Preparatory Postural Adjustments Research Articles

Evidence for constancy in the modularity of trunk muscle activity preceding reaching: implications for the role of preparatory postural activity.

Postural muscle activity precedes voluntary movements of the upper limbs. The traditional view of this activity is that it anticipates perturbations to balance caused by the movement of a limb. However, findings from reach-based paradigms have shown that postural adjustments can initiate center of mass displacement for mobility rather than minimize its displacement for stability. Within this context, altering reaching distance beyond the base of support would place increasing constraints on equilibrium during stance. If the underlying composition of anticipatory postural activity is linked to stability, coordination between muscles (i.e., motor modules) may evolve differently as equilibrium constraints increase. We analyzed the composition of motor modules in functional trunk muscles as participants performed multidirectional reaching movements to targets within and beyond the arm's length. Bilateral trunk and reaching arm muscle activity were recorded. Despite different trunk requirements necessary for successful movement, and the changing biomechanical (i.e., postural) constraints that accompany alterations in reach distance, nonnegative matrix factorization identified functional motor modules derived from preparatory trunk muscle activity that shared common features. Relative similarity in modular weightings (i.e., composition) and spatial activation profiles that reflect movement goals across tasks necessitating differing levels of trunk involvement provides evidence that preparatory postural adjustments are linked to the same task priorities (i.e., movement generation rather than stability).NEW & NOTEWORTHY Reaching within and beyond arm's length places different task constraints upon the required trunk motion necessary for successful movement execution. The identification of constant modular features, including functional muscle weightings and spatial tuning, lend support to the notion that preparatory postural adjustments of the trunk are tied to the same task priorities driving mobility, regardless of the future postural constraints.

Constancy of Preparatory Postural Adjustments for Reaching to Virtual Targets across Different Postural Configurations

Postural and movement components must be coordinated without significant disturbance to balance when reaching from a standing position. Traditional theories propose that muscle activity prior to movement onset create the mechanics to counteract the internal torques generated by the future limb movement, reducing possible instability via centre of mass (CoM) displacement. However, during goal-directed reach movements executed on a fixed base of support (BoS), preparatory postural adjustments (or pPAs) promote movement of the CoM within the BoS. Considering this dichotomy, the current study investigated if pPAs constitute part of a whole-body strategy that is tied to the efficient execution of movement, rather than the constraints of balance. We reasoned that if pPAs were tied primarily to balance control, they would modulate as a function of perceived instability. Alternatively, if tied to dynamics necessary for movement initiation, they would remain unchanged, with feedback-based changes being sufficient to retain balance following volitional arm movement. Participants executed beyond-arm reaching movements in four different postural configurations that altered the quality of the BoS. Quantification of these changes to stability did not drastically alter the tuning or timing of preparatory muscle activity despite modifications to arm and CoM trajectories necessary to complete the reaching movement. In contrast to traditional views, preparatory postural muscle activity is not always tuned for balance maintenance or even as a calculation of upcoming instability but may reflect a requirement of voluntary movement towards a pre-defined location.

\u2018Recoupling\u2019 the attentional and motor control of preparatory postural adjustments to overcome freezing of gait in Parkinson\u2019s

ObjectivesThis study examined if people with Parkinson’s and freezing of gait pathology (FoG) could be trained to increase preparatory weight-shift amplitude, and facilitate step initiation during FoG.MethodsThirty-five people with Parkinson’s and FoG attempted to initiate forward walking from a stationary position caused by a freeze (n = 17, FoG-F) or voluntarily stop (n = 18, FoG-NF) in a Baseline condition and two conditions where an increased weight-shift amplitude was trained via: (i) explicit verbal instruction, and (ii) implicit movement analogies.ResultsAt Baseline, weight-shift amplitudes were smaller during: (i) unsuccessful, compared to successful step initiations (FoG-F group), and (ii) successful step initiations in the FoG-F group compared to FoG-NF. Both Verbal and Analogy training resulted in significant increases in weight-shift amplitude in both groups, and a corresponding pronounced reduction in unsuccessful attempts to initiate stepping (FoG-F group).ConclusionsHypometric preparatory weight-shifting is associated with failure to initiate forward stepping in people with Parkinson’s and FoG. However, impaired weight-shift characteristics are modifiable through conscious strategies. This current study provides a novel and critical evaluation of preparatory weight-shift amplitudes during FoG events. The intervention described represents an attractive ‘rescue’ strategy and should be further scrutinised regarding limitations posed by physical and cognitive deficits.

Concussion is associated with altered preparatory postural adjustments during gait initiation

Gait initiation is a useful surrogate measure of supraspinal motor control mechanisms but has never been evaluated in a cohort following concussion. The aim of this study was to quantify the preparatory postural adjustments (PPAs) of gait initiation (GI) in fifteen concussion patients (4 females, 11 males) in comparison to a group of fifteen age- and sex-matched controls. All participants completed variants of the GI task where their dominant and non-dominant limbs as the ‘stepping’ and ‘support’ limbs. Task performance was quantified using the centre of pressure (COP) trajectory of each foot (computed from a force plate) and a surrogate of the centre of mass (COM) trajectory (estimated from an inertial measurement unit placed on the sacrum).Concussed patients exhibited decreased COP excursion on their dominant foot, both when it was the stepping limb (sagittal plane: 9.71mm [95% CI: 8.14–11.27mm] vs 14.9mm [95% CI: 12.31–17.49mm]; frontal plane: 36.95mm [95% CI: 30.87–43.03mm] vs 54.24mm [95% CI: 46.99–61.50mm]) and when it was the support limb (sagittal plane: 10.43mm [95% CI: 8.73–12.13mm] vs 18.13mm [95% CI: 14.92–21.35mm]; frontal plane: 66.51mm [95% CI: 60.45–72.57mm] vs 88.43mm [95% CI: 78.53–98.32mm]). This was reflected in the trajectory of the COM, wherein concussion patients exhibited lower posterior displacement (19.67mm [95% CI: 19.65mm–19.7mm]) compared with controls (23.62mm [95% CI: 23.6–23.64]). On this basis, we conclude that individuals with concussion display deficits during a GI task which are potentially indicative of supraspinal impairments in motor control.

Preparatory postural adjustments during gait initiation in healthy younger and older adults: Neurophysiological and biomechanical aspects

The most postural adjustments studies have focused on electromyographic and biomechanical events to identify the mechanisms involved in falling, whereas direct recording of central processor system could be an optimal approach to provide new insights into this field. Therefore, the purpose of this study was to determine neurophysiological and biomechanical aspects of the preparatory postural adjustments during gait initiation (GI) in healthy younger and older adults. 16 healthy younger and 15 healthy older adults participated in the study. Stimuli of warning and response were played with an inter-stimulus interval of 2s and subjects were instructed to begin forward stepping with the dominant limb in response to response stimulus. The contingent negative variation (CNV), onset time of electromyographic activity in leading limb muscles and center of pressure (COP) trajectory in the anticipatory phase of GI were measured. Results revealed the peak time of CNV and mean amplitude of Late CNV were earlier and smaller in older group, respectively. However, peak amplitude of CNV did not differ significantly between groups. Moreover, the older group exhibited a delayed onset activity of gluteus medius and tibialis anterior muscles as well as a slower velocity and lesser backward displacement of COP trajectories in comparison with the younger group. Additionally, there were strong relationships between neurophysiological and biomechanical parameters. Findings suggest the age-related changes in the brain activities and preparatory postural adjustments during GI which could be valuable in designing assessment and prevention approaches for falls in this group of people.

Emotional state affects gait initiation in individuals with Parkinson’s disease

The purpose of the present study was to determine the impact of manipulating emotional state on gait initiation in persons with Parkinson's disease (PD) and healthy older adults. Following the presentation of pictures that are known to elicit specific emotional responses, participants initiated gait and continued to walk for several steps at their normal pace. Reaction time, the displacement and velocity of the center of pressure (COP) trajectory during the preparatory postural adjustments, and length and velocity of the first two steps were measured. Analysis of the gait initiation measures revealed that exposure to (1) threatening pictures, relative to all other pictures, speeded the initiation of gait for PD patients and healthy older adults; (2) approach-oriented emotional pictures (erotic and happy people), relative to withdrawal-oriented pictures, facilitated the anticipatory postural adjustments of gait initiation for PD patients and healthy older adults, as evidenced by greater displacement and velocity of the COP movement; and (3) emotional pictures modulated gait initiation parameters in PD patients to the same degree as in healthy older adults. Collectively, these findings hold significant implications for understanding the circuitry underlying the manner by which emotions modulate movement and for the development of emotion-based interventions designed to maximize improvements in gait initiation for individuals with PD.

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.

Step initiation in Parkinson's disease: Influence of initial stance conditions

In this study, we investigated how the size of preparatory postural adjustments prior to step initiation, and step length and velocity depend on initial stance width in patients with Parkinson's disease (PD) both in the ON and OFF levodopa states and in healthy elderly subjects. Twenty-one subjects with idiopathic PD and 24 age-matched healthy control subjects took two steps starting with feet on a two-plate force-platform, from either narrow or wide stance width. We measured how the magnitude of anticipatory postural adjustments (APA) and step characteristics scaled with stance width. Results showed that preparation for step initiation from wide stance was associated with a larger lateral and backward center of pressure (CoP) displacement than from narrow stance. Velocity and length of the first step were also sensitive to initial stance conditions, probably in relation with the differences in the corresponding APA. On the contrary, the duration of APA was not significantly affected by initial stance width, but it was longer in PD compared to healthy subjects, and speeded up by levodopa. Although subjects with PD did scale up the size of their APA with stance width, they had much more difficulty initiating a step from a wide stance than from a narrow stance, as shown by the greater differences from control subjects in the magnitude of the APA. Our results support the hypothesis that PD subjects maintain a narrow stance as a compensation for their inability to sufficiently increase the size of their lateral APA to allow fast step initiation in wide stance.

Developmental changes in compensatory responses to unexpected resistance of leg lift during gait initiation.

The development of the ability to integrate postural adjustments into the gait initiation process was investigated in children, using both kinematic and electromyographic (EMG) analysis. Subjects included children of 1 year of age (1-4 months' walking experience), 2-3 years of age (9-17 months' walking experience), 4-5 years of age (3-4 years' walking experience), and adults. We perturbed the balance of the children during gait initiation to determine the point at which infants begin to develop and finally master the ability to respond to external threats to balance during the gait initiation process. A magnet attached to the force platform on which the child stood was activated and served to resist the child's gait initiation (metal plaques on the soles of the shoes were attracted by the magnet) and thus served as an external perturbation during the gait initiation process. Kinematic and EMG analysis indicated that, while the ability to use preparatory postural adjustments in the gait initiation process emerges early in development, the ability to react efficiently to perturbations during gait initiation does not develop until after about 4-5 years of age. Though even the youngest age groups showed some response to the perturbation, it was highly variable, indicating its primitive form. The main response to the perturbation was a slight decrease in latency and increase in amplitude in the muscles used for push-off for gait initiation. Interestingly there was a shift in response pattern at 4-5 years of age, in both kinematic and EMG patterns. The amplitudes of the lateral and anteroposterior trunk and stance leg oscillations were significantly increased. In addition, the muscle response amplitudes (hamstrings and second quadriceps burst) of the swing leg were significantly increased and delayed (hamstring and gastrocnemius), with coactivation of agonist and antagonist muscles at the knee and ankle joint, concomitant with an exaggerated foot height of the first step.

Influence of pallidal stimulation and levodopa on gait and preparatory postural adjustments in Parkinson's disease.

In order to assess the influence of the bilateral internal globus pallidus (GPi) stimulation on gait and postural instability in Parkinson's disease (PD), we compared gait kinematic parameters and preparatory postural adjustments before and 3 months after stimulation in off- and on-drug conditions for seven patients. Gait kinematic parameters and displacements of centre of pressure (CP) and shoulder computed before a lateral raising task of the leg, were recorded using optoelectric Vicon system. Levodopa (L-dopa) induced a clear benefit for gait velocity (related to an increase of stride length) and also an increase of swing phase duration. GPi stimulation had a limited effect, since the increase of gait velocity was induced by a concomitant increase of stride length and cadence corresponding to a compensatory mechanism. The benefit on swing phase duration was also moderate. Displacements of CP were improved mainly by L-dopa. GPi stimulation and L-dopa had the same beneficial effect on the speed at which the CP was transferred back towards the support side, the ankle velocity, the onset time for ankle displacement, and the decrease of shoulder amplitude towards the support side, which reflects a better postural adjustment phase. This study, based on an objective method, revealed that chronic bilateral GPi stimulation may improve gait and preparatory postural adjustments in severe PD patients with a more limited effect than L-dopa.

Preparatory Postural Adjustments in Parkinsonian Patients with Postural Instability

Postural instability is a common problem in patients with Parkinson's disease. This paper reports results of a study undertaken to investigate some of the possible mechanisms responsible for this instability. Preparatory postural adjustments associated with a lateral leg raising task were studied in five parkinsonian patients and four age-matched controls. Recordings included ground reaction forces, kinematics, and surface EMG activity from multiple leg muscles. In normal subjects there was a well-defined sequence of events preceding the onset of leg elevation, beginning with a transfer of centre of foot pressure (CP), initially toward the moving leg and then back to the support side, followed by displacement of the trunk toward the support side. In the more severely affected parkinsonian patients, the amplitude of the initial displacement of CP was markedly reduced. The interval between the earliest force changes and the onset of leg elevation was prolonged and the relative timing of the kenematic adjustments during this interval was disrupted. In addition the alternating burst and periods of inhibition observed in the EMG recordings from the normal subjects were replaced by continuous tonic EMG activity. These observations suggest that abnormalities in programming preparatory postural adjustments may contribute to postural instability in some patients with advanced Parkinson's disease.

Reaction times of movement preparation in patients with Parkinson's disease.

We studied the onset of preparatory postural responses and subsequent voluntary movements by measuring soleus muscle activities in the standing position in 20 patients with Parkinson's disease. We measured the postural response in preparing to rise on tiptoe as the onset of the premotion silent period (PMSP). Our patients showed no delay when compared with age-matched healthy controls, but did show a significant delay in the onset of voluntary movement. The elongated PMSP (increased duration of the preparatory postural adjustments) was related to the severity of bradykinesia. Results indicate that the conventional reaction time is increased in patients with Parkinson's disease, even though there is no delay of central processing for the preparation of voluntary movements, and that there is bradykinesia of involuntary postural movements.

Bromocriptine treatment of female infertility: report of 13 pregnancies.

Thirteen pregnancies occurred in 12 women who were treated with bromocriptine for infertility. Pretreatment prolactin levels were recorded in 11 patients and were normal in three. Five patients had suspected pituitary tumours, and they received irradiation to prevent swelling of the pituitary and the consequent visual field defects caused by the pressure of the swollen gland on the optic nerve. Ten of the 13 pregnancies have come to term, and all the babies were normal. When a patient with a pituitary tumour developed a visual field defect in the 38th week of pregnancy labour was induced and the defect disappeared after delivery. No multiple pregnancies occurred and there were no major complications.