Central and peripheral fatigue in knee and elbow extensor muscles after a long-distance cross-country ski race.

Although elbow extensors (EE) have a great role in cross-country skiing (XC) propulsion, previous studies on neuromuscular fatigue in long-distance XC have investigated only knee extensor (KE) muscles. In order to investigate the origin and effects of fatigue induced by long-distance XC race, 16 well-trained XC skiers were tested before and after a 56-km classical technique race. Maximal voluntary isometric contraction (MVC) and rate of force development (RFD) were measured for both KE and EE. Furthermore, electrically evoked double twitch during MVC and at rest were measured. MVC decreased more in KE (-13%) than in EE (-6%, P = 0.016), whereas the peak RFD decreased only in EE (-26%, P = 0.02) but not in KE. The two muscles showed similar decrease in voluntary activation (KE -5.0%, EE -4.8%, P = 0.61) and of double twitch amplitude (KE -5%, EE -6%, P = 0.44). A long-distance XC race differently affected the neuromuscular function of lower and upper limbs muscles. Specifically, although the strength loss was greater for lower limbs, the capacity to produce force in short time was more affected in the upper limbs. Nevertheless, both KE and EE showed central and peripheral fatigue, suggesting that the origins of the strength impairments were multifactorial for the two muscles.

Do sweep rowers symmetrically activate their low back muscles during indoor rowing?

This study investigates whether sweep rowers activate their low back muscles asymmetrically when exercising on a rowing ergometer. Given that indoor rowing imposes equal loading demands to left and right back muscles, any side differences in activation are expected to reflect asymmetric adaptations resulting from sweep rowing. In addition to trunk kinematics, surface electromyograms (EMGs) were sampled from multiple skin locations along the lumbar spine of six elite, sweep rowers. The distribution of EMG amplitude along the spine was averaged across strokes and compared between sides. Key results indicate a significant effect of trunk side on EMG amplitude and on the low back region where EMG amplitude was greatest. Such side differences were unlikely because of trunk lateral inclination and rotation, which were smaller than 5° for all rowers tested. Moreover, asymmetries manifested differently between participants; there was not a clear predominance of greater EMG amplitude toward the right/left side in portside/starboard rowers. These results suggest that (a) even during indoor rowing, sweep rowers activate asymmetrically their low back muscles; (b) factors other than rowing side might be associated with low back asymmetries; (c) spatial distribution of surface EMG amplitude is sensitive to bilateral changes in back muscles' activation.

Optimal bipolar system positioning to provide information about the trapezius activity associated with scapular retraction during shoulder exercises for resistance training.

Objective. Recently, the use of EMG biofeedback to make subjects aware of the stabilizer activation associated with scapular retraction during exercise has been of interest, and challenges related to EMG detection have been addressed. Whether there is an optimal bipolar positioning that discriminates the stabilizer activation with retraction from a neutral scapular position during resistance exercises is an open issue that we address here by simultaneously mapping different positions using high-density surface electromyography (HD-sEMG).Approach. Sixteen resistance-trained males performed five pulling exercises with and without scapular retraction, namely barbell rows, dumbbell rows, pull-downs at a lat machine, seated rows, and TRX (total resistance exercises) system rows. HD-sEMG was acquired in a monopolar mode from the medial and lower trapezius (8 × 4 electrodes and inter-electrode distance (ied): 10 mm) and different bipolar systems were simulated in terms of positioning, interelectrode distance, and orientation with respect to the spine: longitudinal with three ieds (20 mm, 30 mm, and 40 mm), one transversal, and two diagonals (ied: 20 mm), totalling six EMG sets. To identify the optimal electrode pair that was able to distinguish between the presence or absence of scapular retraction, we computed: (i) the root mean square (RMS) map for each condition and the difference between them, obtaining a differential RMS map per subject; and (ii) the intersection of cumulative maps, by summing the differential (binary) maps from all subjects.Main results. For the lower trapezius, the results revealed that the diagonal direction (45 degrees; ied: 20 mm) obtained the greater occurrence of intersecting segments within and between exercises than the other electrode configurations, showing low variability for the optimal positioning across exercises. Electrode configuration varied within and between the pulling exercises for the medial trapezius.Significance. This study allows us to identify an optimal bipolar positioning (consistent across subjects and exercises) for lower trapezius activity assessment, representing a guideline for electrode positioning when EMG biofeedback is adopted for selective activation of the lower trapezius during pulling exercises.

Screening, diagnosis and monitoring of sarcopenia: When to use which tool?

BACKGROUND & AIMS: Sarcopenia is a muscle disorder associated with loss of muscle mass, strength and function. Early screening, diagnosis and treatment may improve outcome in different disease conditions. A wide variety of tools for estimation of muscle mass is available and each tool has specific technical requirements. However, different investigational settings and lack of homogeneity of populations influence the definition of gold standards, proving it difficult to systematically adopt these tools. Recently, the European Working Group on Sarcopenia in Older People (EWGSOP) published a revised recommendation (EWGSOP-2) and algorithm for using tools for screening and diagnosing sarcopenia. However, agreement of the EWGSOP2 criteria with other classifications is poor and although an overview of available tools is valuable, for the purpose of clinical decision-making the reverse is useful; a given scenario asks for the most suitable tools. RESULTS: Tools were identified for screening, diagnostics and longitudinal monitoring of muscle mass. For each of these clinical scenarios the most appropriate tools were listed and for each technique their usability is specified based on sensitivity and specificity. Based on this information a specific recommendation is made for each clinical scenario. CONCLUSION: This narrative review provides an overview of currently available tools and future developments for different clinical scenarios such as screening, diagnosis and longitudinal monitoring of alterations in muscle status. It supports clinical decision-making in choosing the right tools for muscle mass quantification depending on the need within a given clinical scenario as well as the local availability and expertise.

Myoelectric manifestations of fatigue in vastus lateralis, medialis obliquus and medialis longus muscles.

The purpose of this study was to determine whether surface electromyography (EMG) assessment of myoelectric manifestations of muscle fatigue is capable of detecting differences between the vastus lateralis and medialis muscles which are consistent with the results of previous biopsy studies. Surface EMG signals were recorded from the vastus medialis longus (VML), vastus medialis obliquus (VMO) and vastus lateralis (VL) muscles during isometric knee extension contractions at 60% and 80% of the maximum voluntary contraction (MVC) for 10s and 60s, respectively. Initial values and rate of change of mean frequency (MNF), average rectified value (ARV) and conduction velocity (CV) of the EMG signal were calculated. Comparisons between the two force levels revealed that the initial values of MNF for the VL muscle were greater at 80% MVC compared to 60% MVC (P<0.01). Comparisons between the vasti muscles demonstrated lower initial values of CV for VMO compared to VL at 60% MVC (P<0.01) and lower than VML and VL at 80% MVC (P<0.01). In addition, initial values of MNF were higher for VL with respect to both VML and VMO at 80% MVC (P<0.01) and initial estimates of ARV were higher for VMO compared to VML at both force levels (P<0.01 at 60% MVC and P<0.05 at 80% MVC). For the sustained contraction at 80% MVC, VL demonstrated a greater decrease in CV over time compared to VMO (P<0.05). These findings suggest that surface EMG signals and their time course during sustained isometric contractions may be useful to non-invasively describe functional differences between the vasti muscles.

Differences in myoelectric manifestations of fatigue in sprinters and long distance runners.

The aim of this study was to determine whether the rate of change of surface EMG variables during a constant force isometric fatiguing contraction (80% MVC, 30 s of duration) of vastus medialis obliquus (VMO) and vastus lateralis (VL) muscles is able to distinguish between nine sprinters and nine long distance runners. Signals were recorded with linear arrays of eight electrodes. Muscle fiber conduction velocity (CV), mean frequency (MNF) of EMG signal power spectrum and average rectified value (ARV) of the EMG signal were calculated both from the whole signal (global approach) and from motor unit action potentials (MUAP distribution approach) extracted from the signal. Results showed significant differences only in the MUAP distribution approach. For this approach, the rates of change of conduction velocity (CV) (both absolute and normalized with respect to the initial value) were found to be always greater in the sprinter (VMO: -0.012 +/- 0.011 m s(-2) and -0.25 +/- 0.21%/s; VL: -0.014 +/- 0.009 m s(-2) and -0.27 +/- 0.17%/s) than in the long distance runner group (VMO: -0.004 +/- 0.006 m s(-2) and -0.08 +/- 0.14%/s; VL: 0.003 +/- 0.012 m s(-2) and 0.08 +/- 0.26%/s) (VL: p = 0.016, VMO: p = 0.034). No differences were observed in the CV initial values and in rates of change of MNF, while MNF initial values recorded from the VMO were found to be greater (119.6 +/- 25.1 Hz) in the sprinter group than in the long distance runner group (99.2 +/- 12.1 Hz, p = 0.016). A correlation was found between initial values and rates of change of CV in the VMO (r = 0.61, p < 0.01, N = 18, Spearman correlation coefficient). The consistency of these findings with the expected fiber type composition between sprinters and long distance runners and the identification of a set of candidated variables for the assessment of muscle adaptation during training and/or rehabilitation programs represent the main results of the study.

Corticotroph axis sensitivity after exercise: comparison between elite athletes and sedentary subjects.

Strenuous exercise activates the hypothalamic-pituitary-adrenal (HPA) axis. Several reports showed that physical training is associated with a decreased efficiency of the feedback control of HPA axis. The aims of the present study were: 1) to evaluate the differences in the mechanical, hormonal, and lactate responses to a high-intensity isokinetic exercise among different groups of competitive athletes (CA, no.=20) of power and endurance disciplines and sedentary controls (SED, no.=10); 2) to determine the effects of the training status on the HPA axis responsiveness following exercise, as indirectly evaluated by the rates of ACTH, cortisol, and DHEA recovery after exercise. CA and SED fulfilled eight sets of twenty concentric contractions of the knee extensors at 180 degrees/sec angular velocity throughout a constant range of motion (100 degrees). There was a rest period of 30 sec between each set and a 3-min rest period between the two legs. Before, immediately after the isokinetic exercise and at different times in the subsequent 120 min of recovery, blood and saliva were sampled to determine plasma ACTH, salivary cortisol, serum DHEA, and serum lactate concentrations. CA showed a higher cortisol response to exercise than SED, whereas no differences were found in the responses of ACTH, DHEA and lactate. In the athlete group the exercise-induced increases of ACTH, cortisol, and lactate were higher in power athletes with respect to endurance athletes. No differences were observed between athletes and SED in the rates of hormonal recovery after exercise: this finding does not support the concept that a reduced feedback control of HPA axis can represent a feature of trained individuals.

Confounding factors in water EMG recordings: an approach to a definitive standard.

This work presents a study on the influence of the aqueous environment on the surface EMG (sEMG) signal recorded in bipolar montage from the abductor pollicis brevis muscle, when only the forearm is immersed in water. Ten men, 30.1+/-4.0 (mean +/- SD) years old, performed ten 2-s 40% MVC isometric contractions of the abductor pollicis brevis muscle in two controlled environments (air and water, at a temperature of 32 degrees C). They were always equipped with electrodes protected with a waterproof adhesive tape. No significant variations (paired Wilcoxon test) due to the environments were observed in the median frequency of the power spectrum (MDF) and in the root mean square (RMS) value of the sEMG signal. These results allow us to assess the methodological criteria to properly record sEMG signals in water and provide the basis to explain different findings obtained by other authors.

The contribution of postural balance analysis in older adult fallers: A narrative review.

OBJECTIVE: Falls are a serious health problem for older adults. Several studies have identified the decline of postural balance as one of the main risk factors for falls. Contrary to what may be believed, the capability of force platform measurements to predict falls remains uncertain. The focus of this narrative review is the identification of postural characteristics of older adults at risk of falling using both static and dynamic postural balance assessments. METHODS: The literature analysis was conducted on Medline/PubMed. The search ended in May 2015. RESULTS: Centre of pressure (CoP) path length, CoP velocity and sway in medial lateral and anterior-posterior are the variables that distinguish older adult fallers from non-fallers. DISCUSSION: Recommendations to medical personnel on how to provide efficient balance training for older adults are offered, discussing the relevance and limitations of postural stability on static and dynamic board in falling risk prevention.

A method for positioning electrodes during surface EMG recordings in lower limb muscles.

PURPOSE: The aim of this work is to provide information about the degree of inter-subject uniformity of location of innervation zone (IZ) in 13 superficial muscles of the lower limb. The availability of such information will allow researchers to standardize and optimize their electrode positioning procedure and to obtain accurate and repeatable estimates of surface electromyography (sEMG) signal amplitude, spectral variables and muscle fiber conduction velocity. METHODS: Surface EMG signals from gluteus maximus, gluteus medius, tensor faciae latae, biceps femoris, semitendinosus, vastus medialis obliquus, vastus lateralis, rectus femoris, tibialis anterior, peroneus longus, soleus, gastrocnemius medialis and lateralis muscles of ten healthy male subjects aged between 25 and 34 years (average = 29.2 years, S.D. = 2.5 years) were recorded to assess individual IZ location and signal quality. RESULTS: Tensor faciae latae, biceps femoris, semitendinosus, vastus lateralis, gastrocnemius medialis and lateralis showed a high level of both signal quality and IZ location uniformity. In contrast, rectus femoris, gluteus medius and peroneus longus were found to show poor results for both indexes. Gluteus maximus, vastus medialis obliquus and tibialis anterior were found to show high signal quality but low IZ location uniformity. Finally, soleus muscle was found to show low signal quality but high IZ location uniformity. CONCLUSIONS: This study identifies optimal electrode sites for muscles in the lower extremity by providing a standard landmarking technique for the localization of the IZ of each muscle so that surface EMG electrodes can be properly positioned between the IZ and a tendon.

Myoelectric manifestations of sternocleidomastoid and anterior scalene muscle fatigue in chronic neck pain patients.

OBJECTIVE: This study compares myoelectric manifestations of fatigue of the sternocleidomastoid (SCM) and anterior scalene (AS) muscles between 10 chronic neck pain subjects and 10 normal matched controls. METHODS: Surface electromyography (sEMG) signals were recorded from the sternal head of SCM and AS muscles bilaterally during sub-maximal isometric cervical flexion contractions at 25 and 50% of the maximum voluntary contraction (MVC). The mean frequency, average rectified value and conduction velocity of the sEMG signal were calculated to quantify myoelectric manifestations of muscle fatigue. RESULTS: For both the SCM and AS muscles, the Mann-Whitney U test indicated that the initial value and slope of the mean frequency in neck pain patients were greater than in healthy subjects (P < 0.05). This was significant both at 25 and 50% of MVC. CONCLUSIONS: These results suggest: (a) a predominance of type-II fibres in the neck pain patients and/or (b) greater fatigability of the superficial cervical flexors in neck pain patients. These results are in agreement with previous muscle biopsy studies in subjects with neck pain, which identified transformation of slow-twitch type-1 fibres to fast-twitch type-IIB fibres, as well as the clinical observation of reduced endurance in the cervical flexors in neck pain patients.

Location of innervation zones of sternocleidomastoid and scalene muscles--a basis for clinical and research electromyography applications.

OBJECTIVES: Advances in surface electromyography (sEMG) techniques provide a clear indication that refinement of electrode location relative to innervation zones (IZ) is required in order to optimise the accuracy, relevance and repeatability of the sEMG signals. The aim of this study was to identify the IZ for the sternocleidomastoid and anterior scalene muscles to provide guidelines for electrode positioning for future clinical and research applications. METHODS: Eleven volunteer subjects participated in this study. Myoelectric signals were detected from the sternal and clavicular heads of the sternocleidomastoid and the anterior scalene muscles bilaterally using a linear array of 8 electrodes during isometric cervical flexion contractions. The signals were reviewed and the IZ(s) were identified, marked on the subjects' skin and measurements were obtained relative to selected anatomical landmarks. RESULTS: The position of the IZ lay consistently around the mid-point or in the superior portion of the muscles studied. CONCLUSIONS: Results suggest that electrodes should be positioned over the lower portion of the muscle and not the mid-point, which has been commonly used in previous studies. Recommendations for sensor placement on these muscles should assist investigators and clinicians to ensure improved validity in future sEMG applications.

Lack of correlation between sternocleidomastoid and scalene muscle fatigability and duration of symptoms in chronic neck pain patients.

AIMS OF THE STUDY: Despite the evidence of greater fatigability of the superficial cervical flexor muscles in neck pain patients, the relationship between duration of neck pain and muscle fatigue has not been investigated. This study examined the relationship between myoelectric manifestations of sternocleidomastoid (SCM) and anterior scalene (AS) muscle fatigue and duration of neck pain in a group of patients with chronic neck pain. MATERIALS AND METHODS: Twenty chronic neck pain patients with a history of pain ranging between 1 and 25 years (mean 6.15, S.D. 5.48 years) participated in this study. Myoelectric signals were recorded from the SCM and AS muscles from the side of greatest pain as patients performed sustained sub-maximal isometric cervical flexion contractions at 25% and 50% of the maximum voluntary contraction (MVC). The initial value and slope of the mean frequency (MNF), average rectified value (ARV) and conduction velocity (CV) of the EMG signals were calculated. Correlation analysis was applied to determine whether a relationship existed between the duration of neck pain and estimates of EMG variables. RESULTS AND CONCLUSIONS: No significant correlation was found between duration of pain and estimates of the MNF, ARV and CV for either SCM or AS contracting at 25% and 50% MVC. The length of history of neck pain does not appear to correlate with the extent of SCM and AS muscle fatigability in patients with chronic neck pain. This finding may be due to an increase of muscle fatigability occurring within the first few years of neck pain, which is not followed by signs of increased fatigue after this time. This was supported by the finding of significant correlations between duration of pain and AS fatigability on a subgroup of patients with pain duration less than 5 years. Future studies are warranted to examine how early signs of cervical muscle fatigability are evident after the onset of neck pain as this will have implications for rehabilitation.

Spatio-temporal evaluation of neck muscle activation during postural perturbations in healthy subjects.

The purpose of this study was to examine the spatio-temporal activation of the sternocleidomastoid (SCM) and cervical extensor (CE) muscles with respect to the deltoid muscle onset during rapid voluntary upper limb movement in healthy volunteers. The repeatability and reliability of the spatio-temporal aspects of the myoelectric signals were also examined. Ten subjects performed bilateral and unilateral rapid upper limb flexion, abduction and extension in response to a visual stimulus. EMG onsets and normalised root mean square (nRMS) values were calculated for the SCM and CE muscles. Subjects attended three testing sessions over non-consecutive days allowing the repeatability and reliability of these measures to be assessed. The SCM and CE muscles demonstrated feed-forward activation (activation within 50 ms of deltoid onset) during rapid arm movements in all directions. The sequence and magnitude of neck muscle activation displayed directional specificity, however, the neck flexor and extensor muscles displayed co-activation during all perturbations. EMG onsets demonstrated high repeatability in terms of repeated measure precision (nSEM in the range 1.9-5.7%). This was less evident for the repeatability of nRMS values. The results of this study provide a greater understanding of cervical neuromotor control strategies. During bilateral and unilateral upper limb perturbations, the SCM and CE muscles demonstrate feed-forward co-activation. It seems apparent that feed-forward activation of neck muscles is a mechanism necessary to achieve stability for the visual and vestibular systems, whilst ensuring stabilisation and protection of the cervical spine.

Surface EMG alterations induced by underwater recording.

BACKGROUND AND PURPOSE: This study aims to verify if amplitude and spectral characteristics of surface EMG signal are modified due to recording in a wet environment. METHODS: Isometric contractions of the biceps brachii muscle of ten subjects were performed in several different set-up combinations, both in dry (D) and in water from hydrotherapy pools (PW), with (PWM) or without moving the pool water and with (T) or without water-resistant adhesive taping. RESULTS: In PW condition the amplitude of the recorded signal is reduced to 5-10% of the corresponding signal recorded in D condition. In PWM the power spectrum is drastically reduced and altered by the water movement that introduces an increase of spectral power in the frequency range 0-20 Hz. The use of T modality allows to record signals with both amplitude and spectral frequencies comparable with those obtained in the D conditions. DISCUSSION AND CONCLUSION: This work demonstrates the need for water resistant taping when EMG signals are recorded in water. Signals recorded without such a protective film are strongly affected in their amplitude and frequency characteristics by the conductivity and the movement of water.

Repeatability of maximal voluntary force and of surface EMG variables during voluntary isometric contraction of quadriceps muscles in healthy subjects.

The repeatability of initial values and rate of change of EMG signal mean spectral frequency (MNF), average rectified values (ARV), muscle fiber conduction velocity (CV) and maximal voluntary contraction (MVC) was investigated in the vastus medialis obliquus (VMO) and vastus lateralis (VL) muscles of both legs of nine healthy male subjects during voluntary, isometric contractions sustained for 50 s at 50% MVC. The values of MVC were recorded for both legs three times on each day and for three subsequent days, while the EMG signals have been recorded twice a day for three subsequent days. The degree of repeatability was investigated using the Fisher test based upon the ANalysis Of VAriance (ANOVA), the Standard Error of the Mean (SEM) and the Intraclass Correlation Coefficient (ICC). Data collected showed a high level of repeatability of MVC measurement (normalized SEM from 1.1% to 6.4% of the mean). MNF and ARV initial values also showed a high level of repeatability (ICC>70% for all muscles and legs except right VMO). At 50% MVC level no relevant pattern of fatigue was observed for the VMO and VL muscles, suggesting that other portions of the quadriceps might have contributed to the generated effort. These observations seem to suggest that in the investigation of muscles belonging to a multi-muscular group at submaximal level, the more selective electrically elicited contractions should be preferred to voluntary contractions.

Geometrical factors in surface EMG of the vastus medialis and lateralis muscles.

Surface EMG signals detected in dynamic conditions are affected by a number of artefacts. Among them geometrical factors play an important role. During movement the muscle slides with respect to the skin because of the variation of its length. Such a shift can considerably modify sEMG amplitude. The purpose of this work is to assess geometrical artefacts on sEMG during isometric contractions at different muscle lengths. The average rectified value (ARV) of 15 single differential signals was obtained by means of a linear array of 16 bar electrodes from the vastus medialis and lateralis muscles. The knee angle was changed from 75 degrees to 165 degrees in steps of 30 degrees and voluntary isometric contractions at a low, medium and high force level were performed for each angle. The ARV pattern was normalized with respect to the mean activity to compare signals from different joint angles. From the data collected it was possible to separate the geometrical changes from the changes due to different intensities of activation. In three out of five subjects, we found (within the resolution of our measures) a 1 cm shift for the vastus medialis muscle while no shift was observed for the other two subjects. For the vastus lateralis muscle a 1 cm shift was found in two out of four subjects. Such a shift produces the main contribution to geometrical artefacts. To avoid such artefacts the innervation zones should be located and the EMG electrodes should not be placed near them.

Repeatability of surface EMG variables during voluntary isometric contractions of the biceps brachii muscle.

The repeatability of initial value and rate of change of mean spectral frequency (MNF), average rectified values (ARV) and muscle fiber conduction velocity (CV) was investigated in the dominant biceps brachii of ten normal subjects during sustained isometric voluntary contractions. Four levels of contraction were studied: 10%, 30%, 50% and 70% of the maximal voluntary contraction level (MVC). Each contraction was repeated three times in each of three different days for a total of nine contractions/level/subject and 90 contractions per level across the ten subjects. Repeatability was investigated using the Intraclass Correlation Coefficient (ICC) and the standard error of the mean (SEM) of the estimates for each subject. Contrary to observations in other muscles, CV estimates appeared to be very repeatable both within and between subjects. CV showed a small but significant increase when contraction force increased from 10% to 50% MVC but no change for further increase of force. As force increased, MNF showed a slight decrease possibly related to a wider spreading of the CV values. The rate of time decrement of MNF and CV increased with the level of contraction. The normalized decrement (% of initial value per second) was in general higher for MNF than for CV and was more repeatable between subjects at 10% MVC than at 70% MVC. A final observation is that a resting time of 5 minutes may not be sufficient after a contraction at 50% or 70% MVC.

Compensation of the effect of sub-cutaneous tissue layers on surface EMG: a simulation study.

A mathematical model of a four-layer medium (muscle+fat+skin+air) is investigated. The system is studied in cartesian coordinates with the hypotheses of muscle anisotropy and isotropy of the fat and skin layers, assuming the fat to be less conductive than the skin. Determination of the potential distribution over the skin, due to sources in the muscle, is based on the solution of the Poisson equation in the spatial frequency domain in the different media. The arbitrary constants are determined imposing the boundary conditions. In this way the transfer function of the fat and skin layers is found and can be used to compute the potential distribution of the muscle-fat interface. The physiological parameters were obtained from literature. The results of simulation studies are proposed; it is clear that the subcutaneous tissue layers produce an attenuation and widening of the potential distribution present at the muscle surface. These effects can be partially compensated using high-pass spatial filters as proposed in the literature. A new class of bidimensional spatial filters is proposed; the filters are defined on the basis of the information about the isotropic layers rather than being general filters. An approximation of the ideal inverse transfer function of the sub-cutaneous layers is proposed as a discrete spatial filter that can be implemented with matrices with a small number of electrodes (maximum of 25 electrodes) and practicable interelectrode distance. A simulated evaluation of the new filter and the limitations of the approach are presented.

Two methods for the measurement of voluntary contraction torque in the biceps brachii muscle.

Appropriate measurement of maximal voluntary contraction force of a single limb muscle or of a muscle group is important in clinical and research situations. To measure muscle force, one segment of an isometric measuring brace is fixed to a support and force is applied to the other. The output of this arrangement is affected by additional contributions such as pushing or pulling with the whole body. This paper quantifies the differences between measurements of torque produced by the biceps brachii with the brace fixed versus those produced when the elbow joint was isolated by suspending the brace from cables. No statistically significant differences were found between MVC values observed with the two methods within subjects. However, a statistically significant difference in EMG fatigue indices was observed and attributed to a different sharing of force production among different muscles in the two conditions. We conclude that different brace arrangements may lead to the same maximal force but to different rates of myoelectric manifestations of muscle fatigue since the effort may be shared differently among the muscles of synergic or stabilizing groups.