Rhythmic motor paradigms are widely used to study sensorimotor timing, yet magnetic resonance imaging (MRI) research has largely focused on central processes, with limited insight into peripheral neuromuscular mechanisms. Motor unit MRI (MUMRI), a motion-sensitive technique in which muscle contraction induces intravoxel water redistribution and transient signal attenuation, enables in vivo visualization of muscle activity. In this study, we developed and validated a combined behavioral-MUMRI paradigm to characterize muscle recruitment during rhythmic foot tapping. Healthy participants performed an auditory-paced tapping task inside an MRI scanner while timing was recorded via an MRI-compatible force transducer and muscle activity was measured using single-slice MUMRI. A variable-latency cueing design systematically sampled the temporal relationship between auditory cues, motor execution, and image acquisition, allowing identification of the optimal latency window for detecting contraction-related signal changes. Fixed-latency acquisitions were then used to assess reproducibility. Behavioral results showed stable performance across conditions, with low variability in tapping accuracy (mean coefficient of variation [CoV] ≈0.078). Transient, localized signal reductions consistent with muscle contraction were observed in anterior lower leg muscles during dorsiflexion. Voxel-wise analyses demonstrated high within-condition reproducibility and latency-dependent spatial patterns, with the greatest average consistency when tapping aligned with scanner rhythm (r ≈0.68). These findings establish a robust framework for integrating rhythmic motor tasks with MUMRI, highlighting the importance of precise temporal alignment for reliable measurement of muscle activity. This approach provides a reproducible method for linking motor behavior to peripheral neuromuscular dynamics and offers potential for advancing both basic and clinical MRI research.

Approximately 97% of patients with amyotrophic lateral sclerosis (ALS) have cytoplasmic mislocalization and aggregation of the ubiquitous nuclear protein, TDP-43. Current rodent models of this disease fail to replicate the progressive motor weakness and characteristic histopathology, possibly because of fundamental neuroanatomical and genetic differences between rodents and humans. In this study, the TDP-43 protein was overexpressed in the motor neuron pool of the brachioradialis muscle unilaterally in two six-year-old female rhesus macaques, using an intersectional genetics approach involving infection with genetically modified adeno-associated virus. Magnetic resonance images demonstrated delayed signal hyperintensities limited to the injected brachioradialis that persisted for 6-7 weeks, consistent with motor neuron degeneration and denervation of the targeted muscle. At post-mortem , the virus-mediated focal protein overexpression event was found to induce widespread deposits of pathological phosphorylated TDP-43 throughout the cervical spinal cord and motor cortex bilaterally, indicating an ALS-like spread of proteinopathy from the transfection site.

Objective:Electrodiagnostic testing plays an important role in diagnosing disorders of neuromuscular transmission (NMT), especially in seronegative myasthenia gravis. However, electrodiagnostic criteria for the diagnosis are sparse. This study aimed at inferring evidence-based recommendations for the electrodiagnostic examination of NMT disorders. Methods:A total of 164 cases with a consensus diagnosis of NMT disorder obtained by peer review by eight experienced neurophysiologists were analysed for differences in examination strategy, the sensitivity of different tests, and inferring minimal criteria. The diagnostic performance of the suggested criteria was validated on 24 MG patients and 50 patients with neuropathy (17), myopathy (15), or fatigue (18). Results:We recommend as minimal electrodiagnostic criteria for NMT disorders, either (a) 2 abnormal repetitive nerve stimulation (RNS), (b) 1 abnormal RNS and 1 abnormal single fiber electromyography (SFEMG) or (c) 2 abnormal SFEMG. These showed a good diagnostic performance with a sensitivity of 87.5 % and a specificity of 100 %. Conclusion:Recommendations with high diagnostic sensitivity and specificity for the minimum number of RNS and SFEMG studies to diagnose NMT disorders developed by an international consensus group are suggested. Significance:The suggested electrodiagnostic recommendations for diagnosing NMT disorders are reliable and suitable for use at different centres.

INTRODUCTION/AIMS:Surviving motor units in neurogenic diseases demonstrate collateral reinnervation. Scanning electromyography (EMG) reveals normal motor unit corridor length, but with "silent regions," suggesting that reinnervation does not result in increased motor unit size but may increase motor unit complexity. Motor unit magnetic resonance imaging (MUMRI) pairs MR imaging with electrical nerve stimulation to visualize individual motor units. This study aimed to assess the motor unit dimensions and complexity in patients with previous poliomyelitis compared to healthy controls. METHODS:Patients with a history of polio were recruited from the British Polio Fellowship, compared to a retrospective cohort of healthy controls. They underwent medical history and examination of lower limb power, fatigue assessment (fatigue severity score, FSS), and a 3 T MUMRI scan of the less-affected lower limb. The cross-sectional area, maximum, and minimum Feret diameter of the motor unit territories in tibialis anterior were calculated. Motor unit complexity was computed using the Hausdorff box-counting method. RESULTS:Of 12 polio survivors, n = 8 (6 female) were suitable for analysis and were compared to 19 controls. The mean motor unit maximum Feret diameter was 10.3 ± 3.1 mm compared to 8.4 ± 5.2 mm in controls (p = 0.34). The mean shape complexity was 0.59 ± 0.12 compared to 0.45 ± 0.2 in controls (p = 0.03). DISCUSSION:Polio survivors demonstrate motor units with normal dimensions but increased shape complexity, indicating nonuniform collateral reinnervation largely limited to existing territories. The size and shape of motor units could help in understanding the physiological processes behind reinnervation, both in polio and other neurogenic diseases such as amyotrophic lateral sclerosis.

Magnetic resonance imaging (MRI) is routinely used in the musculoskeletal system to measure skeletal muscle structure and pathology in health and disease. Recently, it has been shown that MRI also has promise for detecting the functional changes, which occur in muscles, commonly associated with a range of neuromuscular disorders. This review focuses on novel adaptations of MRI, which can detect the activity of the functional sub-units of skeletal muscle, the motor units, referred to as "motor unit MRI (MUMRI)." MUMRI utilizes pulsed gradient spin echo, pulsed gradient stimulated echo and phase contrast MRI sequences and has, so far, been used to investigate spontaneous motor unit activity (fasciculation) and used in combination with electrical nerve stimulation to study motor unit morphology and muscle twitch dynamics. Through detection of disease driven changes in motor unit activity, MUMRI shows promise as a tool to aid in both earlier diagnosis of neuromuscular disorders and to help in furthering our understanding of the underlying mechanisms, which proceed gross structural and anatomical changes within diseased muscle. Here, we summarize evidence for the use of MUMRI in neuromuscular disorders and discuss what future research is required to translate MUMRI toward clinical practice. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY: Stage 3.

The spontaneous contraction of motor units in muscle, i.e. fasciculation, has been recognised as an important diagnostic marker in amyotrophic lateral sclerosis (ALS). Fasciculation can be imaged with a novel MRI technique called motor unit MRI. This technique uses a diffusion weighted sequence on which fasciculation presents as short-living signal voids. We demonstrated an increased fasciculation rate in ALS patients compared to healthy controls by assessing the four body regions relevant in the diagnosis of ALS. The affected body regions differed between patients. This is in line with the heterogeneous disease onset and supports our proposed whole-body approach.

Sarcopenia is commonly associated with ageing, whereby individuals lose muscle mass and strength. One potential contributor to sarcopenia is the degeneration of motor units (MU), defined as a single motor neuron and the muscle fibres it innervates. We used motor unit MRI (MUMRI) to investigate changes to spontaneous MU activity and morphology in a cohort of healthy ageing adults. We found that MU activity and morphology did not appear to change with healthy ageing. We next aim to apply the MUMRI technique in patients with sarcopenia to look for evidence of accelerated MU loss compared to these healthy controls.

The ability to start and stop movements is impaired in movement disorders [ [1] Choudhury S. et al. Slowed movement stopping in Parkinson's disease and focal dystonia is improved by standard treatment. Sci. Rep. 2019; 9: 1-9 Crossref PubMed Scopus (6) Google Scholar ]. Measuring the ability to respond to a cue is relatively easy; stopping is a more hypothetical concept which is harder to quantify. One successful approach relies on the 'horse race' model. This conceptualises a Go and Stop process which race in parallel towards a finish line. If the Go process wins, the movement is executed; if the Stop process wins, movement is inhibited. This theoretical concept was given mathematical expression by Logan and Cowan (1984), leading to the stop signal reaction time (SSRT) [ [2] Logan G.D. Cowan W.B. On the ability to inhibit thought and action: a theory of an act of control. Psychol. Rev. 1984; 91: 295 Crossref Scopus (1925) Google Scholar ]. We measured SSRT through a simple task using a battery-operated portable box (Fig. 1B), ideal for bedside assessment. In the task, the subjects were requested to release a button in response to an intermittently flashing green LED ('go signal') as fast as they could but cease to release the button in a quarter of trials at random when the 'stop signal' (red LED) flashed immediately followed the 'go signal'. The stop signals were delivered in four different delays (Fig. 1A), leading to different proportions of trials where the button was released inappropriately. The probability of error and reaction time during a stop-signal task are used to compute SSRT. Recently, we improved SSRT estimation reliability using a Bayesian mathematical approach (optimum combination stop signal reaction time, ocSSRT) [ [1] Choudhury S. et al. Slowed movement stopping in Parkinson's disease and focal dystonia is improved by standard treatment. Sci. Rep. 2019; 9: 1-9 Crossref PubMed Scopus (6) Google Scholar ]. Essentially, this measure was derived from mean posterior probabilities of SSRTs in four different stop-signal delays and summarised as one mean and 95% confidence interval for individual subjects. Subsequently, we applied ocSSRT to movement disorders patients through a portable battery-operated box with a digital display. We observed that patients with movement disorders e.g. Parkinson's Disease (PD), cervical dystonia (CD), and writer's cramp demonstrated significantly prolonged ocSSRT compared to healthy control [ [1] Choudhury S. et al. Slowed movement stopping in Parkinson's disease and focal dystonia is improved by standard treatment. Sci. Rep. 2019; 9: 1-9 Crossref PubMed Scopus (6) Google Scholar ] (Fig. 1C). Thus, this neurophysiological index may have potential use for classification of various types of movement disorders including psychogenic movement disorders. The prolonged ocSSRT was partially normalised after intake of l-dopa in PD patients and after one month of injecting therapeutic doses of botulinum toxin in the affected muscles in CD patients (Fig. 1D). We also used this box to optimise the settings for deep brain stimulation (DBS) in patients from our Movement Disorders clinic [ [3] Roy A. et al. Stop Signal Reaction Time measured with a portable device validates optimum STN-DBS programming. Brain Stimul.: Basic Translat. Clin. Neuromodul. 2020; 13: 1609-1611 Abstract Full Text Full Text PDF PubMed Scopus (5) Google Scholar ] and potentially this could be further validated as a tool for DBS programming remotely (Fig. 1EF). We also showed that ocSSRT is useful in discriminating between Alzheimer's and Parkinson's disease [ [4] Rahman S. et al. Comparing stop signal reaction times in Alzheimer's and Parkinson's disease. Can. J. Neurol. Sci. 2021; : 1-26 Google Scholar ] (Fig. 1G). SSRT could thus be used for screening and classification of neurodegenerative conditions. Against this backdrop, there is a need for greater accessibility of this tool to movement disorders neurologists and researchers interested in exploring the clinical utility of SSRT. We therefore developed and validated an android application to measure simple visual reaction time (RT) and ocSSRT.

OBJECTIVE:To assess in-vivo cross-sectional and 3D morphology of human motor units in hand, forearm and lower leg muscles using magnetic resonance imaging (MRI). METHODS:Diffusion weighted MRI was used with in-scanner electrical stimulation in healthy controls to image motor units at a single slice in lower leg, forearm and hand muscles (n = 6) and multiple slices in the lower leg for 3D assessment (n = 7). RESULTS:Motor unit cross-sectional area (CSA) and maximum Feret diameter (FDmax) did not differ between the lower leg (CSA: 22.4 ± 8.4 mm2; FDmax: 8.7 ± 2.4 mm), forearm (CSA: 23.6 ± 14.1 mm2; FDmax: 9.0 ± 3.3 mm) and hand (CSA: 26.8 ± 12.8 mm2 and FDmax: 9.6 ± 2.7 mm) (ANOVA; p = 0.487 and p = 0.587, respectively). Lower leg motor units were 8.0 ± 3.8 cm long with largest CSA in the motor unit's middle section. 3D motor unit imaging revealed a complex structure with several units splitting and re-forming along their length. CONCLUSIONS:Motor unit MRI (MUMRI) can be applied to upper limb muscles, and can reveal the 3D structure of human motor units in-vivo. SIGNIFICANCE:MUMRI provides the first in-vivo 2D images of upper limb motor units and 3D images of lower leg motor units. 3D imaging suggest a more complex human motor unit structure than previously thought.

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.

Introduction/Aims Measuring the spatial dimensions of a single motor unit remains a challenging problem, and current techniques, such as scanning electromyography (EMG), tend to underestimate the true dimensions. In this study we aimed to estimate more accurately the dimensions of a single motor unit by developing a clinically applicable scanning EMG protocol that utilizes ultrasound imaging to visualize and target a transect through the center of a single motor unit. Methods Single motor unit twitches in the tibialis anterior muscles of healthy volunteers were elicited via stimulation of the fibular nerve, visualized with ultrasound, and targeted with an intramuscular EMG electrode. The electrode was moved by hand in small steps through the motor unit territory. Ultrasound video output was synchronized to EMG capture, and the needle position was tracked at each step. Results Eight recordings from six participants were collected. The technique was quick and easy to perform (mean time, 6.1 minutes) with reasonable spatial resolution (mean step size, 1.85 mm), yielding motor unit territory sizes between 1.53 and 14.65 mm (mean, 7.15 mm). Discussion Ultrasound-guided motor unit scanning EMG is a quick and accurate method for obtaining a targeted motor unit transect. This combination of two readily available clinical tools provides insights into the dimensions and internal structure of the motor unit as a marker for neuromuscular conditions.

OBJECTIVES:In many neuromuscular diseases, weakness results from a disruption in muscle fibres' arrangement within a motor unit. Limitations in current techniques mean that the spatial distribution of fibres in human motor units remains unknown. METHODS:A flexible multi-channel electrode was developed and bonded to a clinical electromyography (EMG) needle. Muscle fibre action potentials were localised using a novel deconvolution method. This was tested using simulated data, and in recordings collected from the tibialis anterior muscle of healthy subjects. RESULTS:Simulated data indicated good localisation reliability across all sections of the electrode except the end sections. A corrected fibre density was estimated up to 1.4 fibres/mm2. Across five recordings from three individuals, between 4 and 14 motor units were detected. Between 1 and 20 muscle fibres were localised per motor unit within the electrode detection area, with up to 220 muscle fibres localised per recording, with overlapping motor unit territories. CONCLUSIONS:We provide the first direct evidence that human motor units spatially overlap, as well as data related to the spatial arrangement of muscle fibres within a motor unit. SIGNIFICANCE:As well as providing insights into normal human motor physiology, this technology could lead to faster and more accurate diagnosis in patients with neuromuscular diseases.

[Purpose] We have previously shown inversion therapy to be effective in a small prospective randomised controlled trial of patients with lumbar disc protrusions. Our purpose now was to measure symptoms and to compare the surgery rate following inversion for 85 participants with the surgery rate in 3 control groups. [Participants and Methods] Each of the 85 inverted participants acted as their own control for the "symptomatic" part of the study. In the "Need for surgery" part of the study, one control group was made up of similar patients with leg pain and sciatica who were referred to the same clinic in the same year. Two additional control groups were examined: the original control group from the pilot trial and the lumbar disc surgery waiting list patients. [Results] Inversion therapy relieved symptoms: there were improvements in the Visual Analogue Score, Roland Morris and Oswestry Disease indices and Health Utility Score compared with their pre-treatment status. Also, the 2 year surgery rate in the inversion participants in the registry (21%) was significantly lower than in the matched control group (39% at two years and 43% at four years). It was also lower than the surgery rate in the other 2 control groups. [Conclusion] Inversion therapy relieved symptoms and avoided surgery.

Localised signal voids in diffusion‐weighted (DW) images of skeletal muscle have been postulated to occur as a result of muscle fibre contraction and relaxation. We investigated the contrast mechanism of these signal voids using a combination of modelling and experimental measurements by employing DW and phase contrast (PC) imaging sequences. The DW signal and PC signal were simulated for each time point of a theoretical muscle twitch. The model incorporated compaction (simulating actively contracting muscle fibres) and translation (simulating passively moving surrounding fibres). The model suggested that the DW signal depended on contraction time and compaction whereas the PC signal depended on contraction time, compaction and translation. In a retrospective study, we tested this model with subgroup analyses on 10 healthy participants. Electrical nerve stimulation was used to generate muscle twitches in lower leg muscles; the resulting force was measured using an MR‐compatible force transducer. At current levels causing a visible muscle twitch (~13 mA), the width of the first signal drop in the DW signal (mean ± SD: 103 ± 20 ms) was comparable with the force contraction time (93 ± 34 ms; intraclass correlation coefficient [ICC] = 0.717, P = .010). At current levels activating single motor units (~9 mA), the contraction time determined from the DW signal was 75 ± 13 ms and comparable with the PC contraction time (81 ± 15 ms; ICC = 0.925, P = .001). The maximum positive velocity was 0.55 ± 0.26 cm/s and the displacement was 0.20 ± 0.10 mm. Voxel‐wise analysis revealed localised DW changes occurring together with more widespread phase changes. In conclusion, local signal attenuations in DW images following muscle fibre activation are primarily caused by compaction. The PC sequence also detects translating muscle tissue being passively pulled. The magnitude of the changes in DW and PC images depends on the twitch's contractile properties and percentage contraction. DW imaging and PC imaging can therefore measure twitch profiles of skeletal muscle fibres.

Objective To determine the size, shape and distribution of single human motor units in-vivo in healthy controls of different ages. Methods A novel diffusion-weighted magnetic resonance imaging (MRI) technique was used in combination with in-scanner electrical stimulation to quantify the shape, cross-sectional area, and dimensions of individual motor units in 10 healthy subjects. Results Thirty-one discrete motor units were studied. The majority were elliptical or crescent shaped, but occasional split motor units were observed. The mean motor unit cross sectional area was 26.7 +/- 11.2 mm(2), the mean maximum dimension was 10.7 +/- 3.3 mm, and the mean minimum dimension was 4.5 +/- 1.2 mm. Subjects aged over 40 had significantly larger maximum dimensions than those below this age (p < 0.05). Conclusions Motor unit MRI (MUMRI) is a novel technique capable of revealing the size, shape and position of multiple motor units in human muscles. It is reproducible, non-invasive, and sufficiently sensitive to detect physiologically relevant changes in motor unit morphology with age. Significance To our knowledge, these results provide the first imaging assessment of human motor unit morphology. The technique shows promise both as a diagnostic tool and as a biomarker in longitudinal studies of disease progression.

To develop a clinically applicable non-invasive method for detecting motor unit fasciculation in patients with Amyotrophic Lateral Sclerosis (ALS). The lower limbs of 6 healthy controls and 4 patients with confirmed ALS were scanned using a novel diffusion weighted MRI protocol sensitive to micrometer-scale movement of skeletal muscle. Motor unit activity was assessed at rest and during electrical motor nerve stimulation time-locked to scanner acquisition. Incremental stimulation revealed a reproducible pattern of overlapping signal voids with dimensions and temporal profiles consistent with the contraction of single motor units. Patients with ALS showed a significantly higher rate of spontaneous motor unit fasciculation at rest (mean 99.1/min, range 25.7–161 in patients vs 7.7/min, range 4.3–9.7 in controls, p < 0.05) and a significantly higher percentage muscle cross sectional area in which fasciculation was detected (15.9%, SD 2.8 vs 2.9%, SD 1.6, p < 0.05). This study is the first use of imaging to detect fasciculation in ALS patients. The technique is quick to perform and entirely pain-free. The ability to detect changes in motor unit function which precede motor unit degeneration may allow earlier diagnosis and recruitment to clinical trials.

State of the art clinical EMG provides only indirect or inferred measurements of muscle structure. Structural information for motor units has only previously been described using laborious glycogen depletion experiments. Micro-EMG is a novel multi-electrode system that samples EMG signal from 64 electrodes along the surface of the needle in order to localise the muscle fibres of multiple human motor units along the length of a clinical EMG needle. This enables mapping of human motor units from just 5 min recordings. We recorded multi-electrode EMG activity at varying force levels (10–40% of maximum voluntary contraction) in the biceps and tibialis anterior muscle of 6 healthy volunteers. Each EMG channel was decomposed into motor unit action potentials, and fibre action potentials spatially localised using a signal deconvolution method. Features normally only accessible via histology including fibre nearest neighbour distance and all-pairs distance were calculated. Several simulations of muscle electrical activity were created in order to test the accuracy of localisation. For the first time, we were able to produce an electrical localisation map of the human motor unit in vivo. A total of 340 fibres in 31 motor units were isolated and localised. Median nearest neighbour distance varied from 0.44 to 1.02 mm. Simulation findings indicated strong accuracy in fibre number estimation and localisation. Micro-EMG provides unprecedented insights into motor unit structure and function. We anticipate that this method will provide useful biomarkers for disease states such as motor neurone disease.

A novel diffusion‐weighted magnetic resonance imaging protocol sensitive to contraction of individual skeletal motor units was developed. We applied this technique to the lower limb muscles of 4 patients with confirmed amyotrophic lateral sclerosis (ALS) and 6 healthy controls. A 3‐minute scan revealed florid fasciculation in ALS patients, involving both superficial and deep muscles, and at a frequency higher than in healthy controls. This novel imaging technique reveals hitherto unobtainable information on human motor unit structure and function, which may allow earlier diagnosis and recruitment to clinical trials. ANN NEUROL 2019;85:455–459.