Narrow Base Of Support Research Articles

Competing effects of pain and fear of pain on postural control in low back pain?

A cross-sectional, observational study. To determine whether pain and fear of pain have competing effects on postural sway in patients with low back pain (LBP). Competing effects of pain and pain-related fear on postural control can be proposed as the likely explanation for inconsistent results regarding postural sway in the LBP literature. We hypothesized that although pain might increase postural sway, fear of pain might reduce sway through an increased cognitive effort or increased cocontraction to restrict body movement. The cognitive strategy would be less effective under dual-task conditions and the cocontraction strategy was expected to be less effective when standing on a narrow base of support surface. Postural sway was measured in combined conditions of base of support (full and narrow) and cognitive loading (single and dual tasks) in 3 experimental groups with current LBP, recent LBP, and no LBP. Sway amplitude, path length, mean power frequency, and sample entropy were extracted from center-of-pressure data. The current-LBP group and recent-LBP group reported significantly different levels of pain, but similar levels of pain catastrophizing and kinesiophobia. The current-LBP group tended to display larger sway amplitudes in the anteroposterior direction compared with the other 2 groups. Mean power frequency values in mediolateral direction were lower in patients with the current LBP compared with recent LBP. Smaller sample entropy was found in the current-LBP group than the other groups in most experimental conditions, particularly when standing on a narrow base of support. Alterations of postural sway are mostly mediated by pain but not pain-related fear. LBP tends to increase sway amplitude, which seems to be counteracted by increased effort invested in postural control leading to decreased frequency and increased regularity of sway particularly under increased task demands. Cross-sectional study.

Functional gait assessment: Reliability and validity in people with peripheral vestibular disorders

Background/aim Reliable and valid measures evaluating imbalance during walking are important for targeting rehabilitation efforts. This study aimed to investigate the reliability and validity of the Functional Gait Assessment (FGA) in people with peripheral vestibular disorder. Methods This study used a cross-sectional design and tested the reliability and validity of the FGA in 43 participants with peripheral vestibular disorder. The participants' performance on the FGA was filmed. To test intra-rater reliability, three raters reassessed 39 of these participants using the video uptakes. Inter-rater reliability was assessed by five raters analysing the video uptakes. Concurrent validity was investigated using timed measures of dynamic balance and the Activities-specific Balance Confidence scale. Results The intra-rater reliability for the FGA score was high, with a intraclass correlation coefficient (ICC (model 2,1)=0.94 (95% Cl=0.85; 0.97). The intra-rater percentage of agreement for all raters for separate items was moderate to high, and weighted kappa was good to very good, ranging from 0.61 for change in speed to 0.95 for stair climbing. The inter-rater reliability was ICC (model 2,1)=0.73 (95% Cl=0.49; 0.86). Agreement for two random raters was 45–92%. Weighted kappa was fair to very good, ranging from 0.27 for change in speed to 0.87 for walking with a narrow base of support. The internal consistency was high (α=0.88). Gait with eyes closed had the lowest item-to-total (0.33) and item-to-item (<0.30) correlation. Concurrent validity was moderate to strong (rho=0.50–0.76) with the highest correlation coefficient for the Timed Up and Go test. Conclusions The FGA is a reliable and valid measure in people with peripheral vestibular disorder. For improving internal consistency, removal of the item ‘gait with eyes closed’ may be considered.

Estabilidade da marcha na neuropatia diabética periférica

The aim of this study was to evaluate gait stability in diabetic patients with peripheral neuropathy in three conditions: habitual walking with eyes open, walking with eyes closed, and walking with eyes open and narrow base of support. The study included 41 subjects, 18 with neuropathy (NG) and 23 controls. Gait stability was evaluated on a baropodometer using the Footwalk Pro software. The following data were obtained: gait speed and percentage of time spent in double stance and single stance. Significant differences were observed between groups in all three conditions for gait speed and single stance time, which were reduced in NG (p<0.05), and for double stance time, which was increased in NG (p<0.05). For gait speed, double stance time and single stance time, the eyes open condition differed from the eyes closed (p<0.001) and narrow base of support (p<0.001) conditions. In the three conditions studied, patients of NG presented a deficit in gait stability and this performance was even more compromised in the two conditions that required greater postural control. These gait changes resulting from the complexity imposed by the different conditions suggest the inclusion of these conditions in the evaluation and treatment of this population.

The ability of people with Parkinson’s disease to modify dual-task performance in response to instructions during simple and complex walking tasks

Gait impairments are a common and consequential motor symptom in Parkinson's disease (PD). A cognitive strategy that incorporates instructions to concentrate on specific parameters of walking is an effective approach to gait rehabilitation for persons with PD during single-task and simple dual-task walking conditions. This study examined the ability to modify dual-task walking in response to instructions during a complex walking task in people with PD compared to healthy older adults (HOA). Eleven people with PD and twelve HOA performed a cognitive task while walking with either a usual base or a narrow base of support. Dual-task walking and cognitive task performance were characterized under two conditions-when participants were instructed focus on walking and when they were instructed to focus on the cognitive task. During both usual base and narrow base walking, instructions affected cognitive task response latency, with slower performance when instructed to focus on walking compared to the cognitive task. Regardless of task or instructions, cognitive task performance was slower in participants with PD compared to HOA. During usual base walking, instructions influenced gait speed for both people with PD and HOA, with faster gait speed when instructed to focus on walking compared to the cognitive task. In contrast, during the narrow base walking, instructions affected gait speed only for HOA, but not for people with PD. This suggests that among people with PD the ability to modify walking in response to instructions depends on the complexity of the walking task.

ASSESSMENT OF POSTURAL STABILITY IN SUBJECTS WITH LUMBAR DISC HERNIATION

BackgroundChronic spinal disc disease leads to disorders in postural movement coordination and deteriorates postural stability.1 Recent studies suggest that subjects with lumbar disc herniation (LDH) develop visual compensation mechanisms for...

Three simple clinical tests to accurately predict falls in people with Parkinson's disease

Falls are a major cause of morbidity in Parkinson's disease (PD). The objective of this study was to identify predictors of falls in PD and develop a simple prediction tool that would be useful in routine patient care. Potential predictor variables (falls history, disease severity, cognition, leg muscle strength, balance, mobility, freezing of gait [FOG], and fear of falling) were collected for 205 community-dwelling people with PD. Falls were monitored prospectively for 6 months using monthly falls diaries. In total, 125 participants (59%) fell during follow-up. A model that included a history of falls, FOG, impaired postural sway, gait speed, sit-to-stand, standing balance with narrow base of support, and coordinated stability had high discrimination in identifying fallers (area under the receiver-operating characteristic curve [AUC], 0.83; 95% confidence interval [CI], 0.77-0.88). A clinical tool that incorporated 3 predictors easily determined in a clinical setting (falling in the previous year: odds ratio [OR], 5.80; 95% CI, 3.00-11.22; FOG in the past month: OR, 2.39; 95% CI, 1.19-4.80; and self-selected gait speed < 1.1 meters per second: OR, 1.86; 95% CI, 0.96-3.58) had similar discrimination (AUC, 0.80; 95% CI, 0.73-0.86) to the more complex model (P = 0.14 for comparison of AUCs). The absolute probability of falling in the next 6 months for people with low, medium, and high risk using the simple, 3-test tool was 17%, 51%, and 85%, respectively. In people who have PD without significant cognitive impairment, falls can be predicted with a high degree of accuracy using a simple, 3-test clinical tool. This tool enables individualized quantification of the risk of falling. © 2013 Movement Disorder Society.

Motor impairments at presentation of clinically isolated syndrome suggestive of multiple sclerosis: Characterization of different disease subtypes

To evaluate the frequency of motor impairments in patients with recently diagnosed clinically isolated syndrome (CIS) suggestive of MS. Following a neurological examination, patients with CIS were subdivided into two groups according to the number of functional systems involved at onset. Groups consisted of monosymptomatic (n=35, mean EDSS=1.3 ± 0.2) and polysymptomatic (n=17, mean EDSS=2.6 ± 0.1) patients. Motor assessment included peak isometric torque and fatigue index (FI) measured at the knee and ankle bilaterally and spatiotemporal parameters of gait and postural control. Lesion load and atrophy were measured on a 3.0T brain MRI using MSAnalyze computerized software. Twenty eight age- and gender-matched healthy subjects served as controls for motor assessment. Polysymptomatic patients demonstrated reduced peak strength and FI, slower gait velocity, narrower base of support and increased sway rate compared to healthy subjects and mono-symptomatic patients. Furthermore, polysymptomatic and monosymptomatic patient median score in T2-weighted volume lesions (mm<formula>^{3}</formula>) was 4875 and 1628, respectively. Evidence of two subtypes of CIS was established in context with MRI findings and motor deficits. Differentiating between these groups can potentially improve management of MS patients at the initial stages of the disease process.

Equilibrium of the human body and the gravity line: the basics

Bipedalism is a distinguishing feature of the human race and is characterised by a narrow base of support and an ergonomically optimal position thanks to the appearance of lumbar and cervical curves. The pelvis, adapted to bipedalism, may be considered as the pelvic vertebra connecting the spine to the lower limbs. Laterally, the body's line of gravity is situated very slightly behind the femoral heads laterally, and frontally it runs through the middle of the sacrum at a point equidistant from the two femoral heads. Any abnormal change through kyphosis regarding the spinal curves results in compensation, first in the pelvis through rotation and then in the lower limbs via knee flexion. This mechanism maintains the line of gravity within the base of support but is not ergonomic. To analyse sagittal balance, we must thus define the parameters concerned and the relationships between them. These parameters are as follows: for the pelvis: incidence angle, pelvis tilt, sacral slope; for the spine: point of inflexion, apex of lumbar lordosis, lumbar lordosis, spinal tilt at C7; for overall analysis: spino-sacral angle, which is an intrinsic parameter.

Effects of instructed focus and task difficulty on concurrent walking and cognitive task performance in healthy young adults

Dual task paradigms can be used to examine the interactions between cognition and the control of posture and gait. Measuring and interpreting changes in dual task performance is challenging, however, because many factors can influence performance. This study examined the effects of instructed focus and walking task difficulty, and the interaction between these factors, on dual task performance in healthy young adults. Fifteen participants performed a cognitive task while walking with either a usual base or a narrow base of support. Participants were instructed to focus on either the cognitive task or walking. Trade-offs both within and between tasks were assessed using the modified attention allocation index and the performance operating characteristic. Instructed focus influenced both the cognitive task and walking. Performance on the cognitive task was faster with instructions to focus on the cognitive task, and walking was faster (and more accurate in the narrow-base condition) with instructions to focus on walking. Walking task difficulty did not affect cognitive performance but did affect walking, with faster walking in the usual-base versus narrow-base condition. There was evidence of an interaction, with greater effects of instructed focus on the cognitive task during usual versus narrow-base walking. These results support the idea that the ability to flexibly shift attention allocation and task performance in response to instructions depends on the difficulty of the postural control task. The modified attention allocation index and the performance operating characteristic were instrumental in fully characterizing trade-offs between and within tasks in order to understand dual task performance changes. A clearer understanding of the factors that affect dual task walking and the interactions between these factors has important implications for the assessment of dual task performance in both clinical and research settings.

Age-related kinematic changes in late visual-cueing during obstacle circumvention

On a daily basis, we are challenged by common environmental obstacles (e.g. street posts) that require simple and often rapid modifications to our gait patterns to avoid collisions. Poor vision appears to be responsible for important reductions in postural stability during gait; and therefore, individuals with impaired vision, such as the elderly, may be at a greater risk of falling, especially under conditions where stepping avoidance strategies may be constrained by the environment. The purpose of the current study was to examine the body segment and eye-gaze reorientation strategy, role of base of support, as well as visual areas of interest attended to by healthy young (YA) and older adults (OA) when only given limited time, one stride, to prepare for an obstacle circumvention task. Six YA and six OA were asked to perform ten walking trials which required them to circumvent an obstacle in their travel path. Participants used one of two avoidance strategies, either lead leg crossing-over trail leg (narrow base of support) or lead leg stepping-out (wide base of support). Results indicate that base of support constraints did not affect segment reorientation sequence in either age group. The general segment reorientation sequence in YA was initiated by trunk yaw and head yaw, followed by gaze and finally, by M-L foot deviation. No trunk roll deviations were observed. In OA, the general segment reorientation sequence was the following: trunk yaw and trunk roll, gaze and finally, M-L foot deviation. No head yaw deviations were observed. Our findings suggest that YA utilized a foot placement strategy to perform the transient change in travel direction while OA relied on a hip strategy. In addition, YA spent more time gazing straight ahead at the obstacle and the wall, while OA spent more time looking at the ground. This strategy indicates that OA use a more cautious strategy to safely avoid the obstacle. Findings from the present work contribute further knowledge regarding locomotor adjustments during a common, and complex, everyday task in young and older adults.

Step Training Reinforces Specific Spinal Locomotor Circuitry in Adult Spinal Rats

Locomotor training improves function after a spinal cord injury both in experimental and clinical settings. The activity-dependent mechanisms underlying such improvement, however, are sparsely understood. Adult rats received a complete spinal cord transection (T9), and epidural stimulation (ES) electrodes were secured to the dura matter at L2. EMG electrodes were implanted bilaterally in selected muscles. Using a servo-controlled body weight support system for bipedal stepping, five rats were trained 7 d/week for 6 weeks (30 min/d) under quipazine (0.3 mg/kg) and ES (L2; 40 Hz). Nontrained rats were handled as trained rats but did not receive quipazine or ES. At the end of the experiment, a subset of rats was used for c-fos immunohistochemistry. Three trained and three nontrained rats stepped for 1 h (ES; no quipazine) and were returned to their cages for 1 h before intracardiac perfusion. All rats could step with ES and quipazine administration. The trained rats had higher and longer steps, narrower base of support at stance, and lower variability in EMG parameters than nontrained rats, and these properties approached that of noninjured controls. After 1 h of stepping, the number of FOS+ neurons was significantly lower in trained than nontrained rats throughout the extent of the lumbosacral segments. These results suggest that training reinforces the efficacy of specific sensorimotor pathways, resulting in a more selective and stable network of neurons that controls locomotion.

Age-related modifications in steering behaviour: effects of base-of-support constraints at the turn point

This study investigated the effects of altering the base of support (BOS) at the turn point on anticipatory locomotor adjustments during voluntary changes in travel direction in healthy young and older adults. Participants were required to walk at their preferred pace along a 3-m straight travel path and continue to walk straight ahead or turn 40 degrees to the left or right for an additional 2-m. The starting foot and occasionally the gait starting point were adjusted so that participants had to execute the turn using a cross-over step with a narrow BOS or a lead-out step with a wide BOS. Spatial and temporal gait variables, magnitudes of angular segmental movement, and timing and sequencing of body segment reorientation were similar despite executing the turn with a narrow or wide BOS. A narrow BOS during turning generated an increased step width in the step prior to the turn for both young and older adults. Age-related changes when turning included reduced step velocity and step length for older compared to young adults. Age-related changes in the timing and sequencing of body segment reorientation prior to the turn point were also observed. A reduction in walking speed and an increase in step width just prior to the turn, combined with a delay in motion of the center of mass suggests that older adults used a more cautious combined foot placement and hip strategy to execute changes in travel direction compared to young adults. The results of this study provide insight into mobility constraints during a common locomotor task in older adults.

Effects of lateral perturbations and changing stance conditions on anticipatory postural adjustment

The study investigates the role of lateral muscles and changing stance conditions in anticipatory postural adjustments (APAs). Subjects stood laterally to an aluminum pendulum released by an experimenter and were required to stop it with their right or left hand. Stance conditions were manipulated by having the subjects stand in the following positions: on a single limb (SS), with feet together (narrow base of support, NB), and with feet shoulder width apart (regular base of support, RB). Bilateral EMG activity of dorsal, ventral, and lateral trunk and leg muscles and ground reaction forces were recorded and quantified within the time intervals typical of APAs. Anticipatory postural adjustments were seen in all experimental conditions, and their magnitudes depended on the stance and the side of perturbation. Accordingly, APAs in lateral muscles increased on the side of perturbation in SS condition, while simultaneous activation of dorsal muscles occurred on the contralateral side. Smaller APAs were seen in lateral muscles in conditions with a wider base of support (NB, RB) and APAs in dorsal muscles were smaller in NB – in comparison to RB – stance. The results of the present study provide new data on the role of lateral, ventral, and dorsal muscles in anticipatory postural control when dealing with lateral perturbations in conditions of postural instability.

Case Report Vestibular Rehabilitation Decreases Fall Risk and Improves Gaze Stability for an Older Individual with Unilateral Vestibular Hypofunction

Partial or total unilateral vestibular loss is the third most common cause of peripheral vestibular dysfunction. Dysfunction of one or both of the vestibular mechanisms can manifest physically as abnormalities of posture, balance, and/or visual acuity. This case report describes physical therapy examination and individualized intervention with vestibular rehabilitation for a patient with unilateral vestibular hypofunction. The patient was an 80-year-old male with electronystamographically confirmed unilateral vestibular loss of 98.3%. He demonstrated altered balance and gaze stability classifying him as having an increased risk for falling. After 5 weeks of individualized vestibular rehabilitation, the patient significantly decreased his fall risk from 11 to 20 of 24 on the Dynamic Gait Index. His gaze stability also improved from a 4 to 1 line disparity with dynamic visual acuity testing. The patient also had a decrease in perceived disability on the Dizziness Handicap Inventory from 30/100 at evaluation to 12/100 at discharge. Individualized vestibular rehabilitation decreased fall risk and improved gaze stability for a patient with significant unilateral vestibular hypofunction.

Age-related changes in avoidance strategies when negotiating single and multiple obstacles

The aim of this research was to describe age-related changes in locomotor adjustments during obstructed gait and expand and build from the current body of literature describing single obstacle avoidance strategies by including trials in which the subjects stepped over two identical obstacles placed in series. We observed young adults (YA: N = 8; aged 23.1 +/- 2.0 years) and older adults (OA: N = 8; aged 76.1 +/- 4.3 years) as they walked along a 5 m long instrumented pathway (GAITRite) and stepped over one or two obstacles that were scaled to their lower leg length. Infrared markers, tracked using the Optotrak motion analysis system (60 Hz; Northern Digital Inc, Canada), were fixed to subjects' trunk and feet, and several anatomical landmarks were digitized for each segment (e.g. toes). Data analyses included lead and trail toe clearance values, take-off and landing distance, step time, length, width and velocity, and three-dimensional trunk angles. Both age groups were able to successfully complete the obstacle avoidance task, and the presence of a second obstacle did not affect clearance strategies of either OA or YA. OA crossed the obstacles with a reduced step velocity and stepped closer to the trailing edge, although take-off distances were not different between the age groups. Additionally, OA used similar ranges of trunk motion as YA when crossing the obstacle, but did so while using smaller step lengths and step widths compared to YA, effectively, using a narrower base of support. Together, these findings suggest that older adults adopted a more cautious crossing strategy in that they reduced their crossing step velocity. However, other aspects of the avoidance strategy used by the older adults, specifically the shortened landing distances and the use of similar ranges of trunk motion within a narrowed BOS, could potentially put them at risk for tripping or imbalance when stepping over an obstacle.

Measurement of balance in survivors of acute lymphoblastic leukemia in childhood.

Survivors of acute lymphoblastic leukemia (ALL) in childhood have been identified with balance problems. The goals of this study were to objectively quantify these observations by measuring the displacement and velocity of the body centre of pressure, a measure of 'sway'. The subjects included 79 ALL survivors who were at least 1 year post-treatment for ALL and 83 age-matched controls. Testing was carried out using various static balance tests designed to challenge central nervous system maintenance of balance and postural control. Patients were different from controls, across all age groups, in the more difficult task in which they were required to balance over a narrow base of support. However this did not reach statistical significance. When they were asked to perform this task with their eyes closed 32% of the patients tested, as opposed to 2% of controls, were unable to complete the task (significant at P<0.01, chi(2)). The youngest group of patients (children aged 5-7 years) presented consistently with lower (or similar) displacement and lower velocity measures. This phenomenon has been observed in healthy children, however, in patients, these trends were magnified and beyond the normal age range in healthy children. The velocity values were significantly different from the other patient groups (normalized values for velocity; P<0.01). These observations then manifested as higher sway values when patients were challenged with a difficult task with their eyes closed. We propose that survivors of childhood ALL have delayed motor development compared to age-matched controls giving rise to impairment in balance and postural control. It is possible that these impairments are a consequence of intensive multi-agent chemotherapy and cranial irradiation.

Effects of varying task constraints on solutions to joint coordination in a sit-to-stand task.

The question of how multijoint movement is controlled can be studied by discovering how the variance of joint trajectories is structured in relation to important task-related variables. In a previous study of the sit-to-stand task, for instance, variations of body segment postures that leave the position of the body's center of mass (CM) unchanged were significantly greater than variations of body segment posture that varied the CM position. The present experiments tested the hypothesis that such structuring of joint configuration variability is accentuated when the mechanical or perceptual task demands are made more challenging. Six subjects performed the sit-to-stand task without vision (eyes closed), either on a normal or on a narrow support surface. An additional constraint on the postural task was introduced in a third condition by requiring subjects to maintain light touch (less than 1 N) with the fingertips while coming to a standing position on the narrow base of support. The joint configurations observed at each point in normalized time were analyzed with respect to trial-to-trial variability. The task variables CM and head position were used to define goal-equivalent sets of joint configurations ("uncontrolled manifolds," UCMs) within which variation of joint configuration leaves the task variables unchanged. The variability of joint configurations across trials was decomposed into components that did not affect (within the UCM) and that did affect (orthogonal to the UCM) the values of these task variables. Our results replicate the earlier finding of much larger variability in directions of joint space that leave the CM unchanged compared with directions that affect CM position. This effect was even more pronounced here than in the previous experiment, probably because of the more difficult perceptual conditions in the current study (eyes closed). When the mechanical difficulty of the task was increased, the difference between the two types of joint variability was further accentuated, primarily through increase in goal-equivalent variance. This provides evidence for the hypothesis that under challenging task constraints increased variability is selectively directed into task-irrelevant degrees of freedom. Because differential control along different directions of joint space requires coordination among joint angles, this observation supports the view that the CNS responds to increased task difficulty through enhanced coordination among degrees of freedom. The adaptive nature of this coordination is further illustrated by the similar enhanced use of goal-equivalent joint combinations to achieve a stable CM position when subjects stood up under the additional constraint of maintaining light touch with the fingertips. This was achieved by channeling goal-equivalent variability into different directions of joint configuration space.

Age-Related Changes of Postural Control: Effect of Cognitive Tasks

Background: Postural control and falls in the elderly constitute a major health problem. The interest in balance deficits is growing, as concern about the rising costs of health care increases. This issue is particularly relevant to the elderly population in which falls occur most frequently. Postural control in the elderly was studied using a cognitive approach. Objective: The purpose of this study was to study the characteristics of central processing of postural control while performing cognitive tasks. Methods: A dual-task procedure was developed to estimate the level of automaticity of a quiet upright standing task. The effect of a concurrent attention-demanding task (modified Stroop test) on the efficiency of balance control in the elderly was determined using force platform and electromyography measurements. Results: It was found that there is an increase in postural sway in old subjects compared with young subjects when performing single tasks and dual-task tests. The results of the study demonstrate that postural adjustments require cognitive processing; young and old subjects showed similar interference effects on postural steadiness (postural sway) caused by the concurrent attention-demanding task. The results are corroborated by the hypothesis that a dual task gives information on the restoration of automaticity of postural control in old age by a central reorganization process. When performing a dual task tested on a narrow base of support, the old subjects decreased their body sway, while the younger did not. According to electromyography measurements, the older subjects increased their muscle activity in the tibialis anterior and soleus muscles, using slow-twitch motor units compared with the younger subjects. Conclusions: Both alterations (cognitive and base of support) have a substantially greater effect on the elderly than on the young. The older subjects decreased their body sway by activating a cocontraction strategy of postural control around the ankle joint, probably because of the danger to their postural stability.

Egypt: Stable, but for how long?

Hosni Mubarak appears poised to rule for another decade. The present, narrow base of support, however, is unlikely to be viable in the long term. The challenge is whether the system has the durability to survive an economic downturn, social unrest, or both.

Knee position feedback: its effect on management of pelvic instability in a stroke patient

Purpose : This is a case study of improvement in pelvic instability due to a stroke, with treatment. The patient had narrow knee-to-knee distance due to weakness in the pelvic muscles resulting in a narrow base of support. Method : Treatment was focused on pre-gait activities to improve upright posture control followed by gait training involving extrinsic auditory feedback about knee-to-knee distance. Results : After two weeks of therapy with feedback technique incorporated in a functional context of gait therapy, the patient was able to advance his left lower extremity with proper foot placement. Laboratory measurements showed that step width and stride length gotten better after treatment as well. Conclusion : An inclusion of feedback information about knee-to-knee distance could benefit gait training of patients with pelvic instability.