Bite Position Research Articles

A Study of Electromyographic Activity of Masseter and Temporalis Muscles and Maximum Bite Force in Patients with Various Malocclusions

Introduction and objectives: The relationship between form and function of the stomatographic system has been studied by several researchers and it is still not clear, whether a genetically determined facial morphology decides the strength of masticatory muscles, or whether a strong musculature influences the form of the face. The present study was done to relate muscle activity with various malocclusions. Materials and methods: Sixty samples of younger age group were divided according to Angle classification. The electromyography (EMG) activity of masseter and temporalis and the maximum bite force (MBF) at intercuspal (molar) and anterior bite position (incisors) have been studied for different malocclusion groups. Influence of various independent variables like gender, overjet and overbite of the subjects on the bite force was also checked. Results: MBF at intercuspal position (molar) and anterior bite position (incisal) were not significantly different between normal, Class I, Class II Division 1 and Class III malocclusion groups. Conclusion: It was concluded that Class III malocclusion subjects showed the highest masseter and anterior temporalis postural rest activity and Class II Division I malocclusion showed the least anterior temporalis activity at both the intercuspal and interincisal positions during

Feeding in natricines: relationships among feeding morphology, behavior, performance and preferred prey type

AbstractThe diversity of feeding mechanisms among predators reflects phenotypic modifications that may improve feeding performance on a preferred prey type. I compared trophic morphology, feeding performance (time and upper‐jaw walks) and behavior (initial bite and ingestion directions) among three species of natricine snakes that were fed fish and frogs over a broad range of relative prey sizes. Feeding behavior was influenced by prey type but did not differ among the snake species. Both bite and ingestion directions influenced the number of upper‐jaw movements (lateral excursions and protractions) required to consume fish, but only initial bite position significantly affected the number of upper‐jaw movements required to ingest frogs. Within snake species, feeding performance did not differ between fish and frogs for Nerodia fasciata and Nerodia rhombifer; however, Thamnophis proximus consumed fish with fewer upper‐jaw movements. Feeding time differed significantly among snake species when fed both fish and frogs. Trophic morphology did not significantly affect ingestion costs for fish but did influence ingestion when fed frogs. In general, differences in trophic morphology among the three species are not correlated to handling and ingestion performance.

A Study of Bite Force and Various Variables in Children Segregated by Angle's Classification.

ABSTRACTThe stomatographic system has been studied by several researchers, yet it is still unclear, weather a genetically determined facial morphology decides the strength of masticatory muscles,1 or weather a strong musculature influences the form of the face. This formed the basis of present study to relate muscle activity with various malocclusions. Thus, 60 samples of younger age group were divided according to Angle classification and maximum bite force was recorded among the groups. Newly designed bite force recorder was used for recording bite force at molar and at incisal region. Influence of various independent variables like gender, overjet and overbite of the subjects on the bite force was also checked. It was concluded that maximum bite force at intercuspal position (molar) and anterior bite position (incisal) were not significantly different between normal, class I, class II div 1 and class III malocclusion groups. There was no significant correlation between incisal bite force and overjet or overbite, but there was a highly significant difference (p < 0.001) between the males and females for maximum bite force at intercuspal position, with males biting harder than the females.How to cite this article: Singh S, Sandhu N, Kashyap R. A Study of Bite Force and Various Variables in Children Segregated by Angle's Classification. Int J Clin Pediatr Dent 2012;5(2):118-123.

Finite element modelling of squirrel, guinea pig and rat skulls: using geometric morphometrics to assess sensitivity

Rodents are defined by a uniquely specialized dentition and a highly complex arrangement of jaw-closing muscles. Finite element analysis (FEA) is an ideal technique to investigate the biomechanical implications of these specializations, but it is essential to understand fully the degree of influence of the different input parameters of the FE model to have confidence in the model's predictions. This study evaluates the sensitivity of FE models of rodent crania to elastic properties of the materials, loading direction, and the location and orientation of the models' constraints. Three FE models were constructed of squirrel, guinea pig and rat skulls. Each was loaded to simulate biting on the incisors, and the first and the third molars, with the angle of the incisal bite varied over a range of 45°. The Young's moduli of the bone and teeth components were varied between limits defined by findings from our own and previously published tests of material properties. Geometric morphometrics (GMM) was used to analyse the resulting skull deformations. Bone stiffness was found to have the strongest influence on the results in all three rodents, followed by bite position, and then bite angle and muscle orientation. Tooth material properties were shown to have little effect on the deformation of the skull. The effect of bite position varied between species, with the mesiodistal position of the biting tooth being most important in squirrels and guinea pigs, whereas bilateral vs. unilateral biting had the greatest influence in rats. A GMM analysis of isolated incisor deformations showed that, for all rodents, bite angle is the most important parameter, followed by elastic properties of the tooth. The results here elucidate which input parameters are most important when defining the FE models, but also provide interesting glimpses of the biomechanical differences between the three skulls, which will be fully explored in future publications.

Feeding performance in the Western Ribbon Snake (Thamnophis proximus): ontogeny and the effects of prey type and size

It has been suggested that juveniles have exaggerated morphologies or improved performance to compensate for the disadvantages of being small. As gape-limited predators, juvenile snakes are at a particular disadvantage in terms of prey availability, capture, and ingestion. Prey preference often changes with snake size yet previous studies generally compare only one prey type or relative prey size among age classes. Using Western Ribbon Snakes ( Thamnophis proximus (Say in James, 1823)), I investigated the ontogeny of head dimensions and feeding performance on frogs and fish using a range of relative prey sizes. Comparison among snake sizes indicated that smaller individuals had larger relative head sizes than larger conspecifics; however, larger individuals had larger heads. Initial bite position differed between juveniles and adults fed fish but not frogs. Both frogs and fish were generally consumed headfirst by adults and juveniles. Head length did not influence feeding performance in terms of time or pterygoid protractions for snakes fed either fish or frogs. In general, this study suggests that allometry of trophic morphology does not improve feeding performance.

The Mechanical Significance of the Temporal Fasciae in Macaca fascicularis: An Investigation Using Finite Element Analysis

Computational finite element analyses (FEAs) of the skull predict structural deformations under user specified loads and constraints, with results normally presented as stress and strain distributions over the skull's surface. The applied loads are generally a representation of the major adductor musculature, with the skull constrained at bite positions and at the articulating joints. However, virtually all analyses ignore potentially important anatomical structures, such as the fasciae that cover the temporalis muscle and attach onto the zygomatic arch. In vivo experimental studies have shown that removal of the temporal fasciae attachment onto the zygomatic arch in Cebus monkeys results in significant bone adaptation and remodeling in this region, suggesting the fasciae play an important role in stabilising the arch during biting. Here we investigate this potential stabilising role by carrying out FEAs of a macaque skull with and without temporal fasciae included. We explore the extent to which the zygomatic arch might be stabilized during biting by a synchronized tensioning of the temporal fasciae, acting to oppose masseteric contraction forces. According to our models, during temporalis muscle bulging the forces generated within the tensioned temporal fasciae are large enough to oppose the pull of the masseter. Further, a near bending-free state of equilibrium within the arch can be reached, even under forceful biting. We show that it is possible to eliminate the high strain gradients in and around the zygomatic arch that are present in past computational studies, with strains being more uniform in magnitude than previously thought.

Reliability of electromyographic activity vs. bite-force from human masticatory muscles

The reproducibility of electromyographic (EMG) activity in relation to static bite-force from masticatory muscles for a given biting situation is largely unknown. Our aim was to evaluate the reliability of EMG activity in relation to static bite-force in humans. Eighty-four subjects produced five unilateral static bites of different forces at different biting positions on molars and incisors, at two separate sessions, and the surface EMG activities were recorded from temporalis, masseter, and suprahyoid muscles bilaterally. Intraclass correlation coefficients (ICCs) were determined, and an ICC of ≥ 0.60 indicated good reliability of these slopes. The ICCs for jaw-closing muscles during molar biting were: temporalis muscles, ipsilateral 0.58-0.93 and contralateral 0.88-0.91; and masseter muscles, ipsilateral 0.75-0.86 and contralateral 0.69-0.88. The ICCs for jaw-closing muscles during incisor biting were: temporalis muscles, ipsilateral 0.56-0.81 and contralateral 0.34-0.86; and masseter muscles, ipsilateral 0.65-0.78 and contralateral 0.59-0.80. For the suprahyoid muscles the 95% CIs were mostly wide and most included zero. The slopes of the EMG activity vs. bite-force for a given biting situation were reliable for temporalis and masseter muscles. These results support the use of these outcome measurements for the estimation and validation of mechanical models of the masticatory system.

Changes of oral sensibility in subjects with partial anterior open bite and the incorrect position of the tongue

Neuromuscular disorders lead to an imbalance in the position of the jaw. The aim of this study has been to analyse gnostic sensibility in subjects with partial anterior open bite and the incorrect position of the tongue. The study involved 20 subjects with partial anterior open bite and an incorrect tongue position. The control group consisted of 20 individuals with correct occlusion and tongue position. The basic study method was a stereognostic examination using 4 silicon shapes - a square, triangle, circle and semicircle. The accuracy of shape identification and the time that the subjects needed to identify the shapes were analysed before and after the tip of the tongue was anaesthetized. Results showed that correct identification of the shapes was 7.4% worse in the study group than in the control group and that the difference was greatest when the tip of the tongue was anaesthetized - 28.8%. The time needed to identify the shapes was shorter in the study group than in the control group. The results indicate that people with partial anterior open bite and incorrect tongue position exhibit impaired gnostic sensibility, especially at the tip of the tongue. Impaired gnostic sensibility, which is a symptom of the disturbed sensomotoric correlation of the tongue, leads to the tongue's incorrect position in the process of swallowing and speaking.

Neuromuscular Orthotics in the Treatment of Craniomandibular Dysfunction and the Effects on Patients with Multiple Sclerosis: A Pilot Study

The purpose of this pilot study was to identify, measure and document an effect on the subjective multiple sclerosis symptoms and compare it to any objective data changes in the neuromuscular system of the head and neck, following the correction of the jaw position using a neuromuscular orthotic. The hope is to provide clinical evidence of improvement in the disease long-term without relying on the subjective evidence of remissions and exacerbations reported by the patient. The evidence found in the current pilot study measured improvement of head position, jaw position, jaw function, and airway in the neuromuscular bite position, which correlated with the improvement of subjective symptoms of craniomandibular dysfunction and multiple sclerosis. Studies show that the bite affects blood flow in the brain, which may explain the improvement of the patients in the current study.

Craniofacial pain: A neurosurgical outlook

Dear Sir, Myofacial Pain Dysfunction (MPD) is a musculoskeletal dysfunction involving a mal-relationship among the neuromuscular system, temporomandibular joints, and dental occlusion. The disease affects children and adults of all ages and both sexes. Patients complain of pain and / or dysfunction in the mandible, temporomandibular joints, ears, oral cavity, head, and neck. The temporomandibular joint (TMJ) is a paired synovial joint capable of both gliding and hinge movements, articulating mandibular condyle, articular disk, and squamous portions of the temporal bone; and temporomandibular disorders (TMD), as defined by the American Academy of Craniofacial Pain, are a number of clinical problems that involve masticatory muscles, the temporomandibular joint (TMJ) or even both. The etiological factors contributing to TMDs include trauma, anatomical factors, systemic factors, muscular hyperactivity, intra-articular pathology, and psychosocial factors.[1] The most common symptoms of TMJ / TMD include: Headaches (often misdiagnosed as migraines), pain (shoulder, neck, or back), trigeminal neuralgia, Bell's palsy, locking jaw, cracked teeth, numbness in the arms and fingers, facial pain, jaw pain, clicking or popping jaw, clenching or grinding teeth, ringing ears (tinnitus), vertigo, and congested ears. The diagnosis of TMDs includes precise clinical examination, medical and dental history, radiographs, cephalometric views, Computerized Axial Tomography (CAT) scan, and even Magnetic Resonance Imaging (MRI), to visualize more structures including TMJ disk and other soft tissues, dental models, and biometrics. In recent years much attention has been focused on neuromuscular dentistry — a new tool for diagnosing and treating TMD. Neuromuscular dentistry is the science of dentistry that embodies accepted scientific principles of pathophysiology, anatomy, form, and function. It objectively evaluates the complex relationship between teeth, temporomandibular joints, and the masticatory muscles, in order to achieve an occlusion that is based on the optimal relationship between the mandible and the skull — a neuromuscular occlusion. The goal of neuromuscular dentistry is to relax the muscles controlling the jaw position, to establish a true physiological rest position upon which treatment considerations are based.[2] The primary element that sets neuromuscular dentistry apart from traditional dentistry is that neuromuscular dentistry considers the nerves and muscles and the correct positioning of the jaw, whereas, traditional dentistry focuses on just the teeth and joints. The result is a more complete approach, which can resolve painful conditions such as temporomandibular joint disorders, and it provides more comfortable and longer lasting solutions to other dental needs such as dentures and smile makeovers. Neuromuscular dentistry utilizes modern technology to precisely determine the proper positioning of the jaw. It uses computerized instrumentation to measure the patient's jaw movements, via Computerized Mandibular Scanning (CMS) or Jaw Motion Analysis (JMA), muscle activity via electromyography (EMG), and temporomandibular joint sounds via Electro Sonography (ESG) or Joint Vibration Analysis (JVA), to assist in identifying the joint derangements.[2] Surface EMGs are used to verify pre-, mid- and post-treatment conditions before and after ultra-low frequency Transcutaneous Electrical Nerve Stimulator (TENS). By combining both CMS and JMA with ultra-low frequency TENS, the dentist is able to locate a ‘physiological rest’ position as a starting reference position to find a relationship between the upper and lower jaw along an isotonic path of closure up from the physiological rest position, in order to establish a bite position. We still require additional research that would lead toward the evidence necessary for choosing the appropriate treatment techniques. Most pain problems presented to dentists respond to treatment or resolve on their own. However, when pain is chronic and especially if it is associated with significant disability, it is important that the treating dentist screen for psychosocial factors that may affect the patient's pain, functioning, and response to treatment, regardless of the diagnosis and objective findings. Substantial evidence exists that psychosocial dysfunction is prevalent among patients with chronic orofacial pain, most commonly TMD. For chronic orofacial pain, just as for other chronic pain conditions (e.g., tension-type headache, low back pain), psychosocial variables have been found to be more strongly associated with pain intensity and activity interference than have clinical examination findings. Dentists may improve the quality of care provided to patients with oro-facial pain through psychotherapeutic management by referring patients with psychosocial counseling.[3]

Observaciones sobre algunos aspectos del comportamiento depredador en condiciones naturales y en cautividad de la serpiente diurna mastigodryas pleei (dumeril, bibron y dumeril 1854)

Se evaluaron algunos aspectos del comportamiento depredador de la serpiente diurna Mastigodryas pleei frente a dos tipo de presa: mamíferos (ratón de laboratorio) y lagartos (Cnemidophorus lemniscatus), cuantificando la duración de cada paso en la secuencia de depredación y midiendo variables como el número de movimientos de la lengua (tongue flicks), latencia de ataque, distancia de ataque y posición de la mordida sobre el cuerpo de la presa. Se encontró que la máxima distancia de ataque para este colúbrido diurno varia dependiendo de la naturaleza de la presa. La máxima distancia de ataque registrada (12 cm) desplegada durante el ataque a un lagarto y la minima (3 cm) desplegada a un ratón. La distancia de ataque puede estar relacionada con el reconocimiento de un peligro potencial atribuido a las características de una presa, exhibiendo tácticas diferenciales de aproximación y captura para atacar presas inofensivas o presas potencialmente peligrosas. Este hecho se evidencia durante la secuencia de depredación en el lugar específico en donde se propicia la mordida y la distancia relativa del ataque. Los tiempos de cada paso en la secuencia y el número de movimientos de lengua no mostraron diferencias significativas entre tratamientos.

Does long‐term use of unstable dentures weaken jaw muscles?

Although it is well known that conventional denture wearers have lower maximum bite forces than dentate subjects, no previous studies have compared the strength of the jaw muscles between these two groups. This study compared maximum bite forces, electromyographic (EMG) activity and estimated jaw muscle strength among three groups: (i) 17 edentulous subjects using newly acquired implant-retained overdentures (seven men, 10 women; mean age 60.3 +/- 13.0 years); (ii) 10 age-matched, fully dentate subjects (five men, five women; mean age 57.9 +/- 11.0 years); and (iii) 39 young, fully dentate subjects (19 men, 20 women; mean age 24.4 +/- 3.5 years). Electromyographic activity was recorded from subjects' bilateral superficial masseter and anterior temporalis muscles while they generated maximum voluntary bite forces at the right central incisor, right first premolar and right first molar positions. Jaw muscle strength was estimated as the ratio of average EMG activity for all four muscles to the maximum bite force. At all three bite positions, edentulous subjects produced maximum bite forces that were less than half that of dentate subjects. Edentulous subjects also produced significantly less EMG activity and had significantly lower estimated jaw muscle strength. Our results suggest that weakened jaw muscles are one factor contributing to lower maximum bite forces among users of conventional dentures.

Reliability of maximum bite force measurements in age‐varying populations

In order for bite forces to be used clinically, they must be reliable. While bite force transducers are accurate and precise during bench tests, widely varying reliabilities of intra-oral bite forces have been reported when measured in human subjects. Because few studies have reported total reliability, the clinical use of intra-oral bite forces measurements remains questionable. The purposes of this study were to (i) estimate total reliability, including both within- and between-session reliabilities, of repeated maximum incisor and molar bite force measurements and (ii) demonstrate how extraneous variation affects reliability by comparing estimates for which the effects of age have and have not been controlled. A sample of 28 healthy subjects with Class I normal occlusion (seven subjects in each of four age groups: 5, 8, 11 and 14 years) performed two sessions approximately 90 min apart. Each session consisted of three maximum voluntary bites at three bite positions (incisor and right and left molars). For each bite position, between-subject variance (true variance), between-session variance and within-session variance were calculated using Multilevel modelling procedures. The variances were used to estimate between-session reliabilities, within-session reliabilities and total reliabilities. Within-session reliabilities were substantially higher than between-session reliabilities, which in turn was higher than total reliabilities at all bite positions. Reliabilities were highest at the incisor bite position. Not controlling for age effects substantially overestimated total reliability at all bite positions. After controlling for age effects, total reliabilities of repeated maximum bite forces were low to moderate.

Establishing a framework for archosaur cranial mechanics

The aim of this analysis was to establish the basic mechanical principles of simple archosaur cranial form. In particular we estimated the influence of two key archosaur innovations, the secondary palate and the antorbital fenestra, on the optimal resistance of biting-induced loads. Although such simplified models cannot substitute for more complex cranial geometries, they can act as a clearly derived benchmark that can serve as a reference point for future studies incorporating more complex geometry. We created finite element (FE) models comprising either a tall, domed (oreinirostral) snout or a broad, flat (platyrostral) archosaur snout. Peak von Mises stress was recorded in models with and without a secondary palate and/or antorbital fenestra after the application of bite loads to the tooth row. We examined bilateral bending and unilateral torsion-inducing bites for a series of bite positions along the jaw, and conducted a sensitivity analysis of material properties. Pairwise comparison between different FE morphotypes revealed that oreinirostral models are stronger than their platyrostral counterparts. Oreinirostral models are also stronger in bending than in torsion, whereas platyrostral models are equally susceptible to either load type. As expected, we found that models with a fenestra always have greatest peak stresses and by inference are “weaker,” significantly so in oreinirostral forms and anterior biting platyrostral forms. Surprisingly, although adding a palate always lowers peak stress, this is rarely by large magnitudes and is not significant in bilateral bending bites. The palate is more important in unilateral torsion-inducing biting. Two basic principles of archosaur cranial construction can be derived from these simple models: (1) forms with a fenestra are suboptimally constructed with respect to biting, and (2) the presence or absence of a palate is significant to cranial integrity in unilaterally biting animals. Extrapolating these results to archosaur cranial evolution, it appears that if mechanical optimization were the only criterion on which skull form is based, then most archosaurs could in theory strengthen their skulls to increase resistance to biting forces. These strengthened morphotypes are generally not observed in the fossil record, however, and therefore archosaurs appear subject to various non-mechanical morphological constraints. Carnivorous theropod dinosaurs, for example, may retain large suboptimal fenestra despite generating large bite forces, owing to an interplay between craniofacial ossification and pneumatization. Furthermore, living crocodylians appear to strengthen their skull with a palate and filled fenestral opening in the most efficient way possible, despite being constrained perhaps by hydrodynamic factors to the weaker platyrostral morphotype. The future challenge is to ascertain whether these simple predictions are maintained when the biomechanics of complex cranial geometries are explored in more detail.

Determinants of feeding performance in free-ranging pit-vipers (Viperidae: Ovophis okinavensis): key roles for head size and body temperature

Feeding performance (handling time, capture success) in numerous animal species is well known to be influenced by a variety of ecological, functional, and physiological factors. Nonetheless, few studies have tested which factors are the strongest determinants of animal feeding performance in the wild. Using a field-based experiment, we examined the relationships among a number of functionally important variables and the predatory behaviour of free-ranging pit-vipers (Ovophis okinavensis) from Okinawa Island, Japan. Our main findings were: (1) strike latency was negatively related to snake body temperature and, hence, hotter snakes struck at frogs more readily than colder snakes; (2) initial bite position was correlated with ingestion direction (headfirst versus hindfirst) but ingestion direction was not correlated with ingestion duration; and (3) both snake head length and body temperature were negatively related with ingestion duration and, thus, snakes with longer heads and higher body temperatures had shorter ingestion durations. In O. okinavensis, head size and body temperature are therefore likely to have direct ecological consequences in terms of its feeding rate on explosively breeding frogs. More generally, however, this field-based study adds to the growing body of literature demonstrating that temperature has a pervasive influence on the feeding performance of ectotherms in general. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 93, 53–62.

Variations in activities of human jaw muscles depend on tooth-tipping moments

Static mechanical analyses of the masticatory apparatus often assume that jaw muscle activity, as measured using electromyography (EMG), is linearly and constantly related to magnitude of bite force during biting, regardless of bite force-induced tooth-tipping moments. The objective of this study was to test the hypothesis that the relationship between EMG of the jaw muscles and bite force varies with the magnitude and sign of tooth-tipping moments. Seven healthy male subjects produced unilateral static occlusal forces at five biting positions, resulting in sequential changes from buccal (+) to lingual (−) tipping moments on the mandibular first molar. Jaw muscle activities were recorded bilaterally using surface (for temporalis and masseter muscles) and indwelling (for lateral pterygoid muscles) electrodes. Bite forces were recorded and controlled using custom devices. EMG versus bite force data were plotted and regression relationships were calculated for each subject, muscle and biting position. Linear regression analysis, analysis of variance and Bonferroni adjusted least significant difference tests were used to determine the effects of muscle, side (ipsilateral, contralateral) and biting position within subjects. It was found that the relationship between EMG and bite force for different tipping moments differed significantly within a subject and muscle. This was most common in the lateral pterygoid and temporalis muscles (all P ≤ 0.042), where slopes of the EMG:bite force relationship varied between 3:1 and >25:1. In the masseter muscle, the EMG:bite force relationship for different tipping moments differed significantly in one subject ( P < 0.008); slopes varied up to 4.6:1. In conclusion, the relationship between EMG and bite force was linear. However, the slopes of the relationship changed significantly depending on sign (+, −) and magnitude of tipping moments acting on the molars.

Is headfirst ingestion essential in gape-limited predators? Prey-handling behavior of the anurophagous snake Rhabdophis tigrinus (Colubridae)

Headfirst ingestion of prey is a common adaptive behavioral trait in gape-limited predators that swallow large prey whole. To ascertain this tendency during anurophagy, prey handling and direction of prey ingestion were investigated in Rhabdophis tigrinus (Boie, 1826), a snake that feeds mainly on anurans. Examinations of stomach contents of wild R. tigrinus revealed that this snake does not show a tendency for headfirst ingestion of large prey, unlike most other snake species. In the laboratory, direction of ingestion depended largely on initial bite position, and when R. tigrinus swallowed a frog rump-first, the snake grasped both hind limbs of the frog and aligned them side by side so that both were swallowed together simultaneously from their tips. A simple model test suggested that physical resistance in the buccal cavity incurred during transportation of frogs may not differ between headfirst and hind-first ingestion if the hind limbs of frogs are aligned and swallowed simultaneously. Laboratory experiments also demonstrated that ingestively naive hatchlings of R. tigrinus are able to perform the unique manipulation required to swallow frogs hind-first. It is suggested that this unique ingestion mode is an adaptation for anurophagy, and several possible functional advantages are discussed.

Maximal bite force and EMG during bilateral and unilateral clenching

The observed maximal bite force is known to depend on the measuring method employed. Bite position, bite rise and unilateral or bilateral biting influence the observed bite force. The maximal bite force obtained when clenching unilaterally is smaller than the bite force summed for the two sides of a bilateral measurement (Bakke et al., 1989). We determined bite force and muscle activity of the masseter and temporal muscles during maximal voluntary contraction in a group of 81 dentate subjects. The bite force was measured between the first molars both bilaterally and unilaterally. The summed bite force obtained from the bilateral measurement was 569 ± 170 N. The bite force obtained for unilateral clenching was significantly lower: 430 ± 142 N. No significant differences in bite force between the right and left side for both the bilateral and unilateral measurements were observed. The muscle activity of the masseter and temporal muscles obtained from bilateral clenching did not differ significantly (masseter: 248 ± 149 μV and temporalis: 232 ± 105 μV). Also no significant differences were observed in muscle activity between the right and left side during bilateral clenching. We observed a significantly lower muscle activity in both masseter and temporalis for unilateral clenching as compared with bilateral clenching. The results of unilateral clenching showed no differences in muscle activity between the ipsi- and contralateral side for the masseter muscle (186 ± 127 μV). However, the muscle activity at the ipsilateral side of the temporal muscle (197 ± 127 μV) was significantly higher than at the contralateral side (150 ± 81 μV). We may conclude that bilateral clenching yields bite forces that are over 30% larger than those obtained during unilateral clenching. The muscle activity during unilateral clenching is symmetrical in the masseter muscles, but asymmetrical in the temporal muscles.

Influence of methodological parameters on human jaw-stretch reflexes.

The influence of methodological parameters and experimental conditions on the human jaw-stretch reflex was studied in healthy subjects in order to develop a reliable tool for investigation of the excitability of motoneuron pool. Short-latency excitatory reflex responses were evoked by a custom-made stretch device with the subjects biting on a jaw-bar with their front teeth. The displacement and ramp time of the stretches were accurately controlled and automatically triggered by a computer. The reflex responses were measured in the surface electromyogram (EMG) of the masseter and anterior temporalis muscles with online monitoring of the clenching level. The peak-to-peak amplitude of the jaw-stretch reflex was shown to be proportional to the level of EMG activity during isometric contractions, to increase proportionally with increasing stretch displacement at a given ramp time, and to decrease proportionally with increasing ramp time at a given stretch displacement. There were no significant differences in the reflex amplitude between repeated recordings within one session or between different sessions. Local anesthetic around the lower incisors as well as the upper incisors had no significant influence on the reflex amplitude. However, different biting positions on the bars of the stretch device significantly influenced the amplitude of the stretch reflex.

Preliminary Observations on Chemical Preference, Antipredator Responses, and Prey-Handling Behavior of Juvenile Leioheterodon madagascariensis (Colubridae).

Innate responses to prey chemicals, antipredator responses, and prey-handling behavior of a Madagascan colubrid snake, Leioheterodon madagascariensis, were experimentally examined. In a chemical test, ingestively naive hatchlings flicked their tongues frequently to the chemicals prepared from the animal taxa included in their natural diets, suggesting the presence of innate chemical prey preference. In a second test, three different types of stimuli were presented to elicit antipredator responses of the hatchlings. In a nonmoving stimulus session, only a single snake struck and exhibited characteristic displays such as body flattening, neck flattening, head elevation, and jerk. In a moving stimulus session, either no specific responses or a simple flight response was exhibited. In a tactile stimulus session, the above characteristic displays were frequently exhibited. Among these, lateral neck tilting posture accompanying head elevation and neck flattening was unique to L. madagascariensis. In a third test, five types of prey animals were offered to juveniles to examine the effects of prey size and type on prey-handling behavior, but no such effects were detected. Direction of ingestion seemed to depend on initial bite position. Constricting behavior was observed only in a single trial. All but one prey were swallowed alive. This inflexibility of prey-handling methods in the juvenile snakes may reflect the characteristics of generalist feeders, which require ontogenetic experiences to handle prey efficiently. Although L. madagascariensis is considered to have well-developed Duvernoy's glands with enlarged, posterior maxillary teeth, the gland secretion did not seem to cause rapid death of prey.