1,721,121 research outputs found
Anticipatory postural adjustments stabilise the whole upper-limb prior to a gentle index finger tap
Full text linkAbstract Little is known about anticipatory postural adjustments (APAs) developing when body segments of tiny mass are moved. Thus, APAs in the human upper-limb were investigated during a gentle and small index finger tap (35 mm stroke in 50 ms). This task was fulfilled by ten subjects either with prone or supine hand. EMG was recorded from Flexor Digitorum Superficialis (FDS), the prime mover, and from several upper-limb muscles under slight tonic contraction. Regardless of hand posture, EMG was inhibited in Flexor Carpi Radialis and facilitated in Extensor Carpi Radialis well before the FDS burst. With the prone hand, the prime mover activity was preceded by
Biceps inhibition and Triceps facilitation; this effect reverted in sign with the supine hand. A postural reversal was also observed in Anterior Deltoid and Trapezius which were both inhibited with the prone hand. The effect in Trapezius was present only with the unsupported forearm. It is thus demonstrated that a gentle small finger tap produces well-defined anticipatory natural synergies behaving as the most ‘‘classical’’ APAs: (1) they are distributed to several upper-limb muscles creating a postural chain aiming to prevent the effects of the interaction torques generated by the voluntary movement; (2) they change in amplitude according to the level of postural stability and (3) they revert in sign when movement direction is reverted. These results are also corroborated by data obtained from a simple mechanical model simulating finger tapping in a fictive upper-limb. A possible role of APAs in controlling movements’ accuracy is also discussed
Supra-spinal inhibitory circuits shape postural adjustments in upper limb muscles anticipating voluntary index tapping
No full textRecensione di Paolo Cavallari "Pietro Barucci Architetto" Autore: Ruggero Lenci In: Linee n.5, Palombi Editori, Roma 2011, p.66
No full textvedi allegat
Inhibitory APAs observed in several upper limb muscles prior to index tapping are shaped by supraspinal inhibitory circuits
No full textWe previously observed that index tapping is anticipated by inhibitory postural adjustments (iAPA) in the ongoing EMG of several upper limb muscles, including Biceps Brachii (BB) and Superior Trapezius (ST). Aim of the present work is to investigate the spinal and supra-spinal contribution to this effect. This has been tested by probing changes in cortico-spinal excitability, by means of TMS motor evoked potential (MEP) and changes in spinal excitability, by means of T-reflex, during iAPA development. Index tapping was triggered by an acoustic signal, while EMG from the prime mover and from BB or ST was recorded by surface electrodes. MEP or T-reflexes elicited in the relaxed postural muscle (BB or ST), with latency ranging -2000 to +300ms from index flexion, were collected and their peak to peak amplitude measured. In the various subjects (9), with the hand prone and the postural muscle contracted, a clear inhibition of both BB and ST EMG started in average 50+/-23ms before the prime mover onset; in contrast, as expected, when postural muscles were relaxed no inhibitory effect was seen. However, all MEPs falling in the iAPA temporal window were reduced in amplitude, while T-reflexes were unaffected. In each subject, statistical analysis (t-test comparing MEP or T amplitude in the iAPA window versus the responses falling prior to the go-signal) gave significance to this effect (p<0.02). Moreover a high correlation (p<0.01) was found only between the ongoing EMG amplitude and the MEP amplitude in the relaxed muscle. Lastly, on average, MEP inhibition developed 98+/-33ms before the prime mover onset, i.e. about 50ms prior to the iAPA onset. These results show i) that when motoneurons are not firing, iAPAs are still associated to the motor command although under threshold, and ii) that during the iAPA spinal motoneurons are more likely disfacilitated than inhibited. Thus, iAPAs seem to be fully sustained by inhibitory circuits located in the supraspinal centres
Neural compensation for mechanical loading of the hand during coupled oscillations of the hand and foot
No full textThe role of kinaesthetic afferences in controlling coupling of voluntary oscillation of the hand and foot, both in-phase and anti-phase, was investigated by modifying the mechanical properties of one of the two segments (the hand) with applied inertial or elastic loads. Loads consisted of a lead disk, rotating coaxially with the wrist (total inertial momentum 15 g m2), or in two symmetrical rubber bands (elasticity, 4 g deg(-1)) connected 5 cm away from the wrist pivot. Experiments were performed on five male and five female subjects. Both the frequency responses of the hand and foot (i.e. the phase relations between the onset of muscular activation in limb extensors and the onset of the related movement) and the inter-limb phase relations (the phase differences between the hand and foot movement cycles and between the onsets of the electromyographic (EMG) activity in hand and foot extensors) were analysed. The hand frequency-response was fitted with a 2nd-order model, allowing us to describe the loaded and unloaded conditions through the changes in the model response. Inertial loading induced an immediate and steep decay in the frequency response, with a clear-cut reduction of the model resonance frequency, while elastic loading shifted the response to the right and upwards. Inter-limb phase relations were only partially affected by inertial loading of the hand. Despite the fact that the load strongly increased the difference between the frequency-responses of the hand and foot, when hand and foot were oscillated in-phase only about half of this difference remained as an increased phase-lag between hand and foot oscillations. The other half was offset by an advance of the contraction of the hand movers with respect to the foot movers. This compensation mechanism was more effective during anti-phase than during in-phase movements. Elastic loading improved inter-limb synchronisation, since it superimposed the hand frequency-response on that of the foot. In this condition, the requested synchronisation (in-phase or anti-phase) could be achieved by an almost simultaneous (or in strict phase opposition) contraction of the hand and foot movers. In conclusion, the main feedback reaction to the de-coupling effect of hand loading consisted in modifying the timing of activation of the muscles moving the extremities. An advance of hand movers on foot movers is already present in unloaded conditions to compensate for the difference in the natural mechanical properties of the two segments. This advance is enhanced when increasing the inertia of the hand system and attenuated when increasing its elasticity
Modulation de l’excitabilité d’unités motrices isolées du muscle fléchisseur du poignet par les afférences cutanées de la pulpe de l’index chez l’homme
No full textLes effets spinaux produits par une stimulation mécanique ponctuelle appliquée sur la pulpe de l’index ont été précédemment étudiés, chez l’homme, par la technique du réflexe-H (Cavallari et Lalli, Exp Brain Res 120 :345-351, 1998). Les afférences cutanées ainsi stimulées entraînaient une inhibition de courte latence du réflexe-H évoqué dans le muscle fléchisseur du poignet (FCR), immédiatement suivie par une facilitation de longue durée. Par ailleurs, la stimulation électrique des nerfs digitaux de l’index était inefficace.
Le but de cette étude est de montrer que la probabilité de décharge d’unités motrices isolées (UM) du FCR est modulée de la même façon que le réflexe H, après stimulation ponctuelle mécanique, ou électrique, de la pulpe de l’index.
28 unités motrices ont été étudiées par la technique du post stimulus time histogramme (PSTH) sur 5 sujets volontaires sains. Ainsi, après stimulation mécanique de l’index (3xPT, seuil de perception, durée:10ms) une inhibition caractérisée par une latence de 10,4±1,4ms (11/14 UM) est suivie d’une facilitation (14/14 UMs) de longue durée. Le même effet biphasique (latence de l’inhibition 9,6±1,3ms) est observé après stimulation électrique focale (2xPT, durée: 0,8ms) de la pulpe de l’index. Toutes les latences sont exprimées par rapport à la latence de l’excitation monosynaptique homonyme Ia évoquée, dans le FCR, par la stimulation du nerf médian au pli du coude.
Ce travail montre que la stimulation focale électrique ainsi que la stimulation mécanique des afférences cutanées, modulent de manière biphasique la décharge d’une unité motrice isolée, confirmant l’effet observé en réflexe H. Par ailleurs, il est aussi démontré une distribution homogène des voies inhibitrices et excitatrices sur les mêmes motoneurones, écartant, dans ce cas là, l’hypothèse d’une distribution dishomogène des effets produits par les afférences cutanées sur les motoneurones à plus bas seuil (Garnett R, J Physiol 303:351, 1980; Nielsen J, Acta Physiol Scand 147:385, 1993)
Partition of voluntary command to antagonist muscles during cyclic flexion-extension of the hand
No full textActivity distribution between wrist movers during rhythmic flexion-extension of the wrist has been analysed in three different mechanical conditions. Wrist angular position and surface EMG from Extensor Carpi Radialis (ECR) and Flexor Carpi Radialis (FCR) were recorded. In the first condition (hand prone, flexion-extension in a vertical parasagittal plane) the hand passive equilibrium position was ∼50° in flexion. During hand oscillations FCR and ECR were alternatively recruited to move the hand symmetrically away from the equilibrium and de-recruited to allow conservative forces to restore the equilibrium. Switching between antagonists occurred at the centre of the oscillation (equilibrium crossing). In the second condition (hand semi-prone, flexion-extension in a horizontal transversal plane) the hand equilibrium was attained over an angle of about 26°. When the hand was oscillated symmetrically around this equilibrium range, each muscle was recruited when the hand entered the equilibrium range and switching between antagonists therefore occurred in advance of the oscillation centre. Both vertical and horizontal oscillations were also performed all externally to the equilibrium position or range: in these cases only one muscle was recruited over the entire cycle, the EMG burst starting at the onset of the related movement. In the third condition (hand semi-prone, flexion-extension in a horizontal transversal plane) a frictional load added to the platform pivot expanded the equilibrium range to encompass the entire hand oscillation. Now concentric muscle contraction was needed throughout each phase of the movement and switching between antagonists occurred at the movement reversal, i.e. ∼90° in advance of the oscillation centre. The above descriptions held for oscillation frequencies from 0.2 Hz to 3.0 Hz, once the frequency-dependent effects of viscosity and inertia were accounted for. In all the three conditions, contractile forces started developing when an intrinsic or external resistance had to be overcome in order to continue the movement. To account for this control, a neural network is proposed that compares the afferent information about joint position with a position central command, thus detecting the position error caused by the forces that resist to movement. From the sign and amplitude of the error signal the network determines the direction (agonist vs antagonist) and the amount of motor activation
Feedback control of the limbs position during voluntary rhythmic oscillation
No full textThe mechanisms that control the limbs position during rhythmic voluntary oscillations were investigated in ten subjects, who were asked to synchronise the lower peak of their hand or foot rhythmic oscillations to a metronome beat. The efficacy of the "position control" was estimated by measuring the degree of synchronisation between the metronome signal and the requested limb position and how it was affected by changing both the oscillation frequency (between 0.4 and 3.0 Hz) and the limbs inertial properties. With the limbs unloaded, the lower peak of both the hand and foot oscillations lagged the metronome beat of a slight amount that remained constant over the whole frequency range (mean phase delay -13.2° for the hand and -4.7° for the foot). The constancy was obtained by phase-advancing, at each frequency increment, the electromyogram (EMG) activation with respect of the clock beat of the amount necessary to compensate for the simultaneous increase of the lag between the EMG and the movement, produced by the limb mechanical impedance. After loading of either limb, the increase of the oscillation frequency induced larger EMG-movement delays and the anticipatory compensation became insufficient, so that the movement progressively phase-lagged the clock beat. The above results have been accurately simulated by a neural network connected to a pendulum model that shared the same mechanical properties of the moving limb. The network compares a central command (the intended position) to the actual position of the effector and acts as a closed-loop proportional, integrative and derivative controller. It is proposed that the synchronisation of rhythmic oscillations of either the hand or the foot is sustained by a feed-back control that conforms the position of each limb to that encoded in the central voluntary command
Foot phase-response during voluntary oscillations
No full textWhen studying coupled oscillations of hand and foot, we approximated the extremities to pendula forced by sinusoidal inputs and described their input-output phase relations by measuring the phase difference between the onsets of the EMG bursts and the onset of the related movements. However, while hand response was fitted by the simple pendulum model, foot response did not, since below 1.5Hz movement onset led the force onset. This obvious paradox indicated that the initial part of the foot movement was sustained by forces (elastic recoil of antagonists, joint structures, etc) other than contraction of its prime movers. In this perspective, it might be argued that elastic recoil promotes foot motion up to recovery of the equilibrium position, and that contraction starts when equilibrium is crossed. Should this be true, EMG-movement phase relation had to be measured with respect to crossing of equilibrium, not to movement onset. To verify this, the equilibrium position of the foot, resting on a rotating platforms, was measured in 5 subjects (ankle angle = 91±6° SD); then the foot was voluntarily oscillated at various frequencies (0.2 ÷ 3Hz) around its equilibrium. The phase relations were measured between the onset of the tibialis anterior EMG and i) the peak plantar flexion; ii) the equilibrium crossing during dorsiflexion. The frequency responses determined by the two techniques showed a parallel sigmoidal decay. The EMG-equilibrium curve started near zero (-10±10° at 0.2 Hz), reached -95±22° at 3 Hz and could be well fitted by a pendulum model (R2 = 0.93±0.05). The EMG-peak plantar flexion curve started at 59±16° (0.2Hz) and reached –9±13° at 3 Hz. Parallelism of the two curves suggests that referring the EMG onset to the peak plantar flexion introduced a systematic phase shift which may be removed by translating to zero the curve starting point. Approximation of the foot movement by a simple pendulum model seems therefore legitimate
- …
