Sylvain Cremoux

Impaired corticomuscular coherence during isometric elbow flexion contractions in human with cervical Spinal Cord Injury

After spinal cord injury (SCI), the reorganization of the neuromuscular system leads to increased antagonist muscles’ co‐activation—that is, increased antagonist vs. agonist muscles activation ratio—during voluntary contractions. Increased muscle co‐activation is supposed to result from reduced cortical influences on spinal mechanisms inhibiting antagonist muscles. The assessment of the residual interactions between cortical and muscles activity with corticomuscular coherence (CMC) in participants with SCI producing different force levels may shed new lights on the regulation of muscle co‐activation. To achieve this aim, we compared the net joint torque, the muscle co‐activation and the CMC ~ 10 and ~ 20 Hz with both agonist and antagonist muscles in participants with SCI and healthy participants performing actual isometric elbow flexion contractions at three force levels. For all participants, overall CMC and muscle co‐activation decreased with the increase in the net joint torque, but only CMC ~ 10 Hz was correlated with muscle co‐activation. Participants with SCI had greater muscle co‐activation and lower CMC ~ 10 Hz, at the highest force levels. These results emphasize the importance of CMC as a mechanism that could take part in the modulation of muscle co‐activation to maintain a specific force level. Lower CMC ~ 10 Hz in SCI participants may reflect the decreased cortical influence on spinal mechanisms, leading to increased muscle co‐activation, although plasticity of the corticomuscular coupling seems to be preserved after SCI to modulate the force level. Clinically, the CMC may efficiently evaluate the residual integrity of the neuromuscular system after SCI and the effects of rehabilitation.

Effect of training status on beta-range corticomuscular coherence in agonist vs. antagonist muscles during isometric knee contractions

Antagonist muscle co-activation is thought to be partially regulated by cortical influences, but direct motor cortex involvement is not fully understood. Corticomuscular coherence (CMC) measures direct functional coupling of the motor cortex and muscles. As antagonist co-activation differs according to training status, comparison of CMC in agonist and antagonist muscles and in strength-trained and endurance-trained individuals may provide in-depth knowledge of cortical implication in antagonist muscle co-activation. Electroencephalographic and electromyographic signals were recorded, while 10 strength-trained and 11 endurance-trained participants performed isometric knee contractions in flexion and extension at various torque levels. CMC magnitude in 13–21 and 21–31xa0Hz frequency bands was quantified by CMC analysis between Cz electroencephalographic electrode activity and all electromyographic signals. CMC was significant in both 13–21 and 21–31xa0Hz frequency bands in flexor and extensor muscles regardless of participant group, torque level, and direction of contraction. CMC magnitude decreased more in antagonist than in agonist muscles as torque level increased. Finally, CMC magnitude was higher in strength-trained than in endurance-trained participants. These findings provide experimental evidence that the motor cortex directly regulates both agonist and antagonist muscles. Nevertheless, the mechanisms underlying muscle activation may be specific to their function. Between-group modulation of corticomuscular coherence may result from training-induced adaptation, re-emphasizing that corticomuscular coherence analysis may be efficient in characterizing corticospinal adaptations after long-term muscle specialization.

An enhanced experimental procedure to rationalize on the impairment of perception of action capabilities

It is well documented that changes in the physiological states of the perceiver–actor influence the perception of action capabilities. However, because experimental procedures of most studies involved a limitless availability for stimuli visual encoding and perceptual strategies, it remains difficult to adopt a single position among the large range of alternative interpretations for impaired perception. A reaching-to-grasp paradigm under breathing restriction was adapted from Graydon et al. (Cogn Emot 26:1301–1305, 2012) to standardize the time for encoding of stimuli information and narrowed the involvement of perceptual strategies. In the present study, we propose a highly controlled environment where the discrete information is presented during 300xa0ms, congruently with neurophysiological studies focused on visuomotor transformation. An underestimation of the perception of action capabilities is found under breath restriction, suggesting that 300xa0ms for stimuli encoding is sufficient to induce altered visuomotor brain transformations when limiting the involvement of perceptual strategies. This result suggests that such behavior could refer to an impaired brain potentiation of the perceptual occurrence, providing strong hypotheses on the brain dynamics of sensorimotor integration that underlie impaired perception of action capabilities in stressful situations.

Quantification of Movement-Related EEG Correlates Associated with Motor Training: A Study on Movement-Related Cortical Potentials and Sensorimotor Rhythms

The ability to learn motor tasks is important in both healthy and pathological conditions. Measurement tools commonly used to quantify the neurophysiological changes associated with motor training such as transcranial magnetic stimulation and functional magnetic resonance imaging pose some challenges, including safety concerns, utility, and cost. EEG offers an attractive alternative as a quantification tool. Different EEG phenomena, movement-related cortical potentials (MRCPs) and sensorimotor rhythms (event-related desynchronization—ERD, and event-related synchronization—ERS), have been shown to change with motor training, but conflicting results have been reported. The aim of this study was to investigate how the EEG correlates (MRCP and ERD/ERS) from the motor cortex are modulated by short (single session in 14 subjects) and long (six sessions in 18 subjects) motor training. Ninety palmar grasps were performed before and after 1 × 45 (or 6 × 45) min of motor training with the non-dominant hand (laparoscopic surgery simulation). Four channels of EEG were recorded continuously during the experiments. The MRCP and ERD/ERS from the alpha/mu and beta bands were calculated and compared before and after the training. An increase in the MRCP amplitude was observed after a single session of training, and a decrease was observed after six sessions. For the ERD/ERS analysis, a significant change was observed only after the single training session in the beta ERD. In conclusion, the MRCP and ERD change as a result of motor training, but they are subject to a marked intra- and inter-subject variability.

I can’t reach it! Focus on theta sensorimotor rhythm toward a better understanding of impaired action–perception coupling

It is known that anxiety (ANX) impairs action-perception coupling. This study tests whether this impairment could be associated with an alteration of the sensorimotor function. To this aim, the cortical activities underlying the sensorimotor function were recorded in twelve volunteers in a reach-to-grasp paradigm, in which the level of ANX and the position of a glass were manipulated. The experimental manipulation of the ANX-related somatosensory state was expected to prompt participants to underestimate their reaching-to-grasp capabilities while the sensorimotor-related oscillatory brain activities around the 6-Hz (θ) frequency over motor-related and parietal regions were expected to be modulated. We also investigated the oscillatory brain dynamics around the 11.5-Hz (fast-α) frequency as a neural hallmark of ANX manipulation induced by the breath-restriction. Results indeed showed that participants underestimated their reaching-to-grasp maximal performance. Concomittantly, θ-EEG synchronization over the motor cortex contralateral to the dominant hand was higher during glass presentation under breath-restriction condition (+20.1%; p<0.05), and when the glass was perceived as non-reachable (+20.0%; p<0.05). Fast-α-EEG desynchronization was reduced under breath-restriction (-37.7%; p<0.05). The results confirm that ANX-related impairment of action-perception coupling co-modulates with theta-sensorimotor rhythm. This finding is discussed as an altered readiness state in the reaching-related cortical network, while individuals are anxious.

Modulation of intermuscular coherence between homologous muscles reflects different common neural drive regulating bilateral contractions

Objectives In healthy adults, homologous muscle activations appear during bilateral contractions (BI) and can be induced during unilateral contractions (UNI) at high force level. After brain injury, involuntary homologous muscle activations during UNI can have neurological significance. Although homologous muscle activations are supposed to originate from commands sent by the motor cortices [1] , the common neural drive shared across homologous muscles remains unclear. The intermuscular coherence (IMC) between electromyographic (EMG) signals can highlight different neural strategies to control muscle activations [2] , [3] . The IMCxa0∼xa010xa0Hz is supposed to reflect subcortical strategies while IMC in higher frequency bands is supposed to reflect different cortical contributions. This study compared the IMC between homologous muscles according to contraction type. Methods Eleven right-footed subjects performed 3 maximal isometric UNI and BI ankle plantarflexion contractions. Right and left ankle torque and homologous triceps surae EMG signals were recorded. IMC magnitude between homologous EMG signals was calculated in α (5–12xa0Hz), β (13–30xa0Hz), γ (30–60xa0Hz) and γ+ (>xa060xa0Hz) frequency bands during the 2xa0s where the net ankle torque was maximal. Right and left torques and IMC magnitudes were compared between UNI and BI. Results Right torque was not significantly different between conditions. Left torque was significantly different from baseline and between conditions. IMCα was significantly higher during UNI than BI. No significant differences in IMCβ and IMCγ between conditions were observed. IMCγ+ was significantly higher during BI than UNI. Conclusions Our results suggest different neural strategies to control muscle activations according to contraction type. During UNI, higher IMCα suggests subcortical structures involvement to inhibit unsolicited homologous muscle activations [3] . During BI, higher IMCγ+ may suggest an optimization of the neural drive shared across homologous muscles to reduce computational effort [2] , [3] . Similar investigations in people with brain injury may help understanding the underlying neurophysiological mechanisms of motor overflow.

Magnitude of the post-movement beta synchronization correlates with the variability of the ankle torque production

Objectives During and after the completion of a motor task, an event-related desynchronization (ERD) and synchronization (ERS) – i.e., a decrease and increase of the 13–31xa0Hz spectral magnitude–are respectively recorded over the motor cortex (M1). The ERD is supposed to reflect M1 involvement to perform the motor task [1] , [2] while the ERS is supposed to reflect an active “top-down” cortical inhibition of the motor command [1] , [3] . Studies revealed that ERD/S are modulated with the force level [1] . However, it remains to know whether ERD/S are related to the task performance, reflecting force control. We investigated the relationship between the ERD/S and the accuracy and variability of the force production. Methods Eleven healthy participants performed 50xa0×xa010-s isometric right plantar flexion contractions at 20% of their maximal torque. The ankle torque and the M1 EEG activity were recorded. After appropriate preprocessing, the absolute and variable errors (AE/VE) of the ankle torque, respectively reflecting accuracy and variability of the motor task and the ERD were computed during the holding force phase. The ERS was computed 1-s after the end of the contraction. All data were normally distributed. Pearson correlations between ERD/S and AE/VE were computed. Results No significant correlation was found between the ERD and VE (r2xa0=xa00.11; Pxa0=xa00.75) nor between the ERD and AE (r2xa0=xa00.56; Pxa0=xa00.07). The ERS significantly increased as the VE increased (r2xa0=xa00.8; Pxa0 Conclusion These results suggest that the ERD is not linked with fine motor control modulating accuracy and variability of force. However, the significant ERS-VE relationship suggests that the more variable is the force produced, the more M1 is deactivated, probably reflecting an increased inhibition of the motor command.

Functional and Corticomuscular Changes Associated with Early Phase of Motor Training

This study aimed to evaluate the corticomuscular changes associated with motor training. We quantified functional and corticomuscular changes during a four-days motor training. Results revealed better functional performance and increased magnitude of corticomuscular coherence in the 8–13 Hz frequency band over days. These results are supposed to reflect a better integration of sensorimotor integration.

Modeling and Control of Rehabilitation Robotic Device: motoBOTTE

The motoBOTTE is a robot designed for assistive rehabilitation. The identification of a mathematical model for the system is shown; then we design and implement a nonlinear control law for tracking a reference signal. The nonlinear controller design is achieved by combining an exact convex representation with the direct method of Lyapunov. Results are given in terms of linear matrix inequalities (LMI); simulation and real-time results are shown as well.

Effect of the phase of force production on corticomuscular coherence with agonist and antagonist muscles

During isometric contractions, the net joint torque stability is modulated with the force production phases, i.e., increasing (IFP), holding (HFP), and decreasing force (DFP) phases. It was hypothesized that this modulation results from an altered cortical control of agonist and antagonist muscle activations. Eleven healthy participants performed 50 submaximal isometric ankle plantar flexion contractions. The force production phase effect (IFP, HFP and DFP) was assessed on the net joint torque stability, agonist and antagonist muscles activations, cortical activation, and corticomuscular coherence (CMC) with agonist and antagonist muscles. In comparison to HFP, the net joint torque stability, the agonist muscles activation and the CMC with agonist muscles were lower during IFP and even more during DFP. Antagonist muscle activations, cortical activations and CMC with antagonist muscles were higher during HFP than during IFP only. Increased CMC with agonist and antagonist muscles appeared to enhance the fine motor control. At a cortical level, agonist and antagonist muscle activations seemed to be controlled independently according to their muscle function and the phase of force production. Results revealed that CMC was an adequate measure to investigate the cortical regulation of agonist and antagonist muscle activations. This may have potential applications for patients with altered muscle activations.