Paulo Barbosa de Freitas

EFFECTS OF JOINT IMMOBILIZATION ON STANDING BALANCE

We investigated the effect of joint immobilization on the postural sway during quiet standing. We hypothesized that the center of pressure (COP), rambling, and trembling trajectories would be affected by joint immobilization. Ten young adults stood on a force plate during 60 s without and with immobilized joints (only knees constrained, CK; knees and hips, CH; and knees, hips, and trunk, CT), with their eyes open (OE) or closed (CE). The root mean square deviation (RMS, the standard deviation from the mean) and mean speed of COP, rambling, and trembling trajectories in the anterior-posterior and medial-lateral directions were analyzed. Similar effects of vision were observed for both directions: larger amplitudes for all variables were observed in the CE condition. In the anterior-posterior direction, postural sway increased only when the knees, hips, and trunk were immobilized. For the medial-lateral direction, the RMS and the mean speed of the COP, rambling, and trembling displacements decreased after immobilization of knees and hips and knees, hips, and trunk. These findings indicate that the single inverted pendulum model is unable to completely explain the processes involved in the control of the quiet upright stance in the anterior-posterior and medial-lateral directions.

Postural reactions following forward platform perturbation in young, middle-age, and old adults

The aim of the study was to examine how individuals of different ages react to forward balance perturbations. Thirty-six volunteers, divided into four groups [young (YA), middle-age (MA(40) and MA(50)), and old (OA) adults], stood on a platform that was either kept stationary, moved backward, or moved forward. EMG onset, EMG time-to-peak, iEMG, and agonist-antagonist co-activation, as well as cumulative angular excursion, maximum center of mass (CM) backward displacement, and CM time-to-reversal were assessed after forward translations. Postural synergies were assessed using principal component analysis (PCA). The results showed that OA activated their muscles later than YA [TA=25 ms, RF=17 ms] and OA and MA(50) reached the peak of activation later than YA [MA(50):TA=23 ms, RF=32 ms, OA:TA=28 ms, RF=21 ms]. Moreover, OA kept a higher level of activation longer than all younger groups. No differences among groups were observed in co-activation, kinematic, and PCA variables. We conclude that changes in temporal EMG patterns can be seen in the fifth decade. However, such changes have no effect on the CM horizontal displacement and CM time-to-reversal after perturbation, which cannot be justified by the use of different postural synergies, suggesting that temporal aspects of muscle activation could play a minor role in controlling excessive CM displacements after perturbations.

Force coordination in static manipulation tasks performed using standard and non-standard grasping techniques

We evaluated coordination of the hand grip force (GF; normal component of the force acting at the hand–object contact area) and load force (LF; the tangential component) in a variety of grasping techniques and two LF directions. Thirteen participants exerted a continuous sinusoidal LF pattern against externally fixed handles applying both standard (i.e., using either the tips of the digits or the palms; the precision and palm grasps, respectively) and non-standard grasping techniques (using wrists and the dorsal finger areas; the wrist and fist grasp). We hypothesized (1) that the non-standard grasping techniques would provide deteriorated indices of force coordination when compared with the standard ones, and (2) that the nervous system would be able to adjust GF to the differences in friction coefficients of various skin areas used for grasping. However, most of the indices of force coordination remained similar across the tested grasping techniques, while the GF adjustments for the differences in friction coefficients (highest in the palm and the lowest in the fist and wrist grasp) provided inconclusive results. As hypothesized, GF relative to the skin friction was lowest in the precision grasp, but highest in the palm grasp. Therefore, we conclude that (1) the elaborate coordination of GF and LF consistently seen across the standard grasping techniques could be generalized to the non-standard ones, while (2) the ability to adjust GF using the same grasping technique to the differences in friction of various objects cannot be fully generalized to the GF adjustment when different grasps (i.e., hand segments) are used to manipulate the same object. Due to the importance of the studied phenomena for understanding both the functional and neural control aspects of manipulation, future studies should extend the current research to the transient and dynamic tasks, as well as to the general role of friction in our mechanical interactions with the environment.

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.

Effects of varying the load force range and frequency on force coordination in static manipulation

A frequency associated deterioration of the hand grip (GF; normal component of force acting at the digits-object contact area) and load force (LF; the tangential component) coordination has been demonstrated in a variety of repetitive manipulation tasks. However, it remains unknown whether the effect originates from the task frequency per se, or from the rate of LF change (a prediction that could be derived from the minimum jerk hypothesis) which increases with both LF frequency and LF range. The aim of the study was to discern between the effects of LF frequency and LF range (which affects only the rate of LF) in static manipulation tasks. Subjects (N=15) exerted a sinusoidal LF pattern against an externally fixed instrumented handle at 5 different LF frequencies (0.67-3.33Hz) and ranges (6-30N). The results revealed weak and mainly non-significant effects of both LF range and frequency on GF scaling (GF/LF ratio). However, both GF-LF coupling (assessed by correlation coefficients) and GF modulation (change in GF relative to LF variation) demonstrated a prominent decrease associated with LF frequency, but not with LF range. The observed findings were interpreted by switching between hypothetical synergies of GF and LF producing muscles separately employed in the phases of the increasing and decreasing forces. From the practical aspect, however, the results suggest that the frequency, but not the LF range should be taken into account when designing rhythmic manipulation tasks, developing standard tests of hand function based on GF-LF coordination, or comparing the data from different studies.

Grip force adaptation in manipulation activities performed under different coating and grasping conditions

The aim of the study was to evaluate grip force (GF; normal component of hand-object interaction) adaptation across different manipulation conditions. We hypothesized (1) that the absolute safety margin (the difference between the exerted GF and the minimum GF that prevents slippage; absolute SM), rather than the relative SM (the same difference relative to the minimum GF required), could be an invariant feature of manipulation, as well as (2) that the SM would be higher in static than in dynamic tasks. Fourteen participants performed the free holding and the static holding tasks that required a same pulling force. Each task was performed using a variety of grasps and two different object coatings that both provided different frictions acting between the hand and the hand-held object. Both tasks revealed an increase in the relative SM associated with an increase in friction, while the absolute SM either remained unchanged (free holding) or suggested a moderate negative relationship (static holding task). Both relative and absolute SM were also higher in the free holding than in the static holding. The later result could be a consequence of the task mechanical conditions (i.e., dynamic vs. static), rather than of the difference in neural control mechanisms (feedback vs. feed-forward, respectively). The obtained findings suggest that the absolute SM (rather than the relative one) should be used in future studies of hand force coordination in healthy and clinical populations, while GF adaptation obtained from static and dynamic manipulation tasks should be separately assessed.

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.

Sensorimotor Integration in Dyslexic Children under Different Sensory Stimulations

Dyslexic children, besides difficulties in mastering literacy, also show poor postural control that might be related to how sensory cues coming from different sensory channels are integrated into proper motor activity. Therefore, the aim of this study was to examine the relationship between sensory information and body sway, with visual and somatosensory information manipulated independent and concurrently, in dyslexic children. Thirty dyslexic and 30 non-dyslexic children were asked to stand as still as possible inside of a moving room either with eyes closed or open and either lightly touching a moveable surface or not for 60 seconds under five experimental conditions: (1) no vision and no touch; (2) moving room; (3) moving bar; (4) moving room and stationary touch; and (5) stationary room and moving bar. Body sway magnitude and the relationship between room/bar movement and body sway were examined. Results showed that dyslexic children swayed more than non-dyslexic children in all sensory condition. Moreover, in those trials with conflicting vision and touch manipulation, dyslexic children swayed less coherent with the stimulus manipulation compared to non-dyslexic children. Finally, dyslexic children showed higher body sway variability and applied higher force while touching the bar compared to non-dyslexic children. Based upon these results, we can suggest that dyslexic children are able to use visual and somatosensory information to control their posture and use the same underlying neural control processes as non-dyslexic children. However, dyslexic children show poorer performance and more variability while relating visual and somatosensory information and motor action even during a task that does not require an active cognitive and motor involvement. Further, in sensory conflict conditions, dyslexic children showed less coherent and more variable body sway. These results suggest that dyslexic children have difficulties in multisensory integration because they may suffer from integrating sensory cues coming from multiple sources.

Validity of the Manchester Triage System in emergency care: A prospective observational study.

Objectives To determine the validity of the Manchester Triage System (MTS) in emergency care for the general population of patients attending the emergency department, for children and elderly, and for commonly used MTS flowcharts and discriminators across three different emergency care settings. Methods This was a prospective observational study in three European emergency departments. All consecutive patients attending the emergency department during a 1-year study period (2010–2012) were included. Validity of the MTS was assessed by comparing MTS urgency as determined by triage nurses with patient urgency according to a predefined 3-category reference standard as proxy for true patient urgency. Results 288,663 patients were included in the analysis. Sensitivity of the MTS in the three hospitals ranged from 0.47 (95%CI 0.44–0.49) to 0.87 (95%CI 0.85–0.90), and specificity from 0.84 (95%CI 0.84–0.84) to 0.94 (95%CI 0.94–0.94) for the triage of adult patients. In children, sensitivity ranged from 0.65 (95%CI 0.61–0.70) to 0.83 (95%CI 0.79–0.87), and specificity from 0.83 (95%CI 0.82–0.83) to 0.89 (95%CI 0.88–0.90). The diagnostic odds ratio ranged from 13.5 (95%CI 12.1–15.0) to 35.3 (95%CI 28.4–43.9) in adults and from 9.8 (95%CI 6.7–14.5) to 23.8 (95%CI 17.7–32.0) in children, and was lowest in the youngest patients in 2 out of 3 settings and in the oldest patients in all settings. Performance varied considerably between the different emergency departments. Conclusions Validity of the MTS in emergency care is moderate to good, with lowest performance in the young and elderly patients. Future studies on the validity of triage systems should be restricted to large, multicenter studies to define modifications and improve generalizability of the findings.