Short- and long-term changes in joint co-contraction associated with motor learning as revealed from surface EMG.

In the field of motor control, two hypotheses have been controversial: whether the brain acquires internal models that generate accurate motor commands, or whether the brain avoids this by using the viscoelasticity of musculoskeletal system. Recent observations on relatively low stiffness during trained movements support the existence of internal models. However, no study has revealed the decrease in viscoelasticity associated with learning that would imply improvement of internal models as well as synergy between the two hypothetical mechanisms. Previously observed decreases in electromyogram (EMG) might have other explanations, such as trajectory modifications that reduce joint torques. To circumvent such complications, we required strict trajectory control and examined only successful trials having identical trajectory and torque profiles. Subjects were asked to perform a hand movement in unison with a target moving along a specified and unusual trajectory, with shoulder and elbow in the horizontal plane at the shoulder level. To evaluate joint viscoelasticity during the learning of this movement, we proposed an index of muscle co-contraction around the joint (IMCJ). The IMCJ was defined as the summation of the absolute values of antagonistic muscle torques around the joint and computed from the linear relation between surface EMG and joint torque. The IMCJ during isometric contraction, as well as during movements, was confirmed to correlate well with joint stiffness estimated using the conventional method, i.e., applying mechanical perturbations. Accordingly, the IMCJ during the learning of the movement was computed for each joint of each trial using estimated EMG-torque relationship. At the same time, the performance error for each trial was specified as the root mean square of the distance between the target and hand at each time step over the entire trajectory. The time-series data of IMCJ and performance error were decomposed into long-term components that showed decreases in IMCJ in accordance with learning with little change in the trajectory and short-term interactions between the IMCJ and performance error. A cross-correlation analysis and impulse responses both suggested that higher IMCJs follow poor performances, and lower IMCJs follow good performances within a few successive trials. Our results support the hypothesis that viscoelasticity contributes more when internal models are inaccurate, while internal models contribute more after the completion of learning. It is demonstrated that the CNS regulates viscoelasticity on a short- and long-term basis depending on performance error and finally acquires smooth and accurate movements while maintaining stability during the entire learning process.

Proprioceptive acuity predicts muscle co-contraction of the tibalis anterior and gastrocnemius medialis in older adults’ dynamic postural control

Older adults use a different muscle strategy to cope with postural instability, in which they ‘co-contract’ the muscles around the ankle joint. It has been suggested that this is a compensatory response to age-related proprioceptive decline however this view has never been assessed directly. The current study investigated the association between proprioceptive acuity and muscle co-contraction in older adults. We compared muscle activity, by recording surface EMG from the bilateral tibalis anterior and gastrocnemius medialis muscles, in young (aged 18-34) and older adults (aged 65-82) during postural assessment on a fixed and sway-referenced surface at age-equivalent levels of sway. We performed correlations between muscle activity and proprioceptive acuity, which was assessed using an active contralateral matching task. Despite successfully inducing similar levels of sway in the two age groups, older adults still showed higher muscle co-contraction. A stepwise regression analysis showed that proprioceptive acuity measured using variable error was the best predictor of muscle co-contraction in older adults. However, despite suggestions from previous research, proprioceptive error and muscle co-contraction were negatively correlated in older adults, suggesting that better proprioceptive acuity predicts more co-contraction. Overall, these results suggest that although muscle co-contraction may be an age-specific strategy used by older adults, it is not to compensate for age-related proprioceptive deficits.

Influence of neuromuscular fatigue on co-contraction between vastus medialis and vastus lateralis during isometric contractions

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Co-contração e onset muscular durante o desequilíbrio de idosas caidoras e não-caidoras

Pós-graduação em Desenvolvimento Humano e Tecnologias - IBRC

Changes in joint stability with muscle contraction measured from transmission of mechanical vibration

A non-invasive in vivo technique was developed to evaluate changes in wrist joint stability properties induced by increased co-activation of the forearm muscles in a gripping task. Mechanical vibration at 45, 50 and 55 Hz was applied to the radial head in ten healthy volunteers. Vibrations of the styloid process of the radius and the distal end of the metacarpal bone of the index finger were measured with triaxial accelerometers. Joint stability properties were quantified by the transfer function gain between accelerations on either side of the wrist-joint. Gain was calculated with the muscles at rest and at five force levels ranging from 5% to 25% of maximum grip force (%MF). During contraction the gain was significantly greater than in control trial (0%MF) for all contractions levels at 45 and 50 Hz and a trend for 15%MF and higher at 55 Hz. Group means of contraction force and gain were significantly correlated at 45 (R-2 = 0.98) and 50 Hz (R-2 = 0.72), but not at 55 Hz (R-2 = 0.10). In conclusion, vibration transmission gain may provide a method to evaluate changes in joint stability properties. (c) 2005 Published by Elsevier Ltd.

Analyses comparatives des stratégies musculaires et des co-contractions chez des enfants sains et amputés trans-tibiaux lors de tâches dynamiques des membres inférieurs

Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal.

Analyses comparatives des stratégies musculaires et des co-contractions chez des enfants sains et amputés trans-tibiaux lors de tâches dynamiques des membres inférieurs

Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal.

Anticipatory and reactive response to falls: Muscle synergy activation of forearm muscles

We investigated the surface electromyogram response of six forearm muscles to falls onto the outstretched hand. The extensor carpi radialis longus, extensor carpi radialis brevis, extensor carpi ulnaris, abductor pollicis longus, flexor carpi radialis and flexor carpi ulnaris muscles were sampled from eight volunteers who underwent ten self-initiated falls. All muscles initiated prior to impact. Co-contraction is the most obvious surface electromyogram feature. The predominant response is in the radial deviators. The surface electromyogram timing we recorded would appear to be a complex anticipatory response to falling modified by the ef- fect on the forearm muscles following impact. The mitigation of the force of impact is probably more importantly through shoulder abduction and extension and elbow flexion rather than action of the forearm muscles.

Physiological modules for generating discrete and rhythmic movements: Component analysis of EMG signals

A central question in Neuroscience is that of how the nervous system generates the spatiotemporal commands needed to realize complex gestures, such as handwriting. A key postulate is that the central nervous system (CNS) builds up complex movements from a set of simpler motor primitives or control modules. In this study we examined the control modules underlying the generation of muscle activations when performing different types of movement: discrete, point-to-point movements in eight different directions and continuous figure-eight movements in both the normal, upright orientation and rotated 90 degrees. To test for the effects of biomechanical constraints, movements were performed in the frontal-parallel or sagittal planes, corresponding to two different nominal flexion/abduction postures of the shoulder. In all cases we measured limb kinematics and surface electromyographic activity (EMB) signals for seven different muscles acting around the shoulder. We first performed principal component analysis (PCA) of the EMG signals on a movement-by-movement basis. We found a surprisingly consistent pattern of muscle groupings across movement types and movement planes, although we could detect systematic differences between the PCs derived from movements performed in each sholder posture and between the principal components associated with the different orientations of the figure. Unexpectedly we found no systematic differences between the figute eights and the point-to-point movements. The first three principal components could be associated with a general co-contraction of all seven muscles plus two patterns of reciprocal activatoin. From these results, we surmise that both "discrete-rhythmic movements" such as the figure eight, and discrete point-to-point movement may be constructed from three different fundamental modules, one regulating the impedance of the limb over the time span of the movement and two others operating to generate movement, one aligned with the vertical and the other aligned with the horizontal.

Adaptive control of stiffness to stabilize hand position with large loads.

The goal of this work was to investigate stability in relation to the magnitude and direction of forces applied by the hand. The endpoint stiffness and joint stiffness of the arm were measured during a postural task in which subjects exerted up to 30% maximum voluntary force in each of four directions while controlling the position of the hand. All four coefficients of the joint stiffness matrix were found to vary linearly with both elbow and shoulder torque. This contrasts with the results of a previous study, which employed a force control task and concluded that the joint stiffness coefficients varied linearly with either shoulder or elbow torque but not both. Joint stiffness was transformed into endpoint stiffness to compare the effect on stability as endpoint force increased. When the joint stiffness coefficients were modeled as varying with the net torque at only one joint, as in the previous study, we found that hand position became unstable if endpoint force exceeded about 22 N in a specific direction. This did not occur when the joint stiffness coefficients were modeled as varying with the net torque at both joints, as in the present study. Rather, hand position became increasingly more stable as endpoint force increased for all directions of applied force. Our analysis suggests that co-contraction of biarticular muscles was primarily responsible for the increased stability. This clearly demonstrates how the central nervous system can selectively adapt the impedance of the arm in a specific direction to stabilize hand position when the force applied by the hand has a destabilizing effect in that direction.

Role of peripheral afference during acquisition of a complex coordination task

It has long been supposed that the interference observed in certain patterns of coordination is mediated, at least in part, by peripheral afference from the moving limbs. We manipulated the level of afferent input, arising from movement of the opposite limb, during the acquisition of a complex coordination task. Participants learned to generate flexion and extension movements of the right wrist, of 75degrees amplitude, that were a quarter cycle out of phase with a 1-Hz sinusoidal visual reference signal. On separate trials, the left wrist either was at rest, or was moved passively by a torque motor through 50degrees, 75degrees or 100degrees, in synchrony with the reference signal. Five acquisition sessions were conducted on successive days. A retention session was conducted I week later. Performance was initially superior when the opposite limb was moved passively than when it was static. The amplitude and frequency of active movement were lower in the static condition than in the driven conditions and the variation in the relative phase relation across trials was greater than in the driven conditions. In addition, the variability of amplitude, frequency and the relative phase relation during each trial was greater when the opposite limb was static than when driven. Similar effects were expressed in electromyograms. The most marked and consistent differences in the accuracy and consistency of performance (defined in terms of relative phase) were between the static condition and the condition in which the left wrist was moved through 50degrees. These outcomes were exhibited most prominently during initial exposure to the task. Increases in task performance during the acquisition period, as assessed by a number of kinematic variables, were generally well described by power functions. In addition, the recruitment of extensor carpi radialis (ECR), and the degree of co-contraction of flexor carpi radialis and ECR, decreased during acquisition. Our results indicate that, in an appropriate task context, afferent feedback from the opposite limb, even when out of phase with the focal movement, may have a positive influence upon the stability of coordination.

The effect of a segmental, localized lower limb cooling protocol on muscular strength and balance

The human neuromuscular system is susceptible to changes within the thermal environment. Cold extrinsic temperatures can significantly reduce muscle and nervous system function and communication, which can have consequences for motor performance. A repeated measures design protocol exposed participants to a 12°C cold water immersion (CWI) up to the ankle, knee, and hip to determine the effect that reduced skin and muscle temperature had on balance and strength task execution. Although a linear reduction in the ability to perform balance tasks was seen from the control condition through to the hip CWI, results from the study indicated a significant reduction in dynamic balance (Star Excursion Balance Test reach distance) performance from only the hip CWI (P<0.05). This reduced performance could have been due to an increase in joint stiffness, increased agonist-antagonist co-contraction, and/or reduced isokinetic muscular strength. Reduced physical performance due to cold temperature could negatively impact outdoor recreational athletics.

Análise do princípio centrando do método pilates e de indicadores de resistência e estabilidade da coluna lombar

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Análise comparativa de variáveis biomecânicas entre as mulheres idosas caidoras e não caidoras

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Análise das variáveis eletromiográficas e da pressão plantar na marcha em aclive, declive e plano

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)