Age-related changes in the control of whole-body angular momentum during stepping

Anticipatory muscle activations to coordinate balance and movement during motor transitions: A narrative review

BACKGROUND

Maintaining balance while moving is vital for day-to-day activities. A key challenge in the comprehension of human movement is to determine how muscles contribute to balance-movement coordination. Motor transitions, defined as movements executed between two steady balance states, are particularly interesting phases to study balance-movement coordination because a large, discrete change in whole-body momentum may disturb balance. During voluntarily-initiated motor transitions, anticipatory muscle patterns provide the biomechanical conditions that are favourable to both maintaining balance and executing the movement.

RESEARCH QUESTION

What are the mechanical consequences of anticipatory muscle activations for balance-movement coordination during voluntarily-initiated motor transitions?

METHODS

We review the biomechanical contributions of the anticipatory muscle activations identified in the literature during four types of voluntarily-initiated motor transitions, through the prism of three balance mechanisms ('moving the centre of pressure (CoP)', 'counter-rotating segments', and 'applying new external force(s)'). In particular, we investigate how anticipatory muscle activations modulate whole-body centre of mass acceleration.

RESULTS

We show that the mechanical consequences of anticipatory muscle activations have been extensively described, but mainly using the 'moving the CoP' mechanism. Unlike their role during steady balance states, both 'moving the CoP' and 'applying new external force(s)' mechanisms create a required mechanical instability during the anticipatory phase of motor transitions. The 'counter-rotating' mechanism may act as a stabiliser during motor transitions, but additional research is needed to clarify this assumption.

SIGNIFICANCE

This review establishes that muscle activation processes have different mechanical consequences for balance-movement coordination during the anticipatory phases of motor transitions, compared to steady balance states. Because the mechanical instability that is created can lead to falls, a better understanding of the mechanisms underlying motor transitions is needed to enable the design of more effective fall prevention programs and/or devices for population with balance deficits.

A comparison of anticipatory processes between gait and step initiation: The role of the final state

BACKGROUND

Whole-body movement initiation with change of base of support is a common paradigm used to assess balance-movement coordination. During clinical evaluation, two transitions are commonly used: gait or step initiation. During their respective anticipatory periods, two mechanisms regulate the whole-body centre of mass acceleration: a shift in the centre of pressure (CoP); and modulation of internal whole-body angular momentum (HM). These transitions have different final states, and we do not know whether the two mechanisms are used similarly during the anticipatory period.

RESEARCH QUESTION

Is the final state already taken into account during the anticipatory period of a whole-body movement initiation?

METHODS

We compared the time evolution of the CoP and HM in the sagittal plane during the anticipatory period of both gait and step initiation in 13 young, healthy participants.

RESULTS

During gait initiation, we observed a larger backward CoP shift (31-56 % and 75-100 % of the anticipatory period), a larger forward-directed HM (45-68 %), and a lower backward-directed HM (84-100 %) compared to step initiation.

SIGNIFICANCE

Our results show that mechanical instability is larger during gait initiation, while HM modulation is larger during step initiation. These findings suggest that the final state of a transition is taken into account during the anticipatory period. Based on our results, we suggest future research should use gait and step initiation to evaluate the generation of mechanical instability, and the use of free segments, respectively, in populations with balance deficits.

Patients with multiple sclerosis and low disability display cautious rotational behavior during gait initiation

BACKGROUND

Multiple sclerosis induces locomotor impairments. The objective was to characterize the effects of Multiple Sclerosis on whole-body angular momentum control during gait initiation.

METHODS

Fifteen patients with Multiple Sclerosis with Expanded Disability status scale of 2.5 and 16 healthy participants were instructed to perform gait initiation. Spatiotemporal parameters, whole-body angular momentum, net external moment about the body's center of mass and its components were calculated by using a 3D motion capture system and two force plates.

FINDINGS

Patients with Multiple Sclerosis had a significantly smaller whole-body angular momentum range during the double support phase of gait initiation in the transversal plane (p = 0.011), and smaller net external moment at the transition between the initial double support phase and the execution phase in the sagittal plane (p = 0.013). In the transversal plane, patients with Multiple Sclerosis had a smaller net external moment during the double support phase (p = 0.024) and between the double support phase and the execution phase (p < 0.001).

INTERPRETATION

Despite preserved spatiotemporal parameters during gait initiation, patients with Multiple Sclerosis with low disability had reduced net external moments in the transversal and sagittal planes during the critical transitional period of this functional task, which appeared as a compensatory modality to preserve global postural stability. This finding highlights the cautious rotational behaviors in these planes to prevent the risk of falling and preserve dynamic stability. Whole-body angular momentum and net external moment are relevant parameters for functional and disease progression follow-up of the disease.

Balance recovery schemes following mediolateral gyroscopic moment perturbations during walking

Maintaining balance during human walking hinges on the exquisite orchestration of whole-body angular momentum (WBAM). This study delves into the regulation of WBAM during gait by examining balance strategies in response to upper-body moment perturbations in the frontal plane. A portable Angular Momentum Perturbator (AMP) was utilized in this work, capable of generating perturbation torques on the upper body while minimizing the impact on the center of mass (CoM) excursions. Ten participants underwent upper-body perturbations during either the mid-stance or touch-down moment in both ipsilateral and contralateral directions in the frontal plane. Our findings emphasize the predominant role of the hip strategy and foot placement as primary mechanisms for recovering from WBAM perturbations, regardless of the perturbation's timing or direction. Specifically, hip add/abduction torque and step width were significantly modulated following perturbations during the stance and swing phases, respectively, to reject frontal-plane balance threats. The knee and ankle torque modulation were not found to be effective in the recovery process. Additionally, we observed that recovery from WBAM perturbations occurs promptly within the same stride in which the perturbation occurs, unlike other perturbation scenarios, such as platform translation. These insights have the potential to enhance the development of assistive devices and more robust controllers for bipedal robots.

Age related differences in whole body and body part angular momentum in the frontal plane during walking

The contribution of body part angular momentum (BPAM) to whole body angular momentum (WBAM) in the frontal plane during walking differs across age groups. We investigated age related differences in BPAM and WBAM during walking. We used marker coordinate data from a publicly available database for 54 individuals aged 20-30 years and 78 aged 60-70 years. Angular momentum in the frontal plane was calculated as the sum of the translational component and the rotational component for each segment. The angular momentum of each segment was categorized into five BPAM: right and left lower limbs (foot, shank, and thigh), right and left arms (hand, forearm, and upper arm), and torso (head, thorax, and pelvis). BPAM at WBAM peak frames during stride cycles was compared between older and younger adults. The peak WBAM, angular momentum of the stance-and swing side upper limbs, and torso in older adults was significantly larger than that in younger adults, with increases of 74.6% in the stance-side upper limb, 127.5% in the swing-side upper limb, and 30.9% in the torso. These results suggest that interventions aimed at improving torso control could decrease the amplitude of WBAM in the frontal plane in older adults.

The effect of amplitude normalization technique, walking speed, and reporting metric on whole-body angular momentum and its interpretation during normal gait

Whole-body angular momentum (WBAM) represents the cancellations of angular momenta that are produced during a reciprocal gait pattern. WBAM is sensitive to small changes and is used to compare dynamic gait patterns under different walking conditions. Study designs and the normalization techniques used to define WBAM vary and make comparisons between studies difficult. To address this problem, WBAM about each anatomical axis of rotation from a healthy control population during normal gait were investigated within four metrics: 1) range of WBAM, 2) integrated WBAM, 3) statistical parametric mapping (SPM), and 4) principal component analysis (PCA). These data were studied as a function of walking speed and normalization. Normalization techniques included: 1) no normalization, 2) normalization by height, body mass and walking speed, and 3) normalization by height, body mass and a scalar number, gravity×height, that is independent of walking velocity. Significant results were obtained as a function of walking speed regardless of normalization technique. However, the interpretation of significance within each metric was dependent on the normalization technique. Method 3 was the most robust technique as the differences were not altered from the expected relationships within the raw data. Method 2 actually inverted the expected relationship in WBAM amplitude as a function of walking speed, which skewed the results and their interpretation. Overall, SPM and PCA statistical methods provided better insights into differences that may be important. However, depending on the normalization technique used, caution is advised when interpreting significant findings when comparing participants with disparate walking speeds.

Instability during Stepping and Distance between the Center of Mass and the Minimal Moment Axis: Effect of Age and Speed

The goal of this study was to analyze instability during stepping at different speeds in young and older adults. To this aim, the anteroposterior and the mediolateral distances between the body center of mass (COM) and the minimum moment axis (MMA) were computed. A total of 15 young adults (25 y.o. [19–29]) and 15 older adults (68.7 y.o. [63–77]) volunteered for this study. For the computation of the distances, a complete biomechanical protocol combining two force platforms and a 3D motion capture analysis system was setup. The subjects were equipped with 47 reflective markers and were modeled as a frictionless multibody system with 19 segments, 18 joints and 42 degrees of freedom. They were asked to perform a series of stepping tasks at fast and spontaneous speeds. The stepping was divided into five phases, with successive swing and double-stance phases. Greater instability was observed during the swing phases. The distances reveal a significant higher instability at fast speed for both groups (p < 0.001) for all the phases compared with spontaneous speeds. The anteroposterior distance was significantly greater for older adults, highlighting greater instability compared to young adults, while no differences were observed for the mediolateral distance all along the five phases, suggesting higher risks of backward and forward falls during stepping.

Influence of aging on the control of the whole-body angular momentum during volitional stepping: An UCM-based analysis

Evidence suggests that whole-body angular momentum (WBAM) is a highly controlled mechanical variable for performing our daily motor activities safely and efficiently. Recent findings have revealed that, compared to young adults, older adults exhibit larger range of WBAM during various motor tasks, such as walking and stepping. However, it remains unclear whether these age-related changes are ascribed to a poorer control of WBAM with age or not. The purpose of the present study was to examine the effect of normal aging on WBAM control during stepping. Twelve young adults and 14 healthy older adults performed a series of volitional stepping at their preferred selected speed. An Uncontrolled Manifold (UCM) analysis was conducted to explore the presence of synergies among the angular momenta of the body segments (elemental variables) to control WBAM (performance variable); i.e., to stabilize or destabilize it. Results revealed the existence of a stronger synergy destabilizing the WBAM in the sagittal-plane older adults compared to young adults during stepping, while there was no difference between the two groups in the frontal and transversal planes. Although older participants also had a larger range of WBAM in the sagittal plane compared to young adults, we found no significant correlation between synergy index and the range of WBAM in the sagittal plane. We concluded that the age-related changes in WBAM during stepping are not ascribed to alterations in the ability to control this variable with aging.

Toward Safe Wearer-Prosthesis Interaction: Evaluation of Gait Stability and Human Compensation Strategy Under Faults in Robotic Transfemoral Prostheses

Although advanced wearable robots can assist human wearers, their internal faults (i.e., sensors or control errors) also pose a challenge. To ensure safe wearer-robot interactions, how internal errors by the prosthesis limb affect the stability of the user-prosthesis system, and how users react and compensate for the instability elicited by internal errors are imperative. The goals of this study were to 1) systematically investigate the biomechanics of a wearer-robot system reacting to internal errors induced by a powered knee prosthesis (PKP), and 2) quantify the error tolerable bound that does not affect the user's gait stability. Eight non-disabled participants and two unilateral transfemoral amputees walked on a pathway wearing a PKP, as the controller randomly switched the control parameters to disturbance parameters to mimic the errors caused by locomotion mode misrecognition. The size of prosthesis control errors was systematically varied to determine the error tolerable bound that disrupted gait stability. The effect of the error was quantified based on the 1) mechanical change described by the angular impulse applied by the PKP, and 2) overall gait instability quantified using human perception, angular momentum, and compensatory stepping. The results showed that the error tolerable bound is dependent on the gait phase and the direction of torque change. Two balance recovery strategies were also observed to allow participants to successful respond to the induced errors. The outcomes of this study may assist the future design of an auto-tuning algorithm, volitionally-controlled powered prosthetic legs, and training of gait stability.

Human balance control in 3D running based on virtual pivot point concept

Balance control is one of the crucial challenges in bipedal locomotion. Humans need to maintain their trunk upright while the body behaves like an inverted pendulum which is inherently unstable. Instead, the virtual pivot point (VPP) concept introduced a new virtual pendulum model to the human balance control paradigm by analyzing the ground reaction forces (GRF) in the body coordinate frame. This paper presents novel VPP-based analyses of the postural stability of human running in a 3D space. We demonstrate the relation between the VPP position and the gait speed. The experimental results suggest different control strategies in frontal and sagittal planes. The ground reaction forces intersect below the center of mass in the sagittal plane and above the center of mass in the frontal plane. These VPP locations are found for the sagittal and frontal planes at all running speeds, respectively. We introduced a 3D VPP-based model which can replicate the kinematic and kinetic behavior of human running. The similarity between the experimental and simulation results indicates the ability of the VPP concept in predicting human balance control in running and can support its applicability for gait assistance.

Differences in balance control between healthy younger and older adults during steady-state walking

Each year approximately one third of older adults fall and experience extensive musculoskeletal injuries and functional disabilities. An important element in maintaining dynamic balance is the regulation of whole-body angular momentum, which is achieved by proper foot placement with respect to the body center-of-mass as well as generation of appropriate ground reaction forces. Analyzing these quantities in younger and older adults may provide insight into differences in their underlying mechanics for maintaining dynamic balance. This study examined three-dimensional whole-body angular momentum in 13 healthy older (71.8 ± 8.3 years) and 9 younger (23.2 ± 2.8 years) adults walking at their self-selected and fastest-comfortable speeds. The older adults had a significantly higher range of frontal-plane angular momentum compared to the younger adults at both speeds, suggesting poorer mediolateral balance control. This difference was related to the older adults having a wider foot placement with respect to the body center-of-mass, which when combined with the vertical ground reaction force, created a higher destabilizing external moment during single-limb stance that acts to rotate the body towards the contralateral swing leg. To counteract this destabilizing moment, the older adults generated a higher hip abduction moment. There were no differences in the range of sagittal- and transverse-plane angular momentum between age groups at either speed. These results suggest that control of dynamic balance in the frontal-plane is more challenging than in the sagittal-plane for older adults and highlight the importance of proper weight transfer mechanisms and hip abductor force production for maintaining mediolateral balance during walking.

Segmental contribution to whole-body angular momentum during stepping in healthy young and old adults

Recent evidence suggests that during volitional stepping older adults control whole-body angular momentum (H) less effectively than younger adults, which may impose a greater challenge for balance control during this task in the elderly. This study investigated the influence of aging on the segment angular momenta and their contributions to H during stepping. Eighteen old and 15 young healthy adults were instructed to perform a series of stepping at two speed conditions: preferred and as fast as possible. Full-body kinematics were recorded to compute angular momenta of the trunk, arms and legs and their contributions to total absolute H on the entire stepping movement. Results indicated that older adults exhibited larger angular momenta of the trunk and legs in the sagittal plane, which contributed to a higher sagittal plane H range during stepping compared to young adults. Results also revealed that older adults had a greater trunk contribution and lower leg contribution to total absolute H in the sagittal plane compared to young adults, even though there was no difference in the other two planes. These results stress that age-related changes in H control during stepping arise as a result of changes in trunk and leg rotational dynamics.

Effect of increasing speed on whole-body angular momentum during stepping in the elderly

Recent evidence suggests that older adults may have difficulty controlling whole-body angular momentum (H) during volitional stepping, which could impose a major challenge for balance control and result in potential falls. However, it is not known if and how H is influenced by speed when stepping. This study aimed to investigate the effect on H of increasing speed during step initiation in older adults. Twenty-seven healthy individuals over 60 were enrolled in the current study and were instructed to perform a series of step initiations with their dominant leg under two speed conditions: at preferred speed and as fast as possible. Two force plates and a motion-capture system were used to record H and the components of the net external moment (moment arms and ground reaction forces) during the double support and step execution phases of stepping. Results revealed that increasing speed of stepping affected H differently in both stepping phases and in the different planes. H ranges in all three planes increased with speed during the double support phase. During the step execution phase, while H ranges in frontal and transversal planes decreased, sagittal plane H range significantly increased with speed. This increased H range in the sagittal plane, which may result from the task demands, could impose a greater challenge for balance control in the elderly.

Muscle Contributions to Balance Control During Amputee and Nonamputee Stair Ascent

Dynamic balance is controlled by lower-limb muscles and is more difficult to maintain during stair ascent compared to level walking. As a result, individuals with lower-limb amputations often have difficulty ascending stairs and are more susceptible to falls. The purpose of this study was to identify the biomechanical mechanisms used by individuals with and without amputation to control dynamic balance during stair ascent. Three-dimensional muscle-actuated forward dynamics simulations of amputee and nonamputee stair ascent were developed and contributions of individual muscles, the passive prosthesis, and gravity to the time rate of change of angular momentum were determined. The prosthesis replicated the role of nonamputee plantarflexors in the sagittal plane by contributing to forward angular momentum. The prosthesis largely replicated the role of nonamputee plantarflexors in the transverse plane but resulted in a greater change of angular momentum. In the frontal plane, the prosthesis and nonamputee plantarflexors contributed oppositely during the first half of stance while during the second half of stance, the prosthesis contributed to a much smaller extent. This resulted in altered contributions from the intact leg plantarflexors, vastii and hamstrings, and the intact and residual leg hip abductors. Therefore, prosthetic devices with altered contributions to frontal-plane angular momentum could improve balance control during amputee stair ascent and minimize necessary muscle compensations. In addition, targeted training could improve the force production magnitude and timing of muscles that regulate angular momentum to improve balance control.