P. Brugman

Architecture of the human jaw‐closing and jaw‐opening muscles

The human jaw‐closing and jaw‐opening muscles produce forces leading to the development of three‐dimensional bite and chewing forces and to three‐dimensional movements of the jaw. The length of the sarcomeres is a major determinant for both force and velocity, and the maximal work, force, and shortening range each muscle is capable of producing are proportional to the architectural parameter volume, physiological cross‐sectional area, and fiber length, respectively. In addition, the mechanical role the muscles play is strongly related to their three‐dimensional position and orientation in the muscle–bone–joint system. The objective of this study was to compare relevant architectural characteristics for the jaw‐closing and jaw‐opening muscles and to provide a set of data that can be used in biomechanical modeling of the masticatory system.

Coactivation of jaw muscles: Recruitment order and level as a function of bite force direction and magnitude

The aim of this study was to obtain insight into the coactivation behaviour of the jaw muscles under various a priori defined static loading conditions of the mandible. As the masticatory system is mechanically redundant, an infinite number of recruitment patterns is theoretically possible to produce a certain bite force. Using a three-component force transducer and a feedback method, subjects could be instructed to produce a bite force of specific direction and magnitude under simultaneous registration of the EMG activity of anterior and posterior temporal, masseter and digastric muscles on each side. Forces were measured at the second premolars. Vertical, anterior, posterior, lateral and medial force directions were examined; in each direction force levels between 50 N and maximal voluntary force were produced. The results show that for all muscles the bite force-EMG relationship obeys a straight-line fit for forces exceeding 50 N. The relationship varies with bite force direction, except in the case of the digastric muscles. Variation is small for the anterior temporal and large for the posterior temporal and masseter muscles. The relative activation of muscles for a particular force in a particular direction in unique, despite the redundancy.

Amplitude and Timing of EMG Activity in the Human Masseter Muscle during Selected Motor Tasks

Electromyographic (EMG) activity in the human masseter muscle was registered from six different sites, in the anterior, middle, and posterior regions of the superficial and deep layers of the muscle, during static clenching tasks (intercuspal and incisal), selected jaw movements (alternating protrusion/retrusion, right/left latero-deviation, and open/close excursions), and unilateral chewing on right and left sides. Peak-EMG amplitudes and the timing of the peaks were compared. Activity in the regions of the deep masseter was either higher (in mastication and intercuspal open/close excursions) or lower (incisal clenching) than the activities in the superficial masseter. Superficial and deep masseter also differed in their timing of peak EMG: During chewing, peak activity passed from superficial to deep in the balancing-side muscle, and from deep to superficial on the chewing side. During free latero-deviations, peak activity started in the deep masseter, when the jaw moved to the right side (i.e., the side of the muscle), and then passed to the superficial regions, after the jaw movement was reversed to the left side. In addition, within the deep masseter there existed clear anteroposterior differences in activation level (during incisal clenching and open/close excursions) and in timing (during latero-deviation). Such a differentiation of activity was not found in the superficial masseter.

Intermuscular and intramuscular differences in myosin heavy chain composition of the human masticatory muscles.

Among and within the human masticatory muscles a large number of anatomical differences exists indicating that different muscles and muscle portions are specialized for certain functions. In the present study we investigated whether such a specialization is also reflected by intermuscular and intramuscular differences in fibre type composition and fibre cross-sectional area. Fibre type compositions and fibre cross-sectional areas of masticatory muscles were determined in eight cadavers using monoclonal antibodies against myosin heavy chain (MyHC). The temporalis, masseter and pterygoid muscles could be characterized by a relatively large number of fibres containing more than one MyHC isoform (hybrid fibres). In these muscles a large number of fibres expressed MyHC-I, MyHC-fetal and MyHC-cardiac alpha. Furthermore, in these muscles type I fibres had larger cross-sectional areas than type II fibres. In contrast, the mylohyoid, geniohyoid and digastric muscle were characterized by less hybrid fibres, and by less fibres expressing MyHC-I, MyHC-fetal, and MyHC-cardiac alpha, and by more fibres expressing MyHC-IIA; the cross-sectional areas of type I and type II fibres in these muscles did not differ significantly. Compared to the masseter and pterygoid muscles, the temporalis had significantly larger fibres and a notably different fibre type composition. The mylohyoid, geniohyoid, and digastric muscles did not differ significantly in their MyHC composition and fibre cross-sectional areas. Also intramuscular differences in fibre type composition were present, i.e., a regionally higher proportion of MyHC type I fibres was found in the anterior temporalis, the deep masseter, and the anterior medial pterygoid muscle portions; furthermore, significant differences were found between the bellies of the digastric.

Architecture of the Human Pterygoid Muscles

Muscle force is proportional to the physiological cross-sectional area (PCSA), and muscle velocity and excursion are proportional to the fiber length. The length of the sarcomeres is a major determinant of both force and velocity. The goal of this study was to characterize the architecture of the human pterygoid muscles and to evaluate possible functional consequences for muscle force and muscle velocity. For the heads of the lateral and medial pterygoid, the length of sarcomeres and of fiber bundles, the PCSA, and the three-dimensional coordinates of origin and insertion points were determined. Measurements were taken from eight cadavers, and the data were used as input for a model predicting sarcomere length and active muscle force as a function of mandibular position. At the closed-jaw position, sarcomeres in the lateral pterygoid (inferior head, 2.83 ± 0.1 um; superior head, 2.72 ± 0.11 μm) were significantly longer than those in the medial pterygoid (anterior head, 2.48 ± 0.36 um; posterior head, 2.54 ± 0.38 μm). With these initial lengths, the jaw angle at which the muscles were capable of producing maximum active force was estimated to be between 5° and 10°. The lateral pterygoid was characterized by relatively long fibers (inferior, 23 ± 2.7 mm; superior, 21.4 ± 2.2 mm) and a small PCSA (inferior, 2.82 ± 0.66 cm2; superior, 0.95 ± 0.35 cm2), whereas the medial pterygoid had relatively short fibers (anterior, 13.5 ± 1.9 mm; posterior, 12.4 ± 1.5 mm) and a large PCSA (anterior, 2.47 ± 0.57 cm2; posterior, 3.53 ± 0.97 cm2). The mechanical consequence is that the lateral pterygoid is capable of producing 1.7 times larger displacements and velocities than the medial pterygoid, whereas the medial pterygoid is capable of producing 1.6 times higher forces. The model showed that jaw movement had a different effect on active force production in the muscles.

Three-Dimensional Structure of the Human Temporalis Muscle

The maximal force a muscle is capable of producing is proportional to its physiological cross‐sectional area and its excursion range to the length of the muscle fibers. The length of the sarcomeres is a major determinant for both force and excursion range. The human temporalis muscle is an architecturally complex muscle, and little is known regarding the possible heterogeneous distribution of these parameters throughout the muscle. The objective of this study was to determine this distribution for different muscle portions and to examine the functional consequences.

A Feedback Method to Determine the Three-dimensional Bite-force Capabilities of the Human Masticatory System

A feedback procedure is described that enables a subject to exert bite forces in certain specified directions during static contraction of the human jaw muscles. The output of a three-dimensional transducer is fed to a computer. The magnitude and direction of the resultant force are computed and visualized by a cross on the screen of the computer terminal. In a bite experiment, the subject is instructed to match this cross with a point on the screen, representing the desired bite-force direction. The procedure allows for determination of the range of possible bite-force directions and magnitudes for various locations on the dental arch and study of the concomitant recruitement patterns of the jaw muscles. Some examples of measurement are given.

Differences in myosin heavy-chain composition between human jaw-closing muscles and supra- and infrahyoid muscles

Jaw-closing muscles have architectural features suited to force production; supra- and infrahyoid muscles are better adapted to produce velocity and displacement. It was hypothesized that this difference in function would be reflected in myosin heavy-chain (MyHC) composition (equivalent to contraction velocity) and fibre-type cross-sectional area (equivalent to force). MyHC composition was determined in muscles obtained from eight human cadavers, using monoclonal antibodies against MyHC isoforms. Jaw closers contained 4.2 times fewer type IIA fibres and 5.2 times more hybrid fibres than suprahyoid muscles, and 3.9 times fewer type IIA fibres and 3.2 times more hybrid fibres than the infrahyoid muscles. In the jaw closers, MyHC-I was expressed in approx. 70% of all fibres (pure+hybrid), in the suprahyoid muscles in approx. 40%, and in the infrahyoid muscles in approx. 46%. In the jaw closers, type I fibres were 40% larger in diameter than in the supra- and infrahyoid muscles. It can be concluded that the jaw closers have characteristics of slow muscles, and that the supra-/infrahyoid muscles have characteristics of fast muscles.

A longitudinal electromyographic study of the postnatal maturation of mastication in the rabbit

At 2 weeks of age, infant rabbits show chewing movements that resemble those of the adult animal. Previous studies have shown that, at that stage, the accompanying masticatory motor pattern is clearly similar to the suckling motor pattern. As early as 4 weeks, chewing muscle activity is indistinguishable from the adult chewing motor pattern. These reports suggest that the adult chewing motor pattern is developed from the suckling motor pattern. In this study, the chewing motor pattern in the intermediate period (between 2 and 4 weeks of age) was investigated by means of fine-wire electromyography and jaw tracking. Maturation of masticatory movements was found to have two phases. Maximum gape increased in the first few days and was followed by strong development of transverse jaw excursions after the age of 17 days. The increase in jaw excursions was brought about by changes in motor behaviour and facilitated by the development of smooth occlusal surfaces. The changes in motor behaviour were: (1) the level of activity of the balancing-side muscles became more equal to that of the working side; (2) the timing of digastric muscle activity became asymmetrical at the age of 17 days; (3) the peak activity of masseter, temporalis, medial pterygoid and lateral pterygoid muscle portions was gradually shifted or prolonged into the power-stroke phase. It can be concluded that the masticatory contraction pattern shifts from one derived from the suckling contraction pattern at the age of 14 days to one almost similar to the adult chewing pattern at the age of 23 days.

Fibre-type composition of rabbit jaw muscles is related to their daily activity.

Skeletal muscles contain a mixture of fibres with different contractile properties, such as maximum force, contraction velocity and fatigability. Muscles adapt to altered functional demands, for example, by changing their fibre‐type composition. This fibre‐type composition can be changed by the frequency, duration and presumably the intensity of activation. The aim of this study was to analyse the relationship between the spontaneous daily muscle activation and fibre‐type composition in rabbit jaw muscles. Using radio‐telemetry combined with electromyography, the daily activity of five jaw muscles was characterized in terms of the total duration of muscle activity (duty time) and the number of activity bursts. Fibre‐type composition of the muscles was classified by analysing the myosin heavy chain content of the fibres. The amount of slow‐type fibres was positively correlated to the duty time and the number of bursts only for activations exceeding 20–30% of the maximum activity per day. Furthermore, cross‐sectional areas of the slow‐type fibres were positively correlated to the duty time for activations exceeding 30% of the maximum activity. The present data indicate that the amount of activation above a threshold (> 30% peak activity) is important for determining the fibre‐type composition and cross‐sectional area of slow‐type fibres of a muscle. Activation above this threshold occurred only around 2% of the time in the jaw muscles, suggesting that contractile properties of muscle fibres are maintained by a relatively small number of powerful contractions per day.