Motor Unit Action Potential Size Research Articles

Characteristics of motor unit recruitment in boys and men at maximal and submaximal force levels.

The aim of this study was to compare voluntary activation (VA) and motor units (MU) recruitment patterns between boys and men at different contraction levels of the knee extensor muscles. We hypothesized that boys and men would display similar VA and MU recruitment patterns at low submaximal force levels, but that boys would display a lower utilization of their higher-threshold MU and a lower VA at near-maximal and maximal force levels than men. 11 prepubertal boys and 13 men were tested at the optimal knee angle. Next, VA was assessed using the twitch interpolation technique during maximal (MVC) and submaximal isometric voluntary contractions. Mean firing rate (MFR), recruitment threshold (RT) and motor unit action potential size (MUAPSIZE) were extracted to characterize neural strategies. No significant difference between groups was found for VA at every contraction level. Similarly, no significant difference was found for the MFR vs. RT relationship parameters between groups. For the vastus lateralis (VL) muscle, the MUAPSIZE vs. RT relationship differed between boys and men independent of the contraction level (p < 0.05). Boys also displayed a different MFR vs. MUAPSIZE relationship on the VL muscle independent of the contraction level (p < 0.05). To conclude, no difference between boys and men was found for VA regardless of the contraction level investigated. Differences in motor unit recruitment parameters between boys and men seem to be explained by different muscle dimensions between groups.

Surface electromyographic amplitude does not identify differences in neural drive to synergistic muscles.

Surface electromyographic (EMG) signal amplitude is typically used to compare the neural drive to muscles. We experimentally investigated this association by studying the motor unit (MU) behavior and action potentials in the vastus medialis (VM) and vastus lateralis (VL) muscles. Eighteen participants performed isometric knee extensions at four target torques [10, 30, 50, and 70% of the maximum torque (MVC)] while high-density EMG signals were recorded from the VM and VL. The absolute EMG amplitude was greater for VM than VL ( P < 0.001), whereas the EMG amplitude normalized with respect to MVC was greater for VL than VM ( P < 0.04). Because differences in EMG amplitude can be due to both differences in the neural drive and in the size of the MU action potentials, we indirectly inferred the neural drives received by the two muscles by estimating the synaptic inputs received by the corresponding motor neuron pools. For this purpose, we analyzed the increase in discharge rate from recruitment to target torque for motor units matched by recruitment threshold in the two muscles. This analysis indicated that the two muscles received similar levels of neural drive. Nonetheless, the size of the MU action potentials was greater for VM than VL ( P < 0.001), and this difference explained most of the differences in EMG amplitude between the two muscles (~63% of explained variance). These results indicate that EMG amplitude, even following normalization, does not reflect the neural drive to synergistic muscles. Moreover, absolute EMG amplitude is mainly explained by the size of MU action potentials. NEW & NOTEWORTHY Electromyographic (EMG) amplitude is widely used to compare indirectly the strength of neural drive received by synergistic muscles. However, there are no studies validating this approach with motor unit data. Here, we compared between-muscles differences in surface EMG amplitude and motor unit behavior. The results clarify the limitations of surface EMG to interpret differences in neural drive between muscles.

Motor Unit Action Potential Sizes of the First Dorsal Interosseous in Young and Older Individuals

Slopes from the motor unit action potential size (MUAPsize) vs. recruitment threshold (RT) relationships provides a non-invasive measurement of MU sizes. As such, the slopes (APslopes) may be able to quantify age-related MU atrophy. PURPOSE: To determine if APslopes differed between young and old individuals. METHODS: Twenty two young (YG, age = 22.5±2.7 yrs) and ten aged (OG, 61.0±2.0 yrs) subjects completed the investigation. Surface electromyography signals were recorded from the first dorsal interosseous (FDI) during an isometric trapezoidal muscle action with the steady force plateau set at 50% maximal voluntary contraction (MVC). The signals were decomposed to yield MUAPsizes and RTs for each MU. The APslope was calculated via the MUAPsizes vs. RT relationships for MUs recruited between 10 and 50% MVC for each subject. Few MUs with RTs < 10% MVC were observed in young or old and, thus, these MUs were excluded from the slope calculation. FDI cross-sectional area (CSA) and echo intensity (EI) were quantified using ultrasonography. Possible differences in APslope, CSA and EI between groups were examined with independent samples t-tests. RESULTS: OG demonstrated significantly reduced APslopes (OG: 0.033 ± 0.010; YG: 0.048 ± 0.020; p = 0.048) and greater EI (OG: 41.3 ± 7.0 AU; YG: 50.6 ± 7.5 AU; p = 0.002), however, CSA was similar between YG and OG (OG: 2.22 ± 0.47 cm2; YG: 2.09 ± 0.31 cm2; p = 0.438). CONCLUSION: The APslopes suggested non-uniform differences in MUAPsizes in relation to RT, likely due to reduced sizes of higher threshold MUs. Higher threshold MUs have been suggested to contain a greater proportion of type II muscle fibers, which demonstrate greater age induced atrophy than type I fibers commonly associated with lower threshold MUs. As such, the difference in APslopes suggested atrophy of higher threshold MUs in OG. In addition, OG’s elevated EI indicates greater infiltration of adipose and connective tissue into the muscle. Greater EI in combination with similar CSA may indicate a reduced contractile tissue volume in the OG, supporting the speculation that the APslope differences were due to MU atrophy.

Inappropriate interpretation of surface EMG signals and muscle fiber characteristics impedes understanding of the control of neuromuscular function

as the final common pathway from the nervous system to muscle, the motor unit transmits an activation signal generated by the nervous system to engage the contractile proteins and produce the muscle forces needed for reflex responses, automatic behaviors, and voluntary actions ([11][1]). The net

Neuromuscular remodeling and neuromuscular junction instability in human diabetic neuropathy (1168.1)

Diabetic polyneuropathy (DPN) is a progressive axonopathy marked by loss of motor fibers, compensatory collateral reinnervation and reduced stability of neuromuscular transmission. Our objective was to assess the degree of reinnervation and motor unit instability in patients with DPN using decomposition‐based quantitative electromyography (DQEMG). Additionally, relationships between motor unit stability and muscle function were examined. The tibialis anterior (TA) muscle was tested in twelve patients with DPN (65 ± 15 yrs) and 12 age‐matched controls (63 ± 15 yrs). DQEMG was used to analyze surface and intramuscular EMG signals recorded from the TA during moderate voluntary dorsiflexion contractions. Individual motor unit action potential (MUP) trains were identified and analyzed for: MUP size (peak to peak amplitude, area), complexity (turns, fiber dispersion) and stability (near fiber jiggle). DPN patients featured larger (+45% MUP area), more complex (+40% fiber dispersion), and less stable (+30% near fiber jiggle) MUPs (p&lt;0.05). No significant relationships were found between MUP stability and muscular denervation, or strength. MUP complexity and instability were positively related in DPN patients (r=0.46; p&lt;0.05) and controls (r=0.37; p&lt;0.05). DPN is associated with neuromuscular remodeling which leads to increasingly impaired neuromuscular transmission that is detectable using DQEMG.Grant Funding Source: NSERC

Motor unit potential change with contraction level: A concentric macro electromyography study

Motor unit action potential (MUAP) reference values are usually given per muscle and age group. Our aim was to evaluate the change in MUAP size at different contraction levels using the concentric macro (ConMac) EMG technique. ConMac needles were used to record the electrode cannula MUAP and measure its amplitude and area during weak, moderate, and strong muscle contractions. MUAP area and amplitude gradually increased from weak to strong contraction levels in all muscles studied. MUAP amplitudes were larger in distal than in proximal muscles, likely because of a higher fiber density distally. MUAP normal values in the literature per muscle do not take into account the contraction level at which they were recorded and can misrepresent the interpretation of normal. Concentric macro EMG is a simple and useful complement to routine EMG studies and yields additional information on MUAP neurophysiology.

Multichannel surface EMG: Basic aspects and clinical utility

The generation of the surface electromyogram (sEMG) is described with regard to the properties of the single muscle fiber action potential as source, the physical aspects of volume conduction and recording configuration, and the properties and firing pattern of motor units (MUs). The spatial aspect of the motor unit action potential (MUP) is emphasized in relation to the results of high-density, multichannel sEMG measurements. The endplate zone, depth, size, and position of MUs can be estimated. The use of muscle fiber conduction velocity measurements in channelopathies and the changes in pathological fatigue are described. Using the unique patterns of spatial spread of MUPs over the skin (MU fingerprint), MU classification and the determination of firing moments is done noninvasively. Clinical applications of high-density sEMG measurements are reviewed. Emerging possibilities provided by MUP size and fingerprint measurements in neuromuscular disease and motor control are discussed. We conclude that multichannel sEMG adds unique, and sometimes indispensable, spatial information to our knowledge of the motor unit.

Electrophysiological estimation of motor units in Duchenne muscular dystrophy

Electrophysiological estimations of the number and size of motor unit action potentials (MUAP) have been made in the extensor digitorum brevis muscle of 9 patients with Duchenne muscular dystrophy and 9 matched control subjects. A critical analysis of the method used has been made. The number of motor units and the peak terminal latency of the deep peroneal nerve did not differ significantly between the two groups, whilst the size of MUAP was significantly reduced in the patient group. Our findings are in accordance with the traditional concept of Duchenne muscular dystrophy as a primary myopathy, and cast doubts upon the evidence supporting the neurogenic theory of muscular dystrophy.