Isometric agonist and antagonist muscle activation interacts differently with 140 Hz tACS aftereffects at different intensities

Introduction: 1) During tES, increasing intracellular Ca2+ levels beyond those needed for inducing LTP may collapse aftereffects. 2) State-dependent plastic aftereffects are reduced when applied during muscle activation as compared to rest. 3) Cortical surround inhibition by antagonistic muscle activation inhibits the center-innervated agonist. Objectives: To determine the interaction of state dependency of tACS aftereffects at rest and under activation of agonist and antagonist muscles during stimulation with different intensities. Methods: In thirteen healthy participants, we measured MEP amplitudes before and after applying tACS at 140 Hz over the motor cortex in nine single-blinded sessions using sham, 1 mA and 2 mA stimulation intensities during rest and activation of agonist and antagonist muscles. Results: During rest, only 1 mA tACS produced a significant MEP increase, while the 2 mA stimulation produced no significant MEP size shift. During agonist activation 1 mA did not induce MEP changes, after 2 mA first a decrease and later an increase of MEPs were observed. Antagonist activation under sham tACS led to an inhibition, which was restored to baseline by 1 and 2 mA tACS. Conclusions: Increasing stimulation intensity beyond 1 mA does not increase excitability, compatible with too strong intracellular Ca2+increase. Antagonist innervation leads to MEP inhibition supporting the concept of surround inhibition, which can be overcome by tACS at both intensities. During agonist innervation a tACS dose dependent relationship exists. Significance: Our results integrate concepts of "leaky membranes" under activation, surround inhibition, intracellular Ca2+ increase and their role in the aftereffects of tACS.

How Well Do Commonly Used Co-Contraction Indices Approximate Lower Limb Joint Stiffness Trends during Gait?

Muscle co-contraction generates joint stiffness to improve stability and accuracy during limb movement but at the expense of higher energetic cost. The quantification of joint stiffness generated from muscle co-contraction is difficult through both experimental and computational means for its benefit and cost to be assessed. Quantification of muscle co-contraction may offer an alternative path for estimating joint stiffness. By choosing the commonly used Co-Contraction Indices (CCIs) to represent muscle co-contraction, this study investigated the feasibility of using CCI to approximate lower limb joint stiffness trends during gait. A calibrated EMG-driven musculoskeletal model of a hemiparetic individual post-stroke from a previous study was used to generate the quantities required for CCI calculation and model-based estimation of joint stiffness. A total of 14 classes of CCIs for various combinations of antagonistic muscle pairs were calculated based on two common CCI formulations, each with 7 types of quantities that included variations of electromyography (EMG) signals and joint moments from the muscles. Correlations between CCIs and model-based estimates of sagittal plane stiffness of the lower extremity joints (hip, knee, ankle) were computed. Although moderate to strong correlation was observed between some CCI formulations and the corresponding joint stiffness, these associations were highly dependent on the methodological choices made for CCI computation. The overall findings of this study were the following: (1) the formulation proposed by Rudolph et al. (2000), CCI1, was more correlated with joint stiffness than that of Falconer and Winter (1985); (2) Moment-based CCI1 from individual antagonistic muscle pairs was more correlated than EMG-based CCI1; (3) EMG signals with calibrated electromechanical delay and joint moment generated by individual muscle without normalization to a reference value were the most correlated for EMG-based CCI1 and moment-based CCI1, respectively. The combination of antagonistic muscle pairs for most correlated within each CCI class was also identified. By using CCI to approximate joint stiffness trends, this study may open an alternative path to studying joint stiffness.

sEMG Activation of the Flexor Muscles in the Foot during Balance Tasks by Young and Older Women: A Pilot Study

Objective: In this publication, we suggest that young adults and seniors use various defense mechanisms to counteract loss of balance. One of the hypotheses is the change in the coordination of antagonistic muscle groups, especially within the ankles. In this study, we tried to determine if there is a relationship between the condition from resilient, to pre-frail, to frail and the ability to maintain balance during free standing and balance tasks. The aim of the study was to define the importance of muscle activity in the ankle joint, dorsal flexor of the foot for the following: tibialis anterior (TA), plantar flexor of the foot gastrocnemius medialis (GM), gastrocnemius lateralis (GL), and peroneus longus (PER), during balance tasks with eyes open (EO) and closed (EC). We hypothesized that there are differences in the activity and co-activation of the tested muscles in young and older women, which may indicate an increased risk of falls and walking disorders. Materials and methods: A group of 20 women qualified for the study. The group was divided into two subgroups, young (G1) and elderly women (G2). The aim of the study was to define the importance of muscle activity in the ankle joint, dorsal flexor of the foot for the following: tibialis anterior (TA), plantar flexor of the foot gastrocnemius medialis (GM), gastrocnemius lateralis (GL), and peroneus longus (PER), during balance tasks with eyes open (EO) and closed (EC). Results: In this study, we observed significant differences between groups in the maximum and mean values of electromyography activity (EMG) activation of the examined muscles on different types of surfaces and with open and closed eyes. Older women generated higher values of EMG activation in all muscles except the gastrocnemius medialis muscle. The results were significant for co-activation at rest for muscles as follows: tibialis anterior and gastrocnemius medialis with eyes closed (p = 0.01) and peroneus and gastrocnemius lateralis at rest with eyes open (p = 0.03), eyes closed (p = 0.04), and on a foam (p = 0.02). The sEMG amplitude of the tested muscles means that agonist muscle activity changed relative to antagonistic muscle activity. Conclusions: Activation of sEMG and coordination of ankle muscles during balance tasks change with age. It can be hypothesized that assessment of balance during free standing and equivalent tasks can predict the state of frailty, after taking into account other physiological variables that are believed to affect balance control.

Human motor fatigability as evoked by repetitive movements results from a gradual breakdown of surround inhibition

Motor fatigability emerges when demanding tasks are executed over an extended period of time. Here, we used repetitive low-force movements that cause a gradual reduction in movement speed (or ‘motor slowing’) to study the central component of fatigability in healthy adults. We show that motor slowing is associated with a gradual increase of net excitability in the motor network and, specifically, in primary motor cortex (M1), which results from overall disinhibition. Importantly, we link performance decrements to a breakdown of surround inhibition in M1, which is associated with high coactivation of antagonistic muscle groups. This is consistent with the model that a loss of inhibitory control might broaden the tuning of population vectors such that movement patterns become more variable, ill-timed and effortful. We propose that the release of inhibition in M1 is an important mechanism underpinning motor fatigability and, potentially, also pathological fatigue as frequently observed in patients with brain disorders.

Repeat Surgery for Patients Operated in Childhood for Early-Onset Esotropia and Suffering from Persistent Residual, Recurrent or Consecutive Strabismus

Background Patients operated in childhood for early esotropia may suffer from persistent strabismus, whether residual, recurrent, or consecutive, usually with a more complex oculomotor imbalance than in primitive strabismus. History and Signs The clinical data are fundamental, in particular, (1) the basic accommodation-free deviation, (2) the horizontal far/near incomitance, (3) the cyclovertical incomitances and/or A or V pattern, and (4) the motility restrictions. In complicated cases, magnetic resonance tomography of the orbits showed the anomalies of the eye and muscle position. The intraoperative findings have to be taken into account, such as (1) the position of the eyes and (2) the muscle extensibility in antagonistic muscle pairs. Twenty-seven patients, aged 16 to 62, were included in the present retrospective study. All of them had repeat surgery between 2013 to 2018, 15 for residual esotropia and 12 for consecutive exotropia. Therapy and Outcome In five of the eight patients with posterior fixation sutures on the medial recti, the residual exo or estropia was moderate and could be corrected with a recession or tuck of the lateral recti. In the three other patients with pronounced esotropia, revision of the posterior fixation sutures was carried out. In 19 patients (11 esotropia and 8 exotropia) with only prior conventional surgery, the surgical choice for the residual deviation went electively to the muscles that were most responsible for the motor imbalance, without using posterior fixation sutures. In 4 of these 19 patients, the deviation was small due, in particular, to residual incomitances. For any persistent cyclovertical incomitance, the overacting oblique muscles were recessed, and, in four patients with an A or V pattern, the lateral rectus insertions shifted to the horizontal meridian in the same procedure. Discussion The two main difficulties are to assess (1) the imbalance of the active and passive muscle force and (2) the surgically induced incomitances.

Re-description of Azygokeras columbiae Koeller & Littlepage, 1976 (Calanoida: Aetideidae) and musculature of the male grasping antennule

Azygokeras columbiae from Bute Inlet, British Columbia, Canada, is re-described, correcting some details and adding information not available in the original description. Azygokeras columbiae is unique amongst male Aetideidae in having the right antennule modified for grasping but without a true knee joint (geniculation) between segments XX and XXI nor a hiatus in the musculature at this joint, typical of taxa with a geniculate male antennule. Male Azygokeras have wide pivot points and arthrodial membranes between segments XXI and XXII, XXIII and XXIV and XXV and XXVI that allow greater movement in several planes than in homologous segments of Euaugaptilus and Heterorhabdus. Modifications of the terminal antennular joints allow for extensive movement in several planes associated with a series of short muscles in segments XIX to XXVI. These muscles become progressively more massive from proximal to distal on the antennule and are paired with an antagonistic muscle also increasing in mass distally.