The minimum number of contractions required to examine the EMG amplitude versus isometric force relationship for the vastus lateralis and vastus medialis

Studies have demonstrated that the electromyographic (EMG) amplitude versus submaximal isometric force relationship is relatively linear. The purpose of this investigation was to determine the minimum number of contractions required to study this relationship. Eighteen men (mean age = 23 years) performed isometric contractions of the leg extensors at 10–90% of the maximum voluntary contraction (MVC) in 10% increments while surface EMG signals were detected from the vastus lateralis and vastus medialis. Linear regression was used to determine the coefficient of determination, slope coefficient, and y-intercept for each muscle and force combination with successively higher levels included in the model (i.e., 10–30%, … 10–90% MVC). For the slope coefficients, there was a main effect for force combination (P < .001). The pairwise comparisons showed there was no difference from 10–60% through 10–90% MVC. For the y-intercepts, there were main effects for both muscle (vastus lateralis [4.3 μV RMS] > vastus medialis [−3.7 μV RMS]; P = .034) and force combination (P < .001), with similar values shown from 10–50% through 10–90% MVC. The linearity of the absolute EMG amplitude versus isometric force relationship for the vastus lateralis and vastus medialis suggests that investigators may exclude high force contractions from their testing protocol.     

Accuracy of gastrocnemius muscles forces in walking and running goats predicted by one-element and two-element Hill-type models

Hill-type models are commonly used to estimate muscle forces during human and animal movement—yet the accuracy of the forces estimated during walking, running, and other tasks remains largely unknown. Further, most Hill-type models assume a single contractile element, despite evidence that faster and slower motor units, which have different activation–deactivation dynamics, may be independently or collectively excited. This study evaluated a novel, two-element Hill-type model with “differential” activation of fast and slow contractile elements. Model performance was assessed using a comprehensive data set (including measures of EMG intensity, fascicle length, and tendon force) collected from the gastrocnemius muscles of goats during locomotor experiments. Muscle forces predicted by the new two-element model were compared to the forces estimated using traditional one-element models and to the forces measured in vivo using tendon buckle transducers. Overall, the two-element model resulted in the best predictions of in vivo gastrocnemius force. The coefficient of determination, r², was up to 26.9% higher and the root mean square error, RMSE, was up to 37.4% lower for the two-element model than for the one-element models tested. All models captured salient features of the measured muscle force during walking, trotting, and galloping (r²=0.26–0.51), and all exhibited some errors (RMSE=9.63–32.2% of the maximum in vivo force). These comparisons provide important insight into the accuracy of Hill-type models. The results also show that incorporation of fast and slow contractile elements within muscle models can improve estimates of time-varying, whole muscle force during locomotor tasks.     

Muscular sound and force relationship during isometric contraction in man

The contracting muscle generates a low frequency sound detectable at the belly surface, ranging from 11 to 40 Hz. To study the relationship between the muscular sound and the intensity of the contraction a sound myogram (SMG) was recorded by a contact sensor from the biceps brachii of seven young healthy males performing 4-s isometric contractions from 10% to 100% of the maximal voluntary contraction (MVC), in 10% steps. Simultaneously, the electromyogram (EMG) was recorded as an index of muscle activity. SMG and EMG were integrated by conventional methods (iSMG and iEMG). The relationship between iSMG and iEMG vs MVC% is described by parabolic functions up to 80% and 100% MVC respectively. Beyond 80% MVC the iSMG decreases, being about half of its maximal value at 100% MVC. Our results indicate that the motor unit recruitment and firing rate affect the iSMG and iEMG in the same way up to 80% MVC. From 80% to 100% MVC the high motor units' discharge rate and the muscular stiffness together limit the pressure waves generated by the dimensional changes of the active fibres. The muscular sound seems to reflect the intramuscular visco-elastic characteristics and the motor unit activation pattern of a contracting muscle.     

Reliability of a modified motor unit number index (MUNIX) technique

IntroductionThe purpose of this study was to examine the relative and absolute between-day reliability of the motor unit number index (MUNIX).MethodsYoung, healthy adults (n = 19) attended two testing sessions separated by 4-weeks where their maximal pinch-grip strength, MUNIX, and motor unit size index (MUSIX) were assessed in the abductor pollicis brevis muscle. Reliability was assessed by intraclass correlation coefficients (ICC), coefficient of variation (CV) and limits of agreement (LOA).ResultsNo mean differences were observed for MUNIX or MUSIX. The CV for the MUNIX and MUSIX measures were between 13.5% and 17.5%. The ICC for both measures were moderate to moderately-high (0.73–0.76), The LOA for both indicated a homoscedastic relationship.DiscussionOur findings indicate moderate to moderately-high reliability for both MUNIX and MUSIX. Future work is needed to ensure both measures are reliable in other muscles and cohorts, and further investigations are required to examine the validity of MUNIX.     

Estimating the number of motor units using random sums with independently thinned terms

The problem of estimating the numbers of motor units N in a muscle is embedded in a general stochastic model using the notion of thinning from point process theory. In the paper a new moment type estimator for the numbers of motor units in a muscle is denned, which is derived using random sums with independently thinned terms. Asymptotic normality of the estimator is shown and its practical value is demonstrated with bootstrap and approximative confidence intervals for a data set from a 31-year-old healthy right-handed, female volunteer. Moreover simulation results are presented and Monte-Carlo based quantiles, means, and variances are calculated for N in{300,600,1000}.     

Effect of conditioning contraction intensity on postactivation potentiation is muscle dependent

We aimed to examine whether the influence of conditioning contraction intensity on the extent of postactivation potentiation (PAP) is muscle dependent. Eleven healthy males performed both thumb adduction and plantar flexion as a conditioning contraction. The conditioning contraction intensities were set at 20%, 40%, 60%, 80%, or 100% of the maximal voluntary isometric contraction (MVC).Before and after the conditioning contraction, twitch torque was measured for the respective joint to calculate the extent of PAP. In plantar flexion, the extent of PAP became significantly larger as the conditioning contraction intensity increased up to 80% MVC (p < 0.05). In contrast, the extent of PAP in thumb adduction increased significantly only up to 60% MVC (p < 0.05), but not at higher intensities.These results indicate that the influence of the conditioning contraction intensity on the extent of PAP is muscle dependent. Our results suggest that a conditioning contraction with submaximal intensity can sufficiently evoke sizable PAP in the muscle where most of muscle fibers are recruited at submaximal intensities, thereby attenuating muscle fatigue induced by the conditioning contraction.     

Spatial distribution of surface action potentials generated by individual motor units in the human biceps brachii muscle

This study analyses the spatial distribution of individual motor unit potentials (MUPs) over the skin surface and the influence of motor unit depth and recording configuration on this distribution. Multichannel surface (13 × 5 electrode grid) and intramuscular (wire electrodes inserted with needles of lengths 15 and 25 mm) electromyographic (EMG) signals were concurrently recorded with monopolar derivations from the biceps brachii muscle of 10 healthy subjects during 60-s isometric contractions at 20% of the maximum torque. Multichannel monopolar MUPs of the target motor unit were obtained by spike-triggered averaging of the surface EMG. Amplitude and frequency characteristics of monopolar and bipolar MUPs were calculated for locations along the fibers’ direction (longitudinal), and along the direction perpendicular (transverse) to the fibers. In the longitudinal direction, monopolar and bipolar MUPs exhibited marked amplitude changes that extended for 16–32 mm and 16–24 mm over the innervation and tendon zones, respectively. The variation of monopolar and bipolar MUP characteristics was not symmetrical about the innervation zone. Motor unit depth had a considerable influence on the relative longitudinal variation of amplitude for monopolar MUPs, but not for bipolar MUPs. The transverse extension of bipolar MUPs ranged between 24 and 32 mm, whereas that of monopolar MUPs ranged between 72 and 96 mm. The mean power spectral frequency of surface MUPs was highly dependent on the transverse electrode location but not on depth. This study provides a basis for the interpretation of the contribution of individual motor units to the interference surface EMG signal.     

Motor unit activity during long-lasting intermittent muscle contractions in humans

Changes accompanying long-lasting intermittent muscle contractions (30%–50% of the maximal) were investigated by tracing the activity of 38 motor units (MU) of the human biceps brachii muscle recorded from fine-wire branched electrodes. The motor task was a continuous repetition of ramp-and-hold cycles of isometric flexion contractions. During ramp-up phases a significant decline in recruitment thresholds was found with no changes in the discharge pattern. During ramp-down phases the unchanged mean value of derecruitment thresholds during the task was accompanied by increased duration of the last two interspike intervals (ISI). These findings would suggest that during fatigue development the main compensatory mechanism during ramp-up contractions is space coding while for ramp-down contractions it is rate coding. During the steady-state phases the mean value of ISI, as well as the firing variability, had increased by the end of the task in most of the MU investigated . In addition 17 recruited MU were also investigated. These units revealed a lower initial discharge rate and a faster decrease in the mean discharge rate with the development of fatigue. The gradual reduction of the recruitment threshold of already active MU and the recruitment of new units demonstrated an increased excitability of the motorneuron pool during fatigue. A typical recruitment pattern (a first short ISI followed by a long one) was observed during ramp-up contractions in units active from the very beginning of the task, as well as during sustained contractions at the onset of the stable discharge of the additionally recruited MU. Accepted: 23 September 1997     

Motor unit number estimation and quantitative needle electromyography in stroke patients

ObjectiveTo evaluate the effect of upper motor neuron damage upon motor units’ function by means of two separate and supplementary electrophysiological methods.MethodsThe abductor digiti minimi muscle of the non-paretic and the paretic side was studied in forty-six stroke patients with (a) motor unit number estimation (MUNE) – adapted multiple point stimulation method and (b) computerized quantitative needle electromyography (EMG) assessing the configuration of voluntary recruited motor unit potentials. Main outcome comparisons were focused on differences between non-paretic and paretic side.ResultsOn the affected hands mean MUNE value was significantly lower and mean area of the surface recorded single motor unit potentials was significantly larger than the corresponding ones on the non-paretic hands. EMG findings did not reveal remarkable differences between the two sides. Neither severity nor chronicity of stroke was related to MUNE or EMG parameters.DiscussionMUNE results, which suggested reduced motor unit numbers in stroke patients, in conjunction with the normal EMG features in these same muscles has given rise to different interpretations. In a clinical setting, reinnervation type changes in the EMG similar to that occurring in neuronopathies or axonal neuropathies should not be expected in muscles with central neurogenic lesion.     

Experimentally verified model of mechanomyograms recorded during single motor unit contractions

The aim of the paper is to create a model which enables to observe the mechanomyographic (MMG) wave generated during single motor unit contractions in a muscle, while the muscle is immersed in paraffin oil. The muscle model is described as a rheological membrane. Both the muscle and the medium models have been built by using Stiff-Finite-Element-Method (SFEM), which allows one to simulate the muscle surface displacement and the acoustic propagation of this effect in the oil. Such a modelling enables one to determine the impact of the rheological properties of the liquid environment on the shape of the MMG wave. In order to verify the model, the MMG signals and the contraction forces have been recorded in vivo from the medial gastrocnemius muscle of a rat. In these experiments single motor units were stimulated with various stimulation frequencies. A piezotransducer, immersed in paraffin oil, has been used to record the MMG signal recording. The signals recorded during individual twitches of the motor units have been used to estimate the parameters of the model. Subsequently, the model has been experimentally verified. The signals recorded in experiments during unfused and fused tetani have been compared with the simulated model responses in the analogous stimulation program. It has been observed that the MMG signals obtained with the proposed linear model have been consistent with the results of in vivo experiments.     

The site of action of magnetic stimulation of human motor cortex in a patient with motor neuron disease

Cortical and spinal root (C₈-T₁) magnetic stimulation evoked single motor unit potentials in the abductor pollicis brevis and abductor digiti minimi muscles in a patient with chronic motor neuron disease. This patient was unique in that there were few surviving motoneurons in these muscles making it possible to record single motor units. Central motor conduction time was within normal limits. Cortical mapping of motor evoked potentials (MEPs) was carried out using a twin magnetic coil stimulating over the motor cortex at intervals of 1 cm along the coronal axis and 1–2 cm along the sagittal axis. As the site of cortical stimulation was moved from the centre, the latency of the MEPs increased by 0.7–0.8 ms suggesting one synaptic delay. This study provides further data that magnetic stimulation of the human cortex indirectly activates pyramidal cells via interneurons.     

Summation of elementary phonomyograms during isometric twitches in humans

To study its summation principle, the phonomyogram (PMG) from the first interosseus dorsalis muscle was recorded in five subjects during single twitches evoked by electrical stimulation over the motor point. By increasing the current pulse from threshold to maximal intensity, PMG amplitude increased linearly with motor unit recruitment. The twitch amplitude-intensity relationship was also linear. The PMG amplitude was therefore linearly related to the external force. For all these relationships highly significant correlation coefficients were found. These relationships were interpreted as being a consequence of an orderly recruitment, although, contrary to what happens during voluntary contraction, the largest and strongest motor units were recruited before the smallest and weakest ones during axon electrical stimulation. The PMG onset always preceded twitch onsets as indicated by latency measurements [mean 3.2 (SD 1.3) ms versus 11.5 (SD 3.9) ms, respectively]. Moreover, PMG and twitch latencies may have been significantly reduced by recruitment, suggesting that orderly recruitment influenced both PMG amplitude and occurrence. These results were interpreted as being the result of the summation of elementary PMG from every contracting motor unit and the stiffness change of the muscle medium occurring with recruitment. Accepted: 25 August 1997