Short latency stretch reflexes depend on the balance of activity in agonist and antagonist muscles during ballistic elbow movements

The spinal stretch reflex is a fundamental building block of motor function, modulating sensitivity across tasks to augment volitional control. Stretch reflex sensitivity can vary continuously during movement and changes between movement and posture. While there have been many demonstrations of reflex modulation and investigations into the underlying mechanisms, there have been few attempts to provide simple, quantitative descriptions of the relationship between the volitional control and stretch reflex sensitivity throughout tasks that require the coordinated activity of several muscles. Here we develop such a description and use it to test the hypothesis that the modulation of stretch reflex sensitivity during movement can be explained by the balance of activity within the relevant agonist and antagonist muscles better than by the activity only in the muscle homonymous with the elicited reflex. We applied continuous pseudo-random perturbations of elbow angle as subjects completed approximately 500 movements in elbow flexion and extension. Measurements were averaged across the repeated movements to obtain continuous estimates of stretch reflex amplitude and background muscle activity. We also ran a control experiment on a subset of subjects performing postural tasks at muscle activity levels matched to those measured in the movement task. For both experiments, we assessed the relationship between background activity in the agonist and antagonist muscles controlling elbow movement and the stretch reflexes elicited in them. We found that modulation in the stretch reflexes during movement can be described by modulation of the background activity in the agonist and antagonist muscles, and that models incorporating agonists and antagonists are significantly better than those considering only the homonymous muscle. Increases in agonist muscle activity enhanced stretch reflex sensitivity whereas increases in antagonist activity suppressed reflex activity. Surprisingly, the magnitude of these effects was similar, suggesting a balance of control between agonists and antagonist that is very different than the dominance of sensitivity to agonist activity during postural tasks. This greater relative sensitivity to antagonist background activity during movement is due to a large decrease in sensitivity to homonymous muscle activity during movement rather than substantial changes in the influence of antagonist muscle activity.

Temporal Dynamics of Corticomuscular Coherence Reflects Alteration of the Central Mechanisms of Neural Motor Control in Post-Stroke Patients

The neural control of muscular activity during a voluntary movement implies a continuous updating of a mix of afferent and efferent information. Corticomuscular coherence (CMC) is a powerful tool to explore the interactions between the motor cortex and the muscles involved in movement realization. The comparison of the temporal dynamics of CMC between healthy subjects and post-stroke patients could provide new insights into the question of how agonist and antagonist muscles are controlled related to motor performance during active voluntary movements. We recorded scalp electroencephalography activity, electromyography signals from agonist and antagonist muscles, and upper limb kinematics in eight healthy subjects and seventeen chronic post-stroke patients during twenty repeated voluntary elbow extensions and explored whether the modulation of the temporal dynamics of CMC could contribute to motor function impairment. Concomitantly with the alteration of elbow extension kinematics in post-stroke patients, dynamic CMC analysis showed a continuous CMC in both agonist and antagonist muscles during movement and highlighted that instantaneous CMC in antagonist muscles was higher for post-stroke patients compared to controls during the acceleration phase of elbow extension movement. In relation to motor control theories, our findings suggest that CMC could be involved in the online control of voluntary movement through the continuous integration of sensorimotor information. Moreover, specific alterations of CMC in antagonist muscles could reflect central command alterations of the selectivity in post-stroke patients.

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.

A Comparison of Motor Unit Control Strategies between Two Different Isometric Tasks

Background: This study examined the motor unit (MU) control strategies for non-fatiguing isometric elbow flexion tasks at 40% and 70% maximal voluntary isometric contraction. Methods: Nineteen healthy individuals performed two submaximal tasks with similar torque levels: contracting against an immovable object (force task), and maintaining the elbow joint angle against an external load (position task). Surface electromyographic (EMG) signals were collected from the agonist and antagonist muscles. The signals from the agonist were decomposed into individual action potential trains. The linear regression analysis was used to examine the MU recruitment threshold (RT) versus mean firing rates (MFR), and RT versus derecruitment threshold (DT) relationships. Results: Both agonist and antagonist muscles’ EMG amplitudes did not differ between two tasks. The linear slopes of the MU RT versus MFR and RT versus DT relationships during the position task were more negative (p = 0.010) and more positive (p = 0.023), respectively, when compared to the force task. Conclusions: To produce a similar force output, the position task may rely less on the recruitment of relatively high-threshold MUs. Additionally, as the force output decreases, MUs tend to derecruit at a higher force level during the position task.

Tremor

Tremor is broadly classified into physiological tremor and pathological tremor. Depending on the clinical features and the predominant pattern of production, tremor is classified into resting tremor, postural tremor, and kinetic tremor. Tremor is associated with rhythmic contraction of agonist and antagonist muscles, either alternately or simultaneously. Tremor involving muscles in the resting condition is called resting tremor and is seen most commonly in Parkinson disease. Tremor involving muscles during isometric contraction is called postural tremor, and it is most commonly seen in essential tremor. Tremor involving muscles during intended movements (isotonic contraction) is called kinetic tremor, and it is most commonly seen in a lesion of the cerebellar efferent pathway.