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PubMed2006

Performance characterization of the dynamic programming obstacle detection algorithm.

A computer vision-based system using images from an airborne aircraft can increase flight safety by aiding the pilot to detect obstacles in the flight path so as to avoid mid-air collisions. Such a system fits naturally with the development of an external vision system proposed by NASA for use in high-speed civil transport aircraft with limited cockpit visibility. The detection techniques should provide high detection probability for obstacles that can vary from subpixels to a few pixels in size, while maintaining a low false alarm probability in the presence of noise and severe background clutter. Furthermore, the detection algorithms must be able to report such obstacles in a timely fashion, imposing severe constraints on their execution time. For this purpose, we have implemented a number of algorithms to detect airborne obstacles using image sequences obtained from a camera mounted on an aircraft. This paper describes the methodology used for characterizing the performance of the dynamic programming obstacle detection algorithm and its special cases. The experimental results were obtained using several types of image sequences, with simulated and real backgrounds. The approximate performance of the algorithm is also theoretically derived using principles of statistical analysis in terms of the signal-to-noise ration (SNR) required for the probabilities of false alarms and misdetections to be lower than prespecified values. The theoretical and experimental performance are compared in terms of the required SNR.

Algorithms↗
PubMed2001

Obstacles to contraceptive use in Pakistan: a study in Punjab.

The principal aim of this study is to assess the strength in Pakistan of a set of hypothesized obstacles to practicing contraception. Survey data are analyzed that were collected in Punjab province in 1996 and that contain unusually detailed measurement of various perceived costs of practicing contraception, as well as focused measurement of fertility motivation. The framework guiding the research specifies six major obstacles to contraceptive use: the strength of motivation to avoid pregnancy, awareness and knowledge of contraception, the social and cultural acceptability of contraception, perceptions of the husband's preferences and attitudes, health concerns, and perceived access to services. Net effects of each obstacle are estimated through structural equation modeling of the intention to practice contraception in the near future, in which the six obstacles are treated as latent variables. The estimates indicate that the two principal obstacles to using a contraceptive are the woman's perception that such behavior would conflict with her husband's fertility preferences and his attitudes toward family planning and her perception of the social or cultural unacceptability of contraception. The results confirm the value of taking contraceptive costs seriously, and, in particular, of attempting to measure these costs in empirical research on family planning.

Adult↗
PubMed2001

Planning reaching and grasping movements: the problem of obstacle avoidance.

In this article, we review a model of the movement-planning processes that people use for direct reaching, reaching around obstacles, and grasping, and we present observations of subjects' repeated movements of the hand to touch 2 target locations, circumventing an intervening obstacle. The model defines an obstacle as a posture that, if adopted, would intersect with any part of the environment (including the actor himself or herself). The model finds a trajectory that is likely to bring the end-effector to the target by means of a one-or two- stage planning process. Each stage exploits the principles of instance retrieval and instance generation. In the first stage, a goal posture is identified, and the trajectory of a direct transition to that posture is tested for collision. If the direct movement has no collision, the movement to the target is immediately executed in joint space. If, however, the direct movement is foreseen to result in a collision, a second planning stage is invoked. The second planing stage identifies a via posture, movement through which will probably avoid the collision. Movement to and from the via posture is then superimposed on the main movement to the target so that the combined movement reaches the target without colliding with intervening obstacles. We describe the details of instance retrieval and instance generation for each of these planning stages and compare the model's performance with the observed kinematics of direct movements as well as movements around an obstacle. Then we suggest how the model might contribute to the study of movements in people with motor disorders such as spastic hemiparesis.

Adult↗
PubMed2004

Transfer of motor performance in an obstacle avoidance task to different walking conditions.

The aim of this study was to examine whether subjects who have learned a skilled locomotor task can transfer the acquired performance to conditions involving either a change in the external coordinates or in the sensory input from one leg. Subjects were trained to step over an obstacle with minimal foot clearance without visual information about either the obstacle or their legs during treadmill walking. Leg muscle activity and joint kinematics were recorded and analyzed. Acoustic signals provided feedback about foot clearance over the obstacle. After successful training, the transfer of learning between level and downhill walking and to walking with additional weight attached to the leg was examined. It was found that once subjects learned to step over the obstacle at an optimal foot clearance, they could transfer their performance within the first step over the obstacle in the new walking conditions. Closer examination of the transfer between level and downhill walking revealed no consistent kinematic strategy across subjects. To transfer the learned performance to walking with additional weight, subjects consistently and automatically increased biceps femoris muscle activation. The results are discussed in the context of emerging concepts in the neural control of walking and motor learning.

Adolescent↗
PubMed2000

Muscular responses and movement strategies during stumbling over obstacles.

Although many studies have investigated reflexes after stimulation of either cutaneous or proprioceptive afferents, much less is known about responses after more natural perturbations, such as stumbling over an obstacle. In particular, the phase dependency of these responses and their relation to the stumbling behavior has received little attention. Hence response strategies during stumbling reactions after perturbations at different times in the swing phase of gait were studied. While subjects walked on a treadmill, a rigid obstacle unexpectedly obstructed the forward sway of the foot. All subjects showed an "elevating strategy" after early swing perturbations and a "lowering strategy" after late swing perturbations. During the elevating strategy, the foot was directly lifted over the obstacle through extra knee flexion assisted by ipsilateral biceps femoris (iBF) responses and ankle dorsiflexion assisted by tibialis anterior (iTA) responses. Later, large rectus femoris (iRF) activations induced knee extension to place the foot on the treadmill. During the lowering strategy, the foot was quickly placed on the treadmill and was lifted over the obstacle in the subsequent swing. Foot placement was actively controlled by iRF and iBF responses related to knee extension and deceleration of the forward sway. Activations of iTA mostly preceded the main ipsilateral soleus (iSO) responses. For both strategies, four response peaks could be distinguished with latencies of approximately 40 ms (RP1), approximately 75 ms (RP2), approximately 110 ms (RP3), and approximately 160 ms (RP4). The amplitudes of these response peaks depended on the phase in the step cycle. The phase-dependent modulation of the responses could not be accounted for by differences in stimulation or in background activity and therefore is assumed to be premotoneuronal in origin. In mid swing, both the elevating and lowering strategy could occur. For this phase, the responses of the two strategies could be compared in the absence of phase-dependent response modulation. Both strategies had the same initial electromyographic responses till approximately 100 ms (RP1-RP2) after perturbation. The earliest response (RP1) is assumed to be a short-latency stretch reflex evoked by the considerable impact of the collision, whereas the second (RP2) has features reminiscent of cutaneous and proprioceptive responses. Both these responses did not determine the behavioral response strategy. The functionally important response strategies depended on later responses (RP3-RP4). These data suggest that during stumbling reactions, as a first line of defense, the CNS releases a relatively aspecific response, which is followed by an appropriate behavioral response to avoid the obstacle.

Adult↗
PubMed2006

A five-week exercise program can reduce falls and improve obstacle avoidance in the elderly.

BACKGROUND: Falls in the elderly are a major health problem. Although exercise programs have been shown to reduce the risk of falls, the optimal exercise components, as well as the working mechanisms that underlie the effectiveness of these programs, have not yet been established. OBJECTIVE: To test whether the Nijmegen Falls Prevention Program was effective in reducing falls and improving standing balance, balance confidence, and obstacle avoidance performance in community-dwelling elderly people. METHODS: A total of 113 elderly with a history of falls participated in this study (exercise group, n = 79; control group, n = 28; dropouts before randomization, n = 6). Exercise sessions were held twice weekly for 5 weeks. Pre- and post-intervention fall monitoring and quantitative motor control assessments were performed. The outcome measures were the number of falls, standing balance and obstacle avoidance performance, and balance confidence scores. RESULTS: The number of falls in the exercise group decreased by 46% (incidence rate ratio (IRR) 0.54, 95% confidence interval (CI) 0.36-0.79) compared to the number of falls during the baseline period and by 46% (IRR 0.54, 95% CI 0.34-0.86) compared to the control group. Obstacle avoidance success rates improved significantly more in the exercise group (on average 12%) compared to the control group (on average 6%). Quiet stance and weight-shifting measures did not show significant effects of exercise. The exercise group also had a 6% increase of balance confidence scores. CONCLUSION: The Nijmegen Falls Prevention Program was effective in reducing the incidence of falls in otherwise healthy elderly. There was no evidence of improved control of posture as a mechanism underlying this result. In contrast, an obstacle avoidance task indicated that subjects improved their performance. Laboratory obstacle avoidance tests may therefore be better instruments to evaluate future fall prevention studies than posturographic balance assessments.

Accidental Falls↗
PubMedDate not supplied

Overcoming obstacles to behavior change in diabetes self-management.

PURPOSE: This study describes how 5 individuals with type 2 diabetes overcame obstacles that interfered with maintaining behavior changes in diet, exercise, and self-monitoring of blood glucose (SMBG). METHODS: In-depth interviews provided the framework for this descriptive study. During audiotaped, face-to-face, follow-up structured and unstructured interviews, the participants shared their experiences about obstacles encountered and strategies used to overcome them. Each case was examined independently, and all cases were then compared with each other. RESULTS: Obstacles that interfered with maintaining a diet plan were hunger, planned and unplanned meal events, and the desire for new foods. Physical illness and unexpected life events were obstacles to maintaining an exercise and SMBG plan. CONCLUSIONS: Health professionals who assist patients who are beginning behavior change or having difficult maintaining changes need to have an understanding of the various obstacles and how patients can maintain behavior changes over time.

Behavior↗
PubMed1995

Maintenance of balance, gait patterns, and obstacle clearance in Alzheimer's disease.

Patients with cognitive impairment, particularly as a result of Alzheimer's disease (AD), are at increased risk for falls, but it is unclear how, or if, they differ from normal adults in their balance, gait, or ability to clear an obstacle in their path. Using an optoelectronic camera system, we compared body motions and force output at the feet in patients with probable AD (n = 17) with those in healthy older adults (n = 15) while they stood on a force plate or on a beam attached to the force plate that was either stationary or accelerating. Using the same camera system and comparing this AD group with another group of healthy older adults (n = 24), we observed the AD patients during normal walking and while clearing 25- and 152-mm-high obstacles. None of the AD patients had extrapyramidal signs or musculoskeletal impairments. Compared with healthy older adults, normal walking speed was significantly slower in the AD group (p < 0.0001). While clearing either obstacle, the AD patients were significantly slower in their approach (p < 0.0001) and crossing (p < 0.0001) speeds and landed closer to the obstacle after having crossed it (p < 0.02). Moreover, the percent of trials in which a subject made contact with an obstacle was significantly higher in patients with AD (p < 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗
PubMed2004

Stepping over obstacles to improve walking in individuals with poststroke hemiplegia.

For this study, we evaluated two training interventions for improving gait parameters in individuals with poststroke hemiplegia using a training methodology that required them to step over objects. Gait velocity, step length, ability to step over obstacles, and walking endurance were compared before and after 2 weeks of training and 2 weeks after cessation of training. Twenty subjects with poststroke hemiplegia completed six intervention sessions in which they were asked to step over either virtual objects while walking on a motorized treadmill or real foam objects on a 10 m walkway. With the virtual object training, if either foot collided with the virtual object, a tone sounded and a vibrotactile stimulus was applied to the colliding foot. All subjects tolerated the training sessions well, and no incidences of falling or undue cardiovascular responses occurred. The virtual obstacle training generated greater improvements in gait velocity compared with real training (20.5% vs. 12.2% improvement) during the fast walk test (p < 0.01). Improvements in gait velocity for both training methods were similar in the self-selected walk test (33.3% vs. 34.7% improvement). Overall, subjects showed clinically meaningful changes in gait velocity, stride length, walking endurance, and obstacle clearance capacity as a result of either training method. These changes persisted for 2 weeks posttraining. The inclusion of enhanced safety and visual augmentation may be responsible for the effectiveness of the virtual object intervention. These results demonstrate preliminary evidence for clinical effectiveness of obstacle training for improving gait velocity poststroke. In addition, these results provide evidence for enhanced clinical performance with virtual obstacle training.

Adult↗
PubMed1984

Major obstacles to control of irregular migrations: prerequisites to policy.

This paper examines 3 basic obstacles thwarting all attempts to reduce irregular migration. The 1st, rather well known and analyzed, underscores the dependency of all regulation of migratory flows on the system of economic and political relations between developed and developing countries. The 2nd obstacle resides in the persistance and growth of subsequent dependent irregular migration. This obstacle also reveals the relative autonomy of population movements compared with the employment situation in the labor market. The 3rd generally ignored obstacle is the role played by migration itself, particularly the discriminatory status of foreign workers in the labor market, in producing irregular migration.

Demography↗
PubMed1991

Inverse perspective mapping simplifies optical flow computation and obstacle detection.

We present a scheme for obstacle detection from optical flow which is based on strategies of biological information processing. Optical flow is established by a local "voting" (non-maximum suppression) over the outputs of correlation-type motion detectors similar to those found in the fly visual system. The computational theory of obstacle detection is discussed in terms of space-variances of the motion field. An efficient mechanism for the detection of disturbances in the expected motion field is based on "inverse perspective mapping", i.e., a coordinate transform or retinotopic mapping applied to the image. It turns out that besides obstacle detection, inverse perspective mapping has additional advantages for regularizing optical flow algorithms. Psychophysical evidence for body-scaled obstacle detection and related neurophysiological results are discussed.

Algorithms↗
PubMed1995

The role of active forces and intersegmental dynamics in the control of limb trajectory over obstacles during locomotion in humans.

The focus of this paper is to examine the contributions of active and passive forces in the control of limb trajectory over obstacles during locomotion. Kintetic analyses of the swing phase of locomotion were carried out to determine the power profiles at various joints and to parcel the joint moments into moments due to muscle action, gravitational force and motion-dependent terms. The analyses revealed that toe elevation over the obstacles was achieved primarily by flexing at the hip, knee and ankle joint. Power analyses showed that translational energy applied at the hip joint and rotational energy applied at the knee joint were modulated as functions of obstacle height. This demonstrates that increased hip and ankle joint flexion are achieved not through active muscle action but rather through passive forces induced by translational action at the hip (representing contribution by the stance limb muscles) and rotational action at the knee joint. Parcelling the joint moment terms into various components clearly shows how the nervous system exploits intersegmental dynamics to simplify control of limb elevation over obstacles and minimize energy costs.

Adult↗
PubMed1997

Macular holes: migratory gaps and vitreous as obstacles to glial closure.

PURPOSE: Retinal glia may play an important role in the closure of macular holes. This in vitro study examines whether and how the specific pathoanatomy, including foveal eversion and foveal vitreous, may interfere with glial closure of macular holes. METHODS: Culture dishes used to grow glial cells were modified by the placement of slopes, vertical steps, and gaps to mimic the in vivo migratory surface in and surrounding macular holes. In separate experiments, defects were made in a rodent glial monolayer. These defects were exposed to hyaluronic acid (HA) and to rabbit (RV) and bovine (BV) vitreous gel. The migratory behavior and completeness of closure of defects were compared to controls. RESULTS: As expected, glial cells migrated further and in greater numbers on a smooth surface. Slopes and steps were moderate obstacles to migration; gaps in the surface were absolute obstacles. HA modified the pattern of adhesion of cells at the bottom of defects. Defects in the glial monolayer were repaired in 5-7 days. Compared to these controls, repair was inhibited by 11% (n.s.), 28% (P = 0.02), and 58% (P = 0.004) after direct exposure of defects to HA, RV and BV, respectively. CONCLUSION: The elevated and everted margins of macular holes represent slope, step, and gap-like obstacles to the migration of glial cells and hence to the healing of defects. The defect allows extension of extracellular matrix into it and the subretinal space. Our results indicate that gaps in the migratory surface caused and aggravated by eversion and the presence of vitreous present obstacles to glial migration and closure of macular holes.

Animals↗
PubMed2003

Mole rats ( Spalax ehrenbergi) select bypass burrowing strategies in accordance with obstacle size.

Mole rats inhabit extensive individual tunnel systems. Since the energetic cost of burrowing far exceeds that of surface locomotion, excellent orientation ability is crucial. Here we examined whether mole rats can bypass an obstacle (ditch) intersecting an existing tunnel in order to rejoin the two tunnel sections. The mole rats dug two bypass types, depending on the size of the obstacle confronting them: a bypass around the small ditches, parallel and close to the ditch walls; or a bypass beneath the floor of the large ditches. These results demonstrate that the mole rat has the ability to avoid obstacles by digging accurate and energy-conserving bypass tunnels. In order to utilize such a capacity, the mole rat must possess both the means to evaluate the size of the obstacle as well as the ability to perceive its exact position relative to the original tunnel that it will rejoin. Possible mechanisms of orientation that could explain such ability are briefly discussed.

Animals↗
PubMed1998

Reach-to-grasp movements during obstacle avoidance.

The transport and grip components are two controlled components of a prehensile movement. These components are coordinated so that objects of varying size and shape resting in diverse locations can be grasped easily. It has been suggested that the timing between these two components is a specified parameter, although the origin of such timing is unknown. The present study examines the interdependency of the reach and grasp components when the transport component is modified by placing an obstacle of varying height (9 cm and 11 cm) in the hand path between the starting position and the target object location. Subjects were asked to reach over a Plexiglas barrier and grasp a 2-cm dowel. To reach the object, the subject had to elevate the hand. At issue in this experiment is whether changes in hand path trajectory caused by obstacle avoidance produce corresponding changes in the kinematics of grip aperture. The findings showed that reaching in the presence of an obstacle resulted in the prolongation of most transport component time parameters except peak acceleration and a few amplitude parameters. Changes in the transport component also produced systematic prolongation in all time parameters of grip kinematics, including grip closure time. Temporal prolongation was also reflected in a significant decrease in grip opening and closing velocity; only relative time-to-peak closing velocity was maintained. Closure distance and maximum grip aperture were smaller for the obstacle conditions. Together with the observed smaller variability for the distance to peak aperture, these findings suggest that spatial localization of the hand aperture is an important prehensile movement control feature. Parameterization processes for the grip component are closely linked to those of the transport component, and their organization appears to be interdependent.

Adult↗
PubMed2002

Control of obstacle climbing in the cockroach, Blaberus discoidalis. I. Kinematics.

An advantage of legged locomotion is the ability to climb over obstacles. We studied deathhead cockroaches as they climbed over plastic blocks in order to characterize the leg movements associated with climbing. Movements were recorded as animals surmounted 5.5-mm or 11-mm obstacles. The smaller obstacles were scaled with little change in running movements. The higher obstacles required altered gaits, leg positions and body posture. The most frequent sequence used was to first tilt the front of the body upward in a rearing stage, and then elevate the center of mass to the level of the top of the block. A horizontal running posture was re-assumed in a leveling-off stage. The action of the middle legs was redirected by rotations of the leg at the thoracal-coxal and the trochanteral-femoral joints. The subsequent extension movements of the coxal-trochanteral and femoral-tibial joints were within the range seen during horizontal running. The structure of proximal leg joints allows for flexibility in leg use by generating subtle, but effective changes in the direction of leg movement. This architecture, along with the resulting re-direction of movements, provides a range of strategies for both animals and walking machines.

Animals↗
PubMed1998

A model of the neuro-musculo-skeletal system for anticipatory adjustment of human locomotion during obstacle avoidance.

Theoretical studies on human locomotion have shown that a stable and flexible gait emerges from the dynamic interaction between the rhythmic activity of a neural system composed of a neural rhythm generator (RG) and the rhythmic movement of the musculo-skeletal system. This study further explores the mechanism of the anticipatory control of locomotion based on the emergent properties of a neural system that generates the basic pattern of gait. A model of the neuro-musculo-skeletal system to execute the task of stepping over a visible obstacle with both limbs during walking is described. The RG in the neural system was combined with a system referred to as a discrete movement generator (DM), which receives both the output of the RG and visual information regarding the obstacle and generates discrete signals for modification of the basic gait pattern. A series of computer simulations demonstrated that an obstacle placed at an arbitrary position can be cleared by sequential modifications of gait: (1) modulating the step length when approaching the obstacle and (2) modifying the trajectory of the swing limbs while stepping over it. This result suggests that anticipatory adjustments are produced not by the unidirectional flow of the information from visual signals to motor commands but by the bi-directional circulation of information between the DM and the RG. The validity of this model is discussed in relation to motor cortical activity during anticipatory modifications in cats and the ecological psychology of visuo-motor control in humans.

Animals↗
PubMed1996

Traversing step obstacles with manual wheelchairs.

In the public transport system, there are sometimes step obstacles, such as curbs, steps, entrance steps of vehicles, edges of lifting platforms, internal steps in vehicles, etc. This paper describes research work carried out in Sweden to specify the heights of step obstacles possible to traverse with a manual wheelchair manoeuvred by an attendant. The work included trials with the subjects manoeuvring two different kinds of wheelchairs, with dummies as occupants, over a step obstacle with adjustable height. It also included trials with a person in the wheelchair. The subjects were 20 healthy persons in the age range of 23-60 years, with no or only little experience of manoeuvring wheelchairs, chosen to represent bus drivers or other helpers. They rated their perceived feeling of effort on a 10-level rating scale with ratio properties. The occupant rated the feeling of comfort and safety in three grades. All trials were recorded using a video-camera. From the video-film, the situations and series of actions of the subjects were analysed and the length of time for each task was measured. The forces needed when traversing step obstacles with a wheelchair were calculated according to the principles of mechanics and processed by a computer for the different principal traversing situations. The results show that for the planning of systems for public transportation, where the passenger can get assistance, a step height of 50 mm is acceptable, and a step height of 100 mm can be acceptable if there is enough room to manoeuvre the wheelchair, so that it is possible to choose the most convenient way to traverse the step. Higher heights should be avoided.

Journal Article↗