Yves Ballay

Inertial properties of the arm are accurately predicted during motor imagery

In the present study, using the mental chronometry paradigm, we examined the hypothesis that during motor imagery the brain uses a forward internal model of arm inertial properties to predict the motion of the arm in different dynamic states. Seven subjects performed overt and covert arm movements with one (motion around the shoulder joint) and two (motion around both the shoulder and elbow joints) degrees of freedom in the horizontal plane. Arm movements were executed under two loading conditions: without and with an added mass (4kg) attached to the subjects right wrist. Additionally, movements were performed in two different directions, condition which implies changes in the arm inertia due to the inertial anisotropy of the arm. Our analysis was focused on the timing features of overt and covert movements measured by means of an electronic stopwatch. Durations of right-direction arm movements (low inertial resistance) were smaller compared to durations of left-direction arm movements (high inertial resistance). Additionally, loading the arm with an added mass of 4kg significantly changed the dynamics of motion: movements were indeed more prolonged under loaded conditions. In both cases, the duration of simulated movements mirrored that of overtly executed movements. Therefore, neither the inertial anisotropy of the arm nor the addition of an external mass affected the timing correspondence between overt and covert movement execution. These findings suggest that the brain internally represents the inertial properties of the arm and makes use of it both for sensorimotor control and for the generation of motor images.

Mentally represented motor actions in normal aging: III. Electromyographic features of imagined arm movements

Motor imagery is a cognitive process during which subjects mentally simulate movements without actually performing them. Here, we investigated the temporal and electromyographic (EMG) features of imagined arm movements in healthy elderly adults. Twelve young (mean age: 24.0+/-1.3 years) and 12 elderly (mean age: 67.0+/-4.5 years) participants executed and mentally simulated, with their right and left arms and as fast and as accurately as possible, arm pointing movements between three targets located in the frontal plane. We used the mental chronometry paradigm as an indicator of the accuracy of the motor imagery process (i.e. isochrony between executed and imagined movements) and the EMG activity of four arm muscles (anterior deltoid, posterior deltoid, biceps brachii, triceps brachii) during imagined actions as an indicator of the ability to generate purely mental actions. Our findings indicated that young and elderly participants mentally simulated arm movements without activating (i.e. above the baseline level) the muscles of the right or the left arm which are involved in the execution of the same movements. This finding suggests that young and, notably, elderly adults are able to generate covert actions without any overt component. However, we found that motor imagery accuracy (i.e. the temporal correspondence between executed and imagined movements) was significantly deteriorated in elderly adults. We suggest that elderly adults use efferent copies of motor commands to generate motor representations; however, this ability is progressively deteriorated in the aging brain. Therefore, we propose using motor imagery cautiously for motor rehabilitation in the elderly.

Mentally represented motor actions in normal aging: II. The influence of the gravito-inertial context on the duration of overt and covert arm movements

Here, we address the question of whether normal aging influences action representation by comparing the ability of 14 young (age: 23.6 +/- 2.1 years) and 14 older (age: 70.1 +/- 4.5 years) adults to mentally simulate arm movements under a varying dynamic context. We conducted two experiments in which we experimentally manipulated the gravity and inertial components of arm dynamics: (i) unloaded and loaded vertical arm movements, rotation around the shoulder joint, (ii) unloaded and loaded horizontal arm movements, rotations around the shoulder and elbow joints, in two directions (inertial anisotropy phenomenon). The main findings indicated that imagery ability was equivalent between the two groups of age for the unloaded arm movements, but better for the young than the older group, for the loaded arm movements. For the horizontal movements, we found better imagery ability for the young than the older adults for both movement directions and loads. Finally, young and old adults showed low (<8%)-temporal variability for both overt and covert arm movements in all conditions. Our findings showed a specific decline of action representation in the aging brain and suggest that internal models of action become imprecise with advance in age. This is not exact to say that there is a severe impairment of motor prediction in old adults as they can mentally represent their arm movements with high-temporal consistency. Finally, we propose that motor imagery could be used as a therapeutic tool for motor rehabilitation in aged adults.

Kinematic adaptation of locomotor pattern in rheumatoid arthritis patients with forefoot impairment

Rheumatoid arthritis (RA) is a leading cause of disability, which affects primarily the forefoot. Moreover, the forefoot is the final ground body interface for transmitting forces produced by the plantar flexors in order to move the body forward. Therefore, a dysfunction in patients with arthritis might induce important changes in gait, such as modifications in the coordination between legs to correct a reduced range of motion (ROM) and to produce smooth stride motions. First, we wanted to investigate the modifications of gait parameters in order to get a deeper understanding of the locomotor adaptation after a distal joint impairment. Second, we wanted to extract the mechanisms used to compensate for these impairments. In order to carry out this study, RA patients with forefoot impairment and healthy subjects were asked to walk along a straight line at two different velocities and were recorded by a motion analysis system. Patients were able to produce an efficient pattern despite a reduction of the ROM of the forefoot. At normal speed, the substantial modification of the locomotor pattern was linked to the adaptation of the lower-limb segment coordination and to the loss of ROM. Compensative mechanisms are the results of an efficient adaptation that offset the effect of the lesions. In contrast, at high speed, all of the kinematic modifications observed at natural speed vanished. It seems that pain and its associated sensory signals help to update the motor command and compel patients to adjust the descending command to the altered representation of distal mobility. Finally, the mechanical consequences of these changes are of particular interest since different levels of force exerted at the hip, knee and ankle might result in a supplementary structural alteration of these joints.

Pointing to double-step visual stimuli from a standing position: very short latency (express) corrections are observed in upper and lower limbs and may not require cortical involvement.

How fast can we correct a planned movement following an unexpected target jump? Subjects, starting in an upright standing position, were required to point to a target that randomly and unexpectedly jumps forward to a constant spatial location. Rapid motor corrections in the upper and lower limbs, with latency responses of less than 100 ms, were revealed by contrasting electromyographic activities in perturbed and unperturbed trials. The earliest responses were observed primarily in the anterior section of the deltoïdus anterior (shoulder) and the tibialis anterior (leg) muscles. Our findings indicate that visual on-going movement corrections may be accomplished via fast loops at the level of the upper and lower limbs and may not require cortical involvement.

Improvement of motor performance by observational training in elderly people

Action observation influences action execution; this strong coupling is underlined by an overlap of cortical areas activated during observation and execution of action, and is dependent of specific motor experience. The goal of the present study was to verify if action observation can be used for rehabilitation of elderly people. We tested this question with a protocol of observational practice of 2 frequently used movements: walking and sit-to-stand/back-to-sit. Both tasks were performed at normal and maximal speed before and after training, by 8 elderly subjects. Observational practice led to an increase in walking velocity via an increase in step frequency, but without modification of step length. In addition, we noted a reduction in BTS duration, but no modification of STS duration. These results highlight the fact that observational practice induces a reactivation in mental representation of action, and may lead to better movement control. Overall, observational practice offers interesting perspectives for rehabilitation of elderly people.

Asymmetrical after-effects of prism adaptation during goal oriented locomotion

In healthy subjects, sensorimotor after-effects of prism adaptation are known to be symmetric (they appear after using leftward and rightward optical deviations), whereas cognitive after-effects are asymmetric (they appear after using a leftward optical deviation) and rightward oriented. Sensorimotor and cognitive after-effects have been classically studied using different specific tasks. The purpose of the current study was to investigate whether both after-effects may be involved in a same visuo-spatial task. Therefore we compared the amplitude of after-effects following adaptation to a rightward or leftward optical deviation. After-effects were assessed by manual pointing or goal oriented locomotor task. The main result showed a greater amplitude for rightward locomotor after-effects (after adaptation to a leftward deviation) than for leftward locomotor after-effects (after adaptation to a rightward deviation). This means that cognitive after-effects may add to sensorimotor after-effects following adaptation to a leftward optical deviation. This asymmetry challenges the classical distinction between sensorimotor and cognitive after-effects of prism adaptation. Implications for the functional mechanisms and the neuroanatomical substrate of prism adaptation are discussed.

Practice-Related Improvements in Postural Control During Rapid Arm Movement in Older Adults: A Preliminary Study

BACKGROUND Postural control associated with self-paced movement is critical for balance in older adults. The present study aimed to investigate the effects of a virtual reality-based program on the postural control associated with rapid arm movement in this population. METHODS From an upright standing position, participants performed rapid arm-raising movements toward a target. Practice-related changes were assessed by pre- and posttest comparisons of hand kinematics and center of pressure displacement parameters measured in a training group (mean age: 71.50 ± 2.67 years, n = 8) and a control group (mean age: 72.87 ± 3.09 years, n = 8). Training group participants took part in six sessions (35-40 minutes per session, three sessions per week). During the two test sessions, arm raising was analyzed under two conditions of stimuli: choice reaction time and simple reaction time. RESULTS We observed improvements in the arm movement after training under both conditions of stimuli. The initial phase of the center of pressure displacement, especially the anticipatory postural adjustments, was improved in the choice reaction time condition. CONCLUSIONS Our short training program resulted in motor optimization of the postural control associated with rapid arm movements, and this implies central changes in motor programming.

Delayed postural control during self-generated perturbations in the frail older adults

Purpose The aim of this study was to investigate the coordination between posture and movement in pathological aging (frailty) in comparison with normal aging, with the hypothesis that in pathological aging, postural control evolves towards a more reactive mode for which the perturbation induced by the movement is not anticipated and leads to delayed and late postural adjustments. Methods Elderly subjects performed rapid focal arm-raising movements towards a target, from an upright standing position in two stimuli conditions: simple reaction time and choice reaction time (CRT). Hand and center of pressure (CoP) kinematics were compared between a control group and a frail group of the same age. Results In frail individuals, the entire movement was impaired and slowed down. In addition, postural adjustments that classically precede and accompany the focal arm movement were delayed and reduced, especially in the CRT condition in which the motor prediction is more limited. Finally, a correlation between the time to CoP maximal velocity and the timed up- and-go score was observed. Conclusion In these patients, it was concluded that the control of the CoP displacement evolved from a proactive mode in which the perturbation associated with the arm movement is anticipated toward a more reactive mode in which the perturbation is compensated by late and delayed adjustments.

How the ankle joint angle alters the antagonist and agonist torques during maximal efforts in dorsi- and plantar flexion.

The aim of this study was to assess, via an EMG bio‐feedback method, the ankle joint angle effect on the agonist and antagonist torques in plantar‐ (PF) and dorsi‐flexion (DF). The isometric PF and DF maximal voluntary contractions (MVCs) torques were measured simultaneously with surface EMG activity of triceps surae (TS) and tibialis anterior (TA) muscles in 12 young adults (mean age 27) at five different ankle joint angles. Our results showed that: (i) The coactivation level does not properly reflect the mechanical effect of the antagonist muscle, (ii) TS antagonist torque significantly altered the DF MVC–angle relationship, whereas TA antagonist torque did not influence this MVC–angle relationship in PF. The alteration of the MVC with angular position was due, in part, to the coactivation phenomenon in DF, but not in PF. Thenceforth, when investigating the torque at the ankle joint, it is necessary to take into account both agonist and antagonist torque modifications with ankle joint angle.