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At least 19 recordsLinked to original sources

PubMed1998

Increasing obstacle height and decreasing toe-obstacle distance affect the joint moments of the stance limb differently when stepping over an obstacle.

Foot placement during gait is important in regulating the dynamics of the joints of the supporting limb and in maintaining balance of the whole body. We hypothesized that increasing obstacle height and decreasing toe-obstacle distance (distance between the trailing foot and the obstacle during stance of the trailing foot just prior to stepping over the obstacle) would affect the joint moments of the stance limb differently when stepping over an obstacle. A total of 14 healthy young adults stepped over an obstacle 51, 102, 153, and 204 mm in height in a self-selected manner (i.e. toe-obstacle distance was not controlled) and for toe-obstacle distance targets of 10, 20, 30, and 40% of their step lengths measured during unobstructed gait. The adduction and internal rotation moments at the ankle joint increased as toe-obstacle distance decreased. The adduction and internal rotation moments at the hip joint during early stance, the internal rotation moment at the knee joint during late stance, and the dorsiflexion moment at the ankle joint during late stance increased with obstacle height. Reductions in toe-obstacle distance had greater effects on the moments of the ankle joint, and increases in obstacle height had greater effects on the moments of the hip joint. These greater demands on joint moments may affect the abilities of those elderly having decreased muscle strengths to safely step over obstacles. Copyright 1998 Elsevier Science B.V.

Journal Article↗

PubMed2001

Intralimb coordination following obstacle clearance during running: the effect of obstacle height.

The purpose of this study was to investigate the different coordination strategies used following obstacle clearance during running. Ten subjects ran over a level surface and over obstacles of six different heights (10, 12.5, 15, 17.5, 20 and 22.5% of their standing height). Analysis based upon the dynamical systems theory (DST) was used and the phasing relationships between lower extremity segments were examined. The results demonstrated that the increasing obstacle height elicited behavioral changes. The foot and the leg became more independent in their actions, while the leg and the thigh strengthened their already stable relationship. The 15% obstacle height seems to be a critical height for the observed changes.

Adult↗

PubMed1997

Attachment of meandering reentrant wave fronts to anatomic obstacles in the atrium. Role of the obstacle size.

Acetylcholine chloride (ACh) induces nonstationary meandering reentrant wave fronts in the atrium. We hypothesized that an anatomic obstacle of a suitable size prevents meandering by causing attachment of the reentrant wave front tip to the obstacle. Eight isolated canine right atrial tissues (area, 3.8 x 3.2 cm) were mounted in a tissue bath and superfused with Tyrode's solution containing 10 to 15 mumol/L ACh. Holes with 2- to 10-mm diameters were sequentially created in the center of the tissue with biopsy punches. Reentry was induced by a premature stimulus after eight regular stimuli at 400-ms cycle length. The endocardial activation maps and the motion of the induced reentry were visualized dynamically before and after each test lesion using 509 bipolar electrodes. In the absence of a lesion (n = 8), the induced single reentrant wave front, in the form of a spiral wave, meandered irregularly from one site to another before terminating at the tissue border. Holes with 2- to 4-mm diameters (n = 6) had no effect on meandering. However, when the hole diameters were increased to 6 mm (n = 8), 8 mm (n = 8), and 10 mm (n = 6), the tip of the spiral wave attached to the holes, and reentry became stationary. Transition from meandering to an attached state converted the irregular and polymorphic electrogram to a periodic and monomorphic activity with longer cycle lengths (101 +/- 11 versus 131 +/- 9 ms for no hole versus 10-mm hole, respectively; P < .001). Regression analysis showed a significant positive linear correlation between the cycle length of the reentry and the hole diameter (r = .89, P < .01) and between the cycle length of the reentry and the excitable gap (r = .89, P < .05). We conclude that a critically sized anatomic obstacle converts a nonstationary meandering reentrant wave front to a stationary one. This transition converts an irregular "fibrillation-like" activity into regular monomorphic activity.

Acetylcholine↗

PubMedDate not supplied

[Obstacles for recruiting organ donors in intensive care units can be overcomed. Identified obstacles are attitudes, ethics, resources and competence, as shown by an inquiry].

The aim of this study was to identify obstacles for recruiting organ donors in the ICU. All specialists in neurosurgery and half of the specialists in anaesthesiology in Sweden were surveyed. Response rate was 67%. The following obstacles were identified: A neutral approach of the physicians when requesting organ donation of the families instead of a pro-donation approach; ethical problems concerning non-therapeutic ventilation and requesting donation of grieving families; varying competence in diagnosing brain infarction; lack of resources at the ICU. Suggestions for organizational and educational improvements are given.

Adult↗

PubMed1998

Placing the trailing foot closer to an obstacle reduces flexion of the hip, knee, and ankle to increase the risk of tripping.

This study was performed to test the hypothesis that reducing the horizontal distance between the trailing foot (foot crossing the obstacle last) and obstacle, during stance just prior to stepping over the obstacle, would reduce flexion of the hip, knee, and ankle joints of the trailing limb when the toe is over the obstacle to reduce the vertical toe-obstacle clearance and increase the risk of tripping. Fourteen healthy young adults stepped over an obstacle of 51, 102, 153, and 204 mm height in a self-selected manner (i.e., toe-obstacle distance was not controlled) and for toe-obstacle distance targets of 10, 20, 30, and 40% of their step lengths measured during unobstructed gait. The reductions in toe-obstacle distance resulted in linear decreases in flexion of the hip, knee, and ankle when the toe was over the obstacle. Toe-obstacle clearance of the trailing limb decreased significantly as toe-obstacle distance decreased. The reductions in toe-obstacle distance led to contact of the trailing (but not the leading) foot with the obstacle, the closer the obstacle the greater the number of contacts. The reductions also resulted in linear decreases in swing time of the trailing limb from toe-off to when the toe was over the obstacle. The height of the hip was not affected by toe-obstacle distance. Angular velocity of knee flexion was found to increase linearly as toe-obstacle distance decreased and appears to be of primary importance in avoiding obstacle contact.

Accidental Falls↗

PubMed1997

Stepping over an obstacle increases the motions and moments of the joints of the trailing limb in young adults.

Tripping over obstacles is the most frequently mentioned causes of falls. Thus, this study was performed to test the hypotheses that when crossing obstacles, toe-obstacle clearance and the three-dimensional motions and moments at the hip, knee, and ankle of the trailing limb (limb crossing the obstacle last) increase with obstacle height. Data were collected using an optoelectronic digitizing system and force platform. Fourteen healthy young adults were tested during unobstructed level walking and when stepping over obstacles of 51, 102, 153, or 204 mm heights. Toe-obstacle clearances of the trailing foot increased from 31 mm during unobstructed gait to an average of 146 mm when stepping over obstacles of any of these heights. Obstacle height was not found to affect toe-obstacle clearance. When the toe of the trailing limb was over the obstacle, the flexion angles of the hip and knee increased linerly with obstacle height. Compared to flexion of the hip or ankle, flexion of the knee appears to be of primary importance when crossing obstacles with the trailing limb. The maximum extension moment at the hip joint during late stance decreased linearly with obstacle height. At the knee joint, the maximum flexion moment during early stance and the maximum adduction moment during late stance increased linearly with obstacle height. At the ankle joint, the maximum dorsiflexion moment during late stance increased linearly with obstacle height. These greater demands on motions and moments may affect the abilities of those elderly having decreased muscle strengths to step over obstacles.

Adult↗

PubMed1991

Stepping over obstacles: gait patterns of healthy young and old adults.

Falls associated with tripping over an obstacle can be devastating to elderly individuals, yet little is known about the strategies used for stepping over obstacles by either old or young adults. The gait of gender-matched groups of 24 young and 24 old healthy adults (mean ages 22 and 71 years) was studied during a 4 m approach to and while stepping over obstacles of 0, 25, 51, or 152 mm height and in level obstacle-free walking. Optoelectronic cameras and recorders were used to record approach and obstacle crossing speeds as well as bilateral lower extremity kinematic parameters that described foot placement and movement trajectories relative to the obstacle. The results showed that age had no effect on minimum swing foot clearance (FC) over an obstacle. For the 25 mm obstacle, mean FC was 64 mm, or approximately three times that used in level gait; FC increased nonlinearly with obstacle height for all subjects. Although no age differences were found in obstacle-free gait, old adults exhibited a significantly more conservative strategy when crossing obstacles, with slower crossing speed, shorter step length, and shorter obstacle-heel strike distance. In addition, the old adults crossed the obstacle so that it was 10% further forward in their obstacle-crossing step. Although all subjects successfully avoided the riskiest form of obstacle contact, tripping, 4/24 healthy old adults stepped on an obstacle, demonstrating an increased risk for obstacle contact with age.

Adult↗

PubMed2006

Keep looking ahead? Re-direction of visual fixation does not always occur during an unpredictable obstacle avoidance task.

Visual information about the environment, especially fixation of key objects such as obstacles, is critical for safe locomotion. However, in unpredictable situations where an obstacle suddenly appears it is not known whether central vision of the obstacle and/or landing area is required or if peripheral vision is sufficient. We examined whether there is a re-direction of visual fixation from an object fixated ahead to a suddenly appearing obstacle during treadmill walking. Furthermore, we investigated the temporal relationship between the onset of muscle activity to avoid the obstacle and saccadic eye and head movements to shift fixation. Eight females (mean +/- SD; age = 24.8 +/- 2.3 years) participated in this experiment. There were two visual conditions: a central vision condition where participants fixated on two obstacles attached to a bridge on the treadmill and a peripheral vision condition where participants fixated an object two steps ahead. There were two obstacle release conditions: only an obstacle in front of the left foot was released or an obstacle in front of either foot could be released. Only trials when the obstacle was released in front of the left foot were analyzed such that the difference in the two obstacle conditions was whether there was a choice of which foot to step over the obstacle. Obstacles were released randomly in one of three phases during the step cycle corresponding to available response times between 219 and 462 ms. We monitored eye and head movements along with muscle activity and spatial foot parameters. Performance on the task was not different between vision conditions. The results indicated that saccades are rarely made (< 18% of trials) and, when present, are initiated approximately 350 ms after muscle activity for limb elevation, often accompanied by a downward head movement, and always directed to the landing area. Therefore, peripheral vision of a suddenly appearing obstacle in the travel path is sufficient for successful obstacle avoidance during locomotion: visual fixation is generally not re-directed to either the obstacle or landing area.

Adult↗

PubMed1994

Control of human arm movements in two dimensions: paths and joint control in avoiding simple linear obstacles.

In order to examine path planning and the control of redundant degrees of freedom in the human arm, the movements of the shoulder, elbow and wrist were recorded as subjects moved a pointer to a target and avoided a simple obstacle. With respect to joint control, the results show that the extra degree of freedom provided by the wrist is incorporated into target movements in a systematic manner for both large and small obstacles; it is not used only when there is no geometrical alternative. For the wrist, two strategies are apparent, depending upon the length of the obstacle. Wrist extension predominates for shorter obstacles, while flexion or extension and flexion predominate for longer obstacles. These wrist movements shorten the effective length of the distal segments (lower arm plus hand and pointer) and thus reduce the excursion required at the proximal joints. In part, they correspond to assuming the most comfortable arm configuration at each point in the new path necessitated by the obstacle and can be described by static cost functions. However, wrist extension is also used to move the hand and pointer away from the obstacle as shoulder and elbow movements carry the wrist itself towards the obstacle. Wrist flexion is also used to move the pointer tip rapidly past the obstacle. These components, which cannot be explained by static cost functions alone, confirm for the human arm the hypothesized use of redundant degrees of freedom in obstacle avoidance. With respect to path planning, the results show that the minimum distance between pointer and obstacle remains fairly constant over a large range of obstacle lengths; this relative invariance is interpreted to support the hypothesis that workspace coordinates are important for movement planning. However, minimum distance and several other path parameters do depend significantly on the orientation and location of the movement in the workspace. This inhomogeneity implies that movement planning does not occur exclusively in workspace coordinates; it suggests an influence of joint space criteria. In frontal movements, for example, the systematic decline in the minimum distance with increasing obstacle length is interpreted as a compromise reducing the amount of extra joint movement and the discomfort of arm configurations.

Adult↗

PubMed2001

Wave front-obstacle interactions in cardiac tissue: a computational study.

An understanding of wave front-obstacle interactions will greatly enhance our knowledge of the mechanisms involved in cardiac arrhythmias and their therapy. The goal of this computational study is to examine the interactions between wave fronts and various obstacles in a two-dimensional sheet of myocardium. The myocardium is modeled as an isotropic sheet with Luo-Rudy I membrane kinetics. An examination is conducted of wave front interactions with nonconductive and passive-tissue obstacles. Simulations were performed either in environments of reduced myocardial excitability, or with rapid stimulation via a line electrode. The shape of the obstacles and their ability to withdraw current from the active tissue greatly influence wave front-obstacle interactions in each of these environments. The likelihood of wave front detachment from an obstacle corner increases as the curvature of the obstacle corner is increased. A passive-tissue obstacle promotes wave front-obstacle separation in regions of depressed excitability. Under rapid pacing, the presence of the passive obstacle results in wave fragmentation, while the insulator obstacle promotes wave front detachment. The results of this study reveal the importance of obstacle composition and geometry in wave front interactions with cardiac obstacles.

Animals↗

PubMed2004

The effects of distant and on-line visual information on the control of approach phase and step over an obstacle during locomotion.

One of the goals of this study was to examine the nature and role of distant visual information sampled during locomotion in the feedforward control of leading and trailing limb while an individual is required to step over an obstacle in the travel path. In addition we were interested in whether or not on-line visual information available while the limb (lead or trail) is stepping over the obstacle influences limb trajectory control and whether the information provided during lead limb cross would be used to calibrate movement of the trail limb. Towards this end, we manipulated availability of vision following an initial dynamic sampling period during the approach phase in proximity to the obstacle and during the lead and trail limb stepping over the obstacle. Ten participants completed 40 trials of obstacle crossing in 8 testing conditions. Initial dynamic visual sampling was sufficient to ensure successful task performance in the absence of vision in the approach phase and during both lead and trail limb stepping over the obstacle. Despite successful task performance, foot placement of the lead and trail limb before obstacle crossing and limb elevation over the obstacle were increased after withdrawal of vision in the approach area. Furthermore, the correlation between toe clearance and foot placement was diminished. While both limbs require feedforward visual information to control the step over the obstacle, only lead limb elevation was influenced by availability of on-line visual information during obstacle crossing. Results were in agreement with the notion of primacy of information inherent in the optic array over those from static samples of the environment in guiding locomotion. It is suggested that the expected proprioceptive feedback information associated with the limb posture before the obstacle, reconstructed using visual memory from dynamic sampling of the environment, mismatched with those from the actual limb position. Accordingly, participants adopted a different strategy that enabled them to clear the obstacle with a higher safety margin.

Adult↗

PubMed2001

The effect of obstacle position on reach-to-grasp movements.

Numerous everyday tasks require the nervous system to program a prehensile movement towards a target object positioned in a cluttered environment. Adult humans are extremely proficient in avoiding contact with any non-target objects (obstacles) whilst carrying out such movements. A number of recent studies have highlighted the importance of considering the control of reach-to-grasp (prehension) movements in the presence of such obstacles. The current study was constructed with the aim of beginning the task of studying the relative impact on prehension as the position of obstacles is varied within the workspace. The experimental design ensured that the obstacles were positioned within the workspace in locations where they did not interfere physically with the path taken by the hand when no obstacle was present. In all positions, the presence of an obstacle caused the hand to slow down and the maximum grip aperture to decrease. Nonetheless, the effect of the obstacle varied according to its position within the workspace. In the situation where an obstacle was located a small distance to the right of a target object, the obstacle showed a large effect on maximum grip aperture but a relatively small effect on movement time. In contrast, an object positioned in front and to the right of a target object had a large effect on movement speed but a relatively small effect on maximum grip aperture. It was found that the presence of two obstacles caused the system to decrease further the movement speed and maximum grip aperture. The position of the two obstacles dictated the extent to which their presence affected the movement parameters. These results show that the anticipated likelihood of a collision with potential obstacles affects the planning of movement duration and maximum grip aperture in prehension.

Adolescent↗

PubMed2007

Directed cell migration in the presence of obstacles.

BACKGROUND: Chemotactic movement is a common feature of many cells and microscopic organisms. In vivo, chemotactic cells have to follow a chemotactic gradient and simultaneously avoid the numerous obstacles present in their migratory path towards the chemotactic source. It is not clear how cells detect and avoid obstacles, in particular whether they need a specialized biological mechanism to do so. RESULTS: We propose that cells can sense the presence of obstacles and avoid them because obstacles interfere with the chemical field. We build a model to test this hypothesis and find that this naturally enables efficient at-a-distance sensing to be achieved with no need for a specific and active obstacle-sensing mechanism. We find that (i) the efficiency of obstacle avoidance depends strongly on whether the chemotactic chemical reacts or remains unabsorbed at the obstacle surface. In particular, it is found that chemotactic cells generally avoid absorbing barriers much more easily than non-absorbing ones. (ii) The typically low noise in a cell's motion hinders the ability to avoid obstacles. We also derive an expression estimating the typical distance traveled by chemotactic cells in a 3D random distribution of obstacles before capture; this is a measure of the distance over which chemotaxis is viable as a means of directing cells from one point to another in vivo. CONCLUSION: Chemotactic cells, in many cases, can avoid obstacles by simply following the spatially perturbed chemical gradients around obstacles. It is thus unlikely that they have developed specialized mechanisms to cope with environments having low to moderate concentrations of obstacles.

Cell Physiological Phenomena↗

PubMed2006

Stepping over an obstacle on a compliant travel surface reveals adaptive and maladaptive changes in locomotion patterns.

Adaptive human locomotion is dependent on safe clearance of obstacles encountered in the path of locomotion. When the terrain is uneven or compliant, stability along with safe obstacle clearance are competing demands presented to the central nervous system (CNS). To examine how the CNS deals with the two competing demands, six participants walked under four conditions: normal ground walking, normal ground walking with an obstacle in the travel path, compliant surface walking, and compliant surface walking with an obstacle in the travel path. Full body kinematics were measured and swing limb kinetics were derived from these measurements. Results showed that on a compliant surface, the CNS was able to decrease foot placement variability at foot contact when approaching an obstacle, similar to the normal ground terrain. Limb trajectory over the obstacle showed that toe elevation was maintained while clearance over the obstacle was lower in the compliant surface condition due to depression of the surface during push off. This illustrates that the CNS controls toe elevation, not toe clearance when stepping over an obstacle. Work done in the knee during elevation and hip during lowering was similar in the compliant and ground conditions even though a lower clearance over the obstacle was achieved in the complaint condition. This shows the inability of the CNS to account for compression of the surface prior to obstacle clearance and provides further evidence the CNS controls toe elevation, not clearance when stepping over an obstacle.

Adaptation, Physiological↗

PubMed2006

Characteristics of single and double obstacle avoidance strategies: a comparison between adults and children.

Activities of daily living often require us to negotiate several obstacles in the travel path. To date, there is little work investigating how adults accomplish such tasks, and there is even less known about multiple obstacle avoidance strategies used by children. The current work will expand our knowledge about the role of vision in adults and children when avoiding two obstacles placed in their travel path under altered ambient lighting. Healthy 7-year old children (n=10; aged 7.51+/-0.2 years) and adults (n=10; aged 22.76+/-1.7 years) were instrumented with infrared markers (Optotrak, NDI) placed on anatomical landmarks and asked to walk along a ten meter path under three conditions: unobstructed, single obstacle, or double obstacle. These trials were performed under two lighting conditions: Full (simulating standard office lighting) and Low (simulating a dark hallway lit by nightlights). Data analyses included lead and trail clearance values, step length, step width and step velocity, take-off distance and Horizontal toe Displacement at Apex (HDA) which was defined as the distance between the horizontal position of the toe to the leading edge of the obstacle when the toe reaches its peak height. Adults were able to maintain consistent behaviour regardless of the number of obstacles in the travel path. Children, however, adjusted their foot placement for the second obstacle. This indicates that having multiple obstacles in the travel path is a more challenging task for 7-year old, and suggests that children at this age may not have fully developed anticipatory locomotor strategies. Children had larger clearance values than adults for the lead foot crossing the obstacle under all obstacle and lighting conditions, and consistently used larger HDA values than adults. Together, these findings suggest that children adopt more cautious strategies than adults in complex environments. Additionally, children decreased walking velocity, increased step width and decreased their step length in a Low light environment. These changes are all indicators of a more careful avoidance strategy, which implies that children at this age rely heavily on visual information to guide foot placements in a complex environment.

Adult↗

PubMed1989

Obstacles to family practitioners' use of screening tests: determinants of practice?

The effect of nine obstacles on family physicians' screening practices is reported. Family physicians, responding to a survey of periodic health screening practices, were asked to check the obstacles that affected their use of the screening tests listed. In the survey, 129 members of the American Academy of Family Physicians and 146 members of the Society of Teachers of Family Medicine responded. The most frequently reported obstacles to screening were cost to the patient, patient refusal, inconvenience to the patient, and lack of facilities or equipment. The tests for which the most obstacles were reported were sigmoidoscopy, tonometry, and mammography. The Society of Teachers of Family Medicine physicians were more likely to list cost, patient reluctance, inconvenience to the patient, "the literature does not recommend to use the test," time to perform the tests, and high rate of false positives and negatives. American Academy of Family physicians were more likely to report two obstacles, lack of facilities or equipment and risk to the patient. An analysis of the relationship between obstacle report and test utilization indicated that for only five of the nine obstacles were physicians who listed the obstacle less likely to use the test than physicians who did not list the obstacle. Obstacles related with decreased test use were those that affected the physician's ability to perform the test or the efficacy of the test itself while "inactive" obstacles were those that more directly affected and emanated from patients.

Attitude of Health Personnel↗

PubMed2002

The influence of multiple obstacles in the travel path on avoidance strategy.

Obstacle avoidance strategy was analysed as participants were subjected to varied quantities and positions of obstacles in their travel path. Kinematic data were collected with three optoelectric cameras sampling at 100 Hz. Calculated from this data were toe-elevation and take-off distance for both lead and trail limbs. Each subject completed a total of 120 randomly assigned trials (15 repetitions in each of eight test conditions). Test conditions were as follows: no obstacle (C1), a single obstacle at 0 (C2), 1 (C3), 1.5 (C4), and 2 m (C5) positions and double obstacles at 0 and 1 (C6), 0 and 1.5 (C7) and 0 and 2 m (C8) positions. Avoidance strategy was defined in terms of take-off distance and toe-elevation at obstacle crossing. Single obstacle trials demonstrated that take-off distance and toe-elevation are gait parameters, which are controlled in successful obstacle clearance. Double obstacle trials revealed that presence and position of a second obstacle in the travel path influences trail limb take-off distance for both first and second obstacles.

Adult↗

PubMed1994

Effects of age and available response time on ability to step over an obstacle.

BACKGROUND: Falls during walking are often triggered when a foot contacts an obstacle in its path. Yet little is known about the ability of individuals of any age to successfully negotiate obstacles, especially under time-critical conditions. METHODS: The gait of 24 young and 24 old healthy adults (mean ages 23 and 73 years) was studied as they approached and tried to avoid stepping on a band of light, not knowing when or where it might appear on an 8 m-long walkway. This virtual obstacle was placed at the predicted location of the next footfall with available response times (ART) before heel strike that were varied randomly in 50 ms increments from 200 to 450 ms. In addition, their gait was observed as they stepped over a fixed virtual obstacle and over an obstacle that appeared with approximately a 1000 ms ART. RESULTS: The old had an increased risk of obstacle contact while negotiating obstacles under time-critical conditions (p = .082). Mean rates-of-success (RS) in obstacle avoidance for the young ranged from .205 at a 200 ms ART to .969 at a .450 ms ART. Corresponding mean RS for the old were .157 and .920. Lower extremity simple reaction time (SRT) test made under static conditions showed that the mean SRT of the old were approximately 80 ms longer than those of the young. Regression analyses suggested that the old in fact would have needed only 30 ms additional ART to achieve RS equal to that of the young for obstacles appearing with ART from 300 to 450 ms. CONCLUSIONS: Reductions in ART significantly decreased RS. Delays as small as 50 or 100 ms in observing or reacting to obstacles in real-life situations may significantly lower the rate of success that subjects of any age have in avoiding them. Age differences in SRT do not always reliably indicate age differences in obstacle avoidance under time-critical situations.

Accidental Falls↗