Correlation networks of spinal motor neurons that innervate lower limb muscles during a multi-joint isometric task

Movements are reportedly controlled through the combination of synergies that generate specific motor outputs by imposing an activation pattern on a group of muscles. To date, the smallest unit of analysis of these synergies has been the muscle through the measurement of its activation. However, the muscle is not the lowest neural level of movement control. In this human study (n = 10), we used a purely data-driven method grounded on graph theory to extract networks of motor neurons based on their correlated activity during an isometric multi-joint task. Specifically, high-density surface electromyography recordings from six lower limb muscles were decomposed into motor neurons spiking activity. We analyzed these activities by identifying their common low-frequency components, from which networks of correlated activity to the motor neurons were derived and interpreted as networks of common synaptic inputs. The vast majority of the identified motor neurons shared common inputs with other motor neuron(s). In addition, groups of motor neurons were partly decoupled from their innervated muscle, such that motor neurons innervating the same muscle did not necessarily receive common inputs. Conversely, some motor neurons from different muscles – including distant muscles – received common inputs. Our study supports the theory that movements are produced through the control of small numbers of groups of motor neurons via common inputs and that there is a partial mismatch between these groups of motor neurons and muscle anatomy. We provide a new neural framework for a deeper understanding of the structure of common inputs to motor neurons. Abstract figure legend Ten participants performed an isometric multi-joint task, which consisted in producing force on an instrumented pedal. Adhesive grids of 64 electrodes were placed over six lower limb muscles (gastrocnemius medialis [GM] and lateralis [GL], vastus lateralis [VL] and medialis [VM], biceps femoris [BF], semitendinosus [ST]). The high-density EMG signals were decomposed into motor unit spike trains. For each pair of motor neurons, we assessed the correlation between their smoothed discharge rates to determine whether they shared common input. Then, we used a purely data-driven method grounded on graph theory to extract networks of common inputs and we applied a clustering procedure to group the motor neurons according to their positions in the graph (i.e., their correlated activity). Results support the theory that movement is produced through the control of small numbers of groups of motor neurons via common inputs and that there is a partial mismatch between these groups of motor neurons and muscle anatomy

Impact of elbow angular velocity on muscle activation and coactivation during active elbow extension and supination in children with spastic hemiplegic cerebral palsy

Does the increase of angular velocity cause an increase of agonist and antagonist muscles activation and of coactivation during active elbow extension and supination in the involved upper limb of children with spastic hemiplegic cerebral palsy

Pathological and physiological muscle co-activation during active elbow extension in children with unilateral cerebral palsy

Objective To address the roles and mechanisms of co-activation in two flexor/extensor pairs during elbow extension in children with cerebral palsy (CP). Methods 13 Typically Developing (TD) and 13 children with unilateral spastic CP performed elbow extension/flexion at different speeds. Elbow angle and velocity were recorded using a 3D motion analysis system. The acceleration and deceleration phases of extension were analyzed. Co-activation of the brachioradialis/triceps and biceps/triceps pairs was computed for each phase from surface electromyographic signals. Statistical analysis involved linear mixed effects models and Spearman rank correlations. Results During the acceleration phase, there was strong co-activation in both muscle pairs in the children with CP, which increased with speed. Co-activation was weak in the TD children and it was not speed-dependent. During the deceleration phase, co-activation was strong and increased with speed in both groups; co-activation of brachioradialis/triceps was stronger in children with CP, and was negatively correlated with extension range and positively correlated with flexor spasticity. Conclusions Abnormal patterns of co-activation in children with CP were found throughout the entire movement. Co-activation was specific to the movement phase and to each flexor muscle. Significance Co-activation in children with CP is both physiological and pathological

Impact of muscle activation on ranges of motion during active elbow movement in children with spastic hemiplegic cerebral palsy

Background Children with spastic hemiplegic cerebral palsy are restricted in their daily activities due to limited active ranges of motion of their involved upper limb, specifically at the elbow. Their impaired muscles are frequently targeted by anti-spastic treatments that reduce muscle tone. But these treatments do not necessarily improve the limb function. There is a lack of comprehensive knowledge of the quantitative relations between muscle activation and joint active ranges of motion. Consequently, the objective of this study is to quantify the impact of muscle activation on the elbow active ranges of motion. Methods During voluntary elbow pronation/supination and extension/flexion movements, kinematic and electromyographic measurements were collected from the involved upper limb of 15 children with spastic hemiplegic cerebral palsy (mean age = 8.7 years, standard deviation = 2.2) and the dominant upper limb of 15 age-matched children who are typically developing. Representative indicators of the muscle activation, such as the muscle co-activation, were extracted from the electromyographic measurements. Findings Muscle co-activation in the involved upper limb accounted for 78% and 59% of the explained variance of the supination and extension limited active ranges of motion respectively. The agonist and antagonist muscle activations were both longer in the involved upper limb. Interpretations This study succeeded in quantifying the impact of longer antagonist muscle activation on decreased elbow active ranges of motion in children with spastic hemiplegic cerebral palsy. Longer agonist muscle activation suggests that strengthening agonist muscles could increase the extension and supination ranges of motion, which constitutes a perspective of future clinical studies