Motor Unit Morphology Research Articles

Motor Unit Magnetic Resonance Imaging (MUMRI) In Skeletal Muscle.

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.

Near-fiber electromyography

ObjectiveDescribe and evaluate the concepts of near fiber electromyography (NFEMG), the features used, including near fiber motor unit potential (NFMUP) duration and dispersion, which relate to motor unit distal axonal branch and muscle fiber conduction time dispersion, and NFMUP segment jitter, a new measure of the temporal variability of neuromuscular junction transmission (NMJ), and axonal branch and muscle fibre conduction for the near fibres (i.e. NF jitter), and the methods for obtaining their values. MethodsTrains of high-pass filtered motor unit potentials (MUPs) (i.e. NFMUP trains) were extracted from needle-detected EMG signals to assess changes in motor unit (MU) morphology and electrophysiology caused by neuromuscular disorders or ageing. Evaluations using simulated needle-detected EMG data were completed and example human data are presented. ResultsNFEMG feature values can be used to detect axonal sprouting, conduction slowing and NMJ transmission delay as well as changes in MU fiber diameter variability, and NF jitter. These changes can be detected prior to alterations of MU size or numbers. ConclusionsThe evaluations clearly demonstrate and the example data support that NFMUP duration and dispersion reflect MU distal axonal branching, conduction slowing and NMJ transmission delay and/or MU fiber diameter variability and that NFMUP jiggle and segment jitter reflect NF jitter. SignificanceNFEMG can detect early changes in MU morphology and/or electrophysiology and has the potential to augment clinical diagnosis and tracking of neuromuscular disorders.

Non-invasive imaging of single human motor units

ObjectiveTo determine the size, shape and distribution of single human motor units in-vivo in healthy controls of different ages. MethodsA 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. ResultsThirty-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 mm2, 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). ConclusionsMotor 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. SignificanceTo 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.

P36-S Validation of near fiber motor unit potential stability and dispersion measures using simulated signals

Background A motor unit potential (MUP) is the summation of the muscle fiber potentials (MFPs) generated by the fibers of its MU. A near-fiber (NF) MUP is the summation of MFPs from fibers near the recording electrode. The temporal consistency and dispersion of NF MFP contributions can be clinically useful. NF MUP jiggle and NF MUP segment jitter are new measures of MFP temporal consistency and NF duration and NF dispersion are new measures of MFP temporal dispersion. Simulated EMG models provide a controlled scenario for validating the overarching capabilities of these new NF MUP measures. Material and methods A commercial EMG simulator (Karlsson and Stalberg, version 3.6) generated 120 EMG signals each comprised of 4 motor unit potential trains (MUPTs). Across these signals, MU size varied between 100 to 300 fibers, MFP jitter ranged from 20 to 80 μ s and three different recording electrode to endplate distances (5, 20 and 55 mm) were used. MUPTs were extracted from the EMG signals and analyzed using DQEMG. Results NF MUP jiggle and NF MUP segment jitter show high accuracy in evaluating NF MFP jitter. NF-MUP duration and dispersion offer accurate information about MU morphology (i.e. dispersion of fiber end plate locations and varying conduction velocities). Conclusions The new NF MUP measures accurately represent useful electrophysiological and morphological aspects of their corresponding MUs. DQEMG offers a comprehensive evaluation of motor unit electrophysiological stability and morphology that can assist with the diagnosis of neuromuscular diseases.

T39. Diaphragm motor responses to phrenic nerve stimulation: Surface and needle recordings

In studies of phrenic nerve (PN) conduction in amyotrophic lateral sclerosis (ALS) both motor response amplitude and latency have been reported as abnormal. However, correlation with diaphragm motor unit loss, and with diaphragmatic function has not been fully evaluated. We studied 83 patients with ALS, and 21 patients referred with clinically suspected phrenic nerve lesions whose studies were normal. PN responses elicited by percutaneous electrical stimulation in the neck were recorded using superficial electrodes placed at the surface markings of the diaphragm on the chest wall, and a concentric needle electrode inserted into the diaphragmatic costal fibres. Electromyography of diaphragm was performed to analyse motor unit morphology and recruitment. Diaphragm changes were scored as: minor, no fibrillation/sharp-waves (fibs-sw) and mild loss of motor units on activation; moderate, fibs-sw and moderate loss of motor units on activation; severe, fibs-sw and few remaining complex motor units with very increased firing-rate. The 21 controls and 83 ALS patients were matched for age. In controls, the only significant correlation between surface and needle recording was for negative-peak amplitude (p = 0.03). In ALS patients, amplitudes and negative-peak area were highly correlated (p < 0.001), as were PN motor latencies (p = 0.002). Forced vital capacity (FVC) was highly correlated with both amplitude (p < 0.001) and PN latency (p < 0.02), whichever electrode was used. PN amplitude recording with needle electrode was consistent with EMG findings in the diaphragm. In general, patients with peak-to-peak amplitude <1.0 mV (20% percentile of the median value) on needle recording tended to show severe changes on needle EMG of the diaphragm, and those with amplitude ⩾1.0 mV to <1.5 mV (1st IQR of the median value) had moderate changes in diaphragm EMG. Generally, EMG changes in internal intercostal muscles mirrored findings observed in diaphragm. In ALS, PN motor amplitude/area and latency measurements recorded by surface electrodes are highly correlated with needle EMG findings in the diaphragm. CMAP amplitude/area measurements showed high correlation with FVC. In ALS, amplitude/area of the motor PN response, recorded by surface or needle electrodes, correlates with dysfunction of the diaphragm.

Diaphragm motor responses to phrenic nerve stimulation in ALS: Surface and needle recordings

ObjectiveIn studies of phrenic nerve (PN) conduction in amyotrophic lateral sclerosis (ALS) both motor response amplitude and latency have been reported as abnormal. However, correlation with diaphragm motor unit loss, and with diaphragmatic function has not been fully evaluated. MethodsWe studied 83 patients with ALS, and 21 patients referred with clinically suspected phrenic nerve lesions whose studies were normal. PN responses elicited by percutaneous electrical stimulation in the neck were recorded using superficial electrodes placed at the surface markings of the diaphragm on the chest wall, and a concentric needle electrode inserted into the diaphragmatic costal fibres. Electromyography of diaphragm was performed to analyse motor unit morphology and recruitment. ResultsThe 21 controls and 83 ALS patients were matched for age. In controls, the only significant correlation between surface and needle recording was for negative-peak amplitude (p = 0.03). In ALS patients, amplitudes and negative-peak area were highly correlated (p < 0.001), as were PN motor latencies (p = 0.002). Forced vital capacity (FVC) was highly correlated with both amplitude (p < 0.001) and PN latency (p < 0.02), whichever electrode was used. PN amplitude recording with needle electrode was consistent with EMG findings in the diaphragm. ConclusionIn ALS, PN motor amplitude/area and latency measurements recorded by surface electrodes are highly correlated with needle EMG findings in the diaphragm. CMAP amplitude/area measurements showed high correlation with FVC. SignificanceIn ALS, amplitude/area of the motor PN response, recorded by surface or needle electrodes, correlates with dysfunction of the diaphragm.

Correlating motor unit morphology with bioelectrical activity – A simulation study

ObjectivesThe aim was to determine motor unit morphology underpinning the various MUP waveforms using MUP analysis. MethodThe simulation method is based on the decomposition of MUP into single fiber potentials. Number of fibers, fiber diameters and fiber to electrode distances were determined. The impact of each muscle fiber on the MUP waveform was determined and quantified by its percentage contribution. ResultsThe origin of the four examined MUPs of distinct waveforms have been explained by showing the histograms of fiber diameters and their distance to the electrode. In the case of a low amplitude MUP it was found that it originated from fibers of smaller than normal diameters with no dominant fiber. In another case of a MUP of short duration its shape was due to a single fiber close to the electrode which contributed to about 80% of the MUP. In case of polyphasic MUP, muscle fiber diameters variability was responsible for MUP characteristic. MUP from normal muscle originated from few fibers of similar diameters. Correlation between MUP’s characteristic and morphological features has been indicated. Our findings are consistent with the neurophysiological knowledge about the origins of MUP. The approximation method enables MUP analysis that provides quantitative description of motor unit morphology. ConclusionMUP analysis using an approximation method enables to get an insight into motor unit morphology and therefore increases understanding of the way the motor unit structure correlates with MUP waveform. SignificanceExtending the amount of information available from EMG examinations.

O128 Evaluation of muscle fibers diameter using analysis of motor unit potentials

Objectives The study was to develop a method for evaluation of muscle fibers diameter by application an approximation of motor unit morphology using analysis of motor unit potentials (MUPs). Verify the utility and reliability. Method The principle of the approximation method rests upon decomposition of MUP into single fiber potentials to determine the morphology of muscle fibers generating these potentials and their diameters and distribution within motor unit territory. The typical and atypical MUPs of various shapes have been analysed. Results Using approximation method the morphology of motor unit generating the selected MUP has been determined including muscle fibers diameter. Contribution of each fiber to the MUP waveforms has been evaluated. Correlation between some characteristic MUP’s parameters and morphological features has been indicated. Conclusion MUP analysis using an approximation method emerged as a useful tool for the interpretation of MUPs. It enables to get an insight into motor unit morphology and therefore increases understanding of the way the motor unit structure correlates with function. The ability to quantify the fibers diameter can add significantly to the motor unit potentials examination. It may help study of muscle fiber diameter variability and thus compliment the muscle biopsy studies. Significance Extending the amount of information available from EMG examinations.

Do Electrodiagnostic Variables Correlate with Functional Outcomes in Carpal Tunnel Syndrome?

Background: The most common entrapment neuropathy seen in electrodiagnostic (EDX) laboratories is carpal tunnel syndrome (CTS). The diagnostic value of EDX with regard to CTS is well-established, but EDX’s predictive value is unclear. To date, only one study has attempted to establish a relationship between EDX findings and a patient’s clinical status pre- and post-treatment, and there was no significant relationship found. Objective: To establish a relationship between EDX variables and clinical severity assessment with Disabilities of the Arm, Shoulder, and Hand (DASH) scores at a single point in time and over a period of time. Methods: The study was a prospective single group cohort. 41 patients referred to an EDX clinic with suspected diagnoses of CTS were enrolled. Patients underwent EDX studies and completed DASH questionnaires at initial and follow-up visits at 8-month to 12-month intervals. Data collected included median sensory, mixed, and motor latencies, amplitudes, conduction velocities, and needle electromyography (EMG). Correlation coefficients were determined between EDX data variables and severity assessment (independent variables) and patients’ DASH questionnaire measures (dependent variables) at initial and follow-up assessments. Results: Change in DASH score over time positively correlated with left distal median motor latency (DMML) decrement and right transcarpal median sensory conduction block and negatively correlated with left median sensory nerve action potential (SNAP) amplitude decrement percentage, right needle EMG motor unit morphology abnormality, and right median forearm motor conduction velocity increment. The correlations observed are of unclear significance since the DASH scores themselves did not change significantly over time. Conclusions: Objective EDX data coupled with patient-reported outcome (PRO) measures such as the DASH score may be more meaningful in directing clinical care than either of them alone.

The aging neuromuscular system and motor performance.

Age-related changes in the basic functional unit of the neuromuscular system, the motor unit, and its neural inputs have a profound effect on motor function, especially among the expanding number of old (older than ∼60 yr) and very old (older than ∼80 yr) adults. This review presents evidence that age-related changes in motor unit morphology and properties lead to impaired motor performance that includes 1) reduced maximal strength and power, slower contractile velocity, and increased fatigability; and 2) increased variability during and between motor tasks, including decreased force steadiness and increased variability of contraction velocity and torque over repeat contractions. The age-related increase in variability of motor performance with aging appears to involve reduced and more variable synaptic inputs that drive motor neuron activation, fewer and larger motor units, less stable neuromuscular junctions, lower and more variable motor unit action potential discharge rates, and smaller and slower skeletal muscle fibers that coexpress different myosin heavy chain isoforms in the muscle of older adults. Physical activity may modify motor unit properties and function in old men and women, although the effects on variability of motor performance are largely unknown. Many studies are of cross-sectional design, so there is a tremendous opportunity to perform high-impact and longitudinal studies along the continuum of aging that determine 1) the influence and cause of the increased variability with aging on functional performance tasks, and 2) whether lifestyle factors such as physical exercise can minimize this age-related variability in motor performance in the rapidly expanding numbers of very old adults.

The Myotonic Plot Thickens: Electrical Myotonia in Antimuscle-Specific Kinase Myasthenia Gravis.

Electrical myotonia is known to occur in a number of inherited and acquired disorders including myotonic dystrophies, channelopathies, and metabolic, toxic, and inflammatory myopathies. Yet, electrical myotonia in myasthenia gravis associated with antibodies against muscle-specific tyrosine kinase (MuSK) has not been previously reported. We describe two such patients, both of whom had a typical presentation of proximal muscle weakness with respiratory failure in the context of a significant electrodecrement in repetitive nerve stimulation. In both cases, concentric needle examination revealed electrical myotonia combined with myopathic motor unit morphology and early recruitment. These findings suggest that MuSK myasthenia should be included within the differential diagnosis of disorders with electrical myotonia.

Electromyographic studies in mdx and wild‐type C57 mice

The electromyographic (EMG) characteristics of human Duchenne muscular dystrophy (DMD) have been well-described. However, to our knowledge, no prior needle electromyographic (EMG) studies of motor unit morphology have been undertaken in muscles from the mdx mouse, an animal that is genetically homologous to DMD. There are significant phenotypic differences between the human and murine dystrophic conditions, bringing into question whether the mdx mouse is an appropriate animal model for DMD. This study was done in order to characterize the EMG findings in mdx mice, compared to normal wild-type mice, and to assess for similarities to DMD. The tibialis anterior and gastrocnemius/soleus muscles from 34 mice (16 C57 wild-type and 18 mdx), divided into four age groups (3, 12, 18, and 24 months), were examined. Wild-type muscles showed normal insertional activity and no abnormal activity at rest. Motor unit action potential (MUAP) parameters were characterized. In contrast to wild-type muscles, mdx muscles showed increased insertional activity, abnormal spontaneous potentials, and the presence of complex repetitive discharges (CRDs). MUAPs showed increased numbers of phases (4.0 +/- 0.6, P < 0.001) and duration (7.1 +/- 1.2 ms, P < 0.02), as well as late components (15%). These EMG data indicate that mdx muscles display EMG characteristics similar to those found in muscles from boys with DMD, lending credence to the mdx mouse as an animal model for this disease. The data obtained in this study indicate a potential role for EMG as an in vivo, objective measurement tool that could be used longitudinally to monitor the effects of therapeutic interventions in mdx mice. This is important as there are few objective measures of muscle function in murine models that do not require killing the animal.

Physiologically Based Simulation of Clinical EMG Signals

An algorithm that generates electromyographic (EMG) signals consistent with those acquired in a clinical setting is described. Signals are generated using a model constructed to closely resemble the physiology and morphology of skeletal muscle, combined with line source models of commonly used needle electrodes positioned in a way consistent with clinical studies. The validity of the simulation routines is demonstrated by comparing values of statistics calculated from simulated signals with those from clinical EMG studies of normal subjects. The simulated EMG signals may be used to explore the relationships between muscle structure and activation and clinically acquired EMG signals. The effects of motor unit (MU) morphology, activation, and neuromuscular junction activity on acquired signals can be analyzed at the fiber, MU and muscle level. Relationships between quantitative features of EMG signals and muscle structure and activation are discussed.

DISRUPTION OF TONIC DISCHARGE IS A SOURCE OF MOTOR UNIT DISCHARGE VARIABILITY IN OLDER ADULTS

During sustained muscular contractions, motor units discharge about a mean rate with variability (c.f. Galganski et al., 1993; Laidlaw et al. 2000; Patten & Kamen, 2000; Vaillancourt et al., 2002). The nature of the variability may be linked to age-related changes in motor unit morphology and impaired control of muscular force. Typically, only stable segments of motor unit discharge time series are used in variability computations and this selection may result in the loss of information. PURPOSE The purpose was to compare motor unit discharge variability among young and older adults with specific interest in the momentary disruption of tonic discharge. METHODS Young (n = 12, mean 18.8 years) and older (n = 12, mean 72.6 years) adults performed each of two different isometric force-matching tasks involving the first dorsal interosseous muscle (FDI). Motor unit discharge behavior was recorded using a quadrifilar needle electrode. A linear, 20% maximal voluntary contraction (MVC) force-matching task was used to investigate the fluctuations that occur during the maintenance of constant force. A complex sinusoidal force-matching task (20% +/− 3.65%MVC) was used to investigate motor unit discharge during force control requiring substantial modulation of motor unit discharge rates. RESULTS Older adults were less proficient in both force-matching conditions (F= 8.58, p= 0.008) and there was a trend towards greater motor unit discharge variability in this group (F= 2.36, p= 0.118). The most notable difference between the two groups was a greater positive skewness of motor unit inter-spike interval (ISI) distributions in the older adults (F= 11.63, p= 0.003). In both conditions, these skewness results were driven by atypically long ISIs. CONCLUSION Momentary disruptions in tonic motor unit discharge occur more often in older adults. These disruptions could occur due to either the inherent discharge variability associated with activity around the recruitment threshold or some other control mechanism. One of these mechanisms may account for part of the impaired force control.

Action potentials and twitch forces of rabbit masseter motor units at optimum jaw angle

This study examines mutual correlations between electrical and contractile motor-unit properties. Action potentials and twitch force responses of 42 masseter motor units were recorded in 14 rabbits. Motor units were excited by stimulating motoneurones in the trigeminal motor nucleus. Action potentials and twitches were measured at different jaw gapes between 0 and 21°, in steps of 3°. For each motor unit, the jaw angle–active force interrelation was determined and variables for action potential and force were compared at the jaw angle at which the motor unit produced the largest force. The results showed a large variation in variables for action potential and force, possibly related to the variation in motor-unit morphology. A weak correlation was found between the variables for action-potential amplitude and the magnitude of optimum force, indicating that motor units producing larger forces tended to have action potentials with larger amplitudes. Twitch-contraction time and the moment arm of the motor unit correlated positively with both the median frequency and the duration of the action potential. This indicates that slower contracting motor units had longer action potentials and is in accord with the earlier observation that slower motor units are preferentially located in the anterior regions of the masseter.

Motor unit number estimates in idiopathic Parkinson's disease

We previously reported changes in motor unit morphology in patients with Parkinson's disease (PD) using subjective and computerized quantitative electromyography. Now, we present data on motor unit number estimates (MUNE) to address the hypothesis of motor neuron dropout in PD. Twenty patients with PD and 20 age-matched control subjects were screened by clinical criteria and nerve conduction studies to exclude those with neuropathy. Motor unit number estimates in the extensor digitorum brevis and hypothenar group were assessed by three different MUNE techniques. The MUNE technique types included (1) the statistical method developed by Daube, (2) a threshold method, and (3) an F-wave method. The overall multivariate comparison for the six MUNE measurements was significantly lower for the patients than the controls ( P=0.02). The only significant difference in the individual measures was found in the threshold MUNE method of the hypothenar group ( P<0.05). These results are consistent with those of our previous work, and both support the hypothesis that mild motor neuron dropout occurs in idiopathic PD. However, MUNE methods characteristically have large standard deviations which make it difficult to detect small changes. Progress in decreasing the variance of MUNEs will facilitate their use in detecting small motor unit number changes in neurodegenerative disease.

Motor unit changes in sporadic idiopathic Parkinson's disease.

We studied motor unit changes in 20 patients with Parkinson's disease (PD) and 20 age-matched control subjects to look for evidence of motorneuron degeneration in sporadic idiopathic PD. Patients and control subjects were screened by clinical criteria and nerve conduction studies to exclude those with peripheral neuropathic processes. Changes in motor unit morphology were investigated with subjective and computerized quantitative electromyography (QEMG) of the anterior tibialis (AT) and first dorsal interosseous. Multivariate comparisons showed a significant difference in the QEMG analysis for motor unit enlargement in patients with PD versus control subjects. Some of the univariate comparisons for both the subjective and QEMG analyses of the AT were also significant. These results demonstrate that motorneuron drop-out with reinnervation occurs in sporadic idiopathic PD. In summary, our findings provide evidence that clinically silent motorneuron disease occurs in typical cases of sporadic idiopathic PD, suggesting that it may be a normal part of the pathologic picture of PD. Any hypothesis concerning the pathogenic mechanism of PD would need to take into account such a finding.

Morphological features and activation patterns of motor units.

The force that a muscle exerts depends on which motor units have been recruited and at what rate they are discharging action potentials. Because of differences in motor-unit morphology (innervation ratio, average cross-sectional area, specific force, and geometric distribution of muscle fibers), the maximum motor-unit force within a motor-neuron pool is not constant but rather can vary by approximately 50 times. Consequently, muscle force is affected by which motor units have been activated. Similarly, the rate at which a motor neuron discharges action potentials depends on the pattern and quantity of the synaptic input it receives and its intrinsic frequency-current relation. The force that a single motor unit can exert will vary by approximately 3 to 15 times when discharge rate is increased from a minimum to a maximum. In the performance of voluntary contractions, therefore, humans appear to have an infinite number of combinations of motor-unit recruitment and discharge rate that can be used to vary muscle force. However, control strategies have evolved that reduce these options substantially. From experiments on low-force, isometric contractions, it appears that the recruitment order of motor units is relatively fixed and that muscle force is graded by concurrent variations in recruitment and discharge rate over much of the force range. Whereas the recruitment order of motor units appears to be robust in other tasks also, perhaps with the exception of lengthening contractions, the use of discharge rate to grade muscle force seems more variable. When subjected to acute and chronic challenges, motor-unit properties can adapt within limited ranges, and motor-unit discharge rate appears to be more affected than recruitment order, although the extent of recruitment may be affected by changes in use. Because movement is controlled by motor-unit activity, an understanding of motor-unit physiology can have a significant impact on the evaluation and treatment of movement disorders.

A comparison of turns analysis and motor unit analysis in electromyography

We compared the results of turns analysis and motor unit analysis on 4056 electromyographic interference patterns (IPs) from normal subjects and patients with neuromuscular disorders. The motor unit analysis involved decomposing the IPs into their component motor unit action potentials (MUAPs) using automatic decomposition electromyography (ADEMG). We checked the accuracy of the decompositions by attempting to reconstruct some of the IPs from their identified MUAPs using computer simulations. The simulations revealed that ADEMG typically identified more than 60% (but not all) of the MUAPs in a given IP. Both turns and MUAP properties showed regular and related changes with force, age, muscle, and recording electrode type. The number of turns in each IP was highly correlated with the number of active MUAPs ( r = 0.65), the mean MUAP firing rate ( r = 0.72), the mean number of turns per MUAP ( r = 0.34), and the product of these 3 properties ( r = 0.83). The mean amplitude change per turn was highly correlated with the mean MUAP amplitude ( r = 0.82), but also depended on the number of turns per MUAP. Due to the lack of a one-to-one relationship between the turns analysis properties and the MUAP properties, the turns analysis properties by themselves did not provide sufficient information to infer unambiguous physiological information about motor unit morphology or firing behavior.