Intrinsic motoneuron properties in typical human development.

Motoneuron properties and their firing patterns undergo significant changes throughout development and in response to neuromodulators such as serotonin. Here, we examined the age-related development of self-sustained firing and general excitability of tibialis anterior motoneurons in a young development (7-17 years), young adult (18-28 years) and adult (32-53 years) group, as well as in a separate group of participants taking selective serotonin reuptake inhibitors (SSRIs, aged 11-28 years). Self-sustained firing, as measured by ΔF, was larger in the young development (∼5.8 Hz, n = 20) compared to the young adult (∼4.9 Hz, n = 13) and adult (∼4.8 Hz, n = 8) groups, consistent with a developmental decrease in self-sustained firing mediated by persistent inward currents (PIC). ΔF was also larger in participants taking SSRIs (∼6.5 Hz, n = 9) compared to their age-matched controls (∼5.3 Hz, n = 26), consistent with increased levels of spinal serotonin facilitating the motoneuron PIC. Participants in the young development and SSRI groups also had higher firing rates and a steeper acceleration in initial firing rates (secondary ranges), consistent with the PIC producing a steeper acceleration in membrane depolarization at the onset of motoneuron firing. In summary, both the young development and SSRI groups exhibited increased intrinsic motoneuron excitability compared to the adults, which, in the young development group, was also associated with a larger unsteadiness in the dorsiflexion torque profiles. We propose several intrinsic and extrinsic factors that affect both motoneuron PICs and cell discharge which vary during development, with a time course similar to the changes in motoneuron firing behaviour observed in the present study. KEY POINTS: Neurons in the spinal cord that activate muscles in the limbs (motoneurons) undergo increases in excitability shortly after birth to help animals stand and walk. We examined whether the excitability of human ankle flexor motoneurons also continues to change from child to adulthood by recording the activity of the muscle fibres they innervate. Motoneurons in children and adolescents aged 7-17 years (young development group) had higher signatures of excitability that included faster firing rates and more self-sustained activity compared to adults aged ≥18 years. Participants aged 11-28 years of age taking serotonin reuptake inhibitors had the highest measures of motoneuron excitability compared to their age-matched controls. The young development group also had more unstable contractions, which might partly be related to the high excitability of the motoneurons.

Acute intermittent hypoxia enhances strength, and modulates spatial distribution of muscle activation in persons with chronic incomplete spinal cord injury.

Acute intermittent hypoxia (AIH) is an emerging technique for facilitating neural plasticity in individuals with chronic incomplete spinal cord injury (iSCI). A single sequence of AIH enhances hand grip strength and ankle plantarflexion torque, but underlying mechanisms are not yet clear. We sought to examine how AIH-induced changes in magnitude and spatial distribution of the electromyogram (EMG) of the biceps and triceps brachii contributes to improved strength. Seven individuals with iSCI visited the laboratory on two occasions, and received either AIH or Sham AIH intervention in a randomized order. AIH consisted of 15 brief (∼60s) periods of low oxygen (fraction of inspired O2 = 0.09) alternating with 60s of normoxia, whereas Sham AIH consisted of repeated exposures to normoxic air. High-density surface EMG of biceps and triceps brachii was recorded during maximal elbow flexion and extension. We then generated spatial maps which distinguished active muscle regions prior to and 60 min after AIH or Sham AIH. After an AIH sequence, elbow flexion and extension forces increased by 91.7 ± 88.4% and 51.7 ± 57.8% from baseline, respectively, whereas there was no difference after Sham AIH. Changes in strength were associated with an altered spatial distribution of EMG and increased root mean squared EMG amplitude in both biceps and triceps brachii muscles. These data suggest that altered motor unit activation profiles may underlie improved volitional strength after a single dose of AIH and warrant further investigation using single motor unit analysis techniques to further elucidate mechanisms of AIH-induced plasticity.

Post-activation depression from primary afferent depolarization (PAD) produces extensor H-reflex suppression following flexor afferent conditioning.

Suppression of the extensor H-reflex by flexor afferent conditioning is thought to be produced by a long-lasting inhibition of extensor Ia afferent terminals via GABAA receptor-activated primary afferent depolarization (PAD). Given the recent finding that PAD does not produce presynaptic inhibition of Ia afferent terminals, we examined in 28 participants if H-reflex suppression is instead mediated by post-activation depression of the extensor Ia afferents triggered by PAD-evoked spikes and/or by a long-lasting inhibition of the extensor motoneurons. A brief conditioning vibration of the flexor tendon suppressed both the extensor soleus H-reflex and the tonic discharge of soleus motor units out to 150 ms following the vibration, suggesting that part of the H-reflex suppression during this period was mediated by postsynaptic inhibition of the extensor motoneurons. When activating the flexor afferents electrically to produce conditioning, the soleus H-reflex was also suppressed but only when a short-latency reflex was evoked in the soleus muscle by the conditioning input itself. In mice, a similar short-latency reflex was evoked when optogenetic or afferent activation of GABAergic (GAD2+ ) neurons produced a large enough PAD to evoke orthodromic spikes in the test Ia afferents, causing post-activation depression of subsequent monosynaptic EPSPs. The long duration of this post-activation depression and related H-reflex suppression (seconds) was similar to rate-dependent depression that is also due to post-activation depression. We conclude that extensor H-reflex inhibition by brief flexor afferent conditioning is produced by both post-activation depression of extensor Ia afferents and long-lasting inhibition of extensor motoneurons, rather than from PAD inhibiting Ia afferent terminals. KEY POINTS: Suppression of extensor H-reflexes by flexor afferent conditioning was thought to be mediated by GABAA receptor-mediated primary afferent depolarization (PAD) shunting action potentials in the Ia afferent terminal. In line with recent findings that PAD has a facilitatory role in Ia afferent conduction, we show here that when large enough, PAD can evoke orthodromic spikes that travel to the Ia afferent terminal to evoke EPSPs in the motoneuron. These PAD-evoked spikes also produce post-activation depression of Ia afferent terminals and may mediate the short- and long-lasting suppression of extensor H-reflexes in response to flexor afferent conditioning. Our findings highlight that we must re-examine how changes in the activation of GABAergic interneurons and PAD following nervous system injury or disease affects the regulation of Ia afferent transmission to spinal neurons and ultimately motor dysfunction in these disorders.

Facilitation of sensory transmission to motoneurons during cortical or sensory-evoked primary afferent depolarization (PAD) in humans.

Sensory and corticospinal tract (CST) pathways activate spinal GABAergic interneurons that have axoaxonic connections onto proprioceptive (Ia) afferents that cause long-lasting depolarizations (termed primary afferent depolarization, PAD). In rodents, sensory-evoked PAD is produced by GABAA receptors at nodes of Ranvier in Ia afferents, rather than at presynaptic terminals, and facilitates spike propagation to motoneurons by preventing branch-point failures, rather than causing presynaptic inhibition. We examined in 40 human participants whether putative activation of Ia-PAD by sensory or CST pathways can also facilitate Ia afferent activation of motoneurons via the H-reflex. H-reflexes in several leg muscles were facilitated by prior conditioning from low-threshold proprioceptive, cutaneous or CST pathways, with a similar long-lasting time course (∼200 ms) to phasic PAD measured in rodent Ia afferents. Long trains of cutaneous or proprioceptive afferent conditioning produced longer-lasting facilitation of the H-reflex for up to 2 min, consistent with tonic PAD in rodent Ia afferents mediated by nodal α5-GABAA receptors for similar stimulation trains. Facilitation of H-reflexes by this conditioning was likely not mediated by direct facilitation of the motoneurons because isolated stimulation of sensory or CST pathways did not alone facilitate the tonic firing rate of motor units. Furthermore, cutaneous conditioning increased the firing probability of single motor units (motoneurons) during the H-reflex without increasing their firing rate at this time, indicating that the underlying excitatory postsynaptic potential was more probable, but not larger. These results are consistent with sensory and CST pathways activating nodal GABAA receptors that reduce intermittent failure of action potentials propagating into Ia afferent branches. KEY POINTS: Controlled execution of posture and movement requires continually adjusted feedback from peripheral sensory pathways, especially those that carry proprioceptive information about body position, movement and effort. It was previously thought that the flow of proprioceptive feedback from Ia afferents was only reduced by GABAergic neurons in the spinal cord that sent axoaxonic projections to the terminal endings of sensory axons (termed GABAaxo neurons). Based on new findings in rodents, we provide complementary evidence in humans to suggest that sensory and corticospinal pathways known to activate GABAaxo neurons that project to dorsal parts of the Ia afferent also increase the flow of proprioceptive feedback to motoneurons in the spinal cord. These findings support a new role for spinal GABAaxo neurons in facilitating afferent feedback to the spinal cord during voluntary or reflexive movements.

Anomalies of motor unit amplitude and territory after botulinum toxin injection.

Objective. Botulinum toxin (BT) induced cholinergic denervation of hyperactive motor units (MUs) is a clinically accepted and extensively practiced way of managing focal spasticity after stroke. The denervation potentially initiates a temporary reorganization of the MU activation patterns and structures by inducing the emergence of a large number of newly innervated muscle fibers. In this study, we quantify the effect of the BT on MU action potential (MUAP) amplitudes and on the MU territory areas (MUTAs) as seen on the surface of the skin over the biceps brachii (BB) muscle.Approach. We have used a 128-channel high-density surface electromyography (HDsEMG) grid on the spastic and contralateral BB muscle and recorded the myoelectric activity along with the contraction force during isometric contraction of the elbow muscles. We have decomposed the recorded EMG signal into individual MU potentials and estimated the MUAP amplitudes and territory areas before and two weeks after a BT injection.Main result. There were significantly larger median (47 ± 9%) MUAP amplitudes as well as reduction of MUTA (20 ± 2%) two weeks after the injection compared to the respective pre-injection recording.Significance. The observed covariation of the amplitude and the territory area indicates that the large amplitude MUs that appeared after the BT injection have a relatively smaller territory area. These results provide a rare insight into the BT-induced changes of MU characteristics and have the potential to improve spasticity treatment. We discuss the potential contributing factors to these changes subsequent to the injection in the context of the investigated subject cohort.

Characteristic Variation of Electromechanical Delay After the Botulinum Toxin Injection in Spastic Biceps Brachii Muscles.

The objective of this study was to characterize the effects of intramuscular botulinum toxin (BT) injections on the electromechanical delay (EMD) in spastic human biceps muscles. The EMD is calculated as the time lag between the muscle activation onset, as recorded from the surface electromyogram (sEMG), and the onset of recorded force. In a cohort of chronic stroke survivors, we compared the computed EMD derived from the spastic (injected) biceps brachii with that from the contralateral muscle. Eight participants were tested before and up to 3 months after a BT injection. At each session, participants followed an isometric trapezoidal force trajectory at 50 and 30%, respectively, of the tested maximum voluntary contraction (MVC). Joint force and sEMG signals were recorded as well. The number of zero crossings (ZC) of the sEMG during the steady-state portion of the task was also computed. The EMD post-BT was found to increase by 64 ± 10% (at 50% MVC) and 93 ± 18% (at 30% MVC) when compared to pre-BT values, while the number of sEMG-ZC, the mean MVC values, and the force-EMD slope exhibited striking reductions. These parameters, calculated on the contralateral side, remained relatively constant across sessions, with the EMD significantly lower and the MVC values much higher. We discuss potential contributing factors to an increase in EMD values on the affected side, both pre- and post-BT. The observed co-variation across sessions of the increased EMD values with the decreased ZC estimates, a surrogate of motor outflow, and, potentially, more compliant muscle fascicles suggests that the altered motor unit (MU) behavior contributes, at least in part, to the delayed force production.

Performance Evaluation of a Wearable Tattoo Electrode Suitable for High-Resolution Surface Electromyogram Recording.

OBJECTIVE: High-density surface electromyography (HD-sEMG) has been utilized extensively in neuromuscular research. Despite its potential advantages, limitations in electrode design have largely prevented widespread acceptance of the technology. Commercial electrodes have limited spatial fidelity, because of a lack of sharpness of the signal, and variable signal stability. We demonstrate here a novel tattoo electrode that addresses these issues. Our dry HD electrode grid exhibits remarkable deformability which ensures superior conformity with the skin surface, while faithfully recording signals during different levels of muscle contraction. METHOD: We fabricated a 4 cm×3 cm tattoo HD electrode grid on a stretchable electronics membrane for sEMG applications. The grid was placed on the skin overlying the biceps brachii of healthy subjects, and was used to record signals for several hours while tracking different isometric contractions. RESULTS: The sEMG signals were recorded successfully from all 64 electrodes across the grid. These electrodes were able to faithfully record sEMG signals during repeated contractions while maintaining a stable baseline at rest. During voluntary contractions, broad EMG frequency content was preserved, with accurate reproduction of the EMG spectrum across the full signal bandwidth. CONCLUSION: The tattoo grid electrode can potentially be used for recording high-density sEMG from skin overlying major limb muscles. Layout programmability, good signal quality, excellent baseline stability, and easy wearability make this electrode a potentially valuable component of future HD electrode grid applications. SIGNIFICANCE: The tattoo electrode can facilitate high fidelity recording in clinical applications such as tracking the evolution and time-course of challenging neuromuscular degenerative disorders.

Characterization of Differences in the Time Course of Reflex and Voluntary Responses Following Botulinum Toxin Injections in Chronic Stroke Survivors.

Spasticity is a major impairment that can occur following a hemispheric stroke and is often treated with injections of botulinum toxin, a neurotoxin that impairs transmission at the neuromuscular junction. Hyperreflexia is a defining feature of spasticity. Our main objective here was to quantify the time course of changes in the deep tendon reflex (DTR) responses and voluntary activation capacity following BT injection as well as to track changes in a clinical assessment of spasticity. Four chronic stroke survivors, scheduled to receive BT in their Biceps Brachii(BB) as part of their clinical care plan, were recruited for repeated testing sessions over the course of 4 months post injection. Both surface BB EMG reflex response to bicipital tendon taps as well as signals of applied tendon tap forces were recorded before and up to 18 weeks post-BT. Voluntary force and biceps EMG signals were also recorded during maximum voluntary (isometric) contractions (MVC) at each testing session. Our results show major reductions (up to 75%) in voluntary sEMG and force arising between 11 to 35 days post-BT-injection. The stretch reflex gain declined two weeks after the maximal reductions in voluntary EMG and force. Paradoxically, there was a short-term increase in stretch reflex gain, in three out of four participants, approximately 11-35 days post BT. The time course of recovery of voluntary MVC and reflex responses varied considerably with a longer recovery time for the reflex responses.

Estimation of self-sustained activity produced by persistent inward currents using firing rate profiles of multiple motor units in humans.

Persistent inward calcium and sodium currents (IP) activated during motoneuron recruitment help synaptic inputs maintain self-sustained firing until derecruitment. Here, we estimate the contribution of the IP to self-sustained firing in human motoneurons of varying recruitment threshold by measuring the difference in synaptic input needed to maintain minimal firing once the IP is fully activated compared with the larger synaptic input required to initiate firing before full IP activation. Synaptic input to ≈20 dorsiflexor motoneurons simultaneously recorded during ramp contractions was estimated from firing profiles of motor units decomposed from high-density surface electromyography (EMG). To avoid errors introduced when using high-threshold units firing in their nonlinear range, we developed methods where the lowest threshold units firing linearly with force were used to construct a composite (control) unit firing rate profile to estimate synaptic input to higher threshold (test) units. The difference in the composite firing rate (synaptic input) at the time of test unit recruitment and derecruitment (ΔF = Frecruit - Fderecruit) was used to measure IP amplitude that sustained firing. Test units with recruitment thresholds 1-30% of maximum had similar ΔF values, which likely included both slow and fast motor units activated by small and large motoneurons, respectively. This suggests that the portion of the IP that sustains firing is similar across a wide range of motoneuron sizes.NEW & NOTEWORTHY A new method of estimating synaptic drive to multiple, simultaneously recorded motor units provides evidence that the portion of the depolarizing drive from persistent inward currents that contributes to self-sustained firing is similar across motoneurons of different sizes.

Effect of Botulinum Toxin on the Spatial Distribution of Biceps Brachii EMG Activity Using a Grid of Surface Electrodes: A Case Study.

Botulinum toxin (BT) is widely prescribed by physicians for managing spasticity post stroke. In an ongoing study, we examine the spatial pattern of muscle activity in biceps brachii of stroke survivors before and after receiving BT, examined over the course of 11 weeks (2 weeks before - 9 weeks after). We hypothesize that BT alters muscle electrophysiology by disrupting fiber neuromuscular transmission in an inhomogeneous manner and we seek to detect these changes using grid surface electromyography (sEMG). Also, we obtained B-mode ultrasound images to have an accurate interpretation of sEMG data by looking at the fiber angle and subcutaneous fat thickness distribution across muscle. Here, we are reporting a single case where a chronic stroke survivor received BT injection in the biceps brachii (BB). A 16x8 sEMG electrode grid was used to capture the muscle activity distribution of BB during sustained non-fatiguing isometric contraction at 40% of maximal voluntary (MVC) elbow flexion. We obtained the root mean squared (RMS) maps of the signal recorded at each of the $16 \times 8$ electrodes. We observed substantial changes in the RMS pattern of BB muscle after receiving BT. More than 80% decrease in sEMG amplitude (RMS) was observed for the channels around the BT injection site as well as about 74% elbow flexion force reduction at the time point of 3-4 weeks post-injection. We also found significant differences between the spatial voluntary activation pattern of pre and post BT RMS maps. We further observed a non-uniform effect and recovery caused by the BT on the distribution of muscle activity. In conclusion, we observed evidence of alteration of the amplitude and pattern of muscle activity after botulinum toxin injection and can document the capability of grid recordings to detect these pattern changes. Our major goals target further investigation to provide an indepth understanding of the effect of botulinum toxin injection at motor unit level.

Spatial Analysis of Multichannel Surface EMG in Hemiplegic Stroke.

We investigated spatial activation patterns of upper extremity muscles during isometric force generation in both intact persons and in hemispheric stroke survivors. We used a 128-channel surface electromyogram (EMG) grid to record the electrical activity of biceps brachii muscles during these contractions. EMG data were processed to develop 2-D root mean square (RMS) maps of muscle activity. Our objective was to determine whether motor impairments following stroke were associated with changes in the muscle activity maps and in the spatial distribution of muscular activation. We found that, for a given subject, spatial patterns in muscle activity maps were consistent across all measured contraction levels differing only the RMS EMG. However, the maps from opposite arms (stroke-affected versus non-affected) of stroke survivors were significantly different from each other, especially when compared with the differences observed intact participants. Our analyses revealed that chronic stroke altered the size and location of the active region in these maps. The former is potentially related to disruption of fiber and tissue structure, possibly linked to factors such as extracellular fat accumulation, connective tissue infiltration, muscle fiber atrophy, fiber shortening, and fiber loss. Changes in spatial patterns in muscle activity maps may also be linked to a shift in the location of the innervation zone or the endplate region of muscles. Furthermore, the textural analysis of EMG activity maps showed a larger pixel-to-pixel variability in stroke-affected muscles. Alterations in the muscle activity maps were also related to functional impairment (estimated using Fugl-Meyer score) and to the degree of spasticity (estimated using the modified Ashworth scale). Overall, our investigation revealed that the muscle architecture and morphology were significantly altered in the chronic stroke.

Spatial distribution of surface EMG on trapezius and lumbar muscles of violin and cello players in single note playing.

Musicians activate their muscles in different patterns, depending on their posture, the instrument being played, and their experience level. Bipolar surface electrodes have been used in the past to monitor such activity, but this method is highly sensitive to the location of the electrode pair. In this work, the spatial distribution of surface EMG (sEMG) of the right trapezius and right and left erector spinae muscles were studied in 16 violin players and 11 cello players. Musicians played their instrument one string at a time in sitting position with/without backrest support. A 64 sEMG electrode (16×4) grid, 10mm inter-electrode distance (IED), was placed over the middle and lower trapezius (MT and LT) of the bowing arm. Two 16×2 electrode grids (IED=10mm) were placed on the left and right erector spinae muscles. Subjects played each of the four strings of the instrument either in large (1bow/s) or detaché tip/tail (8bows/s) bowing in two sessions (two days). In each of two days, measurements were repeated after half an hour of exercise to see the effect of exercise on the muscle activity and signal stability. A "muscle activity index" (MAI) was defined as the spatial average of the segmented active region of the RMS map. Spatial maps were automatically segmented using the watershed algorithm and thresholding. Results showed that, for violin players, sliding the bow upward from the tip toward the tail results in a higher MAI for the trapezius muscle than a downward bow. On the contrary, in cello players, higher MAI is produced in the tail to tip movement. For both instruments, an increasing MAI in the trapezius was observed as the string position became increasingly lateral, from string 1 (most medial) toward string 4 (most lateral). Half an hour of performance did not cause significant differences between the signal quality and the MAI values measured before and after the exercise. The MAI of the left and right erector spinae was smaller in the case of backrest support, especially for violin players. Back muscles of violin and cello players were activated asymmetrically, specifically in fast movements (detaché tip/tail). These findings demonstrate the sensitivity and stability of the technique and justify more extensive investigation following this proof of concept.

Using surface electromyography to detect changes in innervation zones pattern after human cervical spinal cord injury.

Human spinal cord injuries (SCI) disrupt the pathways between brain and spinal cord, resulting in substantial impairment and loss of function. Currently, we do not have the ability to precisely quantify the "functional" level of motor injury. The aim of this study is to determine if high-density surface electromyography imaging (SEI) can be used to characterize the location and extent of the spinal lesion. SEI is a safe and non-invasive technique, which uses several electrodes to provide a map of muscle activity. We applied the SEI technique to characterize muscle activity in individuals with chronic incomplete cervical SCI. Surface electromyogram signals (sEMG) from Biceps Brachii (BB) were recorded at submaximal levels (20%, 40%, and 60%) of maximum voluntary contractions (MVC) during isometric elbow flexion, shoulder flexion, and elbow abduction in two individuals with SCI. Through time-domain analysis of the collected data, we detected signs of de-innervation and re-innervations by analyzing the innervation zones (IZ) on the left and right BB muscles. We found that the distribution of IZs was different between the two sides. In addition, analysis of sEMG data collected at rest (no voluntary contraction) showed evidence of superficial active motor units that were active during rest (in the absence of spasms). These findings highlight the potential of SEI technique as a potential clinical tool to quantitatively describe the extent of the damage to motor spinal circuitry, and provide added precision to the clinical examinations and radiological findings.

Impairment of muscle force transmission in spastic-paretic muscles of stroke survivors.

Hemispheric stroke survivors tend to have persistent motor impairments, with muscle weakness and muscle spasticity observed concurrently in the affected muscles. The objective of this preliminary study was to identify whether impairment of muscle force transmission could contribute to weakness in spastic-paretic muscles of chronic stroke survivors. To characterize the efficiency of the transmission of muscle forces to the tendon, we activated biceps brachii muscle electrically by stimulating the musculocutaneous nerve with maximum current. The ratio between the elicited maximum twitch force amplitude and the maximum M-wave peak-peak amplitude was calculated as a measure of the efficiency of force transmission. Based on the preliminary results of two stroke survivors, we show that the Force/M-wave ratio was reduced in the affected biceps brachii muscles in comparison with the contralateral muscles, indicating a potential impairment in the muscle force transmission in the affected muscles. Our findings suggest that disrupted muscle force transmission to the tendon could contribute to weakness in spastic muscles of chronic stroke survivors.

Spatial analysis of muscular activations in stroke survivors.

We investigated the spatial patterns of electrical activity in stroke-affected muscles using the high density surface electromyogram (sEMG) grids. We acquired 128-channel sEMG signals from the impaired as well as contralateral Biceps Brachii (BB) muscles of stroke survivors and from healthy participants at various force levels from 20 to 60% of maximum voluntary contraction in an isometric non-fatiguing recording protocol. We found the spatial sEMG pattern to be consistent across force levels in healthy and stroke subjects. However, once compared across sides (left vs right in healthy and impaired vs. contralateral in stroke) we found stroke-affected sides to be significantly different in distribution pattern of sEMG from the contralateral side. The sEMG activity areas were significantly shrunk on the affected sides indicating muscle atrophy due to stroke.

Automatic detection of motor unit innervation zones of the external anal sphincter by multichannel surface EMG.

A method to detect automatically the location of innervation zones (IZs) from 16-channel surface EMG (sEMG) recordings from the external anal sphincter (EAS) muscle is presented in order to guide episiotomy during child delivery. The new algorithm (2DCorr) is applied to individual motor unit action potential (MUAP) templates and is based on bidimensional cross correlation between the interpolated image of each MUAP template and two images obtained by flipping upside-down (around a horizontal axis) and left-right (around a vertical axis) the original one. The method was tested on 640 simulated MUAP templates of the sphincter muscle and compared with previously developed algorithms (Radon Transform, RT; Template Match, TM). Experimental signals were detected from the EAS of 150 subjects using an intra-anal probe with 16 equally spaced circumferential electrodes. The results of the three algorithms were compared with the actual IZ location (simulated signal) and with IZ location provided by visual analysis (VA) (experimental signals). For simulated signals, the inter quartile error range (IQR) between the estimated and the actual locations of the IZ was 0.20, 0.23, 0.42, and 2.32 interelectrode distances (IED) for the VA, 2DCorr, RT and TM methods respectively.

Solving EMG-force relationship using Particle Swarm Optimization.

The Particle Swarm Optimization (PSO) algorithm is applied to the problem of "load sharing" among muscles acting on the same joint for the purpose of estimating their individual mechanical contribution based on their EMG and on the total torque. Compared to the previously tested Interior-Reflective Newton Algorithm (IRNA), PSO is more computationally demanding. The mean square error between the experimental and reconstructed torque is similar for the two algorithms. However, IRNA requires multiple initializations and tighter constraints found by trial-and-errors for the input variables to find a suitable optimum which is not the case for PSO whose initialization is random.