Muscle Units Research Articles

Body Composition of Patients Undergoing Radical Cystectomy for Bladder Cancer: Sarcopenia, Low Psoas Muscle Index, and Myosteatosis Are Independent Risk Factors for Mortality

We assessed a wide array of body composition parameters to identify those most relevant as prognostic tools for patients undergoing radical cystectomy (RC) due to bladder cancer (BC). In this retrospective, single-center study, preoperative computed tomography (CT) scans of 657 patients were measured at the level of the 3rd lumbar vertebra (L3) to determine common body composition indices including sarcopenia, myosteatosis, psoas muscle index (PMI), subcutaneous and visceral fat index (SFI and VFI), visceral-to-subcutaneous fat ratio (VSR), and visceral obesity. Predictors of overall survival (OS) and cancer-specific survival (CSS) were identified in univariate and multivariate survival analysis. Sarcopenia and a low PMI were independently associated with shorter OS (Sarcopenia: HR 1.30; 95% CI 1.02-1.66; p = 0.04 and a low PMI: HR 1.32; 95% CI 1.02-1.70; p = 0.03) and CSS (Sarcopenia: HR 1.64; 95% CI 1.19-2.25; p < 0.01 and a low PMI: HR 1.41; 95% CI 1.02-1.96; p = 0.04). Myosteatosis, measured as decreasing average Hounsfield units of skeletal muscle, was an independent risk factor for OS (HR 0.98; 95% CI 0.97-1.00; p = 0.01) and CSS (HR 0.98; 95% CI 0.96-1.00; p < 0.05). The assessed adipose tissue indices were not significant predictors for OS and CSS. Sarcopenia, a low PMI, and myosteatosis are independent predictors for OS and CSS in patients undergoing radical cystectomy for bladder cancer.

Modeling, Analysis, and Computational Design of Muscle-driven Soft Robots.

Muscle driving is a critical actuation mode of soft or flexible robots and plays a key role in the motion of most animals. Although the system development of soft robots has been extensively investigated, the general kinematic modeling of soft bodies and the design methods used for muscle-driven soft robots (MDSRs) are inadequate. With a focus on homogeneous MDSRs, this article presents a framework for kinematic modeling and computational design. Based on continuum mechanics theory, the mechanical characteristics of soft bodies were first described using a deformation gradient tensor and energy density function. The discretized deformation was then depicted using a triangular meshing tool according to the piecewise linear hypothesis. Deformation models of MDSRs caused by external driving points or internal muscle units were established by the constitutive modeling of hyperelastic materials. The computational design of the MDSR was then addressed based on kinematic models and deformation analysis. Algorithms were proposed to infer the design parameters from the target deformation and to determine the optimal muscles. Several MDSRs were developed, and experiments were conducted to verify the effectiveness of the presented models and design algorithms. The computational and experimental results were compared and evaluated using a quantitative index. The presented framework of deformation modeling and computational design of MDSRs can facilitate the design of soft robots with complex deformations, such as humanoid faces.

Evolutionary convergence of muscle architecture in relation to locomotor ecology in snakes.

The epaxial muscles in snakes are responsible for locomotion and as such can be expected to show adaptations in species living in different environments. Here, we tested whether the structural units that comprise the superficial epaxial muscles (semispinalis-spinalis, SSP; longissimus dorsi, LD; iliocostalis, IC) were different in animals occupying similar habitats. To do so, we analyzed and compared the muscle architecture (mass, fiber length, and physiological cross-sectional area) of the superficial epaxial muscle segments in snakes that differ in their habitat use (e.g., arboreal, terrestrial, and aquatic). Our results showed that arboreal species have on average longer muscles and tendons spanning more segments likely important during gap bridging. Moreover, aquatic snakes show relatively heavier semispinalis-spinalis muscles with a greater cross-sectional area. The longissimus dorsi muscles also showed a greater cross-sectional area compared with terrestrial and especially arboreal snakes. Whereas the more strongly developed muscles in aquatic snakes are likely associated with the dense and viscous environment through which they move, the lighter muscles in arboreal snakes may provide an advantage when climbing. Future studies comparing other ecologies (e.g., burrowing snakes) and additional muscle units (e.g., multifidus; hypaxial muscles) are needed to better understand the structural features driving variation in locomotor performance and efficiency in snakes.

A Wearable Master–Slave Rehabilitation Robot Based on an Epidermal Array Electrode Sleeve and Multichannel Electromyography Network

Investigating electromyography (EMG) signals is vital to promote the development of both rehabilitative robots and understanding of the movement neural mechanism. Interactions between various muscle units are paramount to be measured through network analysis, aiming to reveal how information is propagated and integrated. Herein, an EMG network using an epidermal array electrode sleeve to record multichannel EMGs is constructed. Then, a master–slave rehabilitation robot by adopting the EMG network as a feature for movement intention recognition is built. The results demonstrate that the sleeve can record signals with high quality, characterized by better signal robustness and higher movement recognition performance. The different finger movements evoke the specific spatial network patterns, characterized by the dominated hub at the muscle in charge of the corresponding movement, and the proposed EMG network‐based approach consistently achieves the highest recognition accuracy. Moreover, the proposed approach also shows the relatively less influence of signal length and electrode positions on the movement recognition. Finally, the proposed robot system can achieve 98.21% ± 2.37 accuracy for online control. These results provide a novel theoretical and practical basis for neural prosthesis control and hemiplegic hand rehabilitation.

Tendon Transfers in Rotator Cuff Deficiency

Massive irreparable rotator cuff tears (MIRCT) is a difficult clinical problem to treat in young, active individuals with minimal arthritis. Tendon transfers allow for donation of muscle units that can power the shoulder and restore kinematics. Posterosuperior rotator cuff deficiency can be treated with either a latissimus dorsi tendon or lower trapezius tendon transfer. The authors preferred method for treating irreparable supraspinatus and infraspinatus tears is with an arthroscopic assisted lower trapezius tendon transfer. The advantage of the lower trapezius tendon is that its vector is directly in line with the infraspinatus tendon allowing for better external rotation and abduction moment arms. The tendon is harvested from the spine of the scapula and augmented with an Achilles tendon allograft for length which is then secured at the footprint of the rotator cuff. Anterosuperior rotator cuff deficiency can be treated with a pectoralis major tendon or anterior latissimus tendon transfer. For subscapularis deficiency, our preferred method is an open versus arthroscopic latissimus tendon transfer. The tendon is harvested in the posterior axilla and augmented with an Achilles tendon allograft at the tendon interface to provide bulk. The tendon is passed arthroscopically to the anterior humerus and secured at the lesser tuberosity. Tendon transfers are a safe, reliable and reproducible technique when treating MIRCTs.

Influence of high-velocity low amplitude manipulation of the lumbar spine on the activity of motor units of the erector spinal muscles with functional assessment in healthy young people

Influence of high-velocity low amplitude manipulation of the lumbar spine on the activity of motor units of the erector spinal muscles with functional assessment in healthy young people

A feedforward compensation approach for cable-driven musculoskeletal systems

AbstractThis paper presents a feedforward compensation approach for musculoskeletal systems (MSs). Compared with traditional rigid robots, human arms have the advantages of flexibility and safety in operation in unstructured environments. However, the influence of external unknown disturbances, inner friction effects, and dynamic uncertainties of the MS makes it difficult to model and practically apply. In order to reduce the inner friction effects of the hardware platform and the over-relaxation/tension of the cable-pull drive, a feedforward friction compensation method for the cable-pulled artificial muscle unit is proposed. The method analyzes the friction causes of the hardware structure and establishes a mapping network relationship between the joint variables and the muscle force error in the muscle space. The experimental results show that the method can effectively improve the control accuracy and reduce the artificial muscle over-relaxation/tension instability.

307 Phenotypic-genotypic expansion of plectinopathy in a patient with muscular dystrophy and immune-mediated myasthenia gravis

A 26-year-old female presented with blistering skin lesions since infancy, hair loss, proximal motor weakness, nasal tone speech, swallowing problem, and droopy eyes. Upon neurological examination, normal cognition, inability to walk unassisted, bilateral ptosis with a full range of extraocular movements, weak gag reflex, generalized muscle atrophy, and decreased deep tendon reflexes were detected. Her electromyography revealed small polyphasic muscle unit action potentials, minimal insertional activities, myopathic changes, motor, and sensory increased latency, decreased amplitude, reduced velocity, and normal F wave.

LBSAT238 Teprotumumab Reduces Orbital Fat And Muscle Volume In Patients With Thyroid Eye Disease

Abstract Teprotumumab, a novel IGF-1R antibody has been shown to reduce proptosis in patients with thyroid eye disease (TED) in 2 randomized controlled trials. At present, it is unclear whether the reduction in proptosis is related to a decrease in extraocular muscle or orbital fat tissue. In this prospective longitudinal study, we reviewed the changes to orbital fat and muscle, and compared them, in consecutive patients with TED, who had completed treatment with teprotumumab (8 infusions). The primary outcome was change in orbital fat and muscle volume within 6 weeks of starting and completing therapy. Secondary outcome measures included change in proptosis and the clinical activity score (CAS). All patients had either CT or MRI with ≤ 1 mm slice thickness. Three-dimensional volumetric analysis of the orbital fat and muscle volume was performed using the previously validated 3D image analysis software, MIMICS (Materialise, Leuven, Belgium). Hounsfield units of fat and muscle were defined manually. All extraocular muscle tissue and fat between the orbital septum (eyelid) and the orbital outlet of the optic canal was included. A 3D model created by voxel addition was expressed in millimeters cubed. Segmentation and measurements were performed independently by two graders to review repeatability and observer variability. In line with previous clinical trials, the orbit with the most proptosis was defined as the study orbit and the contralateral orbit was designated the fellow orbit. Twenty-one patients (15 females and 6 males) met the inclusion criteria. Mean (SD) age was 55 (16), while mean (SD) duration of TED was 71 (46) months. There was a reduction of 3 mm in both; study and fellow orbits following treatment (p &amp;lt; 0. 01 for both). Compared to baseline, there was a mean (SD) CAS reduction of 4 (1) in the study orbit and 2.4 (1.1) in the fellow orbit (p &amp;lt; 0. 01 for both). Following therapy, mean (SD) reduction in muscle tissue was 1691 mm3 (1358) in the study orbit and 1714 mm3 (1927) in the fellow orbit (p &amp;lt; 0. 01 when compared to baseline for each). The mean (SD) reduction of fat volume was 1382 mm3 (1318) in the study orbit and 933 mm3 (1699) in the fellow orbit (p &amp;lt; 0. 01 when compared to baseline for each). There was no difference between fat and muscle reduction for the study orbit (p = 0.4) or fellow orbit (p = 0.2). There was a significant correlation between a reduction in proptosis and reduction in orbital soft tissue (fat and muscle) volume. The results of this study suggest that following treatment with teprotumumab, there is a significant decrease in orbital fat and muscle volume. This reduction in soft tissue volume likely drives the reduction in proptosis. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.

Ca2+ entry units in a superfast fish muscle.

Over the past two decades, mounting evidence has demonstrated that a mechanism known as store-operated Ca2+ entry (SOCE) plays a crucial role in sustaining skeletal muscle contractility by facilitating Ca2+ influx from the extracellular space during sarcoplasmic reticulum (SR) Ca2+ depletion. We recently demonstrated that, in exercised fast-twitch muscle from mice, the incidence of Ca2+ entry units (CEUs), newly described intracellular junctions between dead-end longitudinal transverse tubular (T-tubule) extensions and stacks of sarcoplasmic reticulum (SR) flat cisternae, strictly correlate with both the capability of fibers to maintain contractions during fatigue and enhanced Ca2+ influx via SOCE. Here, we tested the broader relevance of this result across vertebrates by searching for the presence of CEUs in the vocal muscles of a teleost fish adapted for extended, high-frequency activity. Specifically, we examined active vs. inactive superfast sonic muscles of plainfin midshipman (Porichthys notatus). Interestingly, muscles from actively humming territorial males had a much higher incidence of CEU SR stacks relative to territorial males that were not actively vocalizing, strengthening the concept that assembly of these structures is dynamic and use-dependent, as recently described in exercised muscles from mice. Our results support the hypothesis that CEUs represent a conserved mechanism, across vertebrates, for enabling high levels of repetitive muscle activity, and also provide new insights into the adaptive mechanisms underlying the unique properties of superfast midshipman sonic muscles.

Sarcomere dynamics revealed by a myofilament integrated FRET-based biosensor in live skeletal muscle fibers

The sarcomere is the functional unit of skeletal muscle, essential for proper contraction. Numerous acquired and inherited myopathies impact sarcomere function causing clinically significant disease. Mechanistic investigations of sarcomere activation have been challenging to undertake in the context of intact, live skeletal muscle fibers during real time physiological twitch contractions. Here, a skeletal muscle specific, intramolecular FRET-based biosensor was designed and engineered into fast skeletal muscle troponin C (TnC) to investigate the dynamics of sarcomere activation. In transgenic animals, the TnC biosensor incorporated into the skeletal muscle fiber sarcomeres by stoichiometric replacement of endogenous TnC and did not alter normal skeletal muscle contractile form or function. In intact single adult skeletal muscle fibers, real time twitch contractile data showed the TnC biosensor transient preceding the peak amplitude of contraction. Importantly, under physiological temperatures, inactivation of the TnC biosensor transient decayed significantly more slowly than the Ca2+ transient and contraction. The uncoupling of the TnC biosensor transient from the Ca2+ transient indicates the biosensor is not functioning as a Ca2+ transient reporter, but rather reports dynamic sarcomere activation/ inactivation that, in turn, is due to the ensemble effects of multiple activating ligands within the myofilaments. Together, these findings provide the foundation for implementing this new biosensor in future physiological studies investigating the mechanism of activation of the skeletal muscle sarcomere in health and disease.

Mechanical and Morphological Changes of the Plantar Flexor Musculotendinous Unit in Children with Unilateral Cerebral Palsy Following 12 Weeks of Plyometric Exercise: A Randomized Controlled Trial.

To investigate how plyometric exercise (PLYO-Ex) affects mechanics and morphometrics of the plantar flexor musculotendinous unit in children with unilateral cerebral palsy, 38 participants (aged 10-16 years) were allocated at random to either the PLYO-Ex group (n = 19; received 24 sessions of plyometric muscle loading, conducted 2 times a week for 3 months in succession) or the control group (n = 19; underwent traditional physical therapy for the same frequency and duration). Measurements were taken pre- and post-intervention. Standard ultrasound imaging was applied to evaluate morphometrics of the gastrocnemius muscle and Achilles tendon unit and an isokinetic dynamometer was used to evaluate maximum voluntary isometric plantar flexors contraction (IVCmax). With controlling for pre-treatment values, significant post-treatment changes favoring the PLYO-Ex group were observed for morphological (tendon (p = 0.003, η2p = 0.23) length; belly length (p = 0.001, η2p = 0.27); tendon thickness (p = 0.035, η2p = 0.35); muscle thickness (p = 0.013, η2p = 0.17); fascicle length (p = 0.009, η2p = 0.18); pennation angle (p = 0.015, η2p = 0.16)) and mechanical and material properties (IVCmax (p = 0.009, η2p = 0.18); tendon's elongation (p = 0.012, η2p = 0.17), stiffness (p = 0.027, η2p = 0.13); stress (p = 0.006, η2p = 0.20); strain (p = 0.004, η2p = 0.21)). In conclusion, plyometric exercise induces significant adaptations within the musculotendinous unit of the plantar flexors in children with unilateral cerebral palsy. These adaptations could improve muscular efficiency and consequently optimize physical/functional performance.

Constitutive assembly of Ca2+ entry units in soleus muscle from calsequestrin knockout mice.

Calcium (Ca2+) entry units (CEUs) are junctions within the I band of the sarcomere between stacks of sarcoplasmic reticulum (SR) cisternae and extensions of the transverse (T)-tubule. CEUs contain STIM1 and Orai1 proteins, the molecular machinery of store-operated Ca2+ entry (SOCE). In extensor digitorum longus (EDL) fibers of wild-type (WT) mice, CEUs transiently assemble during acute exercise and disassemble several hours thereafter. By contrast, calsequestrin-1 (CASQ1) ablation induces a compensatory constitutive assembly of CEUs in EDL fibers, resulting in enhanced constitutive and maximum SOCE that counteracts SR Ca2+ depletion during repetitive activity. However, whether CEUs form in slow-twitch fibers, which express both the skeletal CASQ1 and the cardiac CASQ2 isoforms, is unknown. Herein, we compared the structure and function of soleus muscles from WT and knockout mice that lack either CASQ1 (CASQ1-null) or both CASQs (dCASQ-null). Ultrastructural analyses showed that SR/T-tubule junctions at the I band, virtually identical to CEUs in EDL muscle, were present and more frequent in CASQ1-null than WT mice, with dCASQ-null exhibiting the highest incidence. The greater incidence of CEUs in soleus from dCASQ-null mice correlated with increased specific force production during repetitive, high-frequency stimulation, which depended on Ca2+ entry. Consistent with this, Orai1 expression was significantly increased in soleus of CASQ1-null mice, but even more in dCASQ-null mice, compared with WT. Together, these results strengthen the concept that CEU assembly strongly depends on CASQ expression and provides an alternative source of Ca2+ needed to refill SR Ca2+ stores to maintain specific force production during sustained muscle activity.

Differences in motor unit firing properties of the vastus lateralis muscle during postural and voluntary tasks.

The firing properties of the motor units are usually affected by the motor task. However, it has not been clarified whether the firing properties of the motor units of a specific muscle are different between postural and voluntary tasks. Therefore, this study investigated whether the recruitment and rate coding of the motor units differ between these two motor tasks. Thirteen healthy volunteers performed trapezoidal muscle contraction with a target value of 15% maximum electromyography (EMG) activity by voluntary left knee extension in the sitting position (voluntary task) and postural maintenance in the semi-squatting position (postural task) with a knee flexion angle of 30°. We obtained four channels of surface EMG activity during each task from left vastus lateralis muscle. We extracted the firing properties of individual motor units using the EMG decomposition algorithm. The recruitment threshold and motor unit action potential amplitude were significantly lower in the postural task than in the voluntary task, and conversely, the mean firing rate was significantly higher. These results were explained by the preferential recruitment of motor units with higher recruitment threshold and amplitude in the voluntary task, while motor units with lower recruitment threshold and higher firing rate were preferentially recruited in the postural task. Preferential activation of fatigue-resistant motor units in the postural task is a reasonable strategy as it allows for sustained postural maintenance. We provide the first evidence that motor unit firing properties are clearly different between postural and voluntary tasks, even at the same muscle activity level.

Advanced modeling method for quantifying cumulative subjective fatigue in mid-air interaction

Interaction in mid-air can be fatiguing. A model-based method to quantify cumulative subjective fatigue for such interaction was recently introduced in HCI research. This model separates muscle units into three states: active (MA) fatigued (MF) or rested (MR) and defines transition rules between states. This method demonstrated promising accuracy in predicting subjective fatigue accumulated in mid-air pointing tasks. In this paper, we introduce an improved model that additionally captures the variations of the maximum arm strength based on arm postures and adds linearly-varying model parameters based on current muscle strength. To validate the applicability and capabilities of the new model, we tested its performance in various mid-air interaction conditions, including mid-air pointing/docking tasks, with shorter and longer rest and task periods, and a long-term evaluation with individual participants. We present results from multiple cross-validations and comparisons against the previous model and identify that our new model predicts fatigue more accurately. Our modeling approach showed a 42.5% reduction in fatigue estimation error when the longitudinal experiment data is used for an individual participant’s fatigue. Finally, we discuss the applicability and capabilities of our new approach.

Intraoral Approach to Fat Grafting Based on the Mimetic Muscle Unit.

Recently, the concepts of anatomic-based fat grafting and mid-facial anatomy have been studied. However, there is no clear consensus on the optimal fat grafting technique. We aimed to introduce a novel intraoral approach to fat grafting based on mid-facial mimetic muscles, wherein a substantial amount of fat was injected into appropriate locations to increase patient satisfaction and decrease the need for additional surgical techniques. Sixty-four Asian women (mean age, 41.2 y) who had undergone fat grafting through the intraoral approach between 2014 and 2019 were enrolled. We sequentially performed the following steps: fat harvesting, centrifugation, and fat injection. All patients were followed at an outpatient clinic for 6 to 12 months. We obtained photographs of their face before and at 3 and 6 months postoperatively and conducted a 5-point scale questionnaire survey at the 2 time points postoperatively to evaluate the satisfaction of patients and surgeons. We divided the mid-facial areas into malar eminence, infraorbital, and nasolabial fold areas and evaluated the following items: volume consistency, softness, irregularity, and overall satisfaction. Patients reported "satisfactory" for the postoperative results. There were only 2 cases of complication among the 64 cases which resolved spontaneously. The highest and lowest satisfaction were reported for the malar eminence and nasolabial fold areas, respectively. Fat grafting through the intraoral approach based on the mimetic muscle unit is an effective and safe method for mid-facial rejuvenation.

Kinematics of individual muscle units in natural contractions measured in vivo using ultrafast ultrasound

Objective. The study of human neuromechanical control at the motor unit (MU) level has predominantly focussed on electrical activity and force generation, whilst the link between these, i.e. the muscle deformation, has not been widely studied. To address this gap, we analysed the kinematics of muscle units in natural contractions. Approach. We combined high-density surface electromyography (HDsEMG) and ultrafast ultrasound (US) recordings, at 1000 frames per second, from the tibialis anterior muscle to measure the motion of the muscular tissue caused by individual MU contractions. The MU discharge times were identified online by decomposition of the HDsEMG and provided as biofeedback to 12 subjects who were instructed to keep the MU active at the minimum discharge rate (9.8 ± 4.7 pulses per second; force less than 10% of the maximum). The series of discharge times were used to identify the velocity maps associated with 51 single muscle unit movements with high spatio-temporal precision, by a novel processing method on the concurrently recorded US images. From the individual MU velocity maps, we estimated the region of movement, the duration of the motion, the contraction time, and the excitation–contraction (E–C) coupling delay. Main results. Individual muscle unit motions could be reliably identified from the velocity maps in 10 out of 12 subjects. The duration of the motion, total contraction time, and E–C coupling were 17.9 5.3 ms, 56.6 8.4 ms, and 3.8 3.0 ms (n = 390 across ten participants). The experimental measures also provided the first evidence of muscle unit twisting during voluntary contractions and MU territories with distinct split regions. Significance. The proposed method allows for the study of kinematics of individual MU twitches during natural contractions. The described measurements and characterisations open new avenues for the study of neuromechanics in healthy and pathological conditions.

Temporal Expression of Myogenic Regulatory Genes in Different Chicken Breeds during Embryonic Development

The basic units of skeletal muscle in all vertebrates are multinucleate myofibers, which are formed from the fusion of mononuclear myoblasts during the embryonic period. In order to understand the regulation of embryonic muscle development, we selected four chicken breeds, namely, Cornish (CN), White Plymouth Rock (WPR), White Leghorn (WL), and Beijing-You Chicken (BYC), for evaluation of their temporal expression patterns of known key regulatory genes (Myomaker, MYOD, and MSTN) during pectoral muscle (PM) and thigh muscle (TM) development. The highest expression level of Myomaker occurred from embryonic days E13 to E15 for all breeds, indicating that it was the crucial stage of myoblast fusion. Interestingly, the fast-growing CN showed the highest gene expression level of Myomaker during the crucial stage. The MYOD gene expression at D1 was much higher, implying that MYOD might have an important role after hatching. Histomorphology of PM and TM suggested that the myofibers was largely complete at E17, which was speculated to have occurred because of the expression increase in MSTN and the expression decrease in Myomaker. Our research contributes to lay a foundation for the study of myofiber development during the embryonic period in different chicken breeds.

PPARGC1A Is a Moderator of Skeletal Muscle Development Regulated by miR-193b-3p.

Meat production performance is one of the most important factors in determining the economic value of poultry. Myofiber is the basic unit of skeletal muscle, and its physical and chemical properties determine the meat quality of livestock and poultry to a certain extent. Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PPARGC1A) as a transcriptional coactivator has been found to be widely involved in a series of biological processes. However, PPARGC1A is still poorly understood in chickens. In this manuscript, we reported that PPARGC1A was highly expressed in slow-twitch myofibers. PPARGC1A facilitated mitochondrial biogenesis and regulated skeletal muscle metabolism by mediating the flux of glycolysis and the TCA cycle. Gain- and loss-of-function analyses revealed that PPARGC1A promoted intramuscular fatty acid oxidation, drove the transformation of fast-twitch to slow-twitch myofibers, and increased chicken skeletal muscle mass. Mechanistically, the expression level of PPARGC1A is regulated by miR-193b-3p. Our findings help to understand the genetic regulation of skeletal muscle development and provide a molecular basis for further research on the antagonism of skeletal muscle development and fat deposition in chickens.

Interfacing Motor Units in Nonhuman Primates Identifies a Principal Neural Component for Force Control Constrained by the Size Principle.

Motor units convert the last neural code of movement into muscle forces. The classic view of motor unit control is that the CNS sends common synaptic inputs to motoneuron pools and that motoneurons respond in an orderly fashion dictated by the size principle. This view, however, is in contrast with the large number of dimensions observed in motor cortex, which may allow individual and flexible control of motor units. Evidence for flexible control of motor units may be obtained by tracking motor units longitudinally during tasks with some level of behavioral variability. Here we identified and tracked populations of motor units in the brachioradialis muscle of two macaque monkeys during 10 sessions spanning >1 month with a broad range of rate of force development (1.8-38.6 N · m · s-1). We found a very stable recruitment order and discharge characteristics of the motor units over sessions and contraction trials. The small deviations from orderly recruitment were fully predicted by the motor unit recruitment intervals, so that small shifts in recruitment thresholds happened only during contractions at a high rate of force development. Moreover, we also found that one component explained more than ∼50% of the motor unit discharge rate variance, and that the remaining components represented a time-shifted version of the first. In conclusion, our results show that the recruitment of motoneurons is determined by the interplay of the size principle and common input and that this recruitment scheme is not violated over time or by the speed of the contractions.SIGNIFICANCE STATEMENT With a new noninvasive high-density electromyographic framework, we show the activity of motor unit ensembles in macaques during voluntary contractions. The discharge characteristics of brachioradialis motor units revealed a relatively fixed recruitment order and discharge characteristics across days and rate of force developments. These results were further confirmed through invasive axonal stimulation and recordings of intramuscular electromyographic activity from 16 arm muscles. The study shows for the first time the feasibility of longitudinal noninvasive motor unit interfacing and tracking of the same motor units in nonhuman primates.