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Objective: The contribution of the contralesional motor cortex to the impaired limbs is still controversial. The aim of this study was to investigate the role of descending projections from the contralesional hemisphere during voluntary elbow flexion on the paretic side. Design: Eleven healthy and 10 stroke subjects performed unilateral isometric elbow flexion tasks at various submaximal levels. Transcranialmagnetic stimulation was delivered to the hotspot of bicepsmuscles ipsilateral to the target side (paretic side in stroke subjects or right side in controls) at rest and during elbow flexion tasks. Motor-evoked potential amplitudes of the contralateral resting biceps muscles, transcranial magnetic stimulation--induced ipsilateral force increment, and reflex torque and weakness of spastic elbow flexors were quantified. Results: The normalized motor-evoked potential amplitude increased with force level in both healthy and stroke subjects. However, stroke subjects exhibited significantly higher force increment compared with healthy subjects only at low level of elbow flexion but similar at moderate to high levels. The greater force increment significantly correlated with reflex torque of the spastic elbow flexors, but not weakness. Conclusions: These results provide novel evidence that ipsilateral projections are not likely to contribute to strength but are correlated to spasticity of spastic-paretic elbow flexors after stroke. [ABSTRACT FROM AUTHOR]

Objectives: Botulinum toxin injection remains the first line management for focal spasticity in stroke patients. Botulinum toxin blocks presynaptic release of acetylcholine at the neuromuscular junction. Current knowledge of the innervation zone (IZ) of motor points is based on histological cadaver studies. The specific aim was to localize IZ non-invasively using high-density EMG recordings in healthy (Exp 1) and stroke (Exp 2) subjects. Design: We enrolled 11 young and healthy subjects in Exp 1 and 10 hemiparetic stroke subjects in Exp 2. The subjects had similar procedures in Exp 1 and 2. A high-density linear electrode array was placed from the proximal to distal tendon junction of biceps brachii muscle. EMG recordings were made on each side in a randomized order during the following two conditions: 1) maximum voluntary contraction (MVC) of elbow flexion and 2) electrical stimulation of the musculocutenous nerve at the intensity that generated maximum responses (M-wave) using an electrical stimulator. Stimulus artifacts were removed from EMG recordings offline. The IZ was localized where the polarity of motor unit action potentials changes, i.e., cross-correlation between adjacent two bipolar signals is the lowest. Results: Innervation zones were easily detected visually and were confirmed by cross-correlation analysis in both MVC and M-wave conditions for both healthy and stroke subjects. There were no differences in IZ locations between two sides in healthy and stroke subjects, and between MVC and M-wave methods. Conclusions: This study demonstrated successful and reliable localization of IZ of biceps muscles in both healthy and stroke subjects. The findings suggest that the M-wave technique could be applied to patients with severe spasticity to detect IZ. These patients usually can not produce voluntary contractions. High-density EMG recordings with M-wave technique could be used for accurate IZ location, which could guide botulinum toxin injection for maximum outcomes. [ABSTRACT FROM AUTHOR]

Objectives: Weakness after stroke is widely seen, and it often has adverse functional sequelae. Peripheral factors such as muscle fiber loss, fat infiltration, altered contractile properties, and an inability to fully activate the muscles on the impaired side are all contributing mechanisms. To further understand peripheral and central contributions, we used M-wave EMG recordings to reflect peripheral neuromuscular capabilities, and EMG recordings during maximal voluntary contraction (MVC) to reveal maximal voluntary activation. The ratio of MVC EMG and M-wave EMG provides an estimate of voluntary activation level. Accordingly, our specific aims were 1) to compare peripheral neuromuscular capabilities (M-wave EMG), maximal voluntary activation (MVC EMG), and voluntary activation level (the ratio) of the biceps brachii muscle between impaired and non-impaired side, 2) to correlate voluntary activation level with weakness. Design: Nine chronic hemiparetic stroke subjects (6 male, 3 female; mean: 62.7 years of age; months after stroke: 45.3, ranging from 28 to 93; MAS 0, 1, and 1+) participated in the experiment. Subjects were seated comfortably on a height- adjustable chair with the arm secured firmly on a customized apparatus. A linear electrode array of 20 bars was placed on the biceps brachii muscle for EMG recordings. Two conditions were tested on each side: 1) maximum voluntary contraction (MVC) of elbow flexion and 2) electrical stimulation of the musculocutaneous nerve at the intensity that generated maximum responses (M-wave). Torque during MVC tasks was recorded. The following variables were calculated: EMGM-wave, EMGMVC, peak torque (TorqueMVC), EMGMVC/M-wave on each side. To normalize individual absolute values, the ratios of EMGM-wave/TorqueMVC and EMGMVC/TorqueMVC were also computed. Weakness in defined as the ratio of TorqueMVC on the impaired side to the non-impaired side. Results: As expected, peak torque was significantly smaller on the impaired than non-impaired side (27.9 Nm vs. 44.5Nm, p=0.04). EMGM-wave was also significantly smaller on the impaired than non-impaired side (1743.7 uV vs. 3269.2 uV, pG0.0002). However, normalized EMGM-wave/TorqueMVC was not significantly different between two sides (85.2 vs. 90.0 uV/Nm, p=0.81). The MVC EMG data showed a different pattern. EMGMVC was smaller on the impaired than non-impaired side (588.0 uV vs. 1410.9 uV, p=0.01). Normalized EMGM-wave/TorqueMVC showed the same (19.6 vs. 32.1 uV/Nm, p=0.04). The voluntary activation level, EMGMVC/M-wave, was also smaller on the impaired than non-impaired side (0.27 vs. 0.40, p=0.08). Using linear regression analysis, the voluntary activation level on the impaired side was highly correlated with weakness (R=0.72), but very low (R=0.32) on non-impaired side. Conclusions: Collectively, our findings suggest that both peripheral and central factors contribute to post-stroke weakness, but activation deficit correlates most closely with weakness. These findings also provide further evidence to highlight the potential benefit from high-intensity exercises to enhance central activation for facilitation of motor recovery. [ABSTRACT FROM AUTHOR]