Single Fiber Action Potentials Research Articles

T77. Reference values for jitter recordings with concentric needle electrodes on stimulated frontalis

Single fiber electromyography (SFEMG) serves as the most sensitive technique to assess neuromuscular transmission (NMT). In recent years, disposable concentric needle electrodes (CNE) become an alternative to single fiber electrodes for recordings because of concerns about the risk of transmitting infections. Reference values for this new method should be established first before its clinical use in differential diagnosis of neuromuscular junction disorders. We recruited 73 patients, 32 males and 41 females (18–83 years, mean age 52.2 ± 14.0 years), with eyelid, ocular, or weakness symptoms other than myasthenia gravis (MG). None of them had neuromuscular junction diseases, facial nerve palsy or myopathies. The facial nerve was stimulated at the temporal branch with surface electrodes. CNE recorded 20 apparent single fiber action potentials (ASFAPs) from the frontalis for each subjects. Jitter was measured with amplitude level trigger method and expressed as the mean consecutive difference (MCD). The mean MCD were 17.68 ± 3.30 μs for subjects and 17.66 ± 7.81 μs for individual potentials. For ensuring higher specificity, we set the mean values plus 3 SD as the upper normal limits, and figured out the corresponding cut-offs of 28 μs for mean jitter values and 42 μs for jitter values of individual potentials. Our data derived from disease control subjects are similar to those of previous studies by other authors on healthy controls. It is a helpful supplement to the reference value pool of CNE-SFEMG of the frontalis. Introducing patients with confusing diseases into the reference population will increase the ability of this technique to differentiate MG from non-NMT disorders.

Computational model to investigate the relative contributions of different neuromuscular properties of tibialis anterior on force generated during ankle dorsiflexion.

This study describes a new model of the force generated by tibialis anterior muscle with three new features: single-fiber action potential, twitch force, and pennation angle. This model was used to investigate the relative effects and interaction of ten age-associated neuromuscular parameters. Regression analysis (significance level of 0.05) between the neuromuscular properties and corresponding simulated force produced at the footplate was performed. Standardized slope coefficients were computed to rank the effect of the parameters. The results show that reduction in the average firing rate is the reason for the sharp decline in the force and other factors, such as number of muscle fibers, specific force, pennation angle, and innervation ratio. The fast fiber ratio affects the simulated force through two significant interactions. This study has ranked the individual contributions of the neuromuscular factors to muscle strength decline of the TA and identified firing rate decline as the biggest cause followed by decrease in muscle fiber number and specific force. The strategy for strength preservation for the elderly should focus on improving firing rate. Graphical abstract Neuromuscular properties of Tibialis Anterior on force generated during ankle dorsiflexion.

Determination of Nerve Fiber Diameter Distribution From Compound Action Potential: A Continuous Approach.

When a signal is initiated in the nerve, it is transmitted along each nerve fiber via an action potential (called single fiber action potential (SFAP)) which travels with a velocity that is related with the diameter of the fiber. The additive superposition of SFAPs constitutes the compound action potential (CAP) of the nerve. The fiber diameter distribution (FDD) in the nerve can be computed from the CAP data by solving an inverse problem. This is usually achieved by dividing the fibers into a finite number of diameter groups and solve a corresponding linear system to optimize FDD. However, number of fibers in a nerve can be measured sometimes in thousands and it is possible to assume a continuous distribution for the fiber diameters which leads to a gradient optimization problem. In this paper, we have evaluated this continuous approach to the solution of the inverse problem. We have utilized an analytical function for SFAP and an assumed a polynomial form for FDD. The inverse problem involves the optimization of polynomial coefficients to obtain the best estimate for the FDD. We have observed that an eighth order polynomial for FDD can capture both unimodal and bimodal fiber distributions present in vivo, even in case of noisy CAP data. The assumed FDD distribution regularizes the ill-conditioned inverse problem and produces good results.

S68 SFAP to MUAP: Temporal and spatial characteristics of MU

Motor unit action potential (MUAP) represents the summated activity of single fiber action potentials (SFAP) coming from individual muscle fibers those reside in the pick-up territory of recording needle. Calculated features of MUAP give reliable information concerning the architecture of motor unit under investigation but they also depend on the recording site and technique. In this presentation, features of SFAP will be discussed first as a measure of individual muscle fiber conductivity. Then, how MUAP features are changed by moving the needle near to tendon is questioned. Measuring the bioelectrical activity of motor unit across its territory by scanning EMG will be revisited. SFAP represents the moving quadrupole in near field recordings. The faster the quardupole moves, the quicker the phases of SFAP change and lead to shorter “spike duration” in turn. The distribution of “spike duration” shows less central tendency in myopathic muscle, which can be related to increased variability among the muscle fibers. Furthermore, moving the recording site near to tendon exaggerates this conduction difference and leads to more spiky and temporally dispersed MUAP, a finding that is especially detected in simulation studies. Scanning the electrical activity of motor unit from side to side informs the examiner about the motor unit structure. New algorithms and methods were developed for extracting features from scanning EMG signals. The recorded activity of motor unit during voluntary drive changes by the placement of needle electrode. As it changes, it teaches us a lot!

S67 Special phenomena in the motor unit during voluntary and electrical activation

Different information is obtained about the motor unit (MU) from various EMG methods. Single Fiber EMG (SFEMG) explores individual muscle fibers and endplates, conventional EMG a larger part of the MU and Macro EMG represents the entire MU. The presentation will first discuss consequences for routine EMG of the temporal and spatial variation in the activation of MUs that may lead to misinterpretation of some EMG features. It will also highlight some physiological parameters of conduction along a muscle fiber based on SFEMG. The muscle fiber velocity shows a short-term facilitation by previous activity, which gives rise to variation in arrival time to the recording electrode, seen both at voluntary and electrical stimulation. During long term activity on the other hand, the velocity decreases, seen and heard as change in frequency spectrum of the EMG signals. A special phenomenon is the decrease in action potential amplitude in some myotonic disorders. A surface recorded amplitude drop at repetitive stimulation seen in these cases not sign of neuromuscular junction disturbance, but reflects amplitude changes of individual fiber action potentials. Amplitude of single fiber action potentials also varies in relationship to just previous activity, with lower amplitude for short intervals, also seen in conventional EMG. Different types of reflexes involving just one or a few neurons can be studied with SFEMG. Different single cell phenomena can be studied in situ in man, and many of these details can be seen in routine EMG, once we are aware of them.

Optimizing the design of bipolar nerve cuff electrodes for improved recording of peripheral nerve activity

Objective. Differential measurement of efferent and afferent peripheral nerve activity offers a promising means of improving the clinical utility of implantable neuroprostheses. The tripolar nerve cuff electrode has historically served as the gold standard for achieving high signal-to-noise ratios (SNRs) of the recordings. However, the symmetrical geometry of this electrode array (i.e. electrically-shorted side contacts) precludes it from measuring electrical signals that can be used to obtain directional information. In this study, we investigated the feasibility of using a bipolar nerve cuff electrode to achieve high-SNR of peripheral nerve activity. Approach. A finite element model was implemented to investigate the effects of electrode design parameters—electrode length, electrode edge length (EEL), and a conductive shielding layer (CSL)—on simulated single fiber action potentials (SFAP) and also artifact noise signals (ANS). Main results. Our model revealed that the EEL was particularly effective in increasing the peak-to-peak amplitude of the SFAP (319%) and reducing the common mode ANS (67%) of the bipolar cuff electrode. By adding a CSL to the bipolar cuff electrode, the SNR was found to be 65.2% greater than that of a conventional tripolar cuff electrode. In vivo experiments in anesthetized rats confirmed that a bipolar cuff electrode can achieve a SNR that is 38% greater than that achieved by a conventional tripolar cuff electrode (p < 0.05). Significance. The current study showed that bipolar nerve cuff electrodes can be designed to achieve SNR levels that are comparable to that of tripolar configuration. Further work is needed to confirm that these bipolar design parameters can be used to record bi-directional neural activity in a physiological setting.

A new approach for multi-channel surface EMG signal simulation.

Simulation models are necessary for testing the performance of newly developed approaches before they can be applied to interpreting experimental data, especially when biomedical signals such as surface electromyogram (SEMG) signals are involved. A new and easily implementable surface EMG simulation model was developed in this study to simulate multi-channel SEMG signals. A single fiber action potential (SFAP) is represented by the sum of three Gaussian functions. SFAP waveforms can be modified by adjusting the amplitude and bandwidth of the Gaussian functions. SEMG signals were successfully simulated at different detected locations. Effects of the fiber depth, electrode position and conduction velocity of SFAP on motor unit action potential (MUAP) were illustrated. Results demonstrate that the easily implementable SEMG simulation approach developed in this study can be used to effectively simulate SEMG signals.

A phantom axon setup for validating models of action potential recordings.

Electrode designs and strategies for electroneurogram recordings are often tested first by computer simulations and then by animal models, but they are rarely implanted for long-term evaluation in humans. The models show that the amplitude of the potential at the surface of an axon is higher in front of the nodes of Ranvier than at the internodes; however, this has not been investigated through in vivo measurements. An original experimental method is presented to emulate a single fiber action potential in an infinite conductive volume, allowing the potential of an axon to be recorded at both the nodes of Ranvier and the internodes, for a wide range of electrode-to-fiber radial distances. The paper particularly investigates the differences in the action potential amplitude along the longitudinal axis of an axon. At a short radial distance, the action potential amplitude measured in front of a node of Ranvier is two times larger than in the middle of two nodes. Moreover, farther from the phantom axon, the measured action potential amplitude is almost constant along the longitudinal axis. The results of this new method confirm the computer simulations, with a correlation of 97.6%.

Estimation of the Electrode-Fiber Bioelectrical Coupling From Extracellularly Recorded Single Fiber Action Potentials.

Selective peripheral neural interfaces are currently capable of detecting minute electrical signals from nearby nerve fibers as single fiber action potential (SFAP) waveforms. Each detected single unit has a distinct shape originating from the unique bioelectrical coupling that exists between the neuroprosthetic electrode, the nerve fiber and the extracellular milieu. The bioelectrical coupling manifests itself as a series of low-pass Bessel filters acting on the action currents along the nerve fiber. Here, we present a method to estimate the electrode-fiber bioelectrical coupling through a quantitative analysis of the spectral distribution of the single units extracellularly recorded with the thin-film longitudinal intrafascicular electrode (tfLIFE) in an in vivo mammalian peripheral nerve animal model. The bioelectrical coupling estimate is an estimate of the electrode sensitivity function traversed by the nerve fiber, suggesting that it is as a means to directly measure the spatial relationship between the nerve fiber and electrode. It not only reflects a shape change to the SFAP, but has implications for in situ nerve fiber location tracking, in situ diagnostics of nerves and neuroproshetic electrodes, and assessment of the biocompatibility of neural interfaces and the health of the reporting nerve fibers.

O10: Effect size of quantitative muscle imaging in Duchenne muscular dystrophy exceeds the effect size of clinical scores of muscle function

s of Oral Presentations / Clinical Neurophysiology 125, Supplement 1 (2014) S1–S339 S31 confirmed Duchenne muscular dystrophy. Physical assessment was performed using the motor function measurement (MFM) scale. Quantitative magnet resonance imaging (qMRI) of thigh muscles was performed using the two-point Dixon method. One year functional and imaging changes were different according to the age and walking abilities at inclusion (group 1: below the age of seven; group 2: seven years and older and ambulant; group 3: non-ambulant). While patients of the first group still showed improvement of motor abilities, in group 2 the largest effect size (1.2) was found in the D1 subscore (standing and transfer function) of the MFM, and in group 3 the largest effect size was found for the total MFM score (0.66). In contrast, the effect sizes using qMRI was much larger consisting of 1.6 in group 1 (all thigh muscles), 4.1 in group 2 (hamstrings), and 2.3 in group 3 (knee extensors). These data suggest that qMRI has an added value compared to clinical outcome measures when designing therapeutic clinical trials in patients with DMD. Power analysis suggest that there are only a few needed patients to show an effect of a putative novel treatment when using quantitive muscle MRI in DMD. O11 Estimation of the muscle fiber density from the motor unit action potential I. Goker1, T. Artug2, O. Osman2, B. Baslo3 1Istanbul Arel University, Biomedical Engineering, Istanbul, Turkey; 2Istanbul Arel University, Electrical and Electronics Engineering, Istanbul, Turkey; 3Istanbul University, Medical Faculty, Istanbul, Turkey Introduction: The aim of this study is the estimation of the muscle fiber density (FD) from the Motor Unit Action Potentials (MUAP) to form a method for monitoring the progress of neuromuscular diseases. Methods: Data groups representing motor units of neurogenic (n=5), myopathic (n=5) and healthy cases (n=5) were created via EMG Simulator v3.6. MUAPs generated from each data group have been detected through a concentric needle electrode of a scanning EMG system. The spikes of the single fiber action potentials within the MUAPs have been found by computing the second-order derivative of MUAPs to estimate the detected number of fibers. MUAPs from 20 different locations within a Motor Unit Territory (MUT) have been acquired. The fiber density (FD) has been computed by taking the average of these numbers of fibers. True FDs have been computed for each MUT. Mean Square Errors (MSE) have been computed for each data group to determine the sensitivity of the method. Results: The test statistics of the FDs (mean ± SD) have been found as 4.13±1.31 (Min: 2 and Max: 8), 2.37±0.98 (Min: 1 and Max: 5) and, 2.40±0.70 (Min: 1 and Max: 4) for neurogenic, myopathic and healthy cases respectively. Mean true fiber densities have been 5.66, 2.57 and 1.50 and MSE have been 4.02, 0.15 and, 0.86 for neurogenic, myopathic and healthy cases respectively. These MSE values have been suggested that this method is more selective for myopathic cases compared to neurogenic and healthy cases. Conclusion: Computation of FD via the second-order derivative of MUAPs can be considered as a beneficial preliminary research. Nevertheless, this method might be extended by taking into account other variables such as maximum amplitude to increase the sensitivity for neurogenic cases.

An Empirical Muscle Intracellular Action Potential Model with Multiple Erlang Probability Density Functions based on a Modified Newton Method.

The convolution of the transmembrane current of an excitable cell and a weighting function generates a single fiber action potential (SFAP) model by using the volume conductor theory. Here, we propose an empirical muscle IAP model with multiple Erlang probability density functions (PDFs) based on a modified Newton method. In addition, we generate SFAPs based on our IAP model and referent sources, and use the peak-to-peak ratios (PPRs) of SFAPs for model verification. Through this verification, we find that the relation between an IAP profile and the PPR of its SFAP is consistent with some previous studies, and our IAP model shows close profiles to the referent sources. Moreover, we simulate and discuss some possible ionic activities by using the Erlang PDFs in our IAP model, which might present the underlying activities of ions or their channels during an IAP.

Abnormalities of neuromuscular transmission in patients with Miller–Fisher syndrome

The mechanism of motor weakness in patients with Miller–Fisher syndrome (MFS) remains to be fully elucidated. We performed stimulated single fibre electromyography (sSFEMG) in a clinically weak frontalis muscle in a patient with MFS. Stimulate single fiber EMG revealed increased jitter in over 50% of the apparent single fibre action potentials from the frontalis muscle in addition to increased mean jitter. The findings in the present study suggest dysfunction of neuromuscular transmission in patients with MFS.

The morphology of single muscle fibre potentials – Part II: Experimental findings

Studies dealing with single fibre action potentials (SFAPs) have been more interested in obtaining quantitative data of certain parameters of the SFAP waveform than in the analysis of its morphologic features. The characterization of the SFAP morphology is highly valuable as it will allow to obtain information about in vivo intracellular action potentials (IAPs). However, the SFAP final portion is highly sensitive to distant electrical activity, as shown in Part I of this study. The present paper, Part II, is aimed at analysing the morphologies found in human SFAPs and deriving data of the associated IAPs. It was found that, for most SFAPs (97%), the declining negative phase starts decreasing steeply up to a certain point (slope-discontinuity point), from which it returns more slowly towards the baseline. This return may be further slowed down due to the contamination from distant potentials, but the slope-discontinuity point seems to be an intrinsic feature of human SFAPs. The third phase of SFAPs was, either absent (65%), or of rather small amplitude and prolonged duration. The slope-discontinuity point was apparent for the SFAPs of larger amplitude and vanished gradually as the SFAPs got smaller. Several lines of evidence strongly suggest that the spike duration of human IAPs should be about 1.0ms.

The morphology of single muscle fibre potentials – Part I: Simulation study of the distortion introduced by the distant-interfering potentials

Some morphologic aspects of human single fibre action potentials (SFAPs) are not sufficiently well-known. This uncertainty especially concerns the declining negative phase and the final positive phase (third phase) of SFAPs, as these parts are significantly affected by distant electrical activity. The incomplete characterisation of the SFAP shape is also explained by the limited knowledge of human intracellular action potentials (IAPs). The objectives of this study are to assess the morphologic features of human SFAPs and to derive information about the characteristics of human IAPs. To achieve this, the study has been divided into two parts. The present paper, Part I, aims to analyse the changes in the SFAP time-course introduced by distant-interfering potentials and to evaluate how these changes depend on the spike duration of the corresponding IAP. It was found that, for fibre-to-electrode distances shorter than about 0.2mm, SFAPs generated by short-spike IAPs have a declining negative phase with a steep approximately constant slope that is largely unaffected by the potentials from distant fibres. For the same distances, SFAPs resulting from wide-spike IAPs have a declining negative phase with a slow return towards the baseline that is highly sensitive to distant-interfering potentials. The third phase of an SFAP is considerably distorted by distant potentials irrespective of the spike duration of the IAP.

Influence of unit distance and conduction velocity on the spectra of extracellular action potentials recorded with intrafascicular electrodes

The use of highly selective penetrating electrodes yields multi-unit extracellular action potential (AP) recordings of the nerve fibers in the vicinity of the electrode. Accessing the information carried within the neural data stream further requires discrimination and separation of the multi-unit recording into their constituent multiple single unit spike trains. Shape differences in the single fiber action potentials (SFAPs) are typically used as the criteria for unit separation. The present paper explores the origins of the shape differences through analysis of the SFAP in the frequency domain. We present the derivation and computational model predictions of a method to quantitatively analyse changes in the spectral components of SFAPs with an axially located intrafascicular electrode with non-radially symmetrical sensitivity function. A spatial tissue filter relationship was derived using reciprocity equations in the spatial frequency domain and transformed to time frequency. A three dimensional bioelectrical volume conductor finite element model of a recording electrode residing in a nerve fascicle was developed to explore the potential distribution in the nerve fascicle and further derive the electrode–fiber coupling function in the time-frequency domain. It was found that the spectral distribution of the SFAP was multimodal in nature, similar to empirical reported earlier, and could be predicted by taking the single fiber action currents (SFACs) filtered by the electrode–fiber coupling function. This function manifested itself as a low-pass filter of the SFAC, dependent upon the fiber's location relative to the electrode and conduction velocity. Analysis of the spectral distribution revealed that changes in the landmarks of the distribution could be related to the fiber location and conduction velocity. Moreover, a consistent relationship was found when exploring the distribution of fibers located off the one axis of symmetry.

Influence of the shape of intracellular potentials on the morphology of single-fiber extracellular potentials in human muscle fibers

Attention of the investigators is usually pointed to the peak-to-peak characteristics of single-fiber action potentials (SFAPs) that are mainly determined by the depolarizing phase of the intracellular action potential (IAP). However, the final portion of the SFAP has often specific shape that has to be related to peculiarities of the repolarization phase of IAP and the duration of its spike. A novel piecewise SFAP model is proposed to achieve greater insight into the nature of declining portion of the negative phase and of the third phase of SFAP. It was found that the SFAP third phase is essentially determined by the specific profile of the transition of the IAP falling phase toward the resting voltage, whereas the SFAP declining negative phase is more dependent upon the width of the corresponding IAP spike. We tentatively suggest that the duration of the spike of human IAPs should be over approximately 0.75ms.

Effects of changes in the shape of the intracellular action potential on the peak-to-peak ratio of single muscle fibre potentials

In situ recording of the intracellular action potential (IAP) of human muscle fibres is not yet feasible, and consequently, knowledge about certain IAP characteristics of these IAPs is still limited. The ratio between the amplitudes of the second and first phases (the so-called peak-to-peak ratio, PPR) of a single fibre action potential (SFAP) is known to be closely related to the IAP profile. The PPR of experimentally recorded SFAPs has been found to be largely independent of changes in the fibre-to-electrode (radial) distance. The main goal of this paper is to analyze the effect of changes in different aspects of the IAP spike on the relationship between PPR and radial distance. Based on this analysis, we hypothesize about the characteristics of IAPs obtained experimentally. It was found that the sensitivity of the SFAP PPR to changes in radial distance is essentially governed by the duration of the IAP spike. Assuming that, for mammals, the duration of the IAP rising phase lies within the range 0.2–0.4 ms, we tentatively suggest that the duration of the IAP spike should be over approximately 0.75 ms, with the shape of the spike strongly asymmetric. These IAP characteristics broadly coincide with those observed in mammal IAPs.

Activity-dependent changes in impulse conduction of single human motor axons: A stimulated single fiber electromyography study

Objective The aim of this study is to develop a novel method to assess activity-dependent hyperpolarization in human single motor axons at a constant stimulus frequency by using intra-muscular axonal stimulating single fiber electromyography (s-SFEMG). Methods We performed s-SFEMG in the extensor digitorum communis (EDC) muscle of 10 normal subjects, and measured changes in latencies for single muscle fiber action potentials (MAPs) during 500 stimuli delivered at 5, 10 and 20 Hz. The data were analyzed with a repeated measurement analysis, and multiple comparisons were performed. Results A total of 585 MAPs were examined at 5 Hz ( n = 190), 10 Hz ( n = 210), and 20 Hz ( n = 185) steady stimulation. There was a progressive linear prolongation of latencies, as the stimulus rate increased ( F = 95.6, p < 0.001); the least square means (SEM) of latency change were 100.7 (0.28)% at 5 Hz, 102.3 (0.27)% at 10 Hz and 105.3 (0.28)% at 20 Hz. There were statistically significant differences between frequencies by Tukey–Kramer’s method. Despite the significant latency prolongation, no activity-dependent conduction block developed. A 20 Hz electric stimulation to intramuscular axons was well-tolerated in all the subjects. Conclusions Tetanic stimulation at a constant rate results in significant latency increase in single human motor axons, the extent of which depends on the stimulus frequency. The findings imply that physiological discharge rates will activate the Na +/K + pump and thereby produce axonal hyperpolarization in single motor axons. Significance This technique may detect activity-dependent conduction block if the safety margin of impulse transmission is significantly reduced by demyelination or increased branching due to collateral sprouting in a variety of neuromuscular disorders.

Stimulus level effects on neural excitation and eCAP amplitude

The common assumption that the electrically evoked compound action potential (eCAP) has a linear relationship with the number of excited nerve fibres is derived from the acoustical unitary response concept. This study tests the validity of this hypothesis for electrical stimulation. Five guinea pigs were implanted with the tip of a human HiFocus electrode. eCAPs were measured with the forward masking paradigm, using anodic- and cathodic-leading biphasic current pulses and the inter-pulse interval was varied. Masker and probe amplitudes were varied either individually or simultaneously. Surprisingly, at high levels decreasing eCAP amplitudes were measured with increasing stimulus current. In search for an explanation, the experimental conditions were implemented in our 3D computational model of the implanted guinea pig cochlea to perform a functional comparison. In the final experiment, with fixed inter-pulse interval (IPI) and anodic-leading pulses, increasing stimulus currents showed growing numbers of excited nerve fibres and decreasing eCAP amplitudes at high levels, again. While simulating the relative contribution of single fibres to the overall eCAP, an explanation for this could be found in a waveform change in the modelled single fibre action potentials at high levels. We conclude that highly stimulated nerve fibres have another contribution to the eCAP response than lower stimulated fibres, which leads to a reduction of the eCAP amplitude at high levels.

Effects of 8 wk of voluntary unloaded wheel running on K+ tolerance and excitability of soleus muscles in rat

During intense exercise, efflux of K(+) from working muscles increases extracellular K(+) ([K(+)](o)) to levels that can compromise muscle excitability and hence cause fatigue. In this context, the reduction in the exercise-induced elevation of [K(+)](o) observed after training in humans is suggested to contribute to the increased performance after training. Although a similar effect could be obtained by an increase in the tolerance of muscle to elevated [K(+)](o), this possibility has not been investigated. To examine this, isolated soleus muscles from sedentary (sedentary) rats and from rats that had voluntarily covered 13.1 ± 0.7 km/day in an unloaded running wheel for 8 wk (active) were compared. In muscles from active rats, the loss of force induced by exposure to an elevated [K(+)](o) of 9 mM was 42% lower than in muscles from sedentary rats (P < 0.001). This apparent increase in K(+) tolerance in active rats was associated with an increased excitability as evident from a 33% reduction in the electrical current needed to excite individual muscle fibers (P < 0.0009). Moreover, muscles from active rats had lower Cl(-) conductance, higher maximal rate of rise of single-fiber action potentials (AP), and higher Na(+)/K(+) pump content. When stimulated intermittently at 6.5 mM K(+), muscles from active rats displayed better endurance than muscles from sedentary rats, whereas no difference was found when the muscles were stimulated continuously at 30 or 120 Hz. We conclude that voluntary running increases muscle excitability, leading to improved tolerance to elevated [K(+)](o).