Motor unit territory in relation to tendons in the human masseter muscle

Modelling motor units in 3D: influence on muscle contraction and joint force via a proof of concept simulation

Functional heterogeneity is a skeletal muscle’s ability to generate diverse force vectors through localised motor unit (MU) recruitment. Existing 3D macroscopic continuum-mechanical finite element (FE) muscle models neglect MU anatomy and recruit muscle volume simultaneously, making them unsuitable for studying functional heterogeneity. Here, we develop a method to incorporate MU anatomy and information in 3D models. Virtual fibres in the muscle are grouped into MUs via a novel “virtual innervation” technique, which can control the units’ size, shape, position, and overlap. The discrete MU anatomy is then mapped to the FE mesh via statistical averaging, resulting in a volumetric MU distribution. Mesh dependency is investigated using a 2D idealised model and revealed that the amount of MU overlap is inversely proportional to mesh dependency. Simultaneous recruitment of a MU’s volume implies that action potentials (AP) propagate instantaneously. A 3D idealised model is used to verify this assumption, revealing that neglecting AP propagation results in a slightly less-steady force, advanced in time by approximately 20 ms, at the tendons. Lastly, the method is applied to a 3D, anatomically realistic model of the masticatory system to demonstrate the functional heterogeneity of masseter muscles in producing bite force. We found that the MU anatomy significantly affected bite force direction compared to bite force magnitude. MU position was much more efficacious in bringing about bite force changes than MU overlap. These results highlight the relevance of MU anatomy to muscle function and joint force, particularly for muscles with complex neuromuscular architecture.

Morphology and Physiology of Masticatory Muscle Motor Units

Motor unit territories in masticatory muscles appear to be smaller than territories in limb muscles, and this would suggest a more localized organization of motor control in masticatory muscles. Motor unit cross-sectional areas show a wide range of values, which explains the large variability of motor unit force output. The proportion of motor unit muscle fibers containing more than one myosin heavy-chain (MHC) isoform is considerably larger in masticatory muscles than in limb and trunk muscles. This explains the continuous range of contraction speeds found in masticatory muscle motor units. Hence, in masticatory muscles, a finer gradation of force and contraction speeds is possible than in limb and in trunk muscles. The proportion of slow-type motor units is relatively large in deep and anterior masticatory muscle regions, whereas more fast-type units are more common in the superficial and posterior muscle regions. Muscle portions with a high proportion of slow-type motor units are better equipped for a finer control of muscle force and a larger resistance to fatigue during chewing and biting than muscle portions with a high proportion of fast units. For the force modulation, masticatory muscles rely mostly on recruitment gradation at low force levels and on rate gradation at high force levels. Henneman's principle of an orderly recruitment of motor units has also been reported for various masticatory muscles. The presence of localized motor unit territories and task-specific motor unit activity facilitates differential control of separate muscle portions. This gives the masticatory muscles the capacity of producing a large diversity of mechanical actions. In this review, the properties of masticatory muscle motor units are discussed.

Motor units in the human medial gastrocnemius muscle are not spatially localized or functionally grouped

KEY POINTS

Human medial gastrocnemius (MG) motor units (MUs) are thought to occupy small muscle territories or regions, with low-threshold units preferentially located distally. We used intramuscular recordings to measure the territory of muscle fibres from MG MUs and determine whether these MUs are grouped by recruitment threshold or joint action (ankle plantar flexion and knee flexion). The territory of MUs from the MG muscle varied from somewhat localized to highly distributed, with approximately half the MUs spanning at least half the length and width of the muscle. There was also no evidence of regional muscle activity based on MU recruitment thresholds or joint action. The CNS does not have the means to selectively activate regions of the MG muscle based on task requirements.

ABSTRACT

Human medial gastrocnemius (MG) motor units (MUs) are thought to occupy small muscle territories, with low-threshold units preferentially located distally. In this study, subjects (n = 8) performed ramped and sustained isometric contractions (ankle plantar flexion and knee flexion; range: ∼1-40% maximal voluntary contraction) and we measured MU territory size with spike-triggered averages from fine-wire electrodes inserted along the length (seven electrodes) or across the width (five electrodes) of the MG muscle. Of 69 MUs identified along the length of the muscle, 32 spanned at least half the muscle length (≥ 6.9 cm), 11 of which spanned all recording sites (13.6-17.9 cm). Distal fibres had smaller pennation angles (P < 0.05), which were accompanied by larger territories in MUs with fibres located distally (P < 0.05). There was no distal-to-proximal pattern of muscle activation in ramp contraction (P = 0.93). Of 36 MUs identified across the width of the muscle, 24 spanned at least half the muscle width (≥ 4.0 cm), 13 of which spanned all recording sites (8.0-10.8 cm). MUs were not localized (length or width) based on recruitment threshold or contraction type, nor was there a relationship between MU territory size and recruitment threshold (Spearman's rho = -0.20 and 0.13, P > 0.18). MUs in the human MG have larger territories than previously reported and are not localized based on recruitment threshold or joint action. This indicates that the CNS does not have the means to selectively activate regions of the MG muscle based on task requirements.

Task-dependent control of the jaw during food splitting in humans

Although splitting of food items between the incisors often requires high bite forces, rarely do the teeth harmfully collide when the jaw quickly closes after split. Previous studies indicate that the force-velocity relationship of the jaw closing muscles principally explains the prompt dissipation of jaw closing force. Here, we asked whether people could regulate the dissipation of jaw closing force during food splitting. We hypothesized that such regulation might be implemented via differential recruitment of masseter muscle portions situated along the anteroposterior axis because these portions will experience a different shortening velocity during jaw closure. Study participants performed two different tasks when holding a peanut-half stacked on a chocolate piece between their incisors. In one task, they were asked to split the peanut-half only (single-split trials) and, in the other, to split both the peanut and the chocolate in one action (double-split trials). In double-split trials following the peanut split, the intensity of the tooth impact on the chocolate piece was on average 2.5 times greater than in single-split trials, indicating a substantially greater loss of jaw closing force in the single-split trials. We conclude that control of jaw closing force dissipation following food splitting depends on task demands. Consistent with our hypothesis, converging neurophysiological and morphometric data indicated that this control involved a differential activation of the jaw closing masseter muscle along the anteroposterior axis. These latter findings suggest that the regulation of jaw closing force after sudden unloading of the jaw exploits masseter muscle compartmentalization.

Current computational modelling trends in craniomandibular biomechanics and their clinical implications

Computational models of interactions in the craniomandibular apparatus are used with increasing frequency to study biomechanics in normal and abnormal masticatory systems. Methods and assumptions in these models can be difficult to assess by those unfamiliar with current practices in this field; health professionals are often faced with evaluating the appropriateness, validity and significance of models which are perhaps more familiar to the engineering community. This selective review offers a foundation for assessing the strength and implications of a craniomandibular modelling study. It explores different models used in general science and engineering and focuses on current best practices in biomechanics. The problem of validation is considered at some length, because this is not always fully realisable in living subjects. Rigid-body, finite element and combined approaches are discussed, with examples of their application to basic and clinically relevant problems. Some advanced software platforms currently available for modelling craniomandibular systems are mentioned. Recent studies of the face, masticatory muscles, tongue, craniomandibular skeleton, temporomandibular joint, dentition and dental implants are reviewed, and the significance of non-linear and non-isotropic material properties is emphasised. The unique challenges in clinical application are discussed, and the review concludes by posing some questions which one might reasonably expect to find answered in plausible modelling studies of the masticatory apparatus.

Quantification of myosin heavy chain RNA in human laryngeal muscles: differential expression in the vertical and horizontal posterior cricoarytenoid and thyroarytenoid

BACKGROUND

Human laryngeal muscles are composed of fibers that express type I, IIA, and IIX myosin heavy chains (MyHC), but the presence and quantity of atypical myosins such as perinatal, extraocular, IIB, and alpha (cardiac) remain in question. These characteristics have been determined by biochemical or immunohistologic tissue sampling but with no complementary evidence of gene expression at the molecular level. The distribution of myosin, the main motor protein, in relation to structure-function relationships in this specialized muscle group will be important for understanding laryngeal function in both health and disease.

OBJECTIVES

We determined the quantity of MyHC genes expressed in human posterior cricoarytenoid (PCA) and thyroarytenoid (TA) muscle using real-time quantitative reverse-transcriptase polymerase chain reaction in a large number of samples taken from laryngectomy subjects. The PCA muscle was divided into vertical (V) and horizontal (H) portions for analysis.

RESULTS AND CONCLUSIONS

No extraocular or IIB myosin gene message is present in PCA or TA, but IIB is expressed in human extraocular muscle. Low but detectable amounts of perinatal and alpha gene message are present in both of the intrinsic laryngeal muscles. In H- and V-PCA, MyHC gene amounts were beta greater than IIA greater than IIX, but amounts of fast myosin RNA were greater in V-PCA. In TA, the order was beta greater than IIX greater than IIA. The profiles of RNA determined here indicate that, in humans, neither PCA nor TA intrinsic laryngeal muscles express unique very fast-contracting MyHCs but instead may rely on differential synthesis and use of beta, IIA, and IIX isoforms to perform their specialized contractile functions.

Functional properties and regional differences of human masseter motor units related to three-dimensional bite force

The aim of this study was to estimate numerically the properties of masseter motor units (MUs) in relation to bite force magnitude and direction three-dimensionally and to confirm the hypothesis that the properties differ between different parts of the muscle by means of simultaneous recording of MU activity along with the MU location and three-dimensional (3D) bite force. The MU activity of the right masseter of four healthy men was recorded using a monopolar needle electrode in combination with a surface reference electrode. The location of the needle electrode was estimated stereotactically with the aid of magnetic resonance images and a reference plate. The magnitude and direction of the bite force was recorded with a custom-made 3D bite force transducer. The recorded bite force was displayed on a signal processor, which enabled the participant to adjust the direction and magnitude of the force. The activities of 65 masseter MUs were recorded. Each MU had specific ranges of bite force magnitude and direction (firing range: FR) and an optimum direction for recruitment (minimum firing threshold: MFT). There was a significant negative correlation between MFT and FR width. There were functional differences in MU properties between the superficial and deep masseter and between the superficial layer and deep layer in the superficial masseter. These results indicate that the contribution of human masseter motor units to bite force production is heterogeneous within the muscle.

Heterogeneous activation of the medial pterygoid muscle during simulated clenching

The aim of this study was to investigate whether the medial pterygoid muscle shows differential activation under experimental conditions simulating force generation during jaw clenching. To answer this question, the electromyographic activity of the right medial pterygoid was recorded with two intramuscular electrodes placed in an anterior and posterior muscle region, respectively. Intraoral force transfer and force measurement were achieved by a central bearing pin device equipped with strain gauges. The activity distribution in the muscle was recorded in a central mandibular position during generation of eight different force vectors at a constant amount of force (F=150 N). The investigated muscle regions showed different amounts of EMG activity. The relative intensity of the activation in the two regions changed depending on the task. In other words, the muscle regions demonstrated heterogeneous changes of the EMG pattern for various motor tasks. The results indicate a heterogeneous activation of the medial pterygoid muscle under test conditions simulating force generation during clenching. This muscle behaviour might offer an explanatory model for the therapeutic effects of oral splints.

Differential activity patterns in the masseter muscle under simulated clenching and grinding forces

The aim of this study was to investigate (i) whether the masseter muscle shows differential activation under experimental conditions which simulate force generation during clenching and grinding activities; and (ii) whether there are (a) preferentially active muscle regions or (b) force directions which show enhanced muscle activation. To answer these questions, the electromyographic (EMG) activity of the right masseter muscle was recorded with five intramuscular electrodes placed in two deep muscle areas and in three surface regions. Intraoral force transfer and force measurement were achieved by a central bearing pin device equipped with three strain gauges (SG). The activity distribution in the muscle was recorded in four different mandibular positions (central, left, right, anterior). In each position, maximum voluntary contraction (MVC) was exerted in vertical, posterior, anterior, medial and lateral directions. The investigated muscle regions showed different amount of EMG activity. The relative intensity of the activation, with respect to other regions, changed depending on the task. In other words, the muscle regions demonstrated heterogeneous changes of the EMG pattern for the various motor tasks. The resultant force vectors demonstrated similar amounts in all horizontal bite directions. Protrusive force directions revealed the highest relative activation of the masseter muscle. The posterior deep muscle region seemed to be the most active compartment during the different motor tasks. The results indicate a heterogeneous activation of the masseter muscle under test conditions simulating force generation during clenching and grinding. Protrusively directed bite forces were accompanied by the highest activation in the muscle, with the posterior deep region as the most active area.

Scanning electromyography

A special electromyography (EMG) method, scanning EMG, was introduced by Stålberg and Antoni in 1980 to study the electrophysiological cross sections and sizes of motor units. Scanning EMG gives a new approach for the evaluation of the electrical properties of motor units, providing new data on the normal anatomical distribution of muscle fibers and its changes in different pathologies of the muscle. The description of scanning EMG recordings required the introduction of new parameters (lengths of motor unit cross sections, fractions of motor units, and silent areas), in addition to those used with conventional EMG recordings, and the traditional parameters (duration, amplitude, etc.) acquired new and more accurate explanations. Normal scanning EMG recordings are available for biceps brachii, anterior tibial, and masseter muscles. The findings in normal muscles agree with the nonrandom distribution of muscle fibers in motor units and confirm the suggestion that muscle fibers within motor units tend to be arranged in clusters. In muscular dystrophies, the sizes of motor unit territories do not differ significantly from the normal values. However, the configuration of motor units changes considerably. Abrupt changes in amplitude and duration, segments of short and long duration, increased numbers of fractions, and silent areas have been revealed, showing that dystrophic motor units are definitely fragmented. Scanning EMG supports the assumption that there is clustering of muscle fibers within the dystrophic motor unit, with local grouping of muscle fibers. In neurogenic lesions, the length of motor units is normal or only slightly increased. Reinnervated motor units are restricted to the fascicles in which they are originally found. Reinnervation does not result in an increase in the number of fractions, but the amplitude of the potentials, the length of polyphasic sections, and the duration increase. The increase in the number and length of polyphasic sections can differentiate normal motor units from abnormal ones. However, other features (amplitude, duration, number of fractions, and presence of silent areas) are also necessary to distinguish neurogenic processes from myogenic ones.

Role of motor unit structure in defining function

Motor units, defined as a motoneuron and all of its associated muscle fibers, are the basic functional units of skeletal muscle. Their activity represents the final output of the central nervous system, and their role in motor control has been widely studied. However, there has been relatively little work focused on the mechanical significance of recruiting variable numbers of motor units during different motor tasks. This review focuses on factors ranging from molecular to macroanatomical components that influence the mechanical output of a motor unit in the context of the whole muscle. These factors range from the mechanical properties of different muscle fiber types to the unique morphology of the muscle fibers constituting a motor unit of a given type and to the arrangement of those motor unit fibers in three dimensions within the muscle. We suggest that as a result of the integration of multiple levels of structural and physiological levels of organization, unique mechanical properties of motor units are likely to emerge.

Length changes in the human masseter muscle after jaw movement

The human masseter is a multilayered, complex muscle contributing to jaw motion. Because variations in stretch may cause muscle fibers to function over different portions of their length-tension curves, the aim of this study was to determine how parts of the masseter lengthen or shorten during voluntary jaw movements made by living subjects. Magnetic resonance (MR) imaging and optically-based jaw-tracking were used to measure muscle-insertion positions for four parts of the muscle with six degrees of freedom (DOF), before and after maximum-opening, jaw protrusion and laterotrusion in four adult males. Muscle part lengths and intramuscular tendon lengths were calculated, and these data, with fiber-tendon ratios published previously, were used to estimate putative changes in fiber-length. During maximum jaw-opening, the largest increases in muscle length (34-83%) occurred in the medial part of the deep masseter, whereas the smallest changes occurred in the posterior-most, superficial masseter (2-19%). Smaller changes were found during movement to the ipsilateral side, than during protrusion and movement to the contralateral side. On maximum opening, putative fibers in the deep masseter lengthened up to 83%, whereas those of the superficial masseter stretched up to 72%. The masseter muscle does not stretch uniformly for major jaw movement. Jaw motion to the ipsilateral side causes little length change in any part, and the effect of tendon-stretch on estimated fiber lengths is not substantial. The stretch that occurs infers there are task-related changes in the active and passive tensions produced by different muscle regions.

Skeletal muscle function and fibre types: the relationship between occlusal function and the phenotype of jaw-closing muscles in human

Mammalian skeletal muscle cells are composed of repeated sarcomeric units containing thick and thin filaments of myosin and actin, respectively. Excitation of the myosin ATPase enzyme is possible only with presence of Mg-ATP and Ca(2+). Skeletal muscle fibres may be classified into several types according to the isoform of myosin they contain. Nine isoforms of myosin heavy chain are known to exist in mammalian skeletal muscle including type I, IIA, IIB, IIX, IIM, alpha, neonatal, embryonic, and extra-ocular. Healthy adult human limb skeletal muscle contains type I, IIA, IIB, and IIX myosin heavy chains. The jaw-closing muscles of most carnivores and primates have tissue-specific expression of the type IIM or 'type II masticatory' myosin heavy chain. Adult human jaw-closing muscles, however, do not contain IIM myosin. Rather, they express type I, IIA, IIX (as in human limb muscle), and myosins typically expressed in developing or cardiac muscle. The morphology of human jaw-closing muscle fibres is also unusual in that the type II fibres are of smaller diameter that type I fibres, except in cases of increased function and hypertrophy. This paper describes the relationship of fibre types and motor unit function to changes in human occlusion and masticatory activity. Refereed Scientific Paper

Electromyographic evidence for functional heterogeneity in the inferior head of the human lateral pterygoid muscle: a preliminary multi-unit study

OBJECTIVE

Functional heterogeneity, i.e. regional or selective activation of subpopulations of fibres within a muscle, has been described in some jaw and limb muscles. Each head of the lateral pterygoid muscle may also be functionally heterogeneous. The aims of this investigation were to develop a technique to test this hypothesis, and to use this technique to determine whether there is any multi-unit electromyographic (EMG) evidence for functional heterogeneity within the inferior head of the lateral pterygoid (IHLP).

METHODS

In 3 human subjects without craniomandibular disorders, recordings were made of condylar movement and multi-unit EMG activity from two sites in the IHLP during repeated trials of a contralateral (n = 21) and a protrusive (n = 26) jaw movement. The recording sites within IHLP were approached extraorally (labelled IHLP-extra) and were verified by computer tomography (CT); the other (IHLP-intra) were from sites in IHLP approached intraorally.

RESULTS

In each subject, the time of occurrence of the peak filtered signal from IHLP-extra was significantly different (P<0.05) from IHLP-intra for all protrusion trials but not for contralateral trials.

CONCLUSIONS

The data suggest a task-dependent change in motor unit recruitment order within IHLP and that IHLP is functionally heterogeneous.

Force vectors of single motor units in a multipennate muscle

The masseter muscle of the rabbit has a complex architectural design. Restricted motor unit territories in the muscle provide an anatomic basis for accurate control of the force vector through selective activation. In addition, the muscle shows regional differences in fiber type composition. The main objective of the present study was to measure the force vectors of single motor units within the rabbit masseter muscle by a direct mechanical approach to test the hypothesis that: (1) motor units within the masseter muscle are capable of generating different force vectors; and (2) different motor unit types are distributed heterogeneously throughout the muscle. We used a force transducer, capable of measuring both the magnitude and the position of the line of action of a force in a single plane. Motor units in the masseter muscle showed a large range of twitch contraction times and force magnitudes. There was also a large variation in the direction and moment arm of the lines of action. The variation of the lines of action was (almost) as large as the range of fiber directions found inside the muscle. Largest forces, with relatively slow contraction velocities, were produced by motor units in the anterior masseter. Smaller forces and fastest twitch contractions were produced by motor units in the posterior deep masseter. In addition, motor units in the anterior masseter showed more variability in force production than in the posterior masseter. Our results support the idea that the masseter muscle is divided into functionally different parts.

A flexible high-precision video system for digital recording of motor acts through lightweight reflex markers

This paper describes and evaluates the digital MacReflex system for wireless recording of movements in three dimensions. Up to seven high resolution infra-red sensitive CCD video cameras with electronic shutters register the positions of maximally 40 stroboscopically illuminated retro-reflective tape markers. The system is equipped with real-time video processors for computation of position co-ordinates for the markers and for optimised data transmission and storage. Data are output to any type of computer through a standard serial interface, which also provides possibilities for simultaneous A/D-sampling. Dynamic manipulation of the recorded signals in three-dimensional plots is provided by standard software and transformation and evaluation of recorded data are performed in standard software. The described equipment is found to offer a flexible and easily operated solution for recording of movements with high resolution.

Function-dependent anatomical parameters of rabbit masseter motor units

Rabbit masseter motor units (22) were studied by stimulation of trigeminal motoneurons. We tested the hypotheses that masseter motor units facilitate fine motor control by concentrating fibers in small areas and that the distribution of motor unit fibers depends on the fiber type. The twitch contraction time and the isometric tetanic force were registered. The motor unit fibers were depleted of their glycogen by prolonged stimulation. Serial sections of the entire muscle were stained with the periodic acid Schiff (PAS) and monoclonal antibody stains. The muscle fibers of the motor unit were mapped and identified by four myosin heavy-chain (MHC) isoforms: I, IIA, IID, and cardiac-alpha. In the PAS-stained sections, anatomical parameters of the motor units, affecting the force output, were analyzed: the innervation ratio (IR), motor unit territory area (TA), and relative (R-DENS) and absolute (A-DENS) motor unit fiber densities. The fiber cross-sectional area (F-CSA) was measured for each MHC fiber type. The F-CSA sum of all motor unit fibers, the physiological cross-sectional area (P-CSA), was calculated. The IR ranged between 77 and 720 fibers (mean, 267). The mean TA was 8.71 mm2 (range, 4.45 to 19.58). The mean R-DENS was 10 fibers per 100; the A-DENS was 31 fibers per mm2. Linear correlations were found between the IR and the R-DENS and between the tetanic force and the IR. The F-CSAs showed a stepwise increase in value from type I- to IID-MHC fibers. The mean P-CSA was 0.90 mm2 (range, 0.09 to 2.97). A high linear correlation was noted between the P-CSA and the tetanic force. In conclusion, increase of motor unit size expressed in higher fiber counts and forces is accomplished by increase of the fiber density. Thus, forces can be exerted selectively in restricted regions of the masseter muscle. Differences in fiber orientation due to complex muscle pinnation emphasize the possibility of an accurate muscle performance.

Functional movements of putative jaw muscle insertions

BACKGROUND

The craniomandibular muscles control jaw position and forces at the teeth and temporomandibular joints, but little is known regarding their biomechanical behaviour during dynamic function. The objective of this study was to determine how jaw muscle insertions alter position during different jaw movements in living subjects.

METHODS

Computer 3D reconstruction of MR images and jaw-tracking were combined to permit the examination of movement with six degrees of freedom. Maximum mandibular opening, protrusive and laterotrusive positions were recorded in four subjects, and the translation and rotation of the putative insertions of masseter, temporal, medial, and lateral pterygoid muscles were measured.

RESULTS

The sizes and shapes of regional attachments varied markedly among subjects, and their displacement patterns were different in specific muscles. For instance, when the jaw closed to the dental intercuspal position from maximum gape, the region near the superior insertion site of the masseter moved backward and upward, whereas the region near the inferior insertion site displaced mainly forward. In three subjects, the jaw's rotational center during this act was approximately 26-34 mm below the mandibular condyles.

CONCLUSIONS

Since the movements of each muscle part differ according to variations in the size and shape of insertion areas, individual musculoskeletal form, and patterns of jaw motion during function, the prediction of motion-related muscle mechanics in any one subject is unlikely to be possible without direct measurement of the motion of visualized muscle parts. The present study shows that this information can be obtained.

Electromyographic heterogeneity in the human temporalis and masseter muscles during static biting, open/close excursions, and chewing

The human temporalis and masseter muscles are not activated homogeneously during static bite force tasks. In this study, we studied the possible existence of regional differences in these muscles under dynamic conditions. Electromyographic (EMG) activity was recorded by means of bipolar fine-wire electrodes. Six electrodes were inserted into the temporalis muscle and three into the masseter muscle. Recordings were made during maximal effort intercuspal and incisal static clenches, open/close excursions from both the intercuspal and incisal positions, and unilateral gum and licorice chewing on right and left sides. The EMG peak amplitudes and the peak occurrences were compared. During the static clenches and the open/close excursions, no differences could be demonstrated between the regions of the temporalis muscle. However, during the chewing tasks, the anterior and posterior regions behaved differently. Throughout almost all tasks, both superficial and deep parts could be distinguished in the masseter muscle. A further division of the deep masseter was task-dependent. In both the temporalis and masseter muscles, maximal activity (100%) was reached during intercuspal clenches. The average activity declined to 35% of the maximal activity in the temporalis muscle, to 47% in the deep, and to 86% in the superficial masseter during incisal clenches. During all chewing tasks, the EMG peak activity of the anterior temporalis and the superficial masseter muscles was higher in the working than in the balancing condition. The general finding was that different regions were preferentially activated, according to task. The detailed regional specialization previously observed during static bite force tasks could not be demonstrated in the present study.