Altered control of submaximal bite force during bruxism in humans

The control of bite force during varying submaximal loads was examined in patients suffering from bruxism compared to healthy humans not showing these symptoms. The subjects raised a bar (preload) with their incisor teeth and held it between their upper and lower incisors using the minimal bite force required to keep the bar in a horizontal position. Further loading was added during the preload phase. A sham load was also used. Depending on the session, the teeth were loaded by the experimenter or the subject and in one session the subject did not see the load (no visual feedback). The bite force was measured continuously using a calibrated force transducer. In all the subjects, the bite force increased with increasing load. Following the addition of the load, the level of the tonic bite force was reached rapidly with no marked overshoot. The patients with bruxism used significantly higher bite forces to hold the submaximal loads compared to the control subjects. In the control subjects, the holding forces for each submaximal load were identical in the men and the women and were independent of subject maximal bite force. Sham loading evoked no marked responses in biting force. Whether the subject or the experimenter added the load or whether the subject had visual feedback or not were not significant factors in determining the level of bite force. The results indicated that the patients with bruxism used excessively large biting forces for each given submaximal load. This study showed no evidence that the inappropriate control of bite force by patients with bruxism was due to an abnormality in the higher cortical circuits that regulates the function of trigeminal motoneurons in the brainstem. This was shown by a lack of abnormality in coordination of voluntary hand movement with biting force, a lack of abnormal anticipation response to a sham load and a lack of any effect of visual feedback. The results were in line with the hypothesis that afferent input from oral (periodontal or masticatory muscle) tissues does not provide an appropriate control of motor command in bruxism.     

In vivo 3-dimensional measurement of the force exerted on a tooth during clenching

We have developed a three-dimensional (3D) force-measuring device for teeth and used it to measure functional forces in vivo. It comprises an inner part forming a metal core (abutment), a 3D piezoelectric force transducer, and an outer part forming a metal crown, all joined together with a steel screw. The force transducer can measure +/- 500 N along the z-axis and +/- 150 N along the x- and y-axes. We evaluated the relationship between output and load and the effects of hysteresis and temperature on the output. The transducer had high linearity (r>0.9999), low hysteresis (1.7% at maximum), and high thermal stability (0.05% per degree) along each axis. The measuring device was mounted on the maxillary left second molar of a healthy male subject; the tooth had been endodontically treated (neurovascular bundle removed) and prepared for metal abutment and a crown. The 3D load calculated from the outputs of the transducer was expressed as a vector of the coordinates based on the Frankfort horizontal (x-y) and sagittal (y-z) planes. The force measured during maximum voluntary clenching was about 170 N; the force vector was directed from the crown to the root medially at an angle of about 10 degrees from the y-z plane and posteriorly at an angle of about 3 degrees from the x-z plane. This transducer will enable measurement of forces applied to different types of prosthetic appliances and has the potential to provide important basic in vivo data for analysis using computer simulation.     

Peak power, force, and velocity during jump squats in professional rugby players

Training at the optimal load for peak power output (PPO) has been proposed as a method for enhancing power output, although others argue that the force, velocity, and PPO are of interest across the full range of loads. The aim of this study was to examine the influence of load on PPO, peak barbell velocity (BV), and peak vertical ground reaction force (VGRF) during the jump squat (JS) in a group of professional rugby players. Eleven male professional rugby players (age, 26 ± 3 years; height, 1.83 ± 6.12 m; mass, 97.3 ± 11.6 kg) performed loaded JS at loads of 20-100% of 1 repetition maximum (1RM) JS. A force plate and linear position transducer, with a mechanical braking unit, were used to measure PPO, VGRF, and BV. Load had very large significant effects on PPO (p < 0.001, partial η2 = 0.915); peak VGRF (p < 0.001, partial η2 = 0.854); and peak BV (p < 0.001, partial η2 = 0.973). The PPO and peak BV were the highest at 20% 1RM, though PPO was not significantly greater than that at 30% 1RM. The peak VGRF was significantly greater at 1RM than all other loads, with no significant difference between 20 and 60% 1RM. In resistance trained professional rugby players, the optimal load for eliciting PPO during the loaded JS in the range measured occurs at 20% 1RM JS, with decreases in PPO and BV, and increases in VGRF, as the load is increased, although greater PPO likely occurs without any additional load.     

Factors influencing the output of an implantable force transducer

The objective of this study was to evaluate the performance of the Arthroscopically Implantable Force Probe (AIFP; MicroStrain, Burlington VT) for measuring force in a patellar tendon graft. Transducer drift, reproducibility of output due to the number of loading cycles and device location, and sensitivity to the tendon cross-sectional area were investigated. The AIFP was initialized, and then implanted into five human patellar tendon grafts three times; twice within the same location and once in a different location. The tendons were cyclically loaded in uniaxial tension for 500 cycles in each insertion site. The AIFP was then removed from the tendon and the baseline output was remeasured. It was determined that transducer drift was negligible. The relationship between the tensile load applied to the graft and AIFP output was quadratic and specimen dependent. The cyclic load response of the tendon-AIFP interface demonstrated a 24.9% decrease over the first 20 loading cycles, and subsequent cycling yielded relatively reproducible output. The output of the transducer varied when it was removed from the tendon and then reimplanted in the same location (range 3.7-109. 4% error), as well as in the second location (range 1.5-202.8% error). No correlation was observed between the cross-sectional area of the tendon and transducer output. This study concludes that implantable force probes should be used with caution and calibrated without removing the transducer from the graft.     

Buckle muscle tension transducer: what does it measure?

The question is considered whether the strain of a buckle transducer attached to a muscle tendon provides a proportional measure of the force of the muscle acting directly on that tendon. It is shown that if muscle contains elastic and/or viscous elements in parallel with the force generator, the transducer strain may, under certain conditions, reflect other applied forces acting on the load (limb) in addition to the muscle force.     

Validation of power measurement techniques in dynamic lower body resistance exercises

The objective of this study was to investigate the validity of power measurement techniques utilizing various kinematic and kinetic devices during the jump squat (JS), squat (S) and power clean (PC). Ten Division I male athletes were assessed for power output across various intensities: 0, 12, 27, 42, 56, 71, and 85% of one repetition maximum strength (1RM) in the JS and S and 30, 40, 50, 60, 70, 80, and 90% of 1RM in the PC. During the execution of each lift, six different data collection systems were utilized; (1) one linear position transducer (1-LPT); (2) one linear position transducer with the system mass representing the force (1-LPT+MASS); (3) two linear position transducers (2-LPT); (4) the force plate (FP); (5) one linear position transducer and a force plate (1-LPT+FP); (6) two linear position transducers and a force place (2-LPT+FP). Kinetic and kinematic variables calculated using the six methodologies were compared. Vertical power, force, and velocity differed significantly between 2-LPT+FP and 1-LPT, 1-LPT+MASS, 2-LPT, and FP methodologies across various intensities throughout the JS, S, and PC. These differences affected the load-power relationship and resulted in the transfer of the optimal load to a number of different intensities. This examination clearly indicates that data collection and analysis procedures influence the power output calculated as well as the load-power relationship of dynamic lower body movements.     

In vivo tendon force measurement of 2-week duration in sheep

Tendon tension in vivo may be determined indirectly by measuring intratendinous pressure, by using a buckle transducer or by measuring the tendon strain. All of these methods require appropriate calibration, which is highly dependent on various variables. To measure the tendon load in vivo during a period of 2 weeks in sheep, a measurement technique has been developed using a force sensor interposed serially between the humeral head and the tendon end. Within a supporting frame, a flexion-sensitive force transducer is subjected to three-point bending stress. The load is transmitted by sutures from the tendon end through a hole in the sensor frame, orthogonal to the force transducer. In this configuration, the sensor measures the tensile force acting on the tendon, largely independent of the loading direction. The sensor was screwed to the humeral head and connected to the tendon end which was previously released from its insertion site along with a bone chip, using sutures. Connecting wires passed subcutaneously to a skin outlet about 30 cm away from the transducer. The sensor output was linear to the measured load up to 300 N, with maximum hysteresis of 18% full scale. All sensors worked in vivo without drift over a period of up to 14 days with no change in the calibration data. Forces up to 310 N have been recorded in vivo with daily tension measurements. This study shows that serial tendon tension measurement is feasible and allows for reliable, repeatable recording of the absolute tendon tension at the expense of tendon integrity.     

Loaded shortening and power output in cardiac myocytes are dependent on myosin heavy chain isoform expression

The purpose of this study was to examine the role of myosin heavy chain (MHC) in determining loaded shortening velocities and power output in cardiac myocytes. Cardiac myocytes were obtained from euthyroid rats that expressed alpha-MHC or from thyroidectomized rats that expressed beta-MHC. Skinned myocytes were attached to a force transducer and a position motor, and isotonic shortening velocities were measured at several loads during steady-state maximal Ca(2+) activation (P(pCa4.5)). MHC expression was determined after mechanical measurements using SDS-PAGE. Both alpha-MHC and beta-MHC myocytes generated similar maximal Ca(2+)-activated force, but alpha-MHC myocytes shortened faster at all loads and generated approximately 170% greater peak normalized power output. Additionally, the curvature of force-velocity relationships was less, and therefore the relative load optimal for power output (F(opt)) was greater in alpha-MHC myocytes. F(opt) was 0.31 +/- 0.03 P(pCa4.5) and 0.20 +/- 0.06 P(pCa4.5) for alpha-MHC and beta-MHC myocytes, respectively. These results indicate that MHC expression is a primary determinant of the shape of force-velocity relationships, velocity of loaded shortening, and overall power output-generating capacity of individual cardiac myocytes.     

Influence of sensor size on the accuracy of in-vivo ligament and tendon force measurements.

In-vivo tendon forces are commonly measured using transducers, which detect tension in the tendon fibers. A poorly understood source of measurement errors is the difference in stress distribution within the tendon between experimental and transducer calibration conditions. The objective of this study was to investigate this source of error, and to determine whether these errors could be minimized by proper selection of transducer size. The study was conducted using the infrapatellar ligament (patellar tendon) of New Zealand White rabbits. Tendon force was measured with two different size implantable force transducers (IFTs), one Wide and one Narrow, and by a strain gaged load cell in series with the tendon. Tests were conducted at five different loading conditions selected to produce five different stress distributions within the tendon. One loading condition corresponded to a typical post-experiment calibration, and the data from that condition were used to develop a calibration equation for the transducer. The errors that resulted from using this calibration were determined by comparing the tendon force measured by the in-series load cell with the force predicted from the IFT output using the calibration equation. Changes in stress distribution produced measurement errors up to 64 N with the Narrow IFT but only 24 N with the Wide IFT. We found the measurement error was dependent on sensor width. Our results support the hypothesis that measurement errors can be caused by differences in tendon stress distribution between calibration and experimental conditions. We further showed that these errors can be minimized by using an IFT, which samples the tension in a large percentage of the tendon fibers. Information from this study can be used for selection of an appropriately sized implantable force transducer for measuring tendon and ligament force.     

Force, function and mechanical advantage in the chelae of the American lobster Homarus americanus (Decapoda: Crustacea)

The dimorphic chelae from both sexes and a wide size range of American lobsters, Homarus americanus , were studied with respect to allometry, mechanical advantage, closer muscle apodeme area and occlusive surface morphology. The maximum forces produced by the crusher and cutter chelae were estimated by an in vitro and a static in vivo technique. Another in vivo technique, involving strain gauges, was used to measure the forces delivered by crusher chelae. The latter technique gave data on force pulse duration and frequency, and in combination with a video-recording system could be useful for future studies of predation behaviour. The maximum forces generated increased with chela height for both crusher and cutter chelae. A maximum force of 256 Newtons (N) was recorded near the middle of the crusher dactyl from a 172-mm carapace length lobster, by the strain gauge technique. Crusher chelae developed larger maximum forces than cutter chelae of the same height. This was attributable more to the crusher chela's higher mechanical advantage than to its developing higher input forces. The mean mechanical advantage for male crusher (0.33) was significantly higher than that for female crusher chelae (0.29). Male and female cutter chelae had the same mean mechanical advantage values (0.16). Values for maximum stress developed during contraction in both the crusher and cutter chela closer muscles decreased with chela size. The morphology of the chelae correlated to the forces produced and predation behaviour.     

Muscular force production after concentric contraction

The steady-state force following active shortening does not reach the maximum isometric force associated with the final length. Isolated extensor digitorum longus and soleus muscles from mice (NMRI strain) were used to investigate the force produced by a muscle, and some parameters hypothetically influencing this shortening-induced force depression. The muscles were pre-stimulated at fixed length, shortened and then held isometrically to give maximum post-shortening forces, before de-stimulation. The shortening magnitude was 0.18, 0.36 or 0.72mm (about 2-7% of optimal length), time of shortening was chosen as 0.03, 0.06 and 0.12s, and final length as +0.72, 0 and -0.72mm, related to optimal length. The mechanical work during active shortening was evaluated by integrating the product of force and shortening velocity over the shortening period. The results show a positive correlation between the force depression and the mechanical work, whereas the force depression was not correlated to the velocity of shortening. Depression of the passive force component was also observed following all stimulations. Experiments show that the fully stimulated redevelopment of isometric force following concentric contraction follows a time function similar to the creation of force when isometric muscle is initially stimulated. The conclusion is that the isometric force development after active shortening can be well described by an asymptotic force which is decided by the produced work, and the initial isometric time constant.     

Attachment forces of the hemelytra-locking mechanisms in aquatic bugs (Heteroptera: Belostomatidae)

Combined hemelytra-locking system of Heteroptera, consisting of several locking mechanisms, aids the mechanical stabilisation of the body at rest, resists external loads, and keeps air stored with the option to easily unlock hemelytra prior to flight. The resistance to unlocking of the hemelytron was measured (in mN) with the aid of a load cell force transducer combined with a three-axial micromanipulator. It is shown that macro- and microstructural features of several submechanisms are responsible for their directionality. The highest resistance to unlocking was measured in lateral and dorsal directions. Summarised force of separately measured submechanisms was considerably lower than the force measured in the combined mechanism. Each submechanism is optimised for achieving high resistance to the hemelytron uncoupling in particular direction(s) and to be easily unlocked in another direction. It was demonstrated in the high-speed videorecordings that hemelytra uncoupling is promoted by their short anterior displacement.     

Ambient temperature affects free-flight performance in the fruit fly Drosophila melanogaster

To gain insight into how temperature affects locomotor performance in insects, the limits of flight performance have been estimated in freely flying fruit flies Drosophila melanogaster by determining the maximum load that a fly could carry following take-off. At a low ambient temperature of 15 °C, muscle mechanical power output matches the minimum power requirements for hovering flight. Aerodynamic force production rises with increasing temperature and eventually saturates at a flight force that is roughly equal to 2.1 times the body mass. Within the two-fold range of different body sizes, maximum flight force production during free flight does not decrease with decreasing body size as suggested by standard aerodynamic theories. Estimations of flight muscle mechanical power output yields a peak performance of 110 W kg⁻¹ muscle tissue for short-burst flight that was measured at an ambient temperature of 30 °C. With respect to the uncertainties in estimating muscle mechanical power during free flight, the estimated values are similar to those that were published for flight under tethered flight conditions. Accepted: 5 January 1999     

Influence of loading rate and cable migration on fiberoptic measurement of tendon force

Several investigators have recently used fiberoptic cables to measure tendon forces in situ. The technique may be subject to significant error due to cable migration and differences in the loading rates used for calibration and those experienced during measurement. This in vitro study examined the impact of these potential sources of error on transducer accuracy. A fiberoptic cable was passed perpendicular to the fibers of four Achilles tendons in the mediolateral direction and each specimen was cyclically loaded to 1000 N. The influence of loading rate on transducer output was investigated by comparing results from tests conducted at 20, 200 and 1000 N/s. The effect of cable migration was examined by comparing the outputs obtained after displacing the cable one tendon width medially and laterally along its path in the tendon and then repeating the 200 N/s testing protocol. It was possible to obtain nonlinear specimen-specific relationships between the fiberoptic output and tendon force. Differences in loading rate resulted in root-mean-square (RMS) errors not larger than 17% maximum load. Hysteresis effects caused RMS errors smaller than 5% maximum load. Cable migration errors were less than 27%. The total RMS error due to the combined effects of loading rate difference and cable movement was less than 32%. Fiberoptic measurement of tendon force is attractive due to its low cost, easy implementation and comparable accuracy relative to other implantable force transducers. Although additional factors such as cable placement, edge artifacts due where the transducer exits the skin and non-uniform loading may also influence fiberoptic output, careful control of loading rate and transducer movement during calibration is imperative if maximum accuracy is to be achieved.     

Strength of mid-logarithmic and stationary phase Saccharopolyspora erythraea hyphae during a batch fermentation in defined nitrate-limited medium

A method for measuring mechanical properties of Saccharopolyspora erythraea is reported with data from a batch fermentation. Briefly, hyphae were glued to the end of a tungsten filament mounted horizontally on a sensitive force transducer. Free ends of hyphae were trapped against a flat surface by a second probe. The force transducer and tungsten filament were then moved at a fixed rate, the hypha were strained, and the force resisting motion recorded. From these data the maximum force resisting motion is taken as the force at which breakage occurs. Hyphae from the mid-logarithmic phase of a simple batch fermentation on defined medium were found to have a breaking force of 890 +/- 160 nN (95% confidence), while stationary phase hyphae were weaker at 580 +/- 150 nN. Video recordings of the experiments allowed an approximation of breaking strain, which did not differ significantly between samples at 0.18 +/- 0.03. Electron microscopy was used to measure cell wall thickness, cell diameter, and hence cell wall cross-sectional area. The ultimate tensile strength was estimated to be 24 +/- 3 MPa with no difference between the two samples, the lower breaking force of the stationary phase hyphae being attributed to a thinner cell wall. Assuming a linear relationship between stress and strain, the elastic modulus was estimated to be 140 +/- 30 MPa. These values are comparable with other structural biological materials such as yeast cell walls and collagen.     

The generalized carnivore jaw

A model is presented of the jaw mechanism that relies on the geometrical similarities among mammalian carnivores with carnassial teeth. These similarities, together with estimates of the location of the resultant force of the jaw muscles, allow the model to predict that the mechanical advantage of the jaw lever system is the same in all carnivores with carnassials and, therefore, that the magnitude of the bite force is mainly determined by the absolute amount of jaw musculature.     

Effect of knee musculature on anterior cruciate ligament strain in vivo

Squatting is a commonly prescribed exercise following reconstruction of the anterior cruciate ligament (ACL). The objective of this paper was to measure the in vivo strain patterns of the normal ACL and the load at the knee for the simple squat and for squatting with a “sport cord”. A sport cord is a large elastic rubber tube used for added resistance. Strain patterns were deduced using displacement data from a Hall Effect Strain Transducer (HEST), while joint loads were determined by a mathematical model with inputs from a force plate and electrogoniometers. ACL strain for the free squat in one subject had a maximum of 2% at a knee angle of 10° and was slack for knee angles >17°. In squatting with a sport cord, peak strain was 1% at 10° and was slack at knee angles >14°. Since these peak strains are low, squatting appears to be a safe exercise for conservative rehabilitation of ACL reconstruction patients. In addition, the sport cord is a recommended augmentation to the activity. We believe that the decrease in strain with the sport cord results from added joint stiffness due to greater compressive forces at the tibiofemoral joint. This greater compressive force results from the approximately 10% increase in quadriceps activity. From shear force data predicted by the mathematical model, the maximum anterior drawer force for free squatting (50 N) was considerably less than for sport cord squatting (430 N). Therefore, the value of shear force at the tibiofemoral joint only partially determines the load placed on the ACL.     

A force transducer to measure individual finger loads during activities of daily living.

A new six-degree-of-freedom force transducer has been manufactured, with the sensitivity to measure forces in the range +/-100 N and moments of up to +/-5 Nm. The transducer incorporates two mechanical components: shear forces and bending moments are measured via a strain-gauged tubular section whilst axial forces are transmitted to a cantilevered load cell. Both components are instrumented with 350 ohms strain gauge full bridge circuits and are temperature compensated. After calibration, measurement errors are less than +/-0.3 N for direct forces and +/-0.03 Nm for applied moments. In order to measure sub-maximal finger loads during activities of daily living, the transducer has been incorporated into several housings representing objects in domestic use: a jar, a tap, a key in a lock and a jug kettle.     

Measuring ligament elasticity of the knee joint--elasticity measuring strip and its alternatives]

Those techniques for measuring ligament tension at the knee joint that are most commonly cited and easiest to carry out are discussed. These include four techniques based on the use of strain gauges. Apart from the Omega transducer and the buckle transducer, there is also the tendon force transducer, and the application of strain gauges to the bony ligament insertion sites. Other indirect measuring methods considered are the mercury strain transducer and the Hall effect transducer. The parameter measured with all of these methods is fluctuating current or voltage, which is then correlated with ligament tension. Three direct measurements are also discussed: the separation distances of marked fibres of the ligaments, replacement of fibres by threads, and a load cell/bone plug construction. The measured value is equated with the effective change in ligament length.     

Mechanical properties of rat tail tendon in relation to proximal-distal sampling position and age

The mechanical properties of 3, 15 and 25 month-old rat tail tendons were investigated in relation to proximal-distal sampling location along the fibre length. For the 15 and 25 month-old tendons maximum load as well as collagen content per mm fibre length (unit collagen) increased markedly from the proximal to the distal location. A linear regression analysis of the collagen content and mechanical parameters (maximum load, maximum slope of the load-strain curve and energy absorption) showed that these parameters were linearly correlated to the collagen content. However, normalization of the mechanical parameters with regard to the collagen content did not cancel the dependency of the parameters on proximal-distal sampling location. Normalized load and energy values for the 3 month-old tendons and normalized slope values for the 15 and 25 month-old tendons were found to decrease from proximal to distal location. These findings showed that tail tendons are heterogeneous along their length in respect to mechanical strength. The regression analysis also indicated the existence of an inverse relationship between unit collagen and mechanical quality of the collagen. Alternatively, the mechanical properties of tendon fibres might be influenced by other components than collagen.