Biomedical subjects

A M Wing

Publications and source records attributed to A M Wing.

At least 19 recordsLinked to original sources

PubMed2001

Estimating the minimum grip force required when grasping objects under impulsive loading conditions.

As an aid to studying the efficiency of grip force scaling in the context of collisions, we present a simple cost-effective approach to estimating the slip ratio--that is, the minimum grip-to-load-force ratio needed to prevent object slippage. The grip apparatus comprises a sturdy load cell to measure grip force and two linear potentiometers to provide detailed description of finger movements. The slip ratio was estimated by plotting the magnitude of finger movement against the grip-to-load-force ratio at the time of impact. The slip ratio was dependent on the direction of loading, which stresses the importance of estimating slip ratios in a context similar to that of the experiment in which the efficiency of subjects' behavior is to be assessed.

Adult↗

PubMed2001

Effects of spatial and nonspatial cognitive activity on postural stability.

Is postural stability controlled automatically, or is it affected by concurrent cognitive activity? Are the effects influenced by the nature of the cognitive activity required, and do they increase in old age? To address these questions, 70 participants aged 20-79 years were asked to stand as still as possible on a force platform (postural control task) while performing (a) no cognitive task, (b) a spatial memory task, and (c) a nonspatial memory task. The memory tasks were also performed while seated as a comparison condition. Both spatial and nonspatial memory recall declined with increasing age but were unaffected by position (standing vs. seated). Postural stability declined with age; moreover, there was support for an earlier finding that age decline was greatest when performing the spatial memory task. Each recording period was split into two phases which, for the spatial and nonspatial memory tasks, corresponded to encoding and maintaining the stimuli. In comparison with no task, participants were more stable when encoding stimuli (particularly in the spatial task), but they were less stable when maintaining stimuli (particularly in the nonspatial task). The results suggest that postural stability can be affected by cognitive activity in complex ways, depending on the age of participants, the type of cognitive task (spatial vs. nonspatial), and the cognitive processing required (encoding vs. maintenance).

Adult↗

PubMed2001

Brief bimanual force pulses: correlations between the hands in force and time.

Three experiments assessed coupling phenomena in the coordination of bimanual force pulses. Experiment 1 required symmetric force pulses (equal target forces and rise times for both hands) using the index finger of each hand. As the authors expected, on the basis of bimanual pointing movement results, this experiment revealed positive correlations between both the force rise times and the force amplitudes of the two hands. Experiments 2 and 3 included asymmetric conditions with different target force amplitudes (Experiment 2) or target rise times (Experiment 3). In Experiment 2 force amplitudes but not rise times were fully decoupled in the asymmetric condition. In the asymmetric condition of Experiment 3, however, neither rise times nor force amplitudes were fully decoupled. The results suggest a hierarchical control structure with temporal control dominating nontemporal control of bimanual force coordination.

Adult↗

PubMed2000

Motor control: Mechanisms of motor equivalence in handwriting.

Handwriting is a classic example of how the details of movement can be scale and plane invariant: letter forms reflecting personal style are unchanged, whether one is writing on a piece of paper, on a blackboard or in the sand using the foot. Recent research points to a role for the parietal cortex in such motor equivalence.

Handwriting↗

PubMed2000

Stepping before standing: hip muscle function in stepping and standing balance after stroke.

OBJECTIVE: To compare the pattern of pelvic girdle muscle activation in normal subjects and hemiparetic patients while stepping and maintaining standing balance. DESIGN: Group comparison. METHOD: Seventeen patients who had regained the ability to walk after a single hemiparetic stroke were studied together with 16 normal controls. Median interval between stroke and testing was 17 months. Amplitude and onset latency of surface EMG activity in hip abductors and adductors were recorded in response to sideways pushes in either direction while standing. Similar recordings were made in the same subjects during gait initiation and a single stride. RESULTS: In the standing balance task, normal subjects resisted a sideways push to the left with the left gluteus medius (74 ms) and with the right adductor (111 ms), and vice versa. In hemiparetic patients, the amplitude of activity was reduced in the hemiparetic muscles, the onset latencies of which were delayed (gluteus medius 96 ms, adductor 144 ms). Contralateral, non-paretic, adductor activity was increased after a push towards the hemiparetic side of patients with stroke and the latency was normal (110 ms). During self initiated sideways weight shifts at gait initiation, hemiplegic muscle activation was impaired. By contrast, the pattern and peak amplitude of hip muscle activation in stepping was normal in both hemiparetic and non-hemiparetic muscles of the subjects with stroke. CONCLUSIONS: In ambulant patients with stroke, a normal pattern of activation of hemiparetic muscles is seen in stepping whereas the response of these muscles to a perturbation while standing remains grossly impaired and is compensated by increased activity of the contralateral muscles. This suggests that hemiparetic patients should be able to step before regaining standing balance.

Electromyography↗

PubMed2000

Ground reaction force after a sideways push as a measure of balance in recovery from stroke.

OBJECTIVE: To investigate if measuring ground reaction force after a sideways push at the hips gives a measure of standing balance in stroke subjects. METHODS: Fifteen control subjects and 13 right hemiparetic subjects who were able to stand independently stood with their feet on a single forceplate. Horizontal sideways pushes of 3% body weight were delivered to each side of the pelvis with the subjects held firmly in a semi-rigid belt. Measurements were made of lateral pelvic displacement (sway) and the lateral sheer component of ground reaction force (GRF). RESULTS: Right hemiparetic subjects showed significantly greater sway after a sideways push (p < 0.01) and later onset of GRF (p < 0.01) when pushed to their weak side compared with control subjects. There was also a positive correlation between sway after a sideways push and the onset latency of GRF in both strokes (0.41) and controls (0.61). The hemiparetic subjects swayed more (p < 0.01) when pushed to their weak side compared with their stronger side and their GRF latency was longer, but this latter measurement failed to reach statistical significance. No difference was seen between sides in sway or GRF latency in controls. CONCLUSIONS: The latency of GRF onset after a push at the hips in controls and in stroke subjects is related to sway and both measurements increase after a stroke. This test offers a method of measuring balance after a stroke, and serial testing of an individual after a stroke may prove a useful measure of an individual's recovery of balance.

Adolescent↗

PubMed2000

Changing patterns of postural hip muscle activity during recovery from stroke.

OBJECTIVE: To describe the recovery of neurophysiological responses to perturbation of standing balance after stroke. METHODS: Surface electromyography (EMG) from hip abductors and adductors and ground reaction forces (GRF) were measured in response to 20 sideways pushes applied to the pelvis by a linear motor. Each subject's data from pushes in each direction were averaged and the presence of a muscle EMG response was assessed visually. SUBJECTS: Thirteen acute hemiplegic patients were tested as soon as they could stand after stroke (median six weeks) and serially during recovery. RESULTS: Four patterns of hip muscle activity were seen: (1) no response at all, (2) no response in hemiparetic muscles but compensation by contralateral muscles, (3) an appropriate, if delayed, response in the hemiparetic abductor but not adductor muscles, and (4) a relatively normal pattern in both hemiparetic muscles. Nine of 13 patients showed a change in pattern of hip muscle activity during recovery. All patients who initially resisted the sideways pushes solely with muscles of the unaffected leg later regained use of the hemiparetic hip abductors. CONCLUSIONS: The pattern of hip muscle activation changed towards normal during recovery from stroke in most patients. Use of compensatory strategies early after stroke in these subjects did not prevent return of normal patterns of muscle activation later.

Acute Disease↗

PubMed1999

Lateral balance organisation in human stance in response to a random or predictable perturbation.

The effect of the predictability of perturbation to standing balance was evaluated in terms of the muscle activity and response dynamics of five subjects exposed to horizontal forces at the pelvis producing sideways or forward sway. Rapid (EMG onset latencies of 70-80 ms recorded from the left gluteus medius and gastrocnemius) and qualitatively different patterns of response were produced by forward pushes and pushes to either side. However, the EMG response to left push was constant in pattern and timing, whether the push direction was constant and, therefore, predictable over a block of trials or whether the left push trials were interleaved randomly with right push or forward push trials. Moreover, there were no systematic effects of perturbation direction uncertainty on the latency and rate of increase of ground reaction forces. We conclude that prior information does not speed postural responses that differ quantitatively according to the direction of perturbation to balance.

Adult↗

PubMed1999

Light touch contribution to balance in normal bipedal stance.

It has previously been shown that light contact with the finger tip on a fixed surface reduces centre of pressure (CoP) fluctuations in the frontal plane when standing in an unstable posture with the feet in line (tandem Romberg stance). Positive cross-correlations between horizontal finger forces and CoP fluctuations with finger forces exhibiting a phase lead suggest the hand provides sensory input for postural stability. The present study investigates whether this is the case for normal posture. We report reduced CoP fluctuations in the sagittal plane when light touch is permitted during normal bipedal stance. Moreover, we find positive crosscorrelations between finger tip forces and CoP fluctuations which are of similar magnitude and phase lag to those observed in tandem Romberg stance. This shows the utility of hand touch input for regulation of normal upright posture as well as inherently unstable postures such as tandem Romberg.

Adult↗

PubMed1999

Grip force dynamics in the approach to a collision.

This experiment investigated the prediction of load force (LF) in impulsive collisions inferred from anticipatory adjustments of grip force (GF) used to stabilise a hand-held object. Subjects used a precision grip to hold the object between thumb and index finger of their right hand and used the arm either: (1) to move the object to produce a collision by hitting the lower end of a pendulum, causing it to swing to one of three target angles, or (2) to hold the object still while receiving a collision produced by the experimenter releasing the pendulum from one of three angles. Visual feedback of the pendulum's trajectory was available in the production task only. In all conditions, subjects increased GF in advance of the collision. In receiving the collision without advance information, subjects set GF levels to the mid-range of the experienced forces. When subjects possessed knowledge about the maximum angle of pendulum swing - either because they were going to produce it or because they were verbally informed - magnitude of the anticipatory-GF magnitude response was scaled to the predicted LF magnitude. Furthermore, GF was scaled to LF with a higher gain when producing compared to receiving the collision. This suggests that updating forward models through a semantic route is not as powerful as when the updating is achieved through the more direct route of dynamic exploration.

Adolescent↗

PubMed1998

Anticipating load torques produced by voluntary movements.

The stability of an object held between the finger and thumb depends on friction developed by grip force, normal to the contact surfaces, to overcome tangential load force. Previous research has shown that in lifting an object, grip force rises with the increase in gravitational load force as the hand takes the weight and that in moving an object, grip force is adjusted to meet movement-induced inertial load force. Those results demonstrated the anticipatory nature of coordination of grip force with load force. Whether grip force anticipates load torque was studied in this research. When participants were constrained to use grasp points where the grasp axis was manifestly distant from object center of mass, it was found that they made grip force adjustments in anticipation of load torques that tended to destabilize an object as a result of lifting or moving it. These adjustments imply use of information about object center of mass in movement planning.

Adolescent↗

PubMed1997

The role of internal models in motion planning and control: evidence from grip force adjustments during movements of hand-held loads.

We investigated the issue of whether or not the CNS makes use of an internal model of the motor apparatus in planning and controlling arm movements. In particular, we tested the ability of subjects to predict different hand-held loads by examining grip force adjustments used to stabilize the load in the hand during arm movements. Subjects grasped a manipulandum using a precision grip with the tips of the thumb and index finger on either side. The grip force (normal to the contact surfaces) and the load force (tangential to the surfaces) were measured, along with the trajectory of the hand. The manipulandum was attached to two servo-controlled linear motors used to create inertial and viscous loads as well as a composite load, including inertial, viscous, and elastic components. The form of the hand trajectory was independent of load for some subjects but varied systematically across load conditions in others. Nevertheless, under all load conditions and in all subjects, grip force was modulated in parallel with, and thus anticipated, fluctuations in load force despite the marked variation in the form of the load function. This indicates that the CNS is able to predict the load force and the kinematics of hand movement on which the load depends. We suggest this prediction is based on an internal model of the motor apparatus and external load and is used to determine the grip forces required to stabilize the load.

Adaptation, Physiological↗

PubMed1997

Anticipatory postural adjustments in stance and grip.

The reactive forces and torques associated with moving a hand-held object between two points are potentially destabilising, both for the object's position in the hand and for body posture. Previous work has demonstrated that there are increases in grip force ahead of arm motion that contribute to object stability in the hand. Other studies have shown that early postural adjustments in the legs and trunk minimise the potential perturbing effects on body posture of rapid voluntary arm movement. This paper documents the concurrent evolution of grip force and postural adjustments in anticipation of dynamic and static loads. Subjects held a manipulandum in precision grasp between thumb and index finger and pulled or pushed either a dynamic or a fixed load horizontally towards or away from the body (the grasp axis was orthogonal to the line of the load force). A force plate measured ground reaction torques, and force transducers in the manipulandum measured the load (tangential) and grip (normal) forces acting on the thumb and finger. In all conditions, increases in grip force and ground reaction torque preceded any detectable rise in load force. Rates of change of grip force and ground reaction torque were correlated, even after partialling out a common dependence on load force rate. Moreover, grip force and ground reaction torque rates at the onset of load force were correlated. These results imply the operation of motor planning processes that include anticipation of the dynamic consequences of voluntary action.

Adult↗

PubMed1997

Effects of surface texture and grip force on the discrimination of hand-held loads.

In this paper, we report the results from two experiments in which subjects were required to discriminate horizontal load forces applied to a manipulandum held with a precision grip. The roughness (and hence friction) of the grip surfaces and required grip force were manipulated. In the first experiment, subjects were instructed to judge the load while maintaining hand position and not letting the manipulandum slip. It was found that performance was influenced by surface texture; a given load was judged to be greater when the surface texture was smooth than when it was rough. This result is consistent with a previous study based on lifting objects and indicates that the effect of surface texture applies to loads in general and not just to gravitational loads (i.e., weight). To test whether the load acting on a smooth object is judged to be greater because the grip force required to prevent it from slipping is larger, a second experiment was carried out. Subjects used a visual feedback display to maintain the same grip force for smooth and rough manipulandum surfaces. In this case, there was no effect of surface texture on load perception. These results provide evidence that perceived load depends on the grip force used to resist the load. The implications of these results in terms of central and peripheral factors underlying load discrimination are considered.

Adolescent↗

PubMed1996

Age differences in postural stability are increased by additional cognitive demands.

We report an investigation of postural stability in two groups of volunteers (mean ages of 57 and 77). Participants were required to stand on a force platform while performing five cognitive tasks: (1) random digit generation, (2) Brooks' spatial memory, (3) backward digit recall, (4) silently counting from 1-100, and (5) counting backward in threes (aloud). There was also a control condition in which there was no cognitive task. Postural stability was adversely affected by age in all conditions. Moreover, the difference between the two age groups was significantly greater when performing tasks 2 and 3, in comparison with the age difference in the control condition. Regression analyses revealed that the effect of age on postural stability while performing these particular tasks remained significant even after the following measures were included in the regression: postural stability in the control condition, cognitive performance, intelligence, and speed. We suggest that age differences in postural stability are increased by cognitive tasks requiring use of the visuo-spatial sketchpad component of working memory.

Age Factors↗

PubMed1995

The stability of precision grip forces during cyclic arm movements with a hand-held load.

In this paper we examine the coordination of grip force and load during brisk cyclic arm movements with a hand-held object under a range of conditions. We show that, regardless of the surface texture of the object or movement frequency, grip force is modulated in parallel with load. Thus, the tight coupling between grip force and load observed in short-duration tasks such as lifting or point-to-point movements is also seen in longer-duration cyclic movements. Moreover, the gain of the relation between grip force and load remains essentially constant over time. Across conditions, we find a dissociation between the gain of the relation between grip force and load and the grip force offset. With a more slippery surface texture both the gain and offset increase, whereas increases in frequency lead to an increase in the offset but a decrease in gain. This suggests that these two parameters are under independent high-level control. We also observe that when subjects were instructed to maintain a high-baseline grip force during the movement, grip force was still modulated with load even though an increase in grip was not necessary to prevent slip. This suggests that there is an obligatory coupling between grip force and load. This coupling might be subserved by low-level mechanisms not under high-level control.

Adult↗

PubMed1995

The coordination and consistency of rowers in a racing eight.

Recordings of force developed simultaneously by four rowers in a good club-standard eight over a 20-min training run were analysed in terms of mean and variability. The average force-time profiles showed small but distinctive differences between rowers that were maintained as force declined through the run. The variability was examined further in terms of time-series of features extracted from single force-time profiles, including the peak force, duration and interstroke interval. There were consistent positive correlations between rowers in interstroke intervals even after removal of low-periodicity components (trends) in the time-series.

Adult↗

PubMed1995

Effects of surface texture on weight perception when lifting objects with a precision grip.

In this paper, we show that, when lifting an object using a precision grip with the distal pads of the thumb and index finger at its sides, the perceived weight depends on the object's surface texture. The smoother the surface texture, the greater the perceived weight. We suggest that a smoother object is judged to be heavier because the grip force, normal to the surface, required to prevent it from slipping is greater. The possibility of there being an influence of surface texture per se is excluded by a second experiment that employed a variant of the precision grip in which the thumb supports the weight of the object from underneath. With the grip oriented in this way, there is no need to match grip force to surface texture and, under these conditions, there is no effect of surface texture on weight perception. In the first two experiments, the test and comparison weights were lifted successively by the same hand. In a third experiment, the effect of surface texture was replicated for sequential lifts made with separate hands. Thus, the effect is not restricted to comparisons made with the same hand.

Adolescent↗