THE ONENESS OF POSTURE AND VOLUNTARY MOVEMENT

During 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

Anticipatory Postural Adjustments associated with reaching movements are programmed according to the availability of visual information

During goal-directed arm movements, the eyes, head, and arm are coordinated to look at and reach the target. We examined whether the expectancy of visual information about the target modifies Anticipatory Postural Adjustments (APAs). Ten standing subjects had to (1) move the eyes, head and arm, so as to reach, with both gaze and index-finger, a target of known position placed outside their visual field (Gaze-Reach); (2) look at the target while reaching it (Reach in Full Vision); (3) keep the gaze away until having touched it (Reach then Gaze) and (4) just Gaze without Reach the target. We recorded eye, head, right arm, and acromion kinematics, EMGs from upper- and lower-limb muscles, and forces exerted on the ground. In Gaze-Reach, two coordination strategies were found: when gaze preceded arm muscle recruitment (Gaze-first) and when the opposite occurred (Reach-first). APAs in acromion kinematics, leg muscles, and ground forces started significantly earlier in Gaze-first vs. Reach-first (mean time advance: 44.3 \ub1 8.9 ms), as it was in Reach in Full Vision vs. Reach then Gaze (39.5 \ub1 7.9 ms). The Gaze-first to Reach-first time-shift was similar to that between Reach in Full Vision and Reach then Gaze (p = 0.58). Moreover, Gaze without Reach data witnessed that the head-induced postural actions did not affect the APA onset in Gaze-first and Reach-first. In conclusion, in Gaze-first, the central control of posture considers visual information while planning the movement, like in Reach in Full Vision; while Reach-first is more similar to Reach then Gaze, where vision is not required

Transcranial direct current stimulation of SMA modulates anticipatory postural adjustments without affecting the primary movement

Recent 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

Precision of a pointing movement performed with either the dominant or non-dominant hand is linked to the timing of anticipatory postural adjustments

Introduction: 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\u201330 ms) versus dominant hand (60\u201370 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 \ub1 1.7 mm versus dominant: 10.1 \ub1 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

Intended rather than actual movement velocity determines the latency of anticipatory postural adjustments

The literature reports that anticipatory postural adjustments (APAs) are programmed according to movement velocity. However, the linkage between APAs and velocity has been highlighted within single subjects who were asked to voluntarily change movement velocity; therefore, till now, it has been impossible to discern whether the key factor determining APA latency was the intended movement velocity or the actual one. Aim of this study was to distinguish between these two factors. We analyzed the APA chain that stabilizes the arm during a brisk index finger flexion in two groups of subjects: (1) 29 who composed our database from previous experiments and were asked to "go-as-fast-as-possible" (go-fast), but actually performed the movement with different speeds (238-1180\ub0/s), and (2) ten new subjects who performed the go-fast movement at more than 500\ub0/s and were then asked to go-slow at about 50 % of their initial velocity, thus moving at 300-800\ub0/s. No correlation between APA latency and actual movement speed was observed when all subjects had to go-fast (p > 0.50), while delayed APAs were found in the ten new subjects when they had to go-slow (p < 0.001). Moreover, in the speed range between 300 and 800\ub0/s, the APA latency depended only on movement instruction: subjects going fast showed earlier APAs than those going slow (p < 0.001). These data suggest a stronger role of the intended movement velocity versus the actual one in modifying the timing of postural muscles recruitment with respect to the prime mover. These results also strengthen the idea of a shared postural and voluntary command within the same motor act

Direct-current stimulation of posterior tibial nerve modulates the Soleus H-reflex amplitude

Introduction: 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-re\ufb02ex in the Soleus muscle. Once the H-re\ufb02ex 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 re\ufb02ex amplitude. Then, the H-re\ufb02ex was measured for 30 further minutes, looking for after-effects. Results: Cathodal DCs induced a significant increase of the H-re\ufb02ex 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 re\ufb02ex 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 re\ufb02ex circuit. Moreover it is worth to note that the polarity dependence was opposite to what found for tDCS

Direct current stimulation modulates the excitability of the sensory and motor fibres in the human posterior tibial nerve, with a long-lasting effect on the H-reflex

Several studies demonstrated that transcutaneous direct current stimulation (DCS) may modulate central nervous system excitability. However, much less is known about how DC affects peripheral nerve fibres. We investigated the action of DCS on motor and sensory fibres of the human posterior tibial nerve, with supplementary analysis in acute experiments on rats. In forty human subjects, electric pulses at the popliteal fossa were used to elicit either M-waves or H-reflexes in the Soleus, before (15 min), during (10 min) and after (30 min) DCS. Cathodal or anodal current (2 mA) was applied to the same nerve. Cathodal DCS significantly increased the H-reflex amplitude; the post-polarization effect lasted up to ~ 25 min after the termination of DCS. Anodal DCS instead significantly decreased the reflex amplitude for up to ~ 5 min after DCS end. DCS effects on M-wave showed the same polarity dependence but with considerably shorter after-effects, which never exceeded 5 min. DCS changed the excitability of both motor and sensory fibres. These effects and especially the long-lasting modulation of the H-reflex suggest a possible rehabilitative application of DCS that could be applied either to compensate an altered peripheral excitability or to modulate the afferent transmission to spinal and supraspinal structures. In animal experiments, DCS was applied, under anaesthesia, to either the exposed peroneus nerve or its Dorsal Root, and its effects closely resembled those found in human subjects. They validate therefore the use of the animal models for future investigations on the DCS mechanisms

Cough-Anal Reflex May Be the Expression of a Pre-Programmed Postural Action

When coughing, an involuntary contraction of the external anal sphincter occurs, in order to prevent unwanted leakages or sagging of the pelvis muscular wall. Literature originally described such cough-anal response as a reflex elicited by cough, therefore identifying a precise cause-effect relationship. However, recent studies report that the anal contraction actually precedes the rise in abdominal pressure during cough expiratory effort, so that the sphincter activity should be pre-programmed. In recent years, an important family of pre-programmed muscle activities has been well documented to precede voluntary movements: these anticipatory actions play a fundamental role in whole body and segmental postural control, hence they are referred to as anticipatory postural adjustments (APAs). On these basis, we searched in literature for similarities between APAs and the cough-anal response, observing that both follow the same predictive homeostatic principle, namely that anticipatory collateral actions are needed to prevent the unwanted mechanical consequences induced by the primary movement. We thus propose that the cough-anal response also belongs to the family of pre-programmed actions, as it may be interpreted as an APA acting on the abdominal-thoracic compartment; in other words, the cough-anal response may actually be an Anticipatory Sphincter Adjustment, the visceral counterpart of APAs

Cough-Anal Reflex May Be the Expression of a Pre-Programmed Postural Action

When coughing, an involuntary contraction of the external anal sphincter occurs, in order to prevent unwanted leakages or sagging of the pelvis muscular wall. Literature originally described such cough-anal response as a reflex elicited by cough, therefore identifying a precise cause-effect relationship. However, recent studies report that the anal contraction actually precedes the rise in abdominal pressure during cough expiratory effort, so that the sphincter activity should be pre-programmed. In recent years, an important family of pre-programmed muscle activities has been well documented to precede voluntary movements: these anticipatory actions play a fundamental role in whole body and segmental postural control, hence they are referred to as anticipatory postural adjustments (APAs). On these basis, we searched in literature for similarities between APAs and the cough-anal response, observing that both follow the same predictive homeostatic principle, namely that anticipatory collateral actions are needed to prevent the unwanted mechanical consequences induced by the primary movement. We thus propose that the cough-anal response also belongs to the family of pre-programmed actions, as it may be interpreted as an APA acting on the abdominal-thoracic compartment; in other words, the cough-anal response may actually be an Anticipatory Sphincter Adjustment, the visceral counterpart of APAs