Forearm and Hand Muscles Exhibit High Coactivation and Overlapping of Cortical Motor Representations.

Most of the motor mapping procedures using navigated transcranial magnetic stimulation (nTMS) follow the conventional somatotopic organization of the primary motor cortex (M1) by assessing the representation of a particular target muscle, disregarding the possible coactivation of synergistic muscles. In turn, multiple reports describe a functional organization of the M1 with an overlapping among motor representations acting together to execute movements. In this context, the overlap degree among cortical representations of synergistic hand and forearm muscles remains an open question. This study aimed to evaluate the muscle coactivation and representation overlapping common to the grasping movement and its dependence on the stimulation parameters. The nTMS motor maps were obtained from one carpal muscle and two intrinsic hand muscles during rest. We quantified the overlapping motor maps in size (area and volume overlap degree) and topography (similarity and centroid Euclidean distance) parameters. We demonstrated that these muscle representations are highly overlapped and similar in shape. The overlap degrees involving the forearm muscle were significantly higher than only among the intrinsic hand muscles. Moreover, the stimulation intensity had a stronger effect on the size compared to the topography parameters. Our study contributes to a more detailed cortical motor representation towards a synergistic, functional arrangement of M1. Understanding the muscle group coactivation may provide more accurate motor maps when delineating the eloquent brain tissue during pre-surgical planning.

Effect of the shoulder position on the biceps brachii emg in different dumbbell curls.

Incline Dumbbell Curl (IDC) and Dumbbell Preacher Curl (DPC) are two variations of the standard Dumbbell Biceps Curl (DBC), generally applied to optimize biceps brachii contribution for elbow flexion by fixing shoulder at a specific angle. The aim of this study is to identify changes in the neuromuscular activity of biceps brachii long head for IDC, DPC and DBC exercises, by taking into account the changes in load moment arm and muscle length elicited by each dumbbell curl protocol. A single cycle (concentric-eccentric) of DBC, IDC and DPC, was applied to 22 subjects using a submaximal load of 40% estimated from an isometric MVC test. The neuromuscular activity of biceps brachii long head was compared by further partitioning each contraction into three phases, according to individual elbow joint range of motion. Although all protocols elicited a considerable level of activation of the biceps brachii muscle (at least 50% of maximum RMS), the contribution of this muscle for elbow flexion/extension varied among exercises. The submaximal elbow flexion (concentric) elicited neuro muscular activity up to 95% of the maximum RMS value during the final phase of IDC and DBC and 80% for DPC at the beginning of the movement. All exercises showed significant less muscle activity for the elbow extension (eccentric). The Incline Dumbbell Curl and the classical Dumbbell Biceps Curl resulted in similar patterns of biceps brachii activation for the whole range of motion, whereas Dumbbell Preacher Curl elicited high muscle activation only for a short range of elbow joint angle. Key pointsThe Incline Dumbbell Curl and the Dumbbell Biceps Curl resulted in a considerable neuromuscular effort throughout the whole elbow range of motion.The Incline Dumbbell Curl and the Dumbbell Biceps Curl may be preferable for the improvement of biceps brachii force in training programs.

Lateralized asymmetries in distribution of muscular evoked responses: An evidence of specialized motor control over an intrinsic hand muscle.

Lateralized neural control over hand muscles has been associated with anatomical and physiological asymmetries in the central nervous system. Some studies suggested that the dominant cerebral hemisphere exhibit larger cortical representation areas with lower excitability, while others reported higher cortical excitability in dominant side compared to the contralateral, or even could not find any differences. Thus, neurophysiological lateral asymmetries are still controversial. This study aimed to evaluate differences in dominant and non-dominant sides in motor evoked potentials (MEPs) distribution and investigate whether conventional montages and high-density surface electromyography (HD-sEMG) provide reliable measurements of corticospinal excitability. MEPs elicited by transcranial magnetic stimulation (TMS) were recorded from dominant and non-dominant sides of healthy right-handed participants with an electrode grid over the abductor pollicis brevis muscle. MEPs amplitude distribution, amplitude, latency and resting motor threshold (MT) were evaluated. MEPs distribution significantly shifted towards the lateral direction on the dominant side. MT, amplitude, and latency did not reveal any asymmetries in functional cortical excitability. MEPs amplitude and latency were different for conventional montages and HD-sEMG. Our results suggest that laterality asymmetries manifest in both levels of cortical representation and muscle recruitment, possibly leading to a more pronounced abduction movement on dominant hemisphere compared to the non-dominant side in right-handers. Furthermore, the use of HD-sEMG provided additional insights over conventional electrode montages. A better understanding of laterality asymmetries in fine motor control may help to establish specialized treatments in sensory motor disorders patients.

An alternative approach in muscle fatigue evaluation from the surface EMG signal.

The aim of this study was to compare the intervals of time between adjacent zero crossings (ZCI), an alternative frequency-temporal parameter, with the root-mean-square (RMS) value and the median frequency (F(med)) from the surface EMG (sEMG) signal in muscle fatigue analysis. Twenty right-handed volunteers performed isometric contractions of right biceps brachii muscle while sEMG signals were collected from it at three different and arbitrary load levels until fatigue. The mean ZCI presented a significant correlation with F(med) but not with RMS value and it also presented lower coefficients of variation than others. The results pointed that mean ZCI properties can contribute more than F(med) and RMS value on the interpretation of the muscle function under fatigue conditions.