Effects of postural instability on the coordination between posture and arm reaching

Reaching from standing requires adjustments of hand movement and posture, which are assured by redundant kinematic degrees of freedom. However, the increased demand for postural adjustments may interfere with the stability of reaching. The objective of this study was to investigate the effect of postural instability on the use of kinematic redundancy to stabilize the finger and center-of-mass trajectories during reaching from standing in healthy adults. Sixteen healthy young adults performed reaching movements from standing with and without postural instability induced by small base-of-support. The three-dimensional positions of 48 markers were recorded at 100 Hz. The uncontrolled manifold (UCM) analysis was performed separately with the finger and center-of-mass positions being the performance variables, and joint angles being the elemental variables. ΔV, the normalized difference between the variance in joint angle that does not affect task performance (V_UCM) and the variance that does affect task performance (V_ORT), was calculated separately for finger (ΔV_EP) and center-of-mass (ΔV_COM) positions, and was compared between stable and unstable base-of-support conditions. ΔV_EP decreased after movement onset and reached its minimum value at around 30–50% of the normalized movement time, and increased until movement offset, while ΔV_COM remained stable. At 60%–100% normalized movement time, ΔV_EP was significantly reduced in the unstable base-of-support, compared to the stable base-of-support condition. ΔV_COM remained similar between the two conditions. At movement offset, ΔV_EP was significantly reduced in the unstable base-of-support, compared to the stable base-of-support condition, and was associated with a substantial increase in V_ORT. Postural instability might reduce the ability to use kinematic redundancy to stabilize the reaching movement. The central nervous system may prioritize the maintenance of postural stability over focal movement when postural stability is challenged.     

Anticipatory control of postural and task muscles during rapid arm flexion

A multicomponent pattern of premovement (anticipatory) activity in both task and postural muscles was examined for a unilateral rapid arm flexion movement (Belen'kii, Gurflnkel, & Pal'tsev, 1967) performed under simple visual reaction-time conditions. Subjects performed 30 right- and 30 left-side responses on each of four consecutive days. The anticipatory postural muscle activity may be considered a valid component of voluntary unilateral arm flexion. Invariances and variability in the spatial and temporal characteristics of the neuromuscular pattern are described in relationship to behavioral and motor control models of response organization. It is suggested that some temporal patterning among response components may not be preprogrammed centrally.     

Kinematic information feedback for learning a rapid arm movement

The experiments reported examine the notion that knowledge of results (KR) about the outcome of a response does not provide the necessary information for optimizing performance in many skilled activities. The effect of traditional KR was contrasted with various kinematic feedback parameters in the acquisition of a single degree of freedom response requiring the minimization of movement time. Experiment 1 showed that the presentation of discrete kinematic information feedback (peak accelaration, time to peak accelaration, and velocity at the target location) did not facilitate performance over movement-time KR. Experiment 2 revealed that presentation of a computer generated velocity-time representation of the movement as terminal information feedback improved performance over movement-time KR. This facilitation occured even without knowledge of the kinematics for optimal performance. The findings suggest that the task criterion specifies the appropriate information feedback for skill learning in that the information feedback must match the constraints imposed upon response output.     

Effects of response type on coordinated responses during arm movement

This paper reports some experimental results on the coordination of finger and vocal responses with passing through a target position in multjjoint arm movement. In Experiment 1, we found that the difference in the timing of finger and vocal responses cannot be attributed entirely to efferent or representational effects. Instead, it appears to reflect the extent to which information about the internal stimuli generated by the arm movement are available to the centers controlling these different responses. That is, it is a compatibility effect. In Experiment 2, the case in which a finger response is made on the same side of the body as the moving arm was compared with the case in which it is made with the contralateral hand, which remains static. The interaction effect observed suggests that the path-ways subserving coordinated responses are informationally encapsulated, so that information about arm movement is not shared between the neural centers controlling different coordinated responses.     

Premovement facilitation of corticospinal excitability before simple and sequential movement

The purpose of this study was to investigate whether premovement facilitation of corticospinal excitability before sequential movement was different from that before simple movement. Each of 7 participants who performed choice reaction tasks with the right hand pressed a force transducer with the index finger in response to a start cue or pressed the transducers sequentially with the index finger, little finger, thumb, little finger, and index finger. Transcranial magnetic stimulation was delivered to the left motor cortex before the electromyographic burst in the first dorsal interosseous muscle and motor evoked potentials were recorded from the first dorsal interosseous muscle. The amplitude of the motor-evoked potential increased as its onset got closer to the onset of the electromyographic burst. The increase before the sequential movement was larger and began earlier than that before the simple movement. These findings indicate that premovement facilitation of corticospinal excitability is different in magnitude and timing between sequential and simple movements.     

Effects of response type on coordinated responses during arm movement.

This paper reports some experimental results on the coordination of finger and vocal responses with passing through a target position in multijoint arm movement. In Experiment 1, we found that the difference in the timing of finger and vocal responses cannot be attributed entirely to efferent or representational effects. Instead, it appears to reflect the extent to which information about the internal stimuli generated by the arm movement are available to the centers controlling these different responses. That is, it is a compatibility effect. In Experiment 2, the case in which a finger response is made on the same side of the body as the moving arm was compared with the case in which it is made with the contralateral hand, which remains static. The interaction effect observed suggests that the pathways subserving coordinated responses are informationally encapsulated, so that information about arm movement is not shared between the neural centers controlling different coordinated responses.     

The effects of initial movement dynamics on human responses to postural perturbations

Falls are a major cause of injury, and often occur while turning, reaching, or bending. Yet, we have little understanding of how an ongoing feet-in place activity at the onset of imbalance, and its associated cognitive and biomechanical demands, influence our ability to recover balance. In the current study, we used an ankle-rocking paradigm to determine how the nature of the baseline task influences the balance recovery response to a backward support surface translation. Fourteen participants were instructed to “recover balance without stepping” and were perturbed at vertical while standing quietly (“S”), while ankle rocking and moving forward (“A_f”), or while ankle rocking and moving backward (“A_b”). The results showed that changes in rocking velocity at the time of the perturbation elicited changes in the incidence of stepping, magnitude of trunk angular displacements (p < .01), and the onset latencies of distal muscles (gastrocnemius and soleus, both p < .01) used to recover balance. In addition, plots of onset latencies across all muscles showed that onset latencies appeared to occur earlier in many muscles when participants held a static position compared to when they performed a dynamic task at the onset of the perturbation. The results suggest that muscle activities used to recover balance are tailored to the nature of the perturbation and the ongoing task, and that onset latencies are later when participants are performing a dynamic as opposed to static task at the time of a perturbation. These findings support previous research suggesting that automatic postural responses are highly adaptable to environmental, situational, and task demands.     

Changes in the relationship between movement velocity and movement distance in primary focal hand dystonia

The authors examined the relationship between movement velocity and distance and the associated muscle activation patterns in 18 individuals with focal hand dystonia (FHD) compared with a control group of 18 individuals with no known neuromuscular condition. Participants performed targeted voluntary wrist and elbow flexion movements as fast as possible across 5 movement distances. Individuals with FHD were slower than controls across all distances, and this difference was accentuated for longer movements. Muscle activation patterns were triphasic in the majority of individuals with FHD, and muscle activation scaled with distance in a similar manner to controls. Cocontraction did not explain movement slowing in individuals with dystonia, but there was a trend toward underactivation of the 1st agonist burst in the dystonic group. The authors concluded that slowness is a consistent feature of voluntary movement in FHD and is present even in the absence of dystonic posturing. Underactivation of the 1st agonist burst appears to be the most likely reason to explain slowing.     

The influence of previous movement and posture on subsequent posture

An experiment with 21 subjects has confirmed the findings of other investigators that previous posture affects subsequent posture—the phenomenon of postural persistence. There seems also to be a tendency for the arm to be judged horizontal when in fact above the horizon, no matter which posture has been previously adopted—a “constant upwards effect.” It has also been found that the direction of previous movement affects subsequent posture. After movement, an overshooting effect adds to a “constant upwards effect.”

Various explanations of the phenomenon are discussed, and persistence is considered as an anomaly of postural recognition.     

Standard posture,postural mirroring and client-perceived rapport

The relationship between client-perceived rapport (as measured from a standardized client) and physical mirroring and the standard counsellor posture was investigated with interviews performed by 59 post-graduate students in counselling psychology. Videotaped recordings were used to code counsellor posture in the categories of: total postural mirroring, mirroring of the hands and arms, mirroring of the legs, mirroring of the torso, and the frequency of the standard counsellor posture across each minute of the interviews. These minutes were classified as 'high' in rapport or 'low' in rapport as measured by the standardized client. Results indicated that there was significantly more postural mirroring of the torso during high versus low minutes, but that the counsellor standard posture occurred significantly more frequently during low rapport minutes than in high rapport minutes. However, when examined over the entire length of the interviews, these data were able to be understood in terms of counsellor 'flexibility' of response rather than simply whether these postural behaviours were present or not. Implications for counsellor training are discussed.     

Effects of anticipation certainty on preparatory brain activity and anticipatory postural adjustments associated with voluntary unilateral arm movement while standing

We examined the effects of anticipation certainty concerning which voluntary movement is required in response to a stimulus while standing on preparatory brain activity and anticipatory postural adjustments (APAs). Ten right-handed adults abducted their left or right arm rapidly in response to a visual imperative stimulus, based on the type of stimulus. A warning cue, which did or did not contain information about the side of arm abduction, was presented 2000ms before the imperative stimulus. Preparatory brain activity before arm abduction was quantified by the mean amplitude of the contingent negative variation 100ms before the imperative stimulus (late CNV amplitude). Compared with the low anticipation condition, in the high anticipation condition the following results were obtained only in the case of right arm abduction: (1) larger late CNV amplitude, (2) earlier postural muscle activities with respect to the focal muscle of arm abduction, and (3) smaller peak displacement of center of pressure during the abduction. These findings suggest that high anticipation of voluntary movement of dominant arm to a stimulus while standing influences preparatory brain activity before the movement, resulting in earlier APAs and thus smaller disturbance of postural equilibrium during the movement.     

Attentional focus on supra-postural tasks affects postural control

We examined whether the attentional focus adopted on a supra-postural task has an influence on postural control. Similar to Riley, Stoffregen, Grocki, and Turvey (Human Movement Science 18 (1999) 795), participants were instructed to stand still while lightly touching a loosely hanging sheet with their fingertips. However, instructions varied slightly under two conditions: Participants were either asked to minimize movements of the finger (internal focus) or to minimize movements of the sheet (external focus). In contrast to Riley et al.'s findings, both touch conditions resulted in increased postural sway, compared to a baseline condition (no touch). However, in line with previous findings (e.g., Wulf, McNevin, & Shea, Quarterly Journal of Experimental Psychology 54A (2001) 1143), frequency of responding (fast Fourier transformation) was greater under the external focus condition, compared to both internal focus and baseline conditions. The findings indicate improved static balance responses under external focus conditions and compromised static balance response under internal focus conditions.     

An experimentally confirmed statistical model on arm movement

The purpose of this research work was to develop a methodology to model arm movement in normal subjects and neurologically impaired individuals through the application of a statistical modelling method. Thirteen subjects with Parkinson's disease and 29 normal controls were recruited to participate in an arm motor task. An infrared optoelectronic kinematic movement analysis system was employed to record arm movement at 50 times per second. This study identified the modified extended Freundlich model as one that could be used to describe this task. Results showed that this model fit the data well and that it has a good correspondence between the observed and the predicted data. However, verification of the model showed that the residuals contained a sizeable autocorrelation factor. The Cochrane and Orcutt method was applied to remove this factor, which improved the fit of the model. Results showed that Parkinson's disease subjects had a higher autocorrelation coefficient than the normal subjects for this task. A positive correlation (r(s) = 0.72, p < 0.001) was found between the Langton-Hewer stage and the autocorrelation coefficient of PD subjects. This finding suggests that if autocorrelation is positively correlated with disease progression, clinicians in their clinical practice might use the autocorrelation value as a useful indicator to quantify the progression of a subjects' disease. Significant differences in model parameters were seen between normal and Parkinson's disease subjects. The use of such a model to represent and quantify movement patterns provides an important base for future study.     

How attentional focus on body sway affects postural control during quiet standing

The purpose of this study was to investigate how attentional focus on body sway affects postural control during quiet standing. To address this issue, sixteen young healthy adults were asked to stand upright as immobile as possible on a force platform in both Control and Attention conditions. In the latter condition, participants were instructed to deliberately focus their attention on their body sways and to increase their active intervention into postural control. The critical analysis was focused on elementary motions computed from the centre of pressure (CoP) trajectories: (1) the vertical projection of the centre of gravity (CoG_v) and (2) the difference between CoP and CoG_v (CoP–CoG_v). The former is recognised as an index of performance in this postural task, whilst the latter constitutes a fair expression of the ankle joint stiffness and is linked to the level of neuromuscular activity of the lower limb muscles required for controlling posture. A frequency-domain analysis showed increased amplitudes and frequencies of CoP–CoG_v motions in the Attention relative to the Control condition, whereas non-significant changes were observed for the CoG_v motions. Altogether, the present findings suggest that attentional focus on body sway, induced by the instructions, promoted the use of less automatic control process and hampered the efficiency for controlling posture during quiet standing.     

Differentiated ratings of perceived exertion and physiological responses during aerobic dance steps by impact/type of arm movement

Overall ratings of perceived exertion, i.e., undifferentiated RPE, are often used as indicators of exercise intensity during walking, jogging, and cycling; however, conflicting results concerning RPE during aerobic dance exercise have been reported, and the use of differentiated RPE, i.e., local RPE and central RPE, has not been investigated. The purposes of this study were to assess local, central, and over-all RPE, and physiological responses [heart rate (HR); % HRmax; absolute and relative VO2;% VO2 max, ventilation (VE), ventilatory equivalent (VE.VO2(-1); and oxygen pulse] during aerobic dance exercise varied by Arm Movement (Static Arm vs Dynamic Arm) and Impact (High vs Low). Trained women (N = 25; max VO2 = 50.4 +/- 7.5 ml.kg-1.min.-1) completed four aerobic dance steps. No RPE were significantly correlated with heart rate or VO2; however, for all steps all RPE were significantly (r = .40-.62) correlated with VE.VO2(-1) or VE. No interactions were present for RPE or physiological variables, and main effects were noted for Impact and Arm Movement. All RPE were greater for High Impact and for Static Arm Movement. Because VE and VE.VO2(-1) were correlated with Overall RPE for all steps, this may suggest that participants "attended to" perceived changes in respiratory phenomena during aerobic dance exercise. It appears that during combined arm-and-leg aerobic dance exercise the use of Overall RPE is sufficient to assess perceptual sensations associated with the intensity of the exercise. Changes in Overall RPE were proportionate to objective measures of exercise intensity, i.e., HR and VO2; however, it is recommended that both HR and Overall RPE be used to assess fully a participant's objective and subjective responses during aerobic dance exercise.     

Processes of anticipatory postural adjustment and step movement of gait initiation

The purpose of this study was to elucidate whether the anticipatory postural adjustment (APA) and focal step movement of gait initiation are produced as a single process or different processes and whether the APA receives an inhibitory drive from the ongoing stop process of gait initiation. Healthy humans initiated gait in response to a first visual cue that instructed the initial swing leg. In some trials, a switch or stop cue was also provided after the first cue. When the stop cue was provided, participants withheld gait initiation. When the switch cue was provided, participants immediately switched the initial swing leg. In both the stop and switch tasks, the APA in response to the first cue, represented by the S1 period of the displacement of the center of pressure, appeared in more than half of the trials in which the withholding of gait initiation or switching of the initial swing leg was successfully completed. These findings indicate that the APA and focal step movement of gait initiation are produced as a dual process. In trials in which the APA in response to the first cue appeared, the amplitude and duration of the APA were decreased when the participants switched the initial swing leg or withheld gait initiation. This finding indicates that the ongoing stop process of gait initiation produces an inhibitory drive over the APA. The decreases in the amplitude and duration of the APA during the switching of the initial swing leg were similar to those during the withholding of gait initiation; moreover, the decreases during the switching of the initial swing leg were positively correlated with the decreases during the withholding of gait initiation. Thus, the stop processes during switching the initial swing leg and withholding gait initiation likely share a common inhibitory mechanism over the APA.     

The effect of Velten's mood-induction procedure for depression on hand movement and head-down posture

The purpose of the present study was to extend the behavioral criteria of Velten's mood-induction procedure for depression. Nonverbal behaviors (hand movements and head-down position) associated with clinical depression were measured both before and after a subject read either negative self-referential statements (N=14) or neutral statements (N=12). The measures were unobtrusively obtained by video recordings of the subject's behavior during a standardized interview with an experimenter. The reading of negative self-referential statements led to a significant decrease in the use of hand illustrators and a marginal increase in the use of head-down postures. No significant changes occurred in the control group. The results support the notion that Velten's procedure for depression does alter behavior in a manner similar to the clinical state of depression.This study was conducted as a senior research project at Franklin & Marshall College by the second author under the supervision of the first author.