Arm reactions evoked by the initial exposure to a small balance perturbation: a pilot study

Upper limb contribution during tandem gait in multiple sclerosis: An early marker of balance impairments

Tandem gait is widely used during clinical exams to evaluate dynamic balance in chronic diseases, such as multiple sclerosis (MS). The early detection of balance impairments in MS is challenging to improve the understanding of patients' complaints. The objective was to propose two indexes to quantify the contributions and inefficiency of limb and trunk movements during tandem gait in early-stage MS patients. Fifteen patients with remitting-relapsed MS, with a median Expanded Disability Status Scale of 2.5 [0-4] were compared to 15 matched healthy participants. Three-dimensional motion analysis was performed during tandem gait to calculate spatiotemporal parameters, contribution and inefficiency indexes, based on the linear momentum of body segments. Compared to healthy participants, MS patients at the early stage of disease executed tandem gait with higher speed (p = 0.03) and increased step length (p = 0.03). The contribution indexes of upper limbs were significantly decreased during swing phase in MS patients. The inefficiency index for the upper limbs were around twice higher for MS patients compared to healthy participants. Since the additional movements concerned only light body segments and not contribute to the whole-body forward progression during tandem gait, they could reflected more both upper limb movements alterations and restoring movements to avoid loss of balance during tandem gait around swing phase in MS. These quantified indexes could be used as physical markers to quantify both the balance deterioration and the efficiency of rehabilitation program during the follow up of MS from the early stage of their disease.

Simulating Human Upper and Lower Limb Balance Recovery Responses Using Nonlinear Model Predictive Control

The ability to generate predictive dynamic simulations of human movement using optimal control has been a growing point of interest in the design of medical/assistive devices, e.g. robotic exoskeletons. Despite this, many disseminated simulations of whole-body tasks, such as balance recovery, neglect the role of the upper body instead focusing on postural joints, e.g. ankle, knees, hips. Thus, the purpose of the current study was to use a novel nonlinear model predictive control (NMPC) approach to assess how actuated upper limbs, as well as different individual performance (optimality) criteria, can shape simulated reactive balance recovery responses. A sagittal biomechanical model of a young adult standing was designed and actuated via nonlinear muscle torque generators (rotational single-muscle equivalents). Forward dynamic simulations of balance recovery (NMPCdriven) following an unexpected support-surface perturbation were generated for each unique combination of selected performance criteria (6 total), perturbation direction (forward and backward), and arm joints free/locked. The observed joint trajectories provide insight into the emergence of human elements of postural control from individual optimality criteria, e.g. hip-ankle strategies emerge from single-joint regulation. Quantitative analysis of performance improvements with the arms free suggest that whether arm responses emerge in the simulations may be dependent on the problem's initial guess. Future work should focus on testing further performance criteria and improving NMPC as a model of the nervous system.

Characteristics of upper-extremity reactions to sudden lateral loss of balance in persons with stroke

BACKGROUND

Upper-extremity reactions are part of a whole-body response to counterweight the falling center of mass after unexpected balance loss. Impairments in upper-extremity reactions due to unilateral hemiparesis may contribute to stroke survivors propensity for falling. We aimed to characterize upper-extremity (paretic and non-paretic sides) reactive movements in response to lateral balance perturbations in Persons with Stroke vs. healthy controls.

METHODS

Twenty-six subacute persons with stroke and 15 healthy controls were exposed to multidirectional sudden unannounced surface translations in stance. Spatiotemporal parameters of upper- and lower-extremity balance responses to lateral perturbations were analyzed.

FINDINGS

In both groups reactive upper-extremity movement initiation preceded reactive step initiation. In response to a loss of balance toward the paretic side, persons with stroke demonstrated delayed movement initiation of both upper- and lower-extremity compared with healthy controls (In persons with stroke: 234.7 ± 60.0 msec and 227.1 ± 39.6 msec for upper extremities vs. 272.1 ± 59.1 msec for lower-extremity; and in controls: 180.1 ± 39.9 msec and 197.8 ± 61.3 msec for upper-extremities vs. 219.3 ± 40.8 msec for lower-extremity; p = 0.001, Cohen's d's: 0.59-1.03) and a greater abduction excursion in the ipsilateral upper-extremity compared with the contralateral upper-extremity (In persons with stroke: 39.3 ± 23.6 cm vs. 24.9 ± 10.1 cm, respectively; In Controls: 42.6 ± 21.8 cm vs. 29.3 ± 17.3 cm, respectively).

INTERPRETATION

The faster upper-extremity reactive movement reactions compared to reactive step initiation in both persons with stroke and healthy controls suggests that balance recovery is an automatic "reflex-like" response. Delayed upper-extremity reactive reactions in conditions of surface translation toward the non-paretic side in persons with stroke may increase the risk of falls in the direction of the paretic side.

Evaluation of balance recovery stability from unpredictable perturbations through the compensatory arm and leg movements (CALM) scale

Following unpredictable large-magnitude stance perturbations diverse patterns of arm and leg movements are performed to recover balance stability. Stability of these compensatory movements could be properly estimated through qualitative evaluation. In the present study, we present a scale for evaluation of compensatory arm and leg movements (CALM) in response to unpredictable displacements of the support base in the mediolateral direction. We tested the CALM scale for intra- and inter-rater reliability, correlation with kinematics of arm and leg movement amplitudes, and sensitivity to mode (rotation, translation and combined) and magnitude (velocity) of support base displacements, and also to perturbation-based balance training. Results showed significant intra- and inter-rater coefficients of agreement, ranging from moderate (0.46–0.53) for inter-rater reliability in the arm and global scores, to very high (0.87–0.99) for inter-rater leg scores and all intra-rater scores. Analysis showed significant correlation values between scale scores and the respective movement amplitudes both for arm and leg movements. Assessment of sensitivity revealed that the scale discriminated the responses between perturbation modes, platform velocities, in addition to higher balance recovery stability as a result of perturbation-based balance training. As a conclusion, the CALM scale was shown to provide adequate integrative evaluation of compensatory arm and leg movements for balance recovery stability after challenging stance perturbations, with potential application in fall risk prediction.

Reliability of test procedures for postural reactions in people with acute stroke

Background/Aims:

Regaining and maintaining balance requires postural reactions such as righting reactions, equilibrium reactions, and protective reactions. There is a lack of uniform, standardised, and reliable testing procedures for postural reactions. The aim of the present study was to examine the intra- and interrater reliability of a newly developed postural reactions assessment for use in people with acute stroke.

Methods:

The Postural Reactions Test was developed based on the literature, on previous tests, and on input from an expert panel. A total of 10 physiotherapists assessed a total of 20 video recordings of people with acute stroke performing each postural reaction. These assessments were carried out on two occasions at least 2 weeks apart. The study thus included 400 ratings.

Findings:

For intrarater reliability, the overall proportion of agreement was 86 − 93% for the different postural reactions. For interrater reliability, the most common score for each participant and the number of physiotherapists giving that score were noted. A median of 9–10 out of 10 physiotherapists scored the same value.

Conclusions:

The results indicate that the Postural Reactions Test can be used to reliably assess function in people with acute stroke and that the test can complement the existing assessments for people with affected postural control.

Repeated Exposure to Forward Support-Surface Perturbation During Overground Walking Alters Upper-Body Kinematics and Step Parameters

Locomotion requires both proactive and reactive control strategies to maintain balance. The current study aimed to: (i) ascertain upper body postural responses following first exposure to a forward (slip) support-surface perturbation; (ii) investigate effects of repeated perturbation exposure; (iii) establish relationships between arms and other response components (trunk; center of mass control). Young adults (N = 11) completed 14 walking trials on a robotic platform; six elicited a slip response. Kinematic analyses were focused on extrapolated center of mass position (xCoM), bilateral upper- and forearm elevation velocity, trunk angular velocity, and step parameters. Results demonstrated that postural responses evoked in the first slip exposure were the largest in magnitude (e.g., reduced backward stability, altered reactive stepping, etc.) and preceded by anticipatory anterior adjustments of xCoM. In relation to the perturbed leg, the large contra- and ipsilateral arm responses observed (in first exposure) were characteristically asymmetric and scaled to the degree of peak trunk extension. With repeated exposure, xCoM anticipatory adjustments were altered and in turn, reduced posterior xCoM motion occurred following a slip (changes plateaued at second exposure). The few components of the slip response that persisted across multiple exposures did so at a lesser magnitude (e.g., step length and arms).

Bile acids at the cross-roads of gut microbiome-host cardiometabolic interactions

While basic and clinical research over the last several decades has recognized a number of modifiable risk factors associated with cardiometabolic disease progression, additional and alternative biological perspectives may offer novel targets for prevention and treatment of this disease set. There is mounting preclinical and emerging clinical evidence indicating that the mass of metabolically diverse microorganisms which inhabit the human gastrointestinal tract may be implicated in initiation and modulation of cardiovascular and metabolic disease outcomes. The following review will discuss this gut microbiome-host metabolism axis and address newly proposed bile-mediated signaling pathways through which dysregulation of this homeostatic axis may influence host cardiovascular risk. With a central focus on the major nuclear and membrane-bound bile acid receptor ligands, we aim to review the putative impact of microbial bile acid modification on several major phenotypes of metabolic syndrome, from obesity to heart failure. Finally, attempting to synthesize several separate but complementary hypotheses, we will review current directions in preclinical and clinical investigation in this evolving field.

Holding a Handle for Balance during Continuous Postural Perturbations-Immediate and Transitionary Effects on Whole Body Posture

When balance is exposed to perturbations, hand contacts are often used to assist postural control. We investigated the immediate and the transitionary effects of supportive hand contacts during continuous anteroposterior perturbations of stance by automated waist-pulls. Ten young adults were perturbed for 5 min and required to maintain balance by holding to a stationary, shoulder-high handle and following its removal. Center of pressure (COP) displacement, hip, knee and ankle angles, leg and trunk muscle activity and handle contact forces were acquired. The analysis of results show that COP excursions are significantly smaller when the subjects utilize supportive hand contact and that the displacement of COP is strongly correlated to the perturbation force and significantly larger in the anterior than posterior direction. Regression analysis of hand forces revealed that subjects utilized the hand support significantly more during the posterior than anterior perturbations. Moreover, kinematical analysis showed that utilization of supportive hand contacts alter posture of the whole body and that postural readjustments after the release of the handle, occur at different time scales in the hip, knee and ankle joints. Overall, our findings show that supportive hand contacts are efficiently used for balance control during continuous postural perturbations and that utilization of a handle has significant immediate and transitionary effects on whole body posture.

Calibration of the Leg Muscle Responses Elicited by Predictable Perturbations of Stance and the Effect of Vision

Motor adaptation due to task practice implies a gradual shift from deliberate control of behavior to automatic processing, which is less resource- and effort-demanding. This is true both for deliberate aiming movements and for more stereotyped movements such as locomotion and equilibrium maintenance. Balance control under persisting critical conditions would require large conscious and motor effort in the absence of gradual modification of the behavior. We defined time-course of kinematic and muscle features of the process of adaptation to repeated, predictable perturbations of balance eliciting both reflex and anticipatory responses. Fifty-nine sinusoidal (10 cm, 0.6 Hz) platform displacement cycles were administered to 10 subjects eyes-closed (EC) and eyes-open (EO). Head and Center of Mass (CoM) position, ankle angle and Tibialis Anterior (TA) and Soleus (Sol) EMG were assessed. EMG bursts were classified as reflex or anticipatory based on the relationship between burst amplitude and ankle angular velocity. Muscle activity decreased over time, to a much larger extent for TA than Sol. The attenuation was larger for the reflex than the anticipatory responses. Regardless of muscle activity attenuation, latency of muscle bursts and peak-to-peak CoM displacement did not change across perturbation cycles. Vision more than doubled speed and the amount of EMG adaptation particularly for TA activity, rapidly enhanced body segment coordination, and crucially reduced head displacement. The findings give new insight on the mode of amplitude- and time-modulation of motor output during adaptation in a balancing task, advocate a protocol for assessing flexibility of balance strategies, and provide a reference for addressing balance problems in patients with movement disorders.

Does knee motion contribute to feet-in-place balance recovery?

Although knee motions have been observed at loss of balance, the ankle and hip strategies have remained the focus of past research. The present study aimed to investigate whether knee motions contribute to feet-in-place balance recovery. This was achieved by experimentally monitoring knee motions during recovery from forward falling, and by simulating balance recovery movements with and without knee joint as the main focus of the study. Twelve participants initially held a straight body configuration and were released from different forward leaning positions. Considerable knee motions were observed especially at greater leaning angles. Simulations were performed using 3-segment (feet, shanks+thighs, and head+arms+trunk) and 4-segment (with separate shanks and thighs segments) planar models. Movements were driven by joint torque generators depending on joint angle, angular velocity, and activation level. Optimal joint motions moved the mass center projection to be within the base of support without excessive joint motion. The 3-segment model (without knee motions) generated greater backward linear momentum and had better balance performance, which confirmed the advantage of having only ankle/hip strategies. Knee motions were accompanied with less body angular momentum and a lower body posture, which could be beneficial for posture control and reducing falling impact, respectively.

Anticipatory and Reactive Response to Falls: Muscle Synergy Activation of Forearm Muscles

We investigated the surface electromyogram response of six forearm muscles to falls onto the outstretched hand. The extensor carpi radialis longus, extensor carpi radialis brevis, extensor carpi ulnaris, abductor pollicis longus, flexor carpi radialis and flexor carpi ulnaris muscles were sampled from eight volunteers who underwent ten self-initiated falls. All muscles initiated prior to impact. Co-contraction is the most obvious surface electromyogram feature. The predominant response is in the radial deviators. The surface electromyogram timing we recorded would appear to be a complex anticipatory response to falling modified by the effect on the forearm muscles following impact. The mitigation of the force of impact is probably more importantly through shoulder abduction and extension and elbow flexion rather than action of the forearm muscles.

Effect of arm swing on single-step balance recovery

Balance recovery techniques are useful not only in preventing falls but also in many sports activities. The step strategy plays an important role especially under intense perturbations. However, relatively little is known about the effect of arm swing on stepping balance recovery although considerable arm motions have been observed. The purpose of this study was to examine how the arms influence kinematic and kinetic characteristics in single-step balance recovery. Twelve young male adults were released from three forward-lean angles and asked to regain balance by taking a single step under arm swing (AS) and arm constrained (AC) conditions. It was found that unconstrained arms had initial forward motion and later upward motion causing increased moment of inertia of the body, which decreased falling angular velocity and allowed more time for stepping. The lengthened total balance time included weight transfer and stepping time, although duration increase in the latter was significant only at the largest lean angle. In contrast, step length, step velocity, and vertical ground reaction forces on the stepping foot were unaffected by arm swing. Future studies are required to investigate optimal movement strategies for the arms to coordinate with other body segments in balance recovery and injury reduction.

Maintaining standing balance by handrail grasping

Maintaining balance while standing on a moving bus or subway is challenging, and falls among passengers are a significant source of morbidity. Standing passengers often rely on handrail grasping to resist perturbations to balance. We conducted experiments that simulated vehicle starts, to examine how handrail location (overhead or shoulder-height), perturbation direction (forward, backward, left or right), and perturbation magnitude (1 or 2m/s(2)) affected the biomechanical effort (peak centre-of-pressure (COP) excursion and hand force) and muscle activations (onset and integrated EMG activity) involved in balance maintenance. COP excursions, hand forces and muscle activations were altered in a functional manner based on task constraints and perturbation characteristics. Handrail position affected normalized values of peak COP and hand force during forward and backward, but not sideways perturbations. During backward perturbations, COP excursion was greater when grasping overhead than shoulder-height. During forward perturbations, hand force was greater when grasping shoulder-height than overhead. Biceps activations were earlier during shoulder-height than overhead grasping, while tibialis anterior activity was higher during overhead than shoulder-height grasping. Our results indicate that, when facing forward or backward to the direction of vehicle motion, overhead grasping minimizes hand force, while shoulder-height grasping minimizes COP excursion. In contrast, grasping with a sideways stance eliminates the effect of handrail location, and was associated with equal or lower biomechanical effort. This suggests that, at least for vehicle starts, the most reasonable strategy may be to stand sideways to the direction of the vehicle movement, and grasp either at shoulder-height or overhead.