigate the effects of mild traumatic brain injury (mTBI) on multiple postural indices that characterize body sway behaviour.

METHODS AND PROCEDURES

The body's centre of pressure (COP) displacement was recorded from 11 individuals with a history of mTBI (29.4 ± 6.7 years old) and 11 healthy controls (26.8 ± 3.7 years old) performing bipedal stance on a force platform for 120 seconds. Spatio-temporal (area, amplitude and mean velocity of the COP displacement) and frequency characteristics (frequency containing 80% of the power spectral density) of the body oscillation, as well as its dynamic characteristics (sample entropy estimate of the COP displacement) were extracted from COP signals.

MAIN OUTCOMES AND RESULTS

All postural indices studied were significantly affected by mTBI (p < 0.010). Participants with a history of mTBI presented a larger, slower, and more random body oscillation compared to controls.

CONCLUSION

The results suggest that (a) balance deficits can be recognized as an effect of mTBI; (b) balance deficits induced by mTBI are multi-dimensional, affecting all three domains included in this study; and

A new instrumented method for the evaluation of gait initiation and step climbing based on inertial sensors: a pilot application in Parkinson's disease

Background

Step climbing is a demanding task required for personal autonomy in daily living. Anticipatory Postural Adjustments (APAs) preceding gait initiation have been widely investigated revealing to be hypometric in Parkinson’s disease (PD) with consequences in movement initiation. However, only few studies focused on APAs prior to step climbing. In this work, a novel method based on wearable inertial sensors for the analysis of APAs preceding gait initiation and step climbing was developed to further understand dynamic balance control. Validity and sensitivity of the method have been evaluated.

Methods

Eleven PD and 20 healthy subjects were asked to perform two transitional tasks from quiet standing to level walking, and to step climbing respectively. All the participants wore two inertial sensors, placed on the trunk (L2-L4) and laterally on the shank. In addition, a validation group composed of healthy subjects and 5 PD patients performed the tasks on two force platforms. Correlation between parameters from wearable sensors and force platforms was evaluated. Temporal parameters and trunk acceleration from PD and healthy subjects were analyzed.

Results

Significant correlation was found for the validation group between temporal parameters extracted from wearable sensors and force platforms and between medio-lateral component of trunk acceleration and correspondent COP displacement. These results support the validity of the method for evaluating APAs prior to both gait initiation and step climbing. Comparison between PD subjects and a subgroup of healthy controls confirms a reduction in PD of the medio-lateral acceleration of the trunk during the imbalance phase in the gait initiation task and shows similar trends during the imbalance and unloading phase of the step climbing task. Interestingly, PD subjects presented difficulties in adapting the medio-lateral amplitude of the imbalance phase to the specific task needs.

Conclusions

Validity of the method was confirmed by the significant correlation between parameters extracted from wearable sensors and force platforms. Sensitivity was proved by the capability to discriminate PD subjects from healthy controls. Our findings support the applicability of the method to subjects of different age. This method could be a possible valid instrument for a better understanding of feed-forward anticipatory strategies.

Human standing: does the control strategy preprogram a rigid knee?

Human standing requires control of multisegmental configuration. Does the postural system normally allow flexible adjustment of configuration, or does it minimize degrees of freedom at the ankle, knee, and hip joints? Gentle, external, unpredictable, sagittal, mechanical perturbations (randomized force, 1-10 N; duration, 0.2-2 s; and leg) were applied to either knee of 24 healthy participants who stood symmetrically for 200 s. The translation of knee perturbation force to ankle, knee, and hip joint rotations in the perturbed and unperturbed legs was studied. We assessed whether consequent joint rotations indicated a stiff configuration-conserving or viscous energy-absorbing relationship to the knee perturbation. Two distinctive response patterns were observed. Twenty-two participants showed limited knee flexion and high ankle stiffness, whereas two participants showed substantial knee flexion, low ankle stiffness, measurable internal rotation of the unperturbed hip (0.4 ± 0.3 vs. 3.0 ± 1°, 5.7 ± 17 vs. 0.5 ± 0.3 N/°, 1.1 ± 0.4°, respectively; mean ± SD), and a viscous relationship between perturbation force and subsequent ankle flexion, knee flexion, and perturbed and unperturbed hip internal rotation. The size of knee-flexion response to knee perturbations was uncorrelated with the extent of unperturbed standing sway. Normal standing conceals a large interindividual range in leg control strategies, indicating adaptive potential to progress with development and skill acquisition and decline with age, disease, injury, and fear. Commonly, leg configuration was maintained stiffly. Less commonly, a bilateral, low-stiffness, energy-absorbing strategy utilizing the available degrees of freedom was shown. We propose that identification of individual coordination strategy has diagnostic and prognostic potential in relation to perceptual-posture-movement-fall interactions.