Vennila Krishnan

Anticipatory postural adjustments in individuals with multiple sclerosis

Individuals with multiple sclerosis (MS) frequently exhibit difficulties in balance maintenance. It is known that anticipatory postural adjustments (APAs) play an important role in postural control. However, no information exists on how people living with MS utilize APAs for control of posture. A group of individuals with MS and a group of healthy control subjects performed rapid arm flexion and extension movements while standing on a force platform. Electromyographic (EMG) activity of six trunk and leg muscles and displacement of center of pressure (COP) were recorded and quantified within the time intervals typical of APAs. Individuals with MS demonstrated diminished ability to produce directional specific patterns of anticipatory EMGs as compared to control subjects. In addition, individuals with MS demonstrated smaller magnitudes of anticipatory muscle activation. This was associated with larger displacements of the COP during the balance restoration phase. These results suggest the importance of anticipatory postural control in maintenance of vertical posture in individuals with MS. The outcome of the study could be used while developing rehabilitation strategies focused on balance restoration in individuals with MS.

Hand Function in Multiple Sclerosis: Force Coordination in Manipulation Tasks

OBJECTIVE To evaluate the methodology for exploring the specific aspects of functional impairment in multiple sclerosis (MS) through the pattern of forces exerted in various manipulation tasks. METHODS Twelve mildly involved MS patients (EDSS 2.5-5.5) and 12 healthy controls performed various static and dynamic manipulation tasks with an instrumented device that recorded the grip (G; normal to the digit device contact area) and load force (L; tangential force that causes lifting). RESULTS MS patients consistently displayed lower indices of task performance (as assessed by the ability to produce the required L profiles) and force coordination (as assessed by G/L ratio, coupling of G and L, and G modulation) than the healthy controls across all tested tasks. CONCLUSIONS The applied methodology could be sensitive enough to detect the hand dysfunction in mildly involved individuals with MS. Particularly recommended for future evaluations of the impairment of hand function could be a simple lifting task and the static task of tracing a gradually changing L, as well as the variables depicting both the task performance and G/L ratio. SIGNIFICANCE The applied methodology could be developed into a standard clinical test for the assessment of hand function in MS and, possibly, in other neurological diseases.

Postural control in response to an external perturbation: effect of altered proprioceptive information

The purpose of the study was to investigate the role of altered proprioception on anticipatory (APAs) and compensatory (CPAs) postural adjustments and their interaction. Nine healthy adults were exposed to external perturbations induced at the shoulder level while standing with intact or altered proprioception induced by bilateral Achilles tendon vibration. Visual information was altered (eyes open or closed) in both the conditions. Electrical activity of eight trunk and leg muscles and center of pressure (COP) displacements were recorded and quantified within the time intervals typical for APAs and CPAs. The results showed that when proprioceptive information was altered in eyes-open conditions, anticipatory muscle activity was delayed. Moreover, altered proprioceptive information resulted in smaller magnitudes of compensatory muscle activity as well as smaller COP displacements after the perturbation in both eyes-open and eyes-closed conditions. The outcome of the study provides information on the interaction between APAs and CPAs in the presence of altered proprioception.

Early and Late Components of Feed-forward Postural Adjustments to Predictable Perturbations

OBJECTIVES The purpose was to investigate two types of feed-forward postural adjustments associated with preparation to predictable external perturbations. METHODS Nine subjects stood on a wedge, toes-up or toes-down while a pendulum impacted their shoulders. EMGs of leg and trunk muscles were analyzed within the framework of the uncontrolled manifold hypothesis. RESULTS Early postural adjustments (EPAs) were seen 400-500 ms and anticipatory postural adjustments (APAs), 100-150 ms prior to the impact. EPAs and APAs were also seen in the time profiles of muscle modes representing muscle groups with linear scaling of the activation levels. Center of pressure shifts were stabilized by co-varied adjustments in muscle mode magnitudes across trials. The index of these multi-muscle synergies showed two drops (anticipatory synergy adjustments, ASAs), prior to EPA and APA in each subject. The findings were consistent between the two conditions. CONCLUSIONS The results show that feed-forward postural adjustments represent a sequence of two phenomena, EPAs and APAs. Each of those is preceded by ASAs that reduce stability of a variable that is to be adjusted during the EPAs and APAs. The findings fit a hierarchical scheme with synergic few-to-many mappings at each level of the hierarchy based on the referent body configuration hypothesis. SIGNIFICANCE The results show the complexity of the postural preparation to action. Potentially, they have implications for the current strategies of rehabilitation of patients with neuro-motor disorders characterized by impaired postural control.

The effect of decreased visual acuity on control of posture.

OBJECTIVES The goal of this study was to investigate the effect of visual acuity on the anticipatory (APAs) and compensatory (CPAs) components of postural control. METHODS Ten individuals participated in the experiments involving perturbations induced by a pendulum while their visual acuity was altered. The different visual acuity conditions were no glasses, blurred vision induced by wearing glasses with positive or negative lenses, and no vision. EMG activity of trunk and leg muscles and ground reaction forces were recorded during the typical anticipatory and compensatory periods. RESULTS In the no vision condition the subjects did not generate APAs, which resulted in the largest displacements of the center of pressure (COP) after the perturbation (p<0.01). In all other visual conditions APAs were present showing a distal to proximal order of muscle activation. The subjects wearing positive glasses showed earlier and larger anticipatory EMGs than subjects wearing negative glasses or no glasses at all. CONCLUSIONS The study outcome revealed that changes in visual acuity induced by wearing differently powered eye glasses alter the generation APAs and as a consequence, affect the compensatory components of postural control. SIGNIFICANCE The observed changes in APAs and CPAs in conditions with blurred vision induced by positive and negative glasses suggest the importance of using glasses with an appropriate power. This outcome should be taken into consideration in balance rehabilitation of individuals wearing glasses.

Feedforward postural control in individuals with multiple sclerosis during load release

The aim of the present study was to investigate the organization of anticipatory postural adjustments (APAs) in individuals with multiple sclerosis (MS) during self-initiated perturbation in the sagittal plane. Eleven individuals with MS and eleven age-and-gender matched healthy controls were asked to hold a 2.27 kg load in the extended arms and release it using fast arm abduction movements. Electrical activity of six leg and trunk muscles as well as displacements of the center of pressure (COP) were recorded. The results indicate that individuals with MS (1) demonstrate a reduced magnitude of APAs, (2) delayed latency of APAs and (3) smaller anticipatory COP displacement as compared to healthy control subjects. Moreover, in spite of individuals with MS being mildly affected, their balance capacity was significantly diminished. Thus, the outcome of this study demonstrates the underlying impairment in anticipatory postural control of individuals with MS and provides a background for development of rehabilitation strategies focused on balance restoration in this population.

The effects of age on stabilization of the mediolateral trajectory of the swing foot

To ensure stability during gait, mediolateral placement of the swinging foot must be actively regulated. Logically this occurs through end-point control of the swing limb trajectory, the precision of which is quantified as step-width variability (SWV). Increased SWV with age may reflect reduced precision of this control, but cannot describe if, and how, age-related changes in lower limb kinematic synergies account for reduced precision. We analyzed joint configuration variance across steps within the uncontrolled manifold (UCM) hypothesis, which assumes that redundant sets of elemental variables are organized by the central nervous system to stabilize important performance variables. We tested whether: (1) regardless of age, the swing limb trajectory would be stabilized by a kinematic synergy of the lower limbs, and (2) the strength of the synergy would be weaker in older adults. Ten younger and ten older adults (65+ years) walked on a laboratory walkway at their preferred speed while kinematic data were collected. UCM analysis of segmental configuration variance was performed with respect to the mediolateral trajectory of the swing-limb ankle joint center. Throughout most of swing, the trajectory was stabilized by a kinematic synergy. Despite the greater segmental configuration variance of older adults, the strength of the synergy was not significantly different between groups. Moreover, the synergy index became negative during terminal swing and was not significantly correlated with SWV. Accordingly, co-variation among individual segmental trajectories is more important for stabilization of the swing trajectory during mid-swing, and, throughout swing, aging does not appear to affect this stabilization.

ELABORATE FORCE COORDINATION OF PRECISION GRIP COULD BE GENERALIZED TO BIMANUAL GRASPING TECHNIQUES

Exceptional coordination of grip (G; the normal force that prevents slippage of the grasped object) and load force (L; the tangential force originating from the objects weight and inertia) has been interpreted as a part of evidence that both the anatomy and neural control of human hands have been predominantly designed for manipulation tasks. In the present study, we tested the hypothesis that the precision grasp (uses only the tips of fingers and the thumb of one hand) provides better indices of G and L coordination in static manipulation tasks than two bimanual grasps (palm-palm and fingers-thumb; both using opposing segments of two hands). However, in addition to a subtle difference in relative timing of G and L between the precision and two bimanual grasps, we only found that the fingers-thumb grasp is characterized with higher G/L ratio and somewhat higher modulation of G than not only the precision, but also the bimanual palm-palm grasp. However, all remaining data including the correlation coefficients between G and L demonstrated no difference among three evaluated grasping techniques. Therefore, we concluded that the elaborate G and L coordination associated with uni-manual grasps could be partly generalized to a variety of manipulation tasks including those based on bimanual grasping techniques. Taking into account the importance of manipulation tasks in both everyday life and clinical evaluation, future studies should extend the present research to both other grasping techniques and dynamic manipulation conditions.

Force coordination in static manipulation: discerning the contribution of muscle synergies and cutaneous afferents.

Both an elaborate coordination of the hand grip force (G; normal component of force acting at the digits-object contact area) and load force (L; tangential component), and the role of cutaneous afferents in G-L coordination have been well documented in a variety of manipulation tasks. However, our recent studies revealed that G-L coordination deteriorates when L consecutively changes direction (bidirectional tasks; e.g., when vigorously shaking objects or using tools). The aim of the study was to distinguish between the possible role of the synergy of hand grip and arm muscles (exerting G and L, respectively) and the role of cutaneous afferent input in the observed phenomenon. Subjects (N=14) exerted sinusoidal L pattern in vertical direction against an externally fixed device in trials that gradually changed from uni- to fully bidirectional. In addition, a manipulation of an external arm support decoupled L measured by the device (and, therefore, recorded by the cutaneous receptors) from the action of arm muscles exerting L. The results revealed that switching from uni- to bidirectional tasks, no matter how low and brief L exertion was in the opposite direction, was associated with an abrupt decrease in G-L coordination. This coordination remained unaffected by the manipulation of external support. The first result corroborates our previous conclusion that the force coordination in uni- and bidirectional manipulation tasks could be based on partly different neural control mechanisms. However, the second finding suggests that the studied control mechanisms could depend more on the cutaneous afferent input, rather than on the synergy of the muscles exerting G and L.

Force Coordination in Object Manipulation

Force Coordination in Object Manipulation Purpose: The purpose of this review is to present our recent findings related to the studies of hand function based on the coordination of forces exerted against hand-held objects. Basic procedures: A novel device has been developed for recording grip (GF; acting perpendicularly at the hand-object contact) and load force (LF; acting tangentially) during uni- and bimanual manipulation tasks performed under either static or dynamic conditions. Both healthy participants and neurological patients were tested. The outcome measures were obtained from the task performance (i.e., the ability to exert accurate LF profiles), GF-LF coordination and GF modulation. Main findings: The method applied proved to not only to be both reliable and valid, but also sufficient to detect differences between the dominant and non-dominant hand, as well as between healthy participants and mildly involved neurological patients. Marked differences in most of the depended variables were also detected between unidirectional and bi-direction tasks (i.e., in the tasks where LF acts in one and in two alternating directions). The later finding could not be based neural mechanisms known for their role in manipulative actions, such as on employing ad hoc muscle synergies or on the afferent activity of skin mechanoreceptors. Conclusions: The employed methodological approach can be applied not only to explore various manipulation activities, but also to serve as a basis for future development of specific clinical tests for populations that demonstrate impaired hand function.