Incisal bite force direction in humans and the functional significance of mammalian mandibular translation

Incisal bite force direction was recorded and analyzed in ten human subjects using a specially designed force transducer. In all ten subjects the maxillary incisal bite force was vertically and anteriorly directed both during static biting and during biting associated with simultaneous mandibular translation and rotation. Since the resultant muscle force could not have been equal and opposite to the mandibular bite force, the mandibular condyles must have been loaded. These data demonstrate that the mandible acts as a lever during incisal biting and that there is no consistent relationship between incisal bite force direction and object size. In some individuals the bite force direction was more vertical during biting on a large transducer (30 mm high), while in other subjects it was more vertical during biting on a small transducer (10 mm high).     

Influence of bite force on jaw muscle activity ratios in subject-controlled unilateral isometric biting

Ratios of muscle activities in unilateral isometric biting are assumed to provide information on strategies of muscle activation independently from bite force. If valid, this assumption would facilitate experiments as it would justify subject-control instead of transducer-based force control in biting studies. As force independence of ratios is controversial, we tested whether activity ratios are associated with bite force and whether this could affect findings based on subject-controlled force. In 52 subjects, bite force and bilateral masseter and temporalis electromyograms were recorded during unilateral biting on a transducer with varying force levels and with uniform subject-controlled force. Working/balancing and temporalis/masseter ratios of activity peaks were related to bite force peaks. Activity ratios were significantly but weakly correlated with the bite force. The subject-controlled force varied within ±25% around the prescribed force in 95% of all bites. This scatter could cause a variation of group mean activity ratios of at most ±6% because of the weak correlation between bite force and ratios. As this small variation is negligible in most cases, subject-control of bite force can be considered an appropriate method to obtain group means of relative muscle activation in particular when force control with transducers is not feasible.     

The orientation of the force of the jaw muscles and the length of the mandible in mammals

Ungulates generally have large masseter and pterygoid muscles and a necessarily large angular process provides attachment surface on the mandible. The temporalis muscle tends to be small. It has been suggested that this is an adaptation for enhanced control of the lower jaw and reduction of forces at the jaw joint. I suggest an additional reason: because of the geometry of the jaw, the length of that segment of the lower jaw that spans the distance from the jaw joint to the most posterior tooth is significantly reduced when the masseler and pterygoid are the dominant muscles; this region is necessarily much longer when the temporalis is large.     

Design and construction of a transducer for bite force registration

This study describes the development of a system for quantification of human biting forces by (1) determining the mechanical properties of an epoxy resin reinforced with carbon fiber, (2) establishing the transducer's optimal dimensions to accommodate teeth of various widths while minimizing transducer thickness, and (3) determining the optimal location of strain gages using a series of mechanical resistance and finite element (FE) analyses. The optimal strain gage location was defined as the position that produced the least difference in strain pattern when the load was applied by teeth with two different surface areas. The result is a 7.3-mm-thick transducer with a maximum load capacity beyond any expected maximum bite force (1500 N). This system includes a graphic interface that easily allows acquisition and registration of bite force by any health-sciences or engineering professional.     

Survivorship of an ant-tended membracid as a function of ant recruitment

I used a host-visitor modeling framework to examine the interaction between the treehopper Publilia concava and ants in the genus Formica. In particular, I tested the functional relationship between ant tending, the spatial distribution of treehoppers, and treehopper density. The per-capita density of ants at each host plant was a decreasing function of treehopper density, distance from the ant nest and the neighborhood density of treehoppers. Treehopper survivorship was proportional to the per-capita density of ants and the duration of ant tending. Consequently, treehoppers in low-density aggregations on isolated host plants near the nest received maximum benefit from ant tending. Treehoppers tended by the ant Formica integra were abandoned as the summer progressed, although many of these treehoppers were re-colonized by other species of ants. While F. integra ultimately abandoned all treehoppers, treehoppers on host plants with fewer initial ants were abandoned first. Results from the present study are consistent with previous findings suggesting that patterns of density-dependent benefit for homopterans are a function of the recruitment response of ants. Additionally, results suggest a tradeoff between maximizing the persistence or probability of ant-tending and minimizing competition for ants when tended. In general, host-visitor models of mutualism may provide a theoretical framework for understanding conditional outcomes in ant-homopteran, and other host-visitor mutualisms.     

The structure and function of the jaw muscles in the rat (Rattus norvegicus L.)

'Tonic' and 'phasic' muscle fibre types can be distinguished histologically, using either histochemical techniques or by staining for lipid with Sudan black B. As muscles of mastication not only move the lower jaw of the rat, a 'phasic' action, but also suspend it from the cranium, a 'tonic' activity, some indication of the contribution of the major muscles to these functions has been gained from an examination of the fibre content of transverse frozen sections stained with Sudan black B. The numbers of 'pale' ('phasic') and 'dark' ('tonic') fibres were counted using a montage at a magnification of 60. Results suggest that the anterior temporal, deep masseter and external pterygoid have an important tonic action in stabilizing the position of the lower jaw as well as contributing to the production of movement; and that the superficial masseter and posterior temporal, in particular, have an almost completely phasic action. These conclusions are entirely consistent with the probable functions of the muscles inferred from their anatomy.     

Evolution of bite force in Darwin's finches: a key role for head width

Studies of Darwin's finches of the Galapagos Islands have provided pivotal insights into the interplay of ecological variation, natural selection, and morphological evolution. Here we document, across nine Darwin's finch species, correlations between morphological variation and bite force capacity. We find that bite force correlates strongly with beak depth and width but only weakly or not at all with beak length, a result that is consistent with prior demonstrations of natural selection on finch beak morphology. We also find that bite force is predicted even more strongly by head width, which exceeds all beak dimensions in predictive strength. To explain this result we suggest that head width determines the maximum size, and thus maximum force generation capacity of finch jaw adductor muscles. We suggest that head width is functionally relevant and may be a previously unrecognized locus of natural selection in these birds, because of its close relationship to bite force capacity.     

Ontogenetic scaling of bite force in lizards and turtles

Because selection on juvenile life-history stages is likely strong, disproportionately high levels of performance (e.g., sprint speed, endurance, etc.) might be expected. Whereas this phenomenon has been demonstrated with respect to locomotor performance, data for feeding are scarce. Here, we investigate the relationships among body dimensions, head dimensions, and bite force during growth in lizards and turtles. We also investigate whether ontogenetic changes in bite performance are related to changes in diet. Our analyses show that, for turtles, head dimensions generally increase with negative allometry. For lizards, heads scale as expected for geometrically growing systems. Bite force generally increased isometrically with carapace length in turtles but showed significant positive allometry relative to body dimensions in lizards. However, both lizards and turtles display positive allometric scaling of bite force relative to some measures of head size throughout ontogeny, suggesting (1) strong selection for increased relative bite performance with increasing head size and (2) intrinsic changes in the geometry and/or mass of the jaw adductors during growth. Whereas our data generally do not provide strong evidence of compensation for lower absolute levels of performance, they do show strong links among morphology, bite force, and diet during growth.     

Posterior and anterior components of force during bite loading

Late anterior crowding of teeth has been associated with the anterior component of force (ACF) developed during biting. Possible physiologic mechanisms countering ACF, including the presence of a posterior component of force (PCF), are hypothesized. In this self-controlled study, 60 subjects aged 27.05+/-3.9 years were examined for ACF and PCF that were calculated as the change in tightness of a mandibular dental contact points from non-biting to biting state. Both ACF and PCF were found to develop simultaneously. However, the PCF was 4-7 folds smaller than the ACF (p<0.001). The ACF progressively declined by 10-20 folds (p<0.001) from the posterior to anterior dentition. The lateral incisor-canine contact point had the greatest ACF decline (63-74%). ACF effect on the anterior dentition is counteracted by a protective mechanism consisted of PCF, progressive dissipation of ACF, and canine blockage.     

Changes in gravity rapidly alter the magnitude and direction of a cellular calcium current

In single-celled spores of the fern Ceratopteris richardii, gravity directs polarity of development and induces a directional, trans-cellular calcium (Ca²⁺) current. To clarify how gravity polarizes this electrophysiological process, we measured the kinetics of the cellular response to changes in the gravity vector, which we initially estimated using the self-referencing calcium microsensor. In order to generate more precise and detailed data, we developed a silicon microfabricated sensor array which facilitated a lab-on-a-chip approach to simultaneously measure calcium currents from multiple cells in real time. These experiments revealed that the direction of the gravity-dependent polar calcium current is reversed in less than 25 s when the cells are inverted, and that changes in the magnitude of the calcium current parallel rapidly changing g-forces during parabolic flight on the NASA C-9 aircraft. The data also revealed a hysteresis in the response of cells in the transition from 2g to micro-g in comparison to cells in the micro-g to 2-g transition, a result consistent with a role for mechanosensitive ion channels in the gravity response. The calcium current is suppressed by either nifedipine (calcium-channel blocker) or eosin yellow (plasma membrane calcium pump inhibitor). Nifedipine disrupts gravity-directed cell polarity, but not spore germination. These results indicate that gravity perception in single plant cells may be mediated by mechanosensitive calcium channels, an idea consistent with some previously proposed models of plant gravity perception.     

Double function of aptychi (Ammonoidea) as jaw elements and opercula

Lehmann, U. & Kulicki, C. 1990 10 15: Double function of aptychi (Ammonoidea) as jaw elements and opercula. Lethaia , Vol. 23, pp. 325–331. Oslo. ISSN 0024–1164.
Aptychi are calcitic coverings on the outer surface of organic ammonite lower jaws. They are similar in shape to that of the corresponding ammonite apertures. This observation and additional features of many aptychi are in harmony with their former interpretation as protective opercula. We suggest that they served as opercula in addition to functioning as jaws. The primary function of the lower jaws was thus secondarily extended to that of protective shields when they acquired their calcitic covering, while as lower jaws their importance dwindled to that of a more passive abutment. Phylogenetically, this seems to have started slowly in some anaptychi and became obvious with the first aptychi. ▭ Ammonites, aptychus, operculum, jaw apparatus, evolution, function .     

Seasonal variation of births in rural West Bengal: magnitude, direction and correlates

This paper examines seasonal variation of births in a rural community of West Bengal, India, by exploring data from the 1992-93 National Family Health Survey. Suitable time series analyses were used to determine the seasonal pattern of births and to estimate peaks. The trigonometric regression technique was used to carry out this objective. The study attempted to link the results of the regression analysis to the atmospheric temperature of the region during 1987-91, the distribution of respondents' husbands' occupations and the marriage pattern of the community. It was found that, in the study population, conceptions were numerous in the first quarter of a calendar year and the distribution of conceptions over calendar months was negatively associated with the average monthly temperature. In addition, the marriage pattern of the community and the occupational distribution of the fathers also had a significant effect on the distribution of births over calendar months. It is hoped that the findings will boost the development of needs-based maternal and child health (MCH) and family planning programmes in the community.     

Muscle force recruitment and biomechanical modeling: an analysis of masseter muscle function during mastication in Macaca fascicularis.

The main purpose of this study is to test the hypothesis that as subjects chew with increasing levels of force, the ratio of the working- to balancing-side jaw-muscle force (W/B) decreases and begins to approach 1.0. We did this by analyzing relative masseter force in Macaca fascicularis using both strain gage and surface electromyographic (EMG) techniques. In addition, we also analyzed: 1) the relationship between jaw position using cineradiographic techniques and relative masseter force, 2) the timing differences between relative masseter force from the working and balancing sides, and 3) the loading and unloading characteristics of the masseter muscle. Our findings indicate that when macaques increase the amount of overall masticatory force during chewing, the W/B ratio for masseter force frequently (but not always) decreases and begins to approach 1.0. Therefore, our working hypothesis is not completely supported because the W/B ratio does not decrease with increasing levels of force in all subjects. The data also demonstrate timing differences in masseter force. During apple-skin mastication, the average peak masseter force on the working side occurs immediately at or slightly after the initial occurrence of maximum intercuspation, whereas the average peak masseter force on the balancing side occurs well before maximum intercuspation. On average, we found that peak force from the balancing-side masseter precedes the working-side masseter by about 26 msec. The greater the asynchrony between working- and balancing-side masseter force, the greater the difference in the relative magnitude of these forces. For example, in the subject with the greatest asynchrony, the balancing-side masseter had already fallen to about one-half of peak force when the working-side masseter reached peak force. Our data also indicate that the loading and unloading characteristics of the masseter differ between the working and balancing sides. Loading (from 50 to 100% of peak force) and unloading (from 100 to 50% of peak force) for the balancing-side masseter tends to be rather symmetrical. In contrast, the working-side masseter takes much longer to load from 50 to 100% of peak force than it does to unload from 100 to 50% of peak force. Finally, it takes on average about 35 msec for the working-side zygoma and 42 msec for the balancing-side zygoma to unload from 100 to 50% of peak force during apple-skin mastication, indicating that the unloading characteristics of the macaque masseter during mastication closely approximates its relaxation characteristics (as determined by muscle stimulation).     

Suction force variation in double-hybrid maize as a function of soil moisture

In experiments with double-hybrid maize grown in the field and in a greenhouse, we analysed the value of the suction force as related to the regime of irrigation. The results showed that the level of water supply to the plants is in good connection with the value of the suction force and the coefficient H (the relative degree of water saturation of the cells) analysed at the same time. In the case of the 2 hybrids under investigation the highest value of coefficient H was found to be between 35–48. The data indicate that the use of these two indices should take part in a physiological method for the settlement and the application of water rules in irrigation.     

Development of platelet contractile force as a research and clinical measure of platelet function

This article reviews work performed at the Medical College of Virginia of Virginia Commonwealth University during the development of a whole-blood assay of platelet function. The new assay is capable of assessing platelet function during clotting and thus allows measurement of the contribution of platelets to thrombin generation. Because platelets are monitored in the presence of thrombin, the test gages platelets under conditions of maximal activation. Three parameters are simultaneously assessed on one 700-μL sample of citrated whole blood. Platelet contractile force (PCF), the force produced by platelets during clot retraction, is directly measured as a function of time. This parameter is sensitive to platelet number, platelet metabolic status, glycoprotein IIb/IIIa status, and the presence of antithrombin activities. Clot elastic modulus (CEM), also measured as a function of time, is sensitive to fibrinogen concentration, platelet concentration, the rate of thrombin generation, the flexibility of red cells, and the production of force by platelets. The third parameter, the thrombin generation time (TGT) is determined from the PCF kinetics curve. Because PCF is absolutely thrombin dependent (no thrombin—no force), the initial upswing in PCF occurs at the moment of thrombin production. TGT is sensitive to clotting factor deficiencies, clotting factor inhibitors, and the presence of antithrombins, all of which prolong the TGT and are known to be hemophilic states. Treatment of hemophilic states with hemostatic agents shortens, the TGT toward normal. TGT has been demonstrated to be shorter and PCF to be increased in coronary artery disease, diabetes mellitus, and several other thrombophilic states. Treatment of thrombophilic states with a variety of heparin and nonheparin anticoagulants prolongs the TGT toward normal. The combination of PCF, CEM, and TGT measured on the same sample may allow rapid assessment of global hemostasis and the response to a variety of procoagulant and anticoagulant medications.     

Structure and direction of jaw adductor muscles as herbivorous adaptations in <Emphasis Type="Italic">Neotoma mexicana</Emphasis> (Muridae,Rodentia)

The herbivorous adaptations of the jaw adductor muscles in Neotoma mexicana were clarified by a comparative study with an unspecialized relative, Peromyscus maniculatus. In P. maniculatus, the anterior part of the deep masseter arises entirely from the lateral side of an aponeurosis, i.e., superior zygomatic plate aponeurosis, whereas N. mexicana has an additional aponeurosis for this part of the muscle, and the fibers attach on both sides of the superior zygomatic plate aponeurosis. Although the structure of the temporalis muscle is nearly identical in the two genera, a clear aponeurosis of origin occurs only in N. mexicana. These characteristics allow fibrous tissues to be processed with a large occlusal force. The deep masseter, internal pterygoid, and external pterygoid muscles of N. mexicana incline more anterodorsally than those of P. maniculatus. The transverse force component of these muscles relative to whole muscle force is smaller in N. mexicana than in P. maniculatus, with the exception of the internal pterygoid. The anterior part of the temporalis muscle of N. mexicana is specialized to produce occlusal pressure. These findings suggest that in N. mexicana a large anterior force is required to move the heavy mandible, due to the hypsodont molars, against frictional force from food, and that the posterior pull of the temporalis, which adjusts the forward force by the other jaw adductor muscles to a suitable level, need not be large for the mandibular movement.