Muscular activity during isometric incisal biting

This study attempted to estimate TMJ loading during incisal loading using a custom load-cell device and surface electromyographic (sEMG) recordings of the main jaw closers to assess the outcome correlation. Study participants were 23 healthy volunteers. The incisal loads having submaximal and mean intensity were recorded using a calibrated electronic load cell; simultaneously, surface electromyography (sEMG) of the right and left masseter and temporalis muscles was recorded. Readings of the resting, clenching in maximal and submaximal intercuspal positions and mean (50%) incisal loads were recorded. Clenching sEMG activity was used as a reference for normalization. The mean (SD) submaximal incisal load recorded was 498 (305.78) N, and the mean at 50% of the submaximal load was 268.93 (147.37) N. Mean (SD) sEMG activity during submaximal clenching was 141.23 (87.76) μV, with no significant differences between the four muscles. During submaximal voluntary incisal loading, the normalized mean sEMG activity was 49.99 (34.54) µV %, and 27.17(15.29) µV % during mean (50%) effort. The incisal load was generated mainly by the masseter muscles, as these showed a positive correlation during mean but not during submaximal effort. In the edge-to-edge jaw position, the mean incisal load effort seems to be physiological, but excessive TMJ loads can be expected from chronic or excessive incisal loading. In conclusion, incisal loads require the activity of the masseter muscles, which show a positive correlation between sEMG activity and effective incisal loads during mean, but not during submaximal, effort, and the masseter muscles are dominant over the temporalis muscles during submaximal incisal biting.     

The maximum average bite force for a given jaw length

Major osteological landmarks were used to prepare idealized drawings of mammalian and reptilian lower jaws. Measurements from these drawings allowed the average output or bite force, along the entire jaw, to be calculated for many different anteroposterior positions of the input or muscle force. In the mammalian drawing, the maximum average bite force is exerted when the resultant force is located at about 30% of the way along the jaw from the joints. Because of geometric differences in the reptilian drawing, a resultant positioned at 20% of the way along the jaw exerts the maximum average bite force; a maximum force that is smaller than that in the mammalian case. The estimated location of the muscle resultant in actual cases corresponds to these calculated positions. Therefore, in real animals, the muscles are located in the position that produces the largest average force for any jaw length. The geometric changes necessary to transform the idealized reptilian drawing, with a smaller maximum average bite force, into that of the mammalian drawing, with a larger maximum force, are the same as those changes seen in the fossil record of the reptile/mammal transition. This finding suggests that the morphological changes that occurred in the jaws increased the average bite force in the primitive mammals.     

Effect of an ADP analog on isometric force and ATPase activity of active muscle fibers

The role played by ADP in modulatingcross-bridge function has been difficult to study, because it is hardto buffer ADP concentration in skinned muscle preparations. To solvethis, we used an analog of ADP, spin-labeled ADP (SL-ADP). SL-ADP bindstightly to myosin but is a very poor substrate for creatine kinase orpyruvate kinase. Thus ATP can be regenerated, allowing well-definedconcentrations of both ATP and SL-ADP. We measured isometric ATPaserate and isometric tension as a function of both [SL-ADP], 0.1-2mM, and [ATP], 0.05-0.5 mM, in skinned rabbit psoas muscle,simulating fresh or fatigued states. Saturating levels of SL-ADPincreased isometric tension (by P'), the absolute value of P' beingnearly constant, ~0.04 N/mm², in variable ATP levels, pH7. Tension decreased (50-60%) at pH 6, but upon addition ofSL-ADP, P' was still ~0.04 N/mm². The ATPase wasinhibited competitively by SL-ADP with an inhibition constant,K_i, of ~240 and 280 µM at pH 7 and 6, respectively. Isometric force and ATPase activity could both be fit bya simple model of cross-bridge kinetics.

     

Spatial reorganisation of muscle activity correlates with change in tangential force variability during isometric contractions

The aim of this study was to quantify the effects of spatial reorganisation of muscle activity on task-related and tangential components of force variability during sustained contractions. Three-dimensional forces were measured from isometric elbow flexion during submaximal contractions (50 s, 5–50% of maximal voluntary contraction (MVC)) and total excursion of the centre of pressure was extracted. Spatial electromyographic (EMG) activity was recorded from the biceps brachii muscle. The centroids of the root mean square (RMS) EMG and normalised mutual information (NMI) maps were computed to assess spatial muscle activity and spatial relationship between EMG and task-related force variability, respectively. Result showed that difference between the position of the centroids at the beginning and at the end of the contraction of the RMS EMG and the NMI maps were different in the medial–lateral direction (P < 0.05), reflecting that muscle regions modulate their activity without necessarily modulating the contribution to the task-related force variability over time. Moreover, this difference between shifts of the centroids was positively correlated with the total excursion of the centre of pressure at the higher levels of contractions (>30% MVC, R² > 0.30, P < 0.05), suggesting that changes in spatial muscle activity could impact on the modulation of tangential forces. Therefore, within-muscle adaptations do not necessarily increase force variability, and this interaction can be quantified by analysing the RMS EMG and the NMI map centroids.     

Long-term repeatability of force, endurance time and muscle activity during isometric contractions.

We determined the repeatability and correlations between force, endurance and muscle activity during isometric contractions over three years. Twenty-six subjects, with and without complaints of the shoulder and neck, performed standardized maximal and submaximal shoulder-abduction contractions and wrist extension-contractions at yearly intervals from 1997 to 1999. Peak forces developed during maximal contraction and the endurance times of submaximal contractions during shoulder abduction and wrist extension were measured. Electromyography (EMG) of muscle activity was recorded bilaterally from the upper trapezius, middle deltoid, and forearm extensor muscles. Root mean square EMG amplitudes were calculated. We found statistically significant associations between peak forces developed during wrist extension and shoulder abduction, and between endurance times of submaximal wrist extension and shoulder abduction. No statistically significant changes in peak force and EMG(peak) were found over the measurement years. The responses were not statistically significantly influenced by gender, or neck and shoulder pain. However, we observed considerable intra-individual variation in the inter-year measurements particularly for the responses to submaximal contraction. Such large variations represent a challenge when attempting to use the responses to interpret the effects of therapies.     

Location of the vector of jaw muscle force in mammals

Previous work has suggested that the third molar lies just in front of the point where the resultant vector of jaw muscle force, estimated from dissections, intersects the tooth row. This point meets the jaw such that the vector is 30% of jaw length from the jaw joint. Thus, the vector divides the jaw in the ratio of 3:7 when measurements are taken perpendicular to the vector. In practice, however, distances along mammalian jaws are typically measured on an easily determined line such as a line from one end of the tooth row to the other. The position of the jaw joint is then projected onto this line. As a rule, such a line is not perpendicular to the vector and so the distance from the projection of the joint, out to the rear of the third molar (and the vector's intersection), is different in different mammals. Rarely is this distance 30% of total jaw length. However, when the location of the vector's intersection is measured along the tooth row, this position varies directly with the inclination of the vector; a vector inclined posteriorly intersects the tooth row far from the projection of the joint and an anterior vector's intersection is relatively close. Only a vector perpendicular to the line from one end of the tooth row to the other intersects at 30%. This obvious point suggests a way to test the above hypotheses when the inclination of the vector is not known exactly. The predicted relationship between the distance to the molar, as a percentage of the total jaw length, and the approximate inclination of the vector derived from muscle weights (posterior or anterior depending on whether the temporalis or the masseter/pterygoid, respectively, is dominant) was observed in a sample of 46 different mammals.     

Phylogenetically structured variance in felid bite force: the role of phylogeny in the evolution of biting performance

A key question in evolution is the degree to which morphofunctional complexes are constrained by phylogeny. We investigated the role of phylogeny in the evolution of biting performance, quantified as bite forces, using phylogenetic eigenvector regression. Results indicate that there are strong phylogenetic signals in both absolute and size‐adjusted bite forces, although it is weaker in the latter. This indicates that elimination of size influences reduces the level of phylogenetic inertia and that the majority of the phylogenetic constraint is a result of size. Tracing the evolution of bite force through phylogeny by character optimization also supports this notion, in that relative bite force is randomly distributed across phylogeny whereas absolute bite force diverges according to clade. The nonphylogenetically structured variance in bite force could not be sufficiently explained by species‐unique morphology or by ecology. This study demonstrates the difficulties in identifying causes of nonphylogenetically structured variance in morphofunctional character complexes.     

Influence of temperature upon contractile activation and isometric force production in mechanically skinned muscle fibers of the frog

Increasing temperature (4-22 degrees C) increases the Ca2+ concentration required for activation of mechanically skinned frog muscle fibers. The pCa required for 50% maximal force (pCa50) was inversely proportional to absolute temperature. Assuming that relative force is directly related to fractional occupancy of the Ca2+-binding sites on troponin that regulate force, the shift was consistent with a Gibbs free energy change of binding (delta G) of about -7.8 kcal/mol. This is close to the delta G for Ca2+ binding to the calcium-specific sites on troponin C reported by others. Decreasing Mg2+ from 1 mM to 60 microM shifts the force-pCa curves at either 4 or 22 degrees C to higher pCa, but the shift of pCa50 with temperature over this range (0.4 log units) was the same at low and high Mg2+. Maximal force increased with temperature for the entire range 4-22 degrees C with a Q10 of 1.41, and over the restricted range 4-15 degrees C with a Q10 of 1.20. From the dual effects of temperature on Ca2+ activation and maximal force, one would expect that force would respond differently to temperature change at high or low Ca2+. At high Ca2+, a temperature increase will lead to an increased force. However, at low to intermediate Ca2+ levels (below the intersection of the force-pCa curves for the initial and final temperatures), steady state force should decrease with increasing temperature. The inverse responses should occur with a decrease in temperature. These responses are observed when temperature is changed by rapid solution exchange.     

Bite club: comparative bite force in big biting mammals and the prediction of predatory behaviour in fossil taxa

We provide the first predictions of bite force (BS) in a wide sample of living and fossil mammalian predators. To compare between taxa, we calculated an estimated bite force quotient (BFQ) as the residual of BS regressed on body mass. Estimated BS adjusted for body mass was higher for marsupials than placentals and the Tasmanian devil (Sarcophilus harrisii) had the highest relative BS among extant taxa. The highest overall BS was in two extinct marsupial lions. BFQ in hyaenas were similar to those of related, non-osteophagous taxa challenging the common assumption that osteophagy necessitates extreme jaw muscle forces. High BFQ in living carnivores was associated with greater maximal prey size and hypercarnivory. For fossil taxa anatomically similar to living relatives, BFQ can be directly compared, and high values in the dire wolf (Canis dirus) and thylacine (Thylacinus cynocephalus) suggest that they took relatively large prey. Direct inference may not be appropriate where morphologies depart widely from biomechanical models evident in living predators and must be considered together with evidence from other morphological indicators. Relatively low BFQ values in two extinct carnivores with morphologies not represented among extant species, the sabrecat, Smilodon fatalis, and marsupial sabretooth, Thylacosmilus atrox, support arguments that their killing techniques also differed from extant species and are consistent with 'canine-shear bite' and 'stabbing' models, respectively. Extremely high BFQ in the marsupial lion, Thylacoleo carnifex, indicates that it filled a large-prey hunting niche.     

Influence of bite force and tongue pressure on oro-pharyngeal residue in the elderly

Objective: To investigate the influence of maximal bite force, maximal tongue pressure, number of mastications and swallowing on the oro‐pharyngeal residue in the elderly. Background: Oro‐pharyngeal residue in the elderly is an indication of dysphagia. Pharyngeal residue is especially critical as it may cause aspiration pneumonia, which is one of the major causes of death in elderly. Materials and methods: Videofluorographic recordings were performed on 14 elderly volunteers (six males, eight females, age range 65–93 years) without any history or symptoms of dysphagia. The subjects were instructed to consume 9 g of barium containing bread in two manners; free mastication and swallow (FMS: masticate and swallow freely), and limited mastication and swallow (LMS: swallow once after 30 chewing actions). The amount of oral and pharyngeal residue was evaluated using a 4‐point rating scale. Maximal occlusal force was measured by a pressure sensitive sheet, and maximal tongue pressure using a handy probe. Multiple regression analysis was performed to examine the influence of these items on the amount of oral and pharyngeal residue in FMS and LMS. Results: In FMS, age was found to be a factor which increased oral residue (p = 0.053), and the number of swallowing (p = 0.017) and the state of the prosthesis (p = 0.030) reduced the pharyngeal residue. In LMS, tongue pressure was a factor which reduced oral residue (p = 0.015) and increased pharyngeal residue (p = 0.008). Conclusion: It is suggested that in the elderly tongue pressure contributed to propulsion of the food bolus from oral cavity into the pharynx, and multiple swallowing contributed to the reduction in the amount of pharyngeal residue.     

Equatorial x-ray intensities and isometric force levels in frog sartorius muscle.

Isometric force levels, ranging between 0 and 100% of maximal force P₀ at 2 to 3 °C, were elicited in frog sartorius muscle by means of rapidly cooling a Ringer solution containing 1·25 to 2·0 mm-caffeine. Equatorial X-ray diffraction patterns were obtained in the resting state and during contraction. The ratio of the intensities $\text{I}{}_{11}\text{I}{}_{10}$ increased with force almost linearly, with a slight upward curvature. The individual intensities for the contracting state were normalized relative to both the intensity of the undiffracted beam and the intensity of each reflection in the resting state. These normalized intensities were found to vary in a reciprocal way: I₁₀ decreased while I₁₁ increased throughout the range of forces studied.The gradual change in $\text{I}{}_{11}\text{I}{}_{10}$ with force level indicates that this ratio is a sensitive measure of the number of cross-bridges in the isometric state. A two-state model in which myosin projections are either in a resting or attached state and in which force is proportional to the fraction of projections in the attached state was applied to the experimental data of the individual reflections. I₁₀ deviates from this model in a way that suggests that formation of the first few cross-bridges may decrease the regularity of the remaining unattached myosin projections.     

Effects of palmatine on isometric force and intracellular calcium levels of arterial smooth muscle

The effects of palmatine on isometric force and intracellular free calcium levels ([Ca2+]i) were determined in isolated rat arterial strips. Palmatine dose-dependently relaxed the contractile responses stimulated by phenylephrine (PE) in aortic strips. In contrast, it only partially relaxed aortic strips contracted by 51 mM KCl. Pretreatment with palmatine shifted the dose-response curves of PE both rightwards and downwards in a dose-dependent manner. When Ca2+-free solution and re-addition of Ca2+ were applied to assess PE-induced phasic and tonic contractions, palmatine was found to be effective in inhibiting both contractions. The effects of palmatine on intracellular calcium levels were measured with the bioluminescent calcium indicator aequorin in rat tail artery strips. Palmatine caused a concomitant, dose-dependent decrease in PE-activated isometric force and [Ca2+]i, resulting in small changes in the [Ca2+]i-force relationship. These results suggest that vasodilatory effect of palmatine was mediated by reducing [Ca2+]i as well as affecting [Ca2+]i sensitivity of the contractile apparatus. Palmatine-induced [Ca2+]i decreases appeared to involve decreases in both Ca2+ release from intracellular stores and Ca2+ influx through calcium channels.     

Influence of muscle mass on sympathetic neural activation during isometric exercise

The primary purpose of this study was to determine whether the sympathetic neural activation induced by isometric exercise is influenced by the size of the contracting muscle mass. To address this, in nine healthy subjects (aged 19-27 yr) we measured heart rate, systolic arterial blood pressure, and muscle sympathetic nerve activity in the leg (MSNA; peroneal nerve) before (control) and during 2.5 min of isometric handgrip exercise (30% of maximal voluntary force). Exercise was performed with the right and left arms separately and with both arms simultaneously (random order). During exercise, heart rate, systolic pressure, and MSNA increased above control under all conditions (P less than 0.05). For each variable, the magnitudes of the increases from control to the end of exercise were significantly greater when exercise was performed with two arms compared with either arm alone (P less than 0.05). In general, the increases in heart rate, systolic pressure, and MSNA elicited during two-arm exercise were significantly less than the simple sums of the responses evoked during exercise of each arm separately. These findings indicate that the magnitude of the sympathetic neural activation evoked during isometric exercise in humans is determined in part by the size of the active muscle mass. In addition, our results suggest that the sympathetic cardiovascular adjustments elicited during exercise of separate limbs are not simply additive but instead exhibit an inhibitory interaction (i.e., neural occlusion).     

Multivariate genetic analysis of maximal isometric muscle force at different elbow angles

Thomis, Martine A., Marc Van Leemputte, Hermine H. Maes,Cameron J. R. Blimkie, Albrecht L. Claessens, Guy Marchal, Eustachius Willems, Robert F. Vlietinck, and Gaston P. Beunen. Multivariate genetic analysis of maximal isometric muscle force at different elbowangles. J. Appl. Physiol. 82(3):959-967, 1997.The maximal isometric moment at five differentelbow joint angles was measured in 25 monozygotic and 16 dizygotic maleadult twin pairs (22.4 ± 3.7 yr). Genetic model fittingwas used to quantify the genetic and environmental contributions toindividual differences in isometric strength. Additive genetic factorsexplained 66-78% of the variance in maximal torque at170-140-110 and 80° flexion (extension = 180°). At50° flexion, common and subject-specific environmental factorscontributed equally to the variation. The contribution of uniqueenvironmental factors concurs with the level of variability in muscleactivation and (dis)-comfort of torque production in the specificangle. The relative contribution of lever arm and force-lengthrelationship in torque varies according to the angle. Because thesefactors might be genetic, this variability is reflected in the geneticcontribution at the extreme angles of 170 and 50°. Multivariateanalyses suggested a general set of genes that control muscle area andisometric strength, together with a more specific strength factor.Genetic correlations were high (0.82-0.99). Genes responsible forarm-segment lengths did not contribute to muscle area nor to isometricstrength.

     

Modelling the isometric force response to multiple pulse stimuli in locust skeletal muscle

An improved model of locust skeletal muscle will inform on the general behaviour of invertebrate and mammalian muscle with the eventual aim of improving biomedical models of human muscles, embracing prosthetic construction and muscle therapy. In this article, the isometric response of the locust hind leg extensor muscle to input pulse trains is investigated. Experimental data was collected by stimulating the muscle directly and measuring the force at the tibia. The responses to constant frequency stimulus trains of various frequencies and number of pulses were decomposed into the response to each individual stimulus. Each individual pulse response was then fitted to a model, it being assumed that the response to each pulse could be approximated as an impulse response and was linear, no assumption were made about the model order. When the interpulse frequency (IPF) was low and the number of pulses in the train small, a second-order model provided a good fit to each pulse. For moderate IPF or for long pulse trains a linear third-order model provided a better fit to the response to each pulse. The fit using a second-order model deteriorated with increasing IPF. When the input comprised higher IPFs with a large number of pulses the assumptions that the response was linear could not be confirmed. A generalised model is also presented. This model is second-order, and contains two nonlinear terms. The model is able to capture the force response to a range of inputs. This includes cases where the input comprised of higher frequency pulse trains and the assumption of quasi-linear behaviour could not be confirmed.     

An ultrasensitive isometric force transducer for single smooth muscle cell mechanics.

The principles of operation, design, and performance of a differential photooptical transducer of very high sensitivity are described. Useful range is 10-2,000 mugf with sufficiently low drift to allow recordings of contractile responses of isolated single smooth muscle cells. Comparison with several previous designs is presented.