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THE ONENESS OF POSTURE AND VOLUNTARY MOVEMENT
Full text linkDuring my PhD studies, I have dealt with several aspects of the relationship between posture and voluntary movement. In particular, the main subject of my work has been to analyze the functional role of the Anticipatory Postural Adjustments (APAs) within the motor act and how these are programmed by the central nervous system.
In the past literature, it has been described that APAs are unconscious muscular activities aimed to maintain the equilibrium of the whole body (Massion 1992). The role of the APAs has been first disclosed in movements involving relatively large masses, such as an upper-limb flexion (see for a review, Bouisset and Do 2008). In this case, the shoulder flexion may displace the projection of the center of mass to ground, eventually causing an imbalance of the whole body. Therefore, in order to counteract such a perturbation, the recruitment of the prime mover (Anterior Deltoid) is normally preceded by a specific pattern of EMG activities, defined as APAs, developing in the lower limbs, the hips and the trunk. This inter-limb APA chain is thus able to induce a forward displacement which prevents the backward perturbation caused by the focal movement.
More recently, Caronni and Cavallari (2009a) demonstrated that an APA chain also develop for very simple movements such as a finger flexion, in which small masses are involved and in which the whole-body equilibrium is not threatened. Indeed, the index-finger flexion, performed with the hand prone, has been found being preceded by an excitatory burst in Triceps Brachii, while Biceps Brachii and Anterior Deltoid showed a concomitant inhibition. This APA pattern, shaped in the primary motor cortex (Caronni and Cavallari 2009b), contrasts the flexion of elbow and shoulder induced by the upward perturbation that the index-finger flexion causes on the metacarpo-phalangeal joint.
Caronni and Cavallari papers (2009a, b) contributed to arise three questions: i) do inter-limb and intra-limb APAs share similar control mechanisms? If yes, ii) what is the functional role of intra-limb APAs, since it is hard to keep considering intra-limb APAs simply as a counter-perturbation aiming to maintain the whole-body equilibrium and iii) does the prime mover recruitment and its associated postural adjustments result from two different central commands, as classically proposed (Babinski 1899; Hess 1943; Cordo and Nashner 1982; Brown and Frank 1987) or are they both controlled by a unique motor command, as suggested by more recent evidences (Aruin and Latash 1995; Petersen et al. 2009; Caronni and Cavallari 2009b; Leonard et al. 2011)?
Aim of the present thesis is to shed further light on these questions with the specific target of investigating a possible role of APAs in refining movement accuracy and demonstrating the oneness of the motor command for both anticipatory postural adjustments (APAs) and prime mover recruitment. Thus, I will illustrate: i) the key role of a properly-tailored APA chain on prime mover recruitment in refining movement accuracy, ii) the superposition of the neural network responsible for controlling the voluntary movement and the APAs, focusing on Supplementary Motor Area and Cerebellum, iii) the disruption of both the postural chain and the prime mover recruitment after a period of immobilization and iv) the APAs adaptation to the intended movement, i.e. the expected perturbation. These observations have been already published or are at this moment submitted to publication.
The thesis has been divided in four sections: 1) the Introduction, which includes a review on the existing literature on inter- and intra-limb APAs, and explains my working hypothesis; 2) the Experimental Procedure, which describes how data were collected and analyzed.; 3) the collection of the original papers, and finally 4) the Conclusion, in which I will explain the interpretation of the present results and develop possible conclusions, with special regards on the relationship between postural and prime mover command
The intra-limb anticipatory postural adjustments and their role in movement performance
No full textAnticipatory Postural Adjustments (APAs) are commonly described as unconscious muscular activities aimed to counterbalance the perturbation caused by the primary movement, so as to ensure the whole-body balance. These activities usually create one or more fixation chains which spread over several muscles of different limbs, and may be thus called inter-limb APAs. However, we reported that APAs also precede voluntary movements involving tiny masses, like a flexion/extension of the wrist or even a brisk flexion of the index-finger. In particular, such movements are preceded by an intra-limb APA chain, that involves muscles acting on the proximal joints. Considering the small mass of the moving segments, it is unlikely that the ensuing perturbation could threaten the whole-body balance, so that it is interesting to enquire the physiological role of intra-limb APAs and their organization and control compared to inter-limb APAs. Since several years, our research is focused on intra-limb APAs and highlighted a strict correspondence in their behaviour and temporal/spatial organization with respect to inter-limb APAs. Hence we suggested that both are manifestations of the same phenomenon. Particular emphasis has been given to intra-limb APAs preceding index-finger flexion, because their relatively simple biomechanics and the fact that muscular actions were limited to a single arm allowed peculiar investigations, leading to important conclusions. Indeed, such paradigm provided evidence that APAs and prime mover activation are driven by a shared motor command, and also that by granting a proper fixation of those body segments proximal to the moving one, APAs are involved in refining movement precision
Precision of a pointing movement performed with either the dominant or non-dominant hand is linked to the timing of anticipatory postural adjustments
Full text linkIntroduction: It is a common experience to feel motor awkwardness when performing a pointing movement with the non-preferred limb, which is known to be associated to less precise movements. Here we provide evidence that this last behaviour partly stems from changes in the temporal organization of the Anticipatory Postural Adjustments (APAs) in the non-preferred side. Materials and methods: We investigated the effect of lateralization on APAs in Biceps Brachii, Triceps Brachii and Anterior Deltoid, which stabilize the arm when performing a pen-pointing movement (prime mover Flexor Carpi Radialis). Moreover, we analysed the elbow and wrist kinematics as well as the precision of the pointing movement. Results: The mean kinematics of wrist movement and its latency, with respect to prime mover recruitment, were similar in the two sides, while APAs in Triceps Brachii, Biceps Brachii and Anterior Deltoid were less anticipated when movements were performed with the non-dominant (20–30 ms) versus dominant hand (60–70 ms). APAs in the non-dominant limb were associated with an altered fixation of the elbow, which showed a higher excursion, and with a more scattered pointing error (non-dominant: 16.3 ± 1.7 mm versus dominant: 10.1 ± 0.8 mm). Discussion: By securing the dynamics of the more proximal joints, an appropriate timing of the intra-limb APAs seems necessary for refining the voluntary movement precision. The linkage between APAs, elbow fixation and movement accuracy also agrees with the recent suggestion that APAs and prime mover recruitment are driven by a shared motor command, which strives to obtain an accurate pointing
The ischemic block of the forearm abolishes index-finger’s movements but not its associated APAs
No full textThe voluntary movement induces postural perturbations which are counteracted by unconscious feed-forward motor activities, known as anticipatory postural adjustments (APAs). Thus, for every movement, two motor commands are dispatched: one recruiting the prime mover and one driving APAs. These commands are classically thought to be separately controlled, this study investigates if they could be instead considered as a single element of the motor program. We analyzed APAs in Biceps Brachii, Triceps Brachii and Anterior Deltoid that precede an index-finger flexion (Flexor Digitorum Superficialis). APAs and prime mover activation were recorded in three conditions: before, during and after a long lasting ischemic block of the forearm. Ischemia suppressed both the forearm sensory feed-back and the prime mover activation, thus canceling finger flexion and the ensuing postural perturbation. Thus, a suppression of APAs should be expected, since purposeless and uneconomical. Intriguingly enough, large APAs were instead apparent without significant differences in latency and amplitude, except for Anterior Deltoid that showed smaller APAs during ischemia. The observation that APAs remain tailored to the intended movement in absence of perturbation, supports the idea that postural and voluntary commands cannot be separated each other, as they both belong to an unique motor program
Proper anticipatory postural adjustments lead to an accurate voluntary movement
No full textEquilibrium perturbing forces associated to any voluntary upper-limb movement are strong enough to displace the body centre of mass. In this condition Anticipatory Postural Adjustments (APAs), developing in muscles other than the prime mover, are essential for maintaining the equilibrium stability. Here we test the hypothesis that APAs preceding an upper-limb target reaching could also be crucial in controlling movement accuracy.
Standing subjects (10) flexed their shoulder to reach the centre of a target positioned in front of them. The reaching task was also performed while wearing and after doffing prismatic lenses (shifting the eye field rightward). EMGs from different upper- and lower-limb muscles and mechanical actions to ground were recorded.
We found that: i) before wearing prisms, subjects performed very accurate reaching movements, ii) after wearing prisms, apparent pointing errors occurred, iii) these pointing errors were progressively compensated within each session, and iv) changes in APA amplitude were associated to amplitude of pointing error.
These results demonstrate a strict coupling between voluntary movement and APAs pattern, suggesting that, besides stabilizing the whole body equilibrium, APAs also guarantee movement accuracy. Thus, recalling Bernstein’s “the movement responds as a whole to changes in each small part” we should consider APAs and primary movement as a whole, since they represent two subsequent aspects of the same voluntary motor ac
Ischemic block of the forearm abolishes finger movements but not their associated anticipatory postural adjustments
No full textVoluntary movement is known to induce postural perturbations that are counteracted by unconscious anticipatory postural adjustments (APAs). Thus, for every movement, two motor commands are dispatched: a voluntary command recruiting the prime mover and a postural command driving the APAs. These commands are classically thought to be separated; this study investigates whether they could be instead considered as two elements within the same motor program.
We analyzed the APAs in Biceps Brachii, Triceps Brachii and Anterior Deltoid that stabilize the arm when briskly flexing the index-finger (prime mover Flexor Digitorum Superficialis). APAs and prime mover activation were recorded before, under and after ischemic block of the forearm. Ischemia paralysed the prime mover, thus suppressing the finger movement and the ensuing postural perturbation. If the two commands had been separated, it would have been expected that after a few failed attempts to flex the index-finger, the APAs were suppressed too, being purposeless without postural perturbation.
APAs were still present under ischemia even after 60 movement trials. No significant changes were found in APA amplitude in Biceps and Triceps among different conditions, or in the average APA latency. Inhibitory APA in Anterior Deltoid was reduced but still present under ischemia. In addition, the pharmacologic block of the sole median nerve produced similar effects. APAs were instead almost abolished when applying a fixation point to the wrist.
The observation that APAs remained tailored to the expected perturbation even when that perturbation did not occur supports the idea of a functionally unique motor command driving both the prime mover and the muscles of the APA chain
Direct-current stimulation of posterior tibial nerve modulates the Soleus H-reflex amplitude
Full text linkIntroduction: Several studies demonstrated that transcranial direct current stimulation (tDCs) is a promising non-invasive tool able to modulate the excitability of several CNS structures. Its effect is usually facilitatory when using anodal polarity and inhibitory for the cathodal one. In most studies, DC stimulation was applied on cortical or spinal structures, while little is known about its effect on peripheral nerves fibres. This research aims at highlighting such effect. Methods: In twenty subjects, electrical stimulation of the posterior tibial nerve (1 ms current pulses, 1 shock every 9 s) was used to elicit the H-reflex in the Soleus muscle. Once the H-reflex amplitude was stable for at least 15 min, DCs (either cathodal or anodal) was applied proximally to the same nerve for 10 min, looking for changes in reflex amplitude. Then, the H-reflex was measured for 30 further minutes, looking for after-effects. Results: Cathodal DCs induced a significant increase of the H-reflex amplitude (about +35%) with respect to the control value. In this configuration the after-effect lasted about 25 min. Anodal DCs induced instead a significant decrease (about -25%) of the reflex amplitude. A significant after-effect was observed for just about 5 min. Discussion: This study shows that DCs applied to a peripheral nerve is able to elicit neuromodulation. Its polarity dependence suggests a local change in the excitability of nerve fibres rather than a central modulation of the spinal reflex circuit. Moreover it is worth to note that the polarity dependence was opposite to what found for tDCS
Transcranial direct current stimulation of the SMA enhances intra-limb anticipatory postural adjustments
No full textAnticipatory postural adjustments (APAs) represent a fundamental component of the voluntary movement. APAs circuitry is not fully understood, but several studies suggest that the supplementary motor area (SMA) may be deeply involved.
To confirm its engagement in APA generation, and verify the possibility to interfere with it, we applied trascranial direct current stimulation (tDCs) over the SMA, looking for effects on the postural control of the upper-limb during a brisk index-finger flexion. Indeed, such a movement is preceded by inhibitory APAs carved in Biceps and Anterior Deltoid EMG, and by an excitatory APA on Triceps (Caronni & Cavallari, Exp Brain Res 2009).
Subjects (n=12) performed 3 sessions of 30 index-finger flexions, while sitting on a chair with the right upper-arm along the body, elbow at 90° and hand prone. Before each session, subjects underwent tDCs (1mA, 20min) with CATHODAL, ANODAL or SHAM configuration.
During ANODAL stimulation the average APAs in Biceps and Triceps were greater than during SHAM stimulation (+26.5% and +66%, respectively) and during CATHODAL stimulation (+20.5% and +63.4%). ANOVA p<0.02 Tukey p<0.05, for both muscles. Instead, the APA recorded on Anterior Deltoid was unchanged in all sessions.
These results confirm the SMA involvement in APA generation and suggest that tDCs, which has been shown to be a valuable tool for treating different neurological conditions, may be as well employed to modulate APAs during postural rehabilitation
Transcranial direct current stimulation of SMA modulates anticipatory postural adjustments without affecting the primary movement
Full text linkRecent works provide evidences that anticipatory postural adjustments (APAs) are programmed with the prime mover recruitment as a shared posturo-focal command. However the ability of the CNS to adjust APAs to changes in the postural context implies that the postural and voluntary components should take different pathways before reaching the representation of single muscles in the primary motor cortex. Here we test if such bifurcation takes place at the level of the supplementary motor area (SMA). TDCS was applied over the SMA in 14 subjects, who produced a brisk index-finger flexion. This activity is preceded by inhibitory APAs, carved in the tonic activity of Biceps Brachii and Anterior Deltoid, and by an excitatory APA in Triceps Brachii. Subjects performed a series of 30 flexions before, during and after 20min of tDCS in CATHODAL, ANODAL or SHAM configuration. The inhibitory APA in Biceps and the excitatory APA in Triceps were both greater in ANODAL than in SHAM and CATHODAL configurations, while no difference was found among the latter two (ANODAL vs. SHAM: biceps +26.5%, triceps +66%; ANODAL vs. CATHODAL: biceps +20.5%, triceps: +63.4%; for both muscles, ANOVA p<0.02, Tukey p<0.05). Instead, the APA in anterior deltoid was unchanged in all configurations. No changes were observed in prime mover recruitment and index-finger kinematics. Results show that the SMA is involved in modulating APAs amplitude. Moreover, the differential effect of tDCS observed on postural and voluntary commands suggests that these two components of the motor program are already separated before entering SMA
Hand immobilization affects arm and shoulder postural control
No full textIt is a common experience, immediately after the removal of a cast or a splint, to feel motor awkwardness, which is usually attributed to muscular and joint immobilization. However, the same feeling may also be perceived after a brief period of immobilization. We provide evidence that this last effect stems from changes in the cortical organization of the focal movement as well as in the associated anticipatory postural adjustments. Indeed, these two aspects of the motor act are strongly correlated, although scaled in different manners. In fact, they are both shaped in the primary motor cortex, they both undergo similar amplitude and latency modulation and, as we will show, they are both impaired by the immobilization of the lone prime mover. Neuromuscular effects of limb immobilization are well known; however, most papers focus on changes occurring in the pathways projecting to the prime mover, which acts on the immobilized joint. Conversely, this study investigates the effect of immobilization on anticipatory postural adjustments. Indeed, we show that 12 h of wrist and fingers immobilization effectively modify anticipatory postural adjustments of the elbow and the shoulder, that is, those joints not immobilized within the fixation chain. Accordingly, the motor impairment observed after short-term immobilization most likely stems from the unbalance between anticipatory postural adjustments and the focal movement
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