Center of mass control and multi-segment coordination in children during and after whole-body vibration

Maintaining upright posture under external perturbations requires the coordination between the nervous system and the musculoskeletal system, and it is a milestone of motor development in early childhood. Whole-body vibration (WBV) has acute effect on postural control and muscular activation during standing, resulting in an increased sway velocity and sway area of center-of-mass (COM), and the residual effect usually vanishes within 20 minutes after the vibration exposure. In quiet standing, a human body can be considered as a multi-segment linked system, and an uncontrolled manifold (UCM) approach has been used to examine how this multi-joint motor redundancy is utilized to achieve postural control of the body. The aim of this study was to compare the joint variance structure between children and adults while controlling the COM before, during, and after WBV disturbance

Locomotor adjustments during stair ascent in children with Down Syndrome: Comparison between walking and crawling strategies

Stair negotiation provides an important yet different paradigm to study environment navigation. As one has constantly move himself up while adjusting step length and foot placement due to the stair constraints, this paradigm is ideal to study motor strategy and adaptation in children with and without disabilities. Down syndrome (DS) is the most common genetic condition and causes significant delays in motor and cognitive development. Children with DS show poor postural control and less efficient gait patterns. When negotiating obstacles, they often select a more conservative strategy (i.e. crawling instead of walking). This study aimed to examine motor strategy and spatiotemporal gait patterns in children with DS while ascending stairs of different heights

Effect of whole body vibration on center of mass movement in children and young adults

Whole body vibration (WBV) has acute effects on postural control strategy. The degree of impact depends on the amplitude, frequency, and duration of WBV, and the difficult level of balance task as well as the reliability of sensory information to the individual [1, 2, 3]. Healthy young adults can return to the baseline level of postural sway for a simple balance task within 20 minutes after repeated exposure to WBV [4]. Little is known about the acute and residual effects of WBV on the center-of-mass (COM) movement after a single bout of WBV. This study aimed to understand the COM movements of young adults pre-, during, immediately post-, and 5-minute post 40-second WBV during standing

Transitioning from level surface to stairs in children with and without Down syndrome: Locomotor adjustments during stair ascent

Background: Children with Down syndrome (DS) often show underdeveloped motor ability and adaptation. Stair ascent is a common task to examine locomotor function and external ankle load is often used to perturb the stability of a system and observe the emergence of new patterns. Research question: How do stair height and external ankle load affect locomotor adjustments in 5-to-11-year-old children with typical development (TD) and with DS during stair ascent? Methods: Fourteen children with DS and 14 age- and sex-matched children with TD participated in this study. They walked along a 5-m walkway and ascended 3-step staircases of different heights (low, moderate, and high) with or without ankle load. A 3D motion capture system was used for data collection. Dependent variables included stance time and toe-to-stair distance before stair ascent, and vertical toe clearance and horizontal toe velocity during stair ascent. Mixed ANOVAs with repeated measures were conducted for statistical analysis. Results: The DS group presented a longer stance time and a shorter toe-to-stair distance than the TD group before stair ascent. External ankle load affected, to a greater extent, the DS group than the TD group in stance time and toe-to-stair distance. During stair ascent, while the TD group generally maintained toe clearance and decreased horizontal toe velocity with the increase of stair height, the DS group decreased toe clearance and maintained horizontal toe velocity. Particularly, the DS group displayed a greater toe clearance than the TD group in the LS condition but a smaller toe clearance in the HS condition. In addition, external ankle load increased toe clearance and decreased horizontal toe velocity in both groups. Significance: Children with DS display underdeveloped locomotor adjustments during stair ascent. External ankle load appears to help the DS group regulate toe clearance and horizontal toe velocity for different stair heights

Content Caching and Delivery in Heterogeneous Vehicular Networks

Connected and automated vehicles (CAVs), which enable information exchange and content delivery in real time, are expected to revolutionize current transportation systems for better driving safety, traffic efficiency, and environmental sustainability. However, the emerging CAV applications such as content delivery pose stringent requirements on latency, throughput, reliability, and global connectivity. The current wireless networks face significant challenges to satisfy the requirements due to scarce radio spectrum resources, inflexibility to dynamic traffic demands, and geographic-constrained fixed infrastructure deployment. To empower multifarious CAV content delivery, heterogeneous vehicular networks (HetVNets), which integrate the terrestrial networks with aerial networks formed by unmanned aerial vehicles (UAVs) and space networks constituting of low Earth orbit (LEO) satellites, can guarantee reliable, flexible, cost-effective, and globally seamless service provisioning. In addition, edge caching is a promising solution to facilitate content delivery by caching popular files in the HetVNet access points (APs) to relieve the backhaul traffic with a lower delivery delay. The main technical issues are: 1) to fully reveal the potential of HetVNets for content delivery performance enhancement, content caching scheme design in HetVNets should jointly consider network characteristics, vehicle mobility patterns, content popularity, and APs’ caching capacities; 2) to fully exploit the controllable mobility and agility of UAVs to support dynamic vehicular content demands, the caching scheme and trajectory design for UAVs should be jointly optimized, which has not been well addressed due to their intricate inter-coupling relationships; and 3) for caching-based content delivery in HetVNets, a cooperative content delivery scheme should be designed to enable the cooperation among different network segments with ingenious utilization of heterogeneous network resources. In this thesis, we design the content caching and delivery schemes in the caching-enabled HetVNet to address the three technical issues. First, we study the content caching in HetVNets with fixed terrestrial APs including cellular base stations (CBSs), Wi-Fi roadside units (RSUs), and TV white space (TVWS) stations. To characterize the intermittent network connection caused by limited network coverage and high vehicle mobility, we establish an on-off model with service interruptions to describe the vehicular content delivery process. Content coding then is leveraged to resist the impact of unstable network connections and enhance caching efficiency. By jointly considering file characteristics and network conditions, the content placement is formulated as an integer linear programming (ILP) problem. Adopting the idea of the student admission model, the ILP problem is then transformed into a many-to-one matching problem between content files and HetVNet APs and solved by our proposed stable-matching-based caching scheme. Simulation results demonstrate that the proposed scheme can achieve near-optimal performances in terms of delivery delay and offloading ratio with a low complexity. Second, UAV-aided caching is considered to assist vehicular content delivery in aerial-ground vehicular networks (AGVN) and a joint caching and trajectory optimization (JCTO) problem is investigated to jointly optimize content caching, content delivery, and UAV trajectory. To enable real-time decision-making in highly dynamic vehicular networks, we propose a deep supervised learning scheme to solve the JCTO problem. Specifically, we first devise a clustering-based two-layered (CBTL) algorithm to solve the JCTO problem offline. With a given content caching policy, we design a time-based graph decomposition method to jointly optimize content delivery and UAV trajectory, with which we then leverage the particle swarm optimization algorithm to optimize the content caching. We then design a deep supervised learning architecture of the convolutional neural network (CNN) to make online decisions. With the CNN-based model, a function mapping the input network information to output decisions can be intelligently learnt to make timely inferences. Extensive trace-driven experiments are conducted to demonstrate the efficiency of CBTL in solving the JCTO problem and the superior learning performance with the CNN-based model. Third, we investigate caching-assisted cooperative content delivery in space-air-ground integrated vehicular networks (SAGVNs), where vehicular content requests can be cooperatively served by multiple APs in space, aerial, and terrestrial networks. In specific, a joint optimization problem of vehicle-to-AP association, bandwidth allocation, and content delivery ratio, referred to as the ABC problem, is formulated to minimize the overall content delivery delay while satisfying vehicular quality-of-service (QoS) requirements. To address the tightly-coupled optimization variables, we propose a load- and mobility-aware ABC (LMA-ABC) scheme to solve the joint optimization problem as follows. We first decompose the ABC problem to optimize the content delivery ratio. Then the impact of bandwidth allocation on the achievable delay performance is analyzed, and an effect of diminishing delay performance gain is revealed. Based on the analysis results, the LMA-ABC scheme is designed with the consideration of user fairness, load balancing, and vehicle mobility. Simulation results demonstrate that the proposed LMA-ABC scheme can significantly reduce the cooperative content delivery delay compared to the benchmark schemes. In summary, we have investigated the content caching in terrestrial networks with fixed APs, joint caching and trajectory optimization in the AGVN, and caching-assisted cooperative content delivery in the SAGVN. The proposed schemes and theoretical results should provide useful guidelines for future research in the caching scheme design and efficient utilization of network resources in caching-enabled heterogeneous wireless networks

Spatiotemporal gait pattern in children with and without Down Syndrome while walking from level surface to stairs

Children with Down syndrome (DS) display a delayed motor development, and show clumsy and less coordinated gait pattern. Compared to typically developing children (TD), children with DS walk with a notably slower speed, a shorter stride length, a limited range of motion in ankle joint with increased plantar flexion, and reduced dorsal flexion throughout the gait cycle. When negotiating an obstacle, children with DS show shorter stride length and greater step length variability, and stop in front of the obstacle before stepping over it

Motor strategy and locomotor adjustments in children with and without Down Syndrome while negotiating stairs

Children with Down Syndrome (DS) often show impaired motor control, and walk with a slower speed and a shorter step length than typically developing (TD) children. When negotiating an obstacle, children with DS often stop for a longer duration, choose a more conservative crawling strategy and display a smaller toe clearance than their TD peers

Transitioning from the level surface to stairs in children with and without Down syndrome: Motor strategy and anticipatory locomotor adjustments

Background: Children with Down syndrome (DS) show underdeveloped motor strategy and anticipatory locomotor adjustments (ALA) before crossing an obstacle. Stairs presents another important setting to study environment navigation and motor adaptation. Inclusion of external ankle load is often used to perturb the stability of a system and observe the emergence of new patterns. Research question: How do stair height and external ankle load affect motor strategy and ALA in 5-to-11-year-old children with typical development (TD) and with DS when approaching the stairs? Methods: Fourteen children with DS and 14 age- and sex-matched children with TD participated in the study. They walked along a 5-meter walkway and ascended 3-step staircases. There were three staircases (low, moderate, and high heights) and 2 loading conditions (no load and ankle load). A 3D motion capture system was used to collect data. Motor strategy was coded for each trial. Step length, width, time, and velocity, minimum toe clearance, and horizontal toe velocity were calculated for the last four steps before stair ascent. Mixed ANOVAs with repeated measures were conducted for statistical analysis. Results: The TD group walked up all the stairs, while the DS group displayed a strategy shift from walking to crawling when the stairs became higher. While the TD group maintained the values of most spatiotemporal variables, the DS group continuously decreased step length and velocity but not step width over the last four approaching steps. Ankle load decreased step length, step velocity, minimum toe clearance, and horizontal toe velocity in the DS group, to a greater extent, than in the TD group. Significance: Children with DS show underdeveloped motor strategy and ALA when approaching the stairs, and external ankle load further disrupts these patterns. Stair negotiation appears to be an effective assessment tool for evaluating motor adaptation in children with DS

Children Display Adult-Like Kinetic Pattern in the Time Domain but not in the Frequency Domain While Walking with Ankle Load

While external load is added during locomotion, humans increase the activation of extensors during stance and suppress the initiation of flexors during swing. External load at the ankles, rather than on the waist or thigh, elicits higher activities from ankle extensors, and increases stride length and decreases cadence in adults. Spatiotemporal and kinematic patterns become adult-like in children aged 5-8 years. However, little is known if children show adult like kinetic patterns while walking with external load This study aimed to investigate differences in kinetic patterns between children and adults while walking with external ankle load using both time and frequency domain analyses