Modulation of corticomotor excitability during passive and active wrist flexion and extension

Various mechanisms may alter corticomotor excitability to agonist and antagonist muscles during passive and active limb movement depending on parameters of movement and their functional role. A better understanding of these relationships is important for understanding basic motor control mechanisms, and may be relevant to motor rehabilitation programs after brain injury. The purpose of the present study was to compare changes in corticomotor excitability to wrist flexor and extensor muscles during different phases of movement (flexion/extension), and at rest and during actively or passively-mediated length changes. Motor evoked potentials (MEP) of the flexor carpi radialis (FCR) and extensor carpi radialis (ECR) were recorded from 17 participants during resting and four movement conditions (passive wrist flexion and extension, active wrist flexion and extension) with their palm inserted into a hand piece. Passive and active movements were carried out by moving the hand piece for 22.5° wrist flexion and 22.5° wrist extension from the neutral wrist position of 0° at a cycle rate of 1 Hz. transcranial magnetic stimulation (TMS) was delivered at the neutral position (0°) every ten cycles to obtain 12 MEPs. The mean MEP amplitude was compared across the resting, lengthening and shortening phases for passive and active movements for the FCR and ECR separately by a paired t-test. Comparison was also made between FCR and ECR, and between passive and active movements by a two-way repeated measures ANOVA. The MEP amplitude was significantly (

Cross-activation of the motor cortex during unilateral contractions of the quadriceps

Transcranial magnetic stimulation (TMS) studies have demonstrated that unilateral muscle contractions in the upper limb produce motor cortical activity in both the contralateral and ipsilateral motor cortices. The increase in excitability of the corticomotor pathway activating the resting limb has been termed "cross-activation", and is of importance due to its involvement in cross-education and rehabilitation. To date, very few studies have investigated cross-activation in the lower limb. Sixteen healthy participants (mean age 29 ± 9 years) took part in this study. To determine the effect of varying contraction intensities in the lower limb, we investigated corticomotor excitability and intracortical inhibition of the right rectus femoris (RF) while the left leg performed isometric extension at 0%, 25%, 50%, 75% and 100% of maximum force output. Contraction intensities of 50% maximal force output and greater produced significant cross-activation of the corticomotor pathway. A reduction in silent period duration was observed during 75% and 100% contractions, while the release of short-interval intracortical inhibition (SICI) was only observed during maximal (100%) contractions. We conclude that increasing isometric contraction intensities produce a monotonic increase in cross-activation, which was greatest during 100% force output. Unilateral training programs designed to induce cross-education of strength in the lower limb should therefore be prescribed at the maximal intensity tolerable

Unilateral movement preparation causes task-specific modulation of TMS responses in the passive, opposite limb

Corticospinal excitability is modulated for muscles on both sides of the body during unilateral movement preparation. For the effector, there is a progressive increase in excitability, and a shift in direction of muscle twitches evoked by transcranial magnetic stimulation (TMS) toward the impending movement. By contrast, the directional characteristics of excitability changes in the opposite (passive) limb have not been fully characterized. Here we assessed how preparation of voluntary forces towards four spatially distinct visual targets with the left wrist alters muscle twitches and motor evoked potentials (MEPs) elicited by TMS of left motor cortex. MEPs were facilitated significantly more in muscles homologous to agonist rather than antagonist muscles in the active limb, from 120\ua0ms prior to voluntary EMG onset. Thus, unilateral motor preparation has a directionally-specific influence on pathways projecting to the opposite limb that corresponds to the active muscles rather than the direction of movement in space. The directions of TMS-evoked twitches also deviated toward the impending force direction of the active limb, according to muscle-based coordinates, following the onset of voluntary EMG. The data indicate that preparation of a unilateral movement increases task-dependent excitability in ipsilateral motor cortex, or its downstream projections, that reflect the forces applied by the active limb in an intrinsic (body-centered), rather than an extrinsic (world-centered), coordinate system. The results suggest that ipsilateral motor cortical activity prior to unilateral action reflects the state of the active limb, rather than subliminal motor planning for the passive limb. This article is protected by copyright. All rights reserved