Biomedical subjects

A E Patla

Publications and source records attributed to A E Patla.

At least 37 records · Page 2Linked to original sources

PubMed1999

Low back three-dimensional joint forces, kinematics, and kinetics during walking.

OBJECTIVE: The purpose of this study was to examine the three-dimensional low back loads, spinal motions, and trunk muscular activity during gait. Specific objectives involved assessment of the effects of walking speed, and arm swing on spinal loads, lumbar spine motion, and muscular activation. DESIGN: An in vivo modeling experiment using five male participants. Thirty walking trials were performed by each participant yielding five repeats of each condition (3 walking cadences x 2 arm swing conditions). BACKGROUND: Walking is often prescribed as a rehabilitation task for individuals with low back injuries. However, there are few studies which have examined the joint loading, spinal motions, and muscular activity present when walking. Additionally, the majority of studies examining spine loading during gait have used an inverse dynamics model, commencing at the cranial aspect of the body, approach which does not include the impulsive phases of gait (i.e. heel strikes and toe offs). METHODS: Low back joint forces (bone on bone) and moments were determined using an anatomically complex three-dimensional model (detailing 54 muscles and the passive structures acting at the low back) during three walking cadences and with free arm swing or restricted arm swing. In order to assess the influence of the transient factors such as heel contact on the joint forces a bottom up (from the feet to the lumbar spine) rigid link segment analyses approach was used as one input to the three-dimensional anatomic model. Lumbar spine motion and trunk muscle activation levels were also recorded to assist in partitioning forces amongst the active and passive tissues of the low back. RESULTS: Net joint anterior-posterior shear loading was the only variable significantly affected by walking cadence (fast versus slow P < 0.0003). No variable was significantly affected by the arm swing condition. Trends demonstrated an increase in all variables with increased walking cadence. Similarly, most variables, with the exception of axial twist and lateral bend lumbar spine motion and lateral joint shear, demonstrated increasing trends caused by the restriction of normal arm swing. CONCLUSIONS: Tissue loading during walking appears to be below levels caused by many specific rehabilitation tasks, suggesting that walking is a wise choice for general back exercise and rehabilitation programs. Slow walking with restricted arm swing produced more 'static' lumbar spine loading and motion patterns, which could be detrimental for certain injuries and tissues. Fast walking produced a more cyclic loading pattern.

Adult↗

PubMed1998

Context-dependent reflex control: some insights into the role of balance.

Recent research suggests that the balance requirements of a task dictate the reflexive response. However, these observations were inferred indirectly from either different tasks or different phases of the same task. This study directly tested the hypothesis of balance-dependent control during recovery from an unexpected trip. The subjects were tripped in two different support conditions: unilimb support (provided by the stance limb) or trilimb support (provided by the stance limb and both arms placed on adjacent parallel bars). The subjects exhibited anticipatory changes: they biased the body center of mass toward the support limb in the mediolateral direction and elevated the swing limb higher when there was a possibility of being tripped. The electromyographic (EMG) latencies were not influenced by the threat to equilibrium. The magnitudes of the EMG reflexive response to the trip were clearly modulated as a function of the threat to stability, not in a simple manner, but rather in a complex manner, which optimized the recovery strategy. It is evident that the overriding concern, equilibrium control during locomotion, has a dominant influence on reflex modulation.

Adaptation, Physiological↗

PubMed1998

Simple artificial neural network models can generate basic muscle activity patterns for human locomotion at different speeds.

A neural network model has been developed to represent the shaping function of a central pattern generator (CPG) for human locomotion. The model was based on cadence and electromyographic data obtained from a single human subject who walked on a treadmill. The only input to the model was the fundamental timing of the gait cycle (stride rate) in the form of sine and cosine waveforms whose period was equal to the stride duration. These simple signals were then shaped into the respective muscle activation patterns of eight muscles of the lower limb and trunk. A network with a relatively small number of hidden units trained with back-propagation was able to produce an excellent representation of both the amplitude and timing characteristics of the EMGs over a range of walking speeds. The results are further discussed with respect to the dependence of some muscles upon sensory feedback and other inputs not explicitly presented to the model.

Algorithms↗

PubMed1998

Stepping over obstacles during locomotion: insights from multiobjective optimization on set of input parameters.

In this study we investigate possible objectives that the central nervous system (CNS) may consider in planning a strategy for stepping over an obstacle. A link segment simulation model has been developed based on Lagrangian dynamics, with which muscle force inputs can be optimized to best satisfy the postulated objectives for landing stability, obstacle clearance, and efficiency of the movement. A direct optimization approach with multiobjective criteria based on the kinematic and kinetic characteristics of the swing phase of locomotion is used in the simulation. The role of initial conditions at toe-off and biarticular muscle forces during the swing phase was also investigated. The optimization was performed for both leading limb and the trailing limb during the swing phase. The simulation results demonstrate that the use of biarticular muscles is sufficient to clear a range of obstacles with the trailing limb (obstacle encountered during early swing). Stride length or landing stability objectives need not be specified suggesting a simpler control of trailing limb trajectory by the CNS (one of stride length or landing stability objectives were not necessary). In contrast while the use of biarticular muscles can be sufficient to clear obstacles with the leading limb (obstacle encountered during mid to late swing), a stable landing and smooth toe and knee trajectories are compromised without suitable initial conditions at toe-off. The results suggest that the set of postulated objectives for the lead limb is adequate, although not complete.

Biomechanical Phenomena↗

PubMed1998

Stiffness control of balance in quiet standing.

Our goal was to provide some insights into how the CNS controls and maintains an upright standing posture, which is an integral part of activities of daily living. Although researchers have used simple performance measures of maintenance of this posture quite effectively in clinical decision making, the mechanisms and control principles involved have not been clear. We propose a relatively simple control scheme for regulation of upright posture that provides almost instantaneous corrective response and reduces the operating demands on the CNS. The analytic model is derived and experimentally validated. A stiffness model was developed for quiet standing. The model assumes that muscles act as springs to cause the center-of-pressure (COP) to move in phase with the center-of-mass (COM) as the body sways about some desired position. In the sagittal plane this stiffness control exists at the ankle plantarflexors, in the frontal plane by the hip abductors/adductors. On the basis of observations that the COP-COM error signal continuously oscillates, it is evident that the inverted pendulum model is severely underdamped, approaching the undamped condition. The spectrum of this error signal is seen to match that of a tuned mass, spring, damper system, and a curve fit of this "tuned circuit" yields omega n the undamped natural frequency of the system. The effective stiffness of the system, Ke, is then estimated from Ke = I omega n2, and the damping B is estimated from B = BW X I, where BW is the bandwidth of the tuned response (in rad/s), and I is the moment of inertia of the body about the ankle joint. Ten adult subjects were assessed while standing quietly at three stance widths: 50% hip-to-hip distance, 100 and 150%. Subjects stood for 2 min in each position with eyes open; the 100% stance width was repeated with eyes closed. In all trials and in both planes, the COP oscillated virtually in phase (within 6 ms) with COM, which was predicted by a simple 0th order spring model. Sway amplitude decreased as stance width increased, and Ke increased with stance width. A stiffness model would predict sway to vary as Ke-0.5. The experimental results were close to this prediction: sway was proportional to Ke(-0.55). Reactive control of balance was not evident for several reasons. The visual system does not appear to contribute because no significant difference between eyes open and eyes closed results was found at 100% stance width. Vestibular (otolith) and joint proprioceptive reactive control were discounted because the necessary head accelerations, joint displacements, and velocities were well below reported thresholds. Besides, any reactive control would predict that COP would considerably lag (150-250 ms) behind the COM. Because the average COP was only 4 ms delayed behind the COM, reactive control was not evident; this small delay was accounted for by the damping in the tuned mechanical system.

Adult↗

PubMed1998

Is backward stepping over obstacles achieved through a simple temporal reversal of forward stepping?

The main purpose of the study was to examine whether backward stepping over obstacles was a simple temporal reversal of kinematic and muscle activation patterns found in forward obstacle avoidance. Obstacle avoidance was used as a probe to represent one aspect of walking over variable terrain. Kinematics, trajectories and muscle activation profiles for forward versus backward stepping over obstacles revealed that the simple reversal of locomotor patterns observed for level walking cannot be applied to obstacle avoidance. However, key kinematic data and limb trajectories for backward leading limb stepping were found to be similar to existing forward trailing limb data. Therefore, it appears that stepping over obstacles requires a complex upper level reorganization of the basic locomotor pattern based on biomechanical and sensory feedback.

Adult↗

PubMed1997

Where and when do we look as we approach and step over an obstacle in the travel path?

Spatio-temporal gaze behaviour patterns were analysed as normal participants wearing a mobile eye tracker approached and stepped over obstacles of varying height in the travel path. We examined the frequency and duration of three types of gaze fixation with respect to the participants' stepping patterns: obstacle fixation (ObsFix); travel fixation (TravFix) (when the gaze is stable and travelling at the speed of whole body) and fixation in the 4-6m region (Fix4-6). During the approach phase to the obstacle, participants fixated on the obstacle for approximately 20% of the travel time. Only Fix4-6 duration was modulated as a function of obstacle height by regulating the frequency and reflected the increased time needed for detection of the small low contrast obstacle in the travel path. Frequency of ObsFix increased significantly as a function of obstacle height and reflected visuo-motor transformation needed for limb elevation control. Participants did not fixate on the obstacle as they were stepping over, but did the planning in the steps before. TravFix duration and frequency was constant while Fix4-6 duration was higher in the step before and step over the obstacle reflecting visual search of the landing area for the lead limb following obstacle avoidance. These results clearly show that obstacle information provided by vision is used in a feed-forward rather than on-line control mode to regulate locomotion. Information about self-motion acquired from optic flow during TravFix can be used to control velocity of locomotion.

Fixation, Ocular↗

PubMed1997

Intralimb dynamics simplify reactive control strategies during locomotion.

The utilization of passive dynamics to control the swing trajectory is one mechanism which serves to minimize energy costs during locomotion, in addition to reducing the complexity of the neural control. In a reactive situation (e.g. trip or slip during walking), the energy cost may not be a major determinant of the locomotor activity as there is a need for quick corrective action under the threat of a fall. Therefore, we addressed the following question: does the nervous system utilize passive dynamics during the reactive control of locomotion? An unexpected mechanical perturbation was applied to the foot during early and late swing during walking. Video data were input into an inverse dynamics routine to obtain the joint moment and mechanical power profiles and to partition the joint moments into active and passive components. The nervous system still utilized the passive dynamics of the effector system; active control of a single joint, the knee joint, passively facilitated the flexor action at the proximal hip and distal ankle joint following the early swing perturbation. The minimization of the mechanical energy cost was not a major determinant for this task since the total mechanical work during the perturbed steps was greater than during normal steps. A neuromuscular constraint was observed following the late swing perturbation; the active control of the hip and knee joints were increased but the magnitude of the hip extensor/knee flexor moment was invariant and equal to 1.6. The intralimb dynamics identified during these responses may serve to simplify the complexity of the active control of the nervous system.

Adult↗

PubMed1997

A methodology for definition of neuromuscular electrical stimulation sequences: an application toward overcoming small obstacles.

Neuromuscular electrical stimulation (NMES) is an important tool for research in the rehabilitation of lost movements in spinal cord injured patients. However, for NMES to be used outside the laboratory setup, the stimulation controller needs to be able to cope with challenging environmental conditions. A methodology to allow conversion of any specific motor task used to cope with one of these environmental conditions toward a control algorithm for NMES is proposed. The methodology was used in the definition of a stimulation sequence that allows overcoming small obstacles by the spinal cord injured patient. Simulations have proven the method feasibility, and clinical trials have confirmed these findings. Motor task conversion using the kinematic approach has shown better results than that of muscle activity.

Adolescent↗

PubMed1996

Obstacle avoidance during locomotion is unaffected in a patient with visual form agnosia.

A patient (D.F.) who developed visual form agnosia following carbon monoxide-induced anoxia was assessed on three tests designed to measure her sensitivity to obstacle height in a locomotor task. Although her verbal estimates of the height of the obstacles were correlated with their actual height, the slope of the line relating estimated and actual obstacle height was much shallower than for control subjects. Similarly, when asked to estimate the height of the obstacle by raising one leg while standing nearby, the slope of line relating toe elevation and obstacle height was shallower than in control subjects. In contrast, D.F. was able to negotiate the same obstacles during locomotion as well as control subjects: toe elevation increased linearly as a function of obstacle height with similar slopes and correlation for the line relating toe elevation and obstacle height. These results provide additional support for the proposal by Goodale and Milner that the cortical pathways mediating the required transformations for the visual control of skilled actions are separate from those mediating experiential perception of the visual world.

Adult↗

PubMed1996

Characteristics of voluntary visual sampling of the environment for safe locomotion over different terrains.

The characteristics of visual sampling required for successful locomotion over various terrains is the focus of this work. In the first experiment we directly address the role of continuous visual monitoring of the environment in guiding locomotion by allowing the subjects to choose when and where to take a visual sample of the terrain and examine the effects of different terrains on characteristics of visual sampling. Young subjects walked over travel paths of varying difficulties while wearing opaque liquid crystal eyeglasses and pressed a hand-held switch to make the glasses transparent when they needed to sample the environment. Travel time and visual sampling characteristics were recorded. Results show that intermittent sampling (less than 50%) of the environment is adequate for safe locomotion, even over a novel travel path. The frequency, duration and timing of visual samples are dependent on terrain characteristics. Visual sampling of the environment is unaffected by preview restriction of the travel path and is increased when specific foot placement is required and there is a potential hazard in the travel path. In the second experiment we dissociated steering control and obstacle avoidance from specific foot placement and examined visual sampling demands prior to the initiation of the swing phase and during the swing phase. The results show that steering control and obstacle avoidance do influence the visual sampling time, which is scaled to the magnitude of change. Vision was used in a feedforward control mode to plan for and initiate appropriate changes in the swing limb trajectory: its use during the swing phase to provide on-line control was minimal.

Adult↗

PubMed1996

Effect of a cataract simulation on clinical and real world vision.

AIMS/BACKGROUND: Many reports have indicated that some patients with cataract can retain good visual acuity but complain of significant visual problems. This is the first in a series of papers trying to determine what causes these symptoms and whether other clinical tests can predict the real world vision loss. METHODS: The effect of a cataract simulation with a similar angular distribution of light scatter as real cataract on clinical (visual acuity, contrast sensitivity, and disability glare) and real world vision (face recognition, reading speed, and mobility orientation) was investigated. RESULTS: The simulation had a relatively small effect on visual acuity (6/6 with the simulation), but much larger effects on contrast sensitivity and low contrast acuity with and without glare. The simulation had no effect on high luminance and high contrast real world tasks, such as mobility orientation in room light and optimal reading speed. A small, but significant deterioration was found for the slightly lower contrast task of face and expression recognition. However, under low luminance conditions, substantial defects in mobility orientation were obtained (despite 6/6 acuity). CONCLUSIONS: Although the relative effect of the cataract simulation on acuity and contrast tasks is not typical of the average cataract, it can be found in those cataract patients with visual problems despite good visual acuity. This corroborates the suggestion that it is large amounts of wide angle light scatter (forward and/or backward) which are at least partly responsible for visual disability in cataract patients with good visual acuity. A patient's reported visual disability may depend on the percentage of time he or she spends under low contrast and/or low luminance conditions, such as walking or reading in dim illumination, and walking or driving at night, in fog, or heavy rain.

Adult↗

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↗

PubMed1995

Modelling the time-keeping function of the central pattern generator for locomotion using artificial sequential neural network.

The paper investigates the ability of a sequential neural network to model the time-keeping function (fundamental frequency oscillation) of a central pattern generator for locomotion. The intention is not to strive for biological fidelity, but rather to ensure that the network obeys the organisational and operational principles of central pattern generators developed through empirical research. The timing function serves to produce the underlying locomotor rhythm which can be transformed by nonlinear static shaping functions to construct the necessary locomotor activation patterns. Using two levels of tonic activations in the form of a step increase, a network consisting of nine processing units was successfully trained to output both sine and cosine waveforms, whose frequencies were modified in response to the level of input activation. The network's ability to generalise was demonstrated by appropriately scaling the frequency of oscillation in response to a range of input amplitudes, both within and outside the values on which it was trained. A notable and fortunate result was the model's failure to oscillate in the absence of input activation, which is a necessary property of the CPG model. It was further demonstrated that the oscillation frequency of the output waveforms exhibited both a high temporal stability and a very low sensitivity to input noise. The results indicate that the sequential neural network is a suitable candidate to model the time-keeping functions of the central pattern generator for locomotion.

Humans↗

PubMed1995

The Waterloo Vision and Mobility Study: postural control strategies in subjects with ARM.

Binocular vision function and standing balance control was assessed in 16 subjects with age-related maculopathy (ARM) (mean age 73.9 +/- 7.4 years) and 19 controls (mean age 69.1 +/- 5.5 years). Balance control was assessed using the center of pressure signal from force plate data. It was quantified using the root mean square (RMS) error of the amplitude, sampled over a 1 min period. This was evaluated during normal standing, and while the input from the kinesthetic and/or visual systems were disrupted. The two subject groups showed significantly different RMS values across conditions (P < 0.005). The differences in RMS between ARM and control groups were only significant when the input to the kinesthetic sensory system was disrupted. Our results suggest that in the normal standing condition, the kinesthetic and vestibular systems compensated for the lack of useful information from the visual system in ARM subjects. When kinesthetic system input was disrupted, balance control of the ARM group was significantly poorer than the control. There was a weak correlation between postural control and contrast sensitivity. Various strategies for preventing falls in elderly patients with low vision are suggested.

Aged↗

PubMed1994

Strategies for recovery from a trip in early and late swing during human walking.

The movement strategies and the underlying organization of the muscular responses for recovery from a tripping perturbation applied in early and late swing during walking were studied in humans. The latencies of the reflex response (60-140 ms) suggested that polysynaptic pathways are involved. The most common movement outcome was an elevating strategy of the swing limb in response to the early swing perturbation and a lowering strategy in response to the late swing perturbation. The elevating strategy comprised a flexor component of the swing limb and an extensor component of the stance limb. There was a temporal sequencing of the swing limb biceps femoris prior to the swing limb rectus femoris response to remove the limb from the obstacle prior to accelerating the limb over the obstacle. The extensor response of the stance limb generated an early heel-off to increase the height of the body. Thus, the lower limb joints collaborated to increase the height of the centre of mass and provide extra time to extend the swing limb in preparation for the landing. Flexion of the swing limb would be dangerous in response to the late swing perturbation as the swing limb is approaching the ground and the body mass has passed forward of the stance foot. Instead, a lowering strategy was accomplished by inhibitory responses of the swing limb vastus lateralis and/or excitatory responses of the swing limb biceps femoris. Both these responses resulted in a rapid lowering of the limb to the ground with a flat foot or forefoot landing and a shortening of the step length. Thus, in response to the late swing perturbation, the same recovery strategy was achieved by different patterns of muscle activation. These results demonstrate that the recovery strategies provided a functionally appropriate response for overcoming the obstacle and maintaining the ongoing locomotion.

Accidental Falls↗

PubMed1994

Waterloo Vision and Mobility Study: gait adaptations to altered surfaces in individuals with age-related maculopathy.

Walking is an extremely complex task that can become seriously challenged if one of the sensory systems which provides input to the motor system is compromised. The present study evaluated gait adaptations to altered surface characteristics and high and low ambient light conditions by subjects with age-related maculopathy (ARM). Twenty subjects with ARM and 20 control subjects walked along a 6 m path, along which they met 1 of 3 altered surfaces (compliant, uneven, or shiny). Kinematic data and ground reaction forces information were analyzed to discern gait adaptation strategies used by the ARM group. Ten trials on each surface were collected under both high and low ambient light levels. The ARM subjects were found to be generally more cautious when walking on the altered surfaces. For example, they walked more slowly, with a longer swing time. However, gait adaptations in the ARM group were not merely scaled versions of normal gait but were adjustments to adapt to environmental changes. Gait was modified to avoid tripping over a surface edge, to prevent slipping at heel contact, and to balance during stance. These adaptations enabled subjects to maintain safe mobility when walking in a challenging environment.

Adaptation, Physiological↗

PubMed1993

Models of recruitment and rate coding organization in motor-unit pools.

1. Isometric muscle force and the surface electromyogram (EMG) were simulated from a model that predicted recruitment and firing times in a pool of 120 motor units under different levels of excitatory drive. The EMG-force relationships that emerged from simulations using various schedules of recruitment and rate coding were compared with those observed experimentally to determine which of the modeled schemes were plausible representations of the actual organization in motor-unit pools. 2. The model was comprised of three elements: a motoneuron model, a motor-unit force model, and a model of the surface EMG. Input to the neuron model was an excitatory drive function representing the net synaptic input to motoneurons during voluntary muscle contractions. Recruitment thresholds were assigned such that many motoneurons had low thresholds and relatively few neurons had high thresholds. Motoneuron firing rate increased as a linear function of excitatory drive between recruitment threshold and peak firing rate levels. The sequence of discharge times for each motoneuron was simulated as a random renewal process. 3. Motor-unit twitch force was estimated as an impulse response of a critically damped, second-order system. Twitch amplitudes were assigned according to rank in the recruitment order, and twitch contraction times were inversely related to twitch amplitude. Nonlinear force-firing rate behavior was simulated by varying motor-unit force gain as a function of the instantaneous firing rate and the contraction time of the unit. The total force exerted by the muscle was computed as the sum of the motor-unit forces. 4. Motor-unit action potentials were simulated on the basis of estimates of the number and location of motor-unit muscle fibers and the propagation velocity of the fiber action potentials. The number of fibers innervated by each unit was assumed to be directly proportional to the twitch force. The area of muscle encompassing unit fibers was proportional to the number of fibers innervated, and the location of motor-unit territories were randomly assigned within the muscle cross section. Action-potential propagation velocities were estimated from an inverse function of contraction time. The train of discharge times predicted from the motoneuron model determined the occurrence of each motor-unit action potential. The surface EMG was synthesized as the sum of all motor-unit action-potential trains. 5. Two recruitment conditions were tested: narrow (limit of recruitment < 50% maximum excitation) and broad recruitment range conditions (limit of recruitment > 70% maximum excitation).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗