Asymmetric cross-strain protection for amphibians exposed to a fungal-metabolite prophylactic treatment.

Chytridiomycosis, an infectious disease of amphibians caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), poses an imminent conservation threat. The global spread of Bd has led to mass mortality events in many amphibian species, resulting in at least 90 species' extinctions to date. Exposure to Bd metabolites (i.e. non-infectious antigenic chemicals released by Bd) partially protects frogs during subsequent challenges with live Bd, suggesting its use as a prophylactic treatment and potential vaccine. However, we do not know whether Bd metabolite exposure protects against strains beyond the one used for treatment. To address this knowledge gap, we conducted a 3 × 2 experiment where we exposed adult Cuban treefrogs, Osteopilus septentrionalis, to one of three treatments (Bd metabolites from California-isolated strain JEL-270, Panamá-isolated strain JEL-419, or an artificial spring water control) and then challenged individuals with live Bd from either strain. We found that exposure to Bd metabolites from the California-isolated strain significantly reduced Bd loads of frogs challenged with the live Panamá-isolated strain, but no other treatments were found to confer protective effects. These findings demonstrate asymmetric cross-protection of a Bd metabolite prophylaxis and suggest that work investigating multiple, diverse strains is urgently needed.

Biomechanics of the movable pretarsal adhesive organ in ants and bees.

Hymenoptera attach to smooth surfaces with a flexible pad, the arolium, between the claws. Here we investigate its movement in Asian weaver ants (Oecophylla smaragdina) and honeybees (Apis mellifera). When ants run upside down on a smooth surface, the arolium is unfolded and folded back with each step. Its extension is strictly coupled with the retraction of the claws. Experimental pull on the claw-flexor tendon revealed that the claw-flexor muscle not only retracts the claws, but also moves the arolium. The elicited arolium movement comprises (i) about a 90 degrees rotation (extension) mediated by the interaction of the two rigid pretarsal sclerites arcus and manubrium and (ii) a lateral expansion and increase in volume. In severed legs of O. smaragdina ants, an increase in hemolymph pressure of 15 kPa was sufficient to inflate the arolium to its full size. Apart from being actively extended, an arolium in contact also can unfold passively when the leg is subject to a pull toward the body. We propose a combined mechanical-hydraulic model for arolium movement: (i) the arolium is engaged by the action of the unguitractor, which mechanically extends the arolium; (ii) compression of the arolium gland reservoir pumps liquid into the arolium; (iii) arolia partly in contact with the surface are unfolded passively when the legs are pulled toward the body; and (iv) the arolium deflates and moves back to its default position by elastic recoil of the cuticle.

The threshold trip duration for which recovery is no longer possible is associated with strength and reaction time.

The recovery of young adults from trips of increasing severity was studied. Our null hypothesis was that lower extremity strength, and reaction time, step time, step distance and step velocity measured in a volitional stepping task would not explain a significant portion of the variance in the magnitude of the threshold trip duration for which recovery is no longer possible. Ten males and 11 females (average age 26.8 and 28.4 years old, respectively) were subjected to trips of increasing duration until recovery was no longer possible with a single step. The average threshold trip duration for which subjects were no longer able to recover with a single step was 681+/-169ms. The threshold trip duration significantly increased as lower extremity strength increased and volitional reaction time decreased (multiple stepwise linear regression: R(2)=0.52, p=0.001). The other volitional step parameters and the subject characteristics were not significantly associated with the magnitude of the threshold trip duration. These results suggest that some trip-related falls may be due to slower reaction times and/or reduced lower extremity strengths.

Disturbance type and gait speed affect fall direction and impact location.

Since falling to the side and impacting on or near the hip increase hip fracture risk, we examined the fall direction and pelvis impact location resulting from four disturbances (faint, slip, step down, trip) at three gait speeds (fast, normal, slow) in 14 young adults instructed not to attempt recovery. We hypothesized that certain disturbances such as faints and slips and slow walking speed were more likely to result in an impact on the hip. For each trial, the fall direction, impact location and pelvis impact velocity were measured. The results showed that both disturbance type and gait speed significantly affected fall direction and impact location (analysis of covariance with repeated measures, p< or =0.0001) with a significant interaction (p<0.05). Trips and steps down usually resulted in forward falls, with frontal impacts regardless of gait speed. At fast gait speed, slips and faints also usually resulted in forward falls, with frontal impacts. As gait speed decreased, however, slips usually resulted in sideways or backward falls, with impact on the hip or buttocks, and faints resulted in a greater number of sideways falls, with impact near the hip. Therefore, compared to other disturbances and gait speeds, slipping or fainting while walking slowly was more likely to result in an impact on the hip, suggesting a greater risk for hip fracture. Furthermore, 56% of the impact velocities generated were within one standard deviation of the estimate of the mean impact velocity needed to fracture an elderly femur.

Recurrent laryngeal nerve transposition in guinea pigs.

Improved control of prosthetic voice aids for laryngectomees might be possible to obtain with residual laryngeal motor nerve signals. We were able to recover motor signals from the recurrent laryngeal nerve (RLN) by transposing it into the ipsilateral denervated sternohyoid muscle (SH) in 8 guinea pigs. Reinnervation was monitored by electromyographic recordings from surface and intramuscular needle electrodes in awake animals. Within 4 to 14 weeks after surgery, all animals demonstrated laryngeal-like motor activity in the reinnervated SH, including activity during respiration, sniffing, swallowing, and/or vocalizing. After 3 to 6 months, the animals were reanesthetized, and nerve stimulation and section experiments confirmed the RLN as the source of reinnervation in all cases. In several animals, activity of the RLN-innervated SH was demonstrated to be correlated with that of contralateral laryngeal muscles. Histochemical analysis of the SH indicated a unilateral transformation from mostly fatigable to mostly fatigue-resistant fiber types ipsilateral to the RLN transposition, a phenotype more typical of laryngeal muscles. Thus, RLN transposition at the time of laryngectomy may be a method for salvaging laryngeal control signals that could be used to control prosthetic voice devices.

Results from demineralized bone creep tests suggest that collagen is responsible for the creep behavior of bone.

Cortical and trabecular bone have similar creep behaviors that have been described by power-law relationships, with increases in temperature resulting in faster creep damage accumulation according to the usual Arrhenius (damage rate approximately exp (-Temp.-1)) relationship. In an attempt to determine the phase (collagen or hydroxyapatite) responsible for these similar creep behaviors, we investigated the creep behavior of demineralized cortical bone, recognizing that the organic (i.e., demineralized) matrix of both cortical and trabecular bone is composed primarily of type I collagen. We prepared waisted specimens of bovine cortical bone and demineralized them according to an established protocol. Creep tests were conducted on 18 specimens at various normalized stresses sigma/E0 and temperatures using a noninvasive optical technique to measure strain. Denaturation tests were also conducted to investigate the effect of temperature on the structure of demineralized bone. The creep behavior was characterized by the three classical stages of decreasing, constant, and increasing creep rates at all applied normalized stresses and temperatures. Strong (r2 > 0.79) and significant (p < 0.01) power-law relationships were found between the damage accumulation parameters (steady-state creep rate d epsilon/dt and time-to-failure tf) and the applied normalized stress sigma/E0. The creep behavior was also a function of temperature, following an Arrhenius creep relationship with an activation energy Q = 113 kJ/mole, within the range of activation energies for cortical (44 kJ/mole) and trabecular (136 kJ/mole) bone. The denaturation behavior was characterized by axial shrinkage at temperatures greater than approximately 56 degrees C. Lastly an analysis of covariance (ANCOVA) of our demineralized cortical bone regressions with those found in the literature for cortical and trabecular bone indicates than all three tissues creep with the same power-law exponents. These similar creep activation energies and exponents suggest that collagen is the phase responsible for creep in bone.

An engineering model of dynamic cardiomyoplasty. I.

Dynamic cardiomyoplasty (DCM) is an emerging surgical procedure for heart failure in which the patient's latissimus dorsi (LD) muscle is wrapped around the heart and stimulated to contract in synchrony with the heartbeat as a cardiac assist measure. A 6 week training protocol of progressive electrical stimulation renders the normally fatigueable skeletal muscle fatigue-resistant and suitable for chronic stimulation. To date, over 500 procedures have been performed in worldwide clinical trials. Investigators typically report symptomatic improvement and modest hemodynamic improvement in patients. Controversy exists regarding the exact mechanism of DCM. To test the hypothesis that DCM augments cardiac stroke volume through improvement in systolic function, we formulated an engineering model of dynamic cardiomyoplasty to predict stroke volume. The heart and the LD were modeled as nested (series) elastance chambers, and the vasculature was represented by a two-element Windkessel model. Using five healthy goats, we verified model predictions of stroke volume for both stimulator ON beats (y = 1.00x-0.08, r = 0.87, p < 0.0001) and OFF beats (y = 1.01x+1.06, r = 0.91, p < 0.0001), where x and y are the measured and predicted stroke volumes, respectively. The model confirms that using untrained latissimus dorsi applied to the normal myocardium produces only moderate increases in stroke volume and suggests that future research should focus on increasing LD strength after training.

An engineering model of dynamic cardiomyoplasty. II. Clinical applications.

Previously, a modification to the Sunagawa engineering model for the isolated left ventricle and arterial system was proposed and validated for dynamic cardiomyoplasty in an acute goat preparation. To test the hypothesis that this model may be applied to the clinical scenario in cardiomyoplasty patients, we predicted human stroke volume using the model with human clinical data from the literature. Predicted stroke volume correlated well with published stroke volume in patients who have had the dynamic cardiomyoplasty procedure. These results suggest that the modest hemodynamic improvement commonly reported after the procedure is performed may be due to diminished latissimus dorsi strength after transformation. The validity of both the original Sunagawa model and the previously proposed modification for dynamic cardiomyoplasty is further supported with these results. A nomogram methodology for predicting stroke volume after dynamic cardiomyoplasty for any particular patient is presented.

Creep contributes to the fatigue behavior of bovine trabecular bone.

Repetitive, low-intensity loading from normal daily activities can generate fatigue damage in trabecular bone, a potential cause of spontaneous fractures of the hip and spine. Finite element models of trabecular bone (Guo et al., 1994) suggest that both creep and slow crack growth contribute to fatigue failure. In an effort to characterize these damage mechanisms experimentally, we conducted fatigue and creep tests on 85 waisted specimens of trabecular bone obtained from 76 bovine proximal tibiae. All applied stresses were normalized by the previously measured specimen modulus. Fatigue tests were conducted at room temperature; creep tests were conducted at 4, 15, 25, 37, 45, and 53 degrees C in a custom-designed apparatus. The fatigue behavior was characterized by decreasing modulus and increasing hysteresis prior to failure. The hysteresis loops progressively displaced along the strain axis, indicating that creep was also involved in the fatigue process. The creep behavior was characterized by the three classical stages of decreasing, constant, and increasing creep rates. Strong and highly significant power-law relationships were found between cycles-to-failure, time-to-failure, steady-state creep rate, and the applied loads. Creep analyses of the fatigue hysteresis loops also generated strong and highly significant power law relationships for time-to-failure and steady-state creep rate. Lastly, the products of creep rate and time-to-failure were constant for both the fatigue and creep tests and were equal to the measured failure strains, suggesting that creep plays a fundamental role in the fatigue behavior of trabecular bone. Additional analysis of the fatigue strain data suggests that creep and slow crack growth are not separate processes that dominate at high and low loads, respectively, but are present throughout all stages of fatigue.

Predicting the impact response of a nonlinear single-degree-of-freedom shock-absorbing system from the measured step response.

We measured the step response of a surrogate human pelvis/impact pendulum system at force levels between 50 and 350 N. We then fit measured response curves with four different single-degree-of-freedom models, each possessing a single mass, and supports of the following types: standard linear solid, Voigt, Maxwell, and spring. We then compared model predictions of impact force during high-energy collisions (pendulum impact velocity ranging from 1.16 to 2.58 m/s) to force traces from actual impacts to the surrogate pelvis. We found that measured peak impact forces, which ranged from 1700 to 5600 N, were best predicted by the mass-spring, Maxwell, and standard linear solid models, each of which had average errors less than 3 percent. Reduced accuracy was observed for the commonly used Voigt model, which exhibited an average error of 10 percent. Considering that the surrogate pelvis system used in this study exhibited nonlinear stiffness and damping similar to that observed in simulated fall impact experiments with human volunteers, our results suggest that these simple models allow-impact forces in potentially traumatic falls to be predicted to within reasonable accuracy from the measured response of the body in safe, simulated collisions.

Distribution of contact force during impact to the hip.

Hip fracture is a common, costly, and debilitating injury occurring primarily in the elderly. Commonly viewed as a consequence of osteoporosis, it is less often appreciated that > 90% of hip fractures are caused by falls, and that fracture risk is governed not only by bone fragility, but also by the mechanics of the fall. Our goal is to develop experimental and mathematical models that describe the dynamics of impact to the hip during a fall, and explain the factors that influence hip contact force and fracture risk during a fall. In the current study, we used "pelvis release experiments" to test the hypothesis that, during a fall on the hip, two pathways exist for energy absorption and force generation at contact: a compressive load path directly in line with the hip, and a flexural load path due to deformation of muscles and ligaments peripheral to the hip. We also explored whether trunk position or muscle contraction influence the body's impact response and the magnitude of force applied to the hip during a fall. Our results suggest that only 15% of total impact force is distributed to structures peripheral to the hip and that peak forces directly applied to the hip are well within the fracture range of the elderly femur. We also found that impacting with the trunk upright significantly increases peak force applied to the hip, whereas muscle contraction has little effect. These results should have application in the development of fracture risk indices that incorporate both fall severity and bone fragility, and the design of interventions such as hip pads and energy-absorbing floors that attempt to reduce fracture risk by decreasing in-line stiffness and hip contact force during a fall.

The tensile behavior of demineralized bovine cortical bone.

Bone is frequently modeled as a two-phase composite of hydroxyapatite mineral crystals dispersed throughout an organic collagen matrix. However, because of the numerous limitations (e.g. small sample size, poor strain measuring techniques, rapid demineralization with acids) of previous mechanical tests of bone with its hydroxyapatite chemically removed, we have determined new, accurate data on the material properties of the demineralized bone matrix for use in these composite models. We performed tensile tests on waisted specimens of demineralized bovine cortical bone from six humeral diaphyses. Specimens were demineralized over 14 days with a 0.5 M disodium EDTA solution that was replaced daily. Atomic absorption spectrophotometry was used to track the demineralization process and to determine the effectiveness of our demineralization protocol. Mechanical tests were performed at room temperature under displacement control at an approximate strain rate of 0.5% per s. We imposed nine preconditioning cycles before a final ramp to failure, and measured gauge length displacements using a non-invasive optical technique. The resulting stress-strain curves were similar to the tensile behavior observed in mechanical tests of other collagenous tissues, exhibiting an initial non-linear 'toe' region, followed by a linear region and subsequent failure without evidence of yielding. We found an average modulus, ultimate stress, and ultimate strain of 613 MPa (S.D. = 113 MPa), 61.5 MPa (S.D. = 13.1 MPa), and 12.3% (S.D. = 0.5%), respectively. Our average modulus is approximately half the value frequently used in current composite bone analyses. These data should also have clinical relevance because the early strength of healing fractured bone depends largely on the material properties of the collagen matrix.

Decreased myocardial oxygen consumption indices in dynamic cardiomyoplasty.

BACKGROUND: To investigate the theory of decreased myocardial oxygen consumption (MVo2) in dynamic cardiomyoplasty (DCM), previous studies have calculated indices of MVo2 in DCM. These previous studies, however, used left ventricular pressure in formulas that assumed the assumed the heart to be in its native state, with the reference pressure at the epicardium assumed to be atmospheric. In DCM, however, the reference pressure at the epicardium is no longer atmospheric but rather is the compressive pressure generated by the latissimus dorsi (LD). We therefore used the transmural myocardial pressure, Pt, to calculate indices of MVo2 in DCM. METHODS AND RESULTS: A half-ellipsoidal, fluid-filled balloon was interposed between the LD and myocardium in a balloon-mediated cardiomyoplasty procedure in five goats. With commonly used LD stimulation parameters, Pt was calculated as left ventricular pressure minus balloon luminal pressure. Using Pt, the transmural tension time index (TtTI) and transmural pressure volume area (PtVA) were calculated. In another series of four goats, LD stimulation parameters were optimized and the TtTI and PtVA recalculated. With standard LD stimulation parameters, the TtTI decreased by 48%, from 15.8 to 8.2 mm Hg.s, and the PtVA by 21%, from 775 to 612 mm Hg.mL, as the LD was stimulated to contract. When the optimized parameters were used, the TtTI decreased by 45%, from 11.2 to 6.2 mm Hg.s, and the PtVA by 33%, from 1984 to 1371 mm Hg.mL. CONCLUSIONS: Our results suggest that DCM with a fluid-filled balloon decreases MVo2 as the LD contracts and that LD stimulation parameters have a determining effect on this benefit.

Hip impact velocities and body configurations for voluntary falls from standing height.

Fall dynamics have largely been ignored in the study of hip fracture etiology and in the development of hip fracture prevention strategies. In this study, we asked the following questions: (1) What are the ranges of hip impact velocities associated with a sideways fall from standing height? (2) What are the ranges of body configurations at impact? and (3) How do protective reflexes such as muscle activation or using an outstretched hand influence fall kinematics? To answer these questions, we recruited six young healthy athletes who performed voluntary sideways falls on a thick foam mattress. Several categories of falls were investigated: (a) muscle-active vs muscle-relaxed falls; (b) falls from a standing position or from walking; and (c) falls in which an outstretched arm was used to break the fall. Each fall was videotaped at 60 frames s(-1). Fall kinematics parameters were obtained by digitizing markers placed on anatomical points of interest. The mean value for vertical hip impact velocity was 2.75 ms(-1) (+ or - 0.42 ms(-1) [S.D.]). The mean value for trunk angle (the angle between the trunk and the vertical) was 17.3 degrees (+ or - 11.5 degrees [S.D.]). We found a 38 percent reduction in the trunk angle at impact, and a 7 percent reduction in hip impact velocity for relaxed vs muscle-active falls. Finally, regarding the. falls in which an outstretched arm was used, only two out of the six subjects were able to break the fall with their arm or hand. For the remaining subjects hip impact occurred first, followed by contact of the arm or hand.

Etiology and prevention of age-related hip fractures.

Falls and fall-related injuries are among the most serious and common medical problems experienced by the elderly. Hip fracture, one of the most severe consequences of falling in the elderly, occurs in only about 1% of falls. Despite this, hip fracture accounts for a large share of the disability, death, and medical costs associated with falls. As measured by their frequency, influence on quality of life, and economic cost, hip fractures are a public health problem of crisis proportions. Without successful international initiatives aimed at reducing the incidence of falls and hip fractures, the implications for allocations of health resources in this and the next century are staggering. Identifying those at risk for harmful falls requires an understanding of what kinds of falls result in injury and fracture. In elderly persons who fall, in most of whom hip bone mineral density is already several standard deviations below peak values, fall severity (as reflected in falling to the side and impacting the hip) and body habitus are important risk factors for hip fracture and touch on a domain of risk entirely missed by knowledge of bone mineral density. These findings clearly suggest that factors related to both loading and bone fragility play important roles in the etiology of hip fracture. We provide a strategy, based on engineering approaches to fracture risk prediction, for determining the relative etiologic importance of loading and bone fragility and to summarize some of what is known about both sets of factors. We define a factor of risk, phi, as the ratio of the loads applied to the hip divided by the loads necessary to cause fracture and summarize available data on the numerator and the denominator of phi. We then provide an overview of the complex interplay between the risks associated with the initiation, descent, and impact phases of a fall, thereby suggesting an organized approach for evaluating intervention efforts being used to prevent hip fractures. The findings emphasize the continuing need for combined intervention strategies that focus on fall prevention, reductions in fall severity, and maintaining or increasing femoral bone mass and strength, either through targeted exercise programs, optimal nutrition (Ca, Vitamin D), and/or in the use of osteodynamic agents. By developing and refining the factor of risk, a property that captures both the contributions of bone density and the confounding influences of body habitus and fall severity, we believe these intervention strategies can be targeted more appropriately.

New technique measures decreased transmural myocardial pressure in cardiomyoplasty.

BACKGROUND: We introduce the use of a fluid-filled balloon, interposed between myocardium and latissimus dorsi (LD), as a new technique to measure transmural myocardial pressure in an acute goat model of dynamic cardiomyoplasty. METHODS: A half-ellipsoidal balloon, composed of polychloryl vinyl layers, was sutured to the atrioventricular groove in 5 goats, thereby completely enveloping both ventricles. Left LD dynamic cardiomyoplasty was then performed, anchoring the LD to the felt sewing skirt of the balloon so that the LD completely covered the balloon. Left ventricular pressure and balloon pressure were measured with the stimulator in the 1:2 mode as balloon volume was varied. RESULTS: Average transmural myocardial pressure, defined as left ventricular pressure minus balloon pressure, decreased from 34.4 mm Hg to 15.6 mm Hg during stimulator-on beats (p < 0.05). CONCLUSION: These results support the conclusion that dynamic cardiomyoplasty unloads the left ventricle by decreasing wall stress. Furthermore, transmural myocardial pressure decreased more when balloon volume was increased, implying that the LD sarcomere length has an effect on wall stress. A balloon may therefore allow optimization of LD sarcomere length and thus assisted cardiac performance.

Energy-shunting hip padding system attenuates femoral impact force in a simulated fall.

Recent studies suggest that hip padding systems reduce the incidence of hip fractures during falls. However, no data exist on the force attenuating capacity of hip pads under realistic fall impact conditions, and thus it is difficult to compare the protective merit of various pad designs. Our goal is to design a comfortable hip padding system which reduces femoral impact force in a fall below the mean force required to fracture the elderly cadaveric femur. In pursuit of this objective, we designed and constructed a hip pad testing system consisting of an impact pendulum and surrogate human pelvis. We then developed a hip pad containing a shear-thickening material which allows for shunting of the impact energy away from the femur and into the surrounding soft tissue. Finally, we conducted experiments to assess whether the surrogate pelvis accurately represents the impact behavior of the human female pelvis in a fall, and to determine whether our energy-shunting pad attenuates femoral impact force in a fall more effectively than seven available padding systems. We found the surrogate pelvis accurately represented the human female pelvis in regional variation in soft tissue stiffness, total effective stiffness and damping, and impact force attenuation provided by trochanteric soft tissues. We also found that our padding system attenuated femoral impact force by 65 percent, thereby providing two times the force attenuation of the next best system. Moreover, the energy-shunting pad was the only system capable of lowering femoral impact force well below the mean force required to fracture the elderly femur in a fall loading configuration. These results suggest that the force attenuating potential of hip pads which focus on shunting energy away from the femur is superior to those which rely on absorbing energy in the pad material. While these in-vitro results are encouraging, carefully designed prospective clinical trials will be necessary to determine the efficacy of these approaches to hip fracture prevention.

Force attenuation in trochanteric soft tissues during impact from a fall.

The risk for hip fracture from a fall is known to decrease with increased body mass index (weight/height2), a relative measure of obesity. To explore whether this reduced risk is due to the protective effect of increased soft-tissue cushioning in obese individuals, we used an impact pendulum and surrogate human pelvis to conduct simulated fall impact experiments on trochanteric soft tissues harvested from the cadavers of nine elderly individuals. For each impact, the total applied energy was 140 J. Peak forces ranged from 4,050 to 6,420 N, and tissue energy absorption ranged from 8.4 to 81.6 J. Increased tissue thickness correlated strongly with both decreased peak force (r2 = 0.91) and increased tissue energy absorption (r2 = 0.76). However, peak forces in all cases were within 1 SD of previously reported average fracture forces for elderly cadaveric femora. This suggests that force attenuation in trochanteric soft tissues alone is insufficient to prevent hip fracture in falls in which an elderly person lands directly on the hip. In such falls, additional energy-absorbing mechanisms, such as breaking the fall with an outstretched hand and eccentric contraction of the quadriceps during descent, are likely to be involved if fracture does not occur.

The force-velocity curve in passive whole muscle is asymmetric about zero velocity.

The force-velocity property of passive muscle was investigated to determine if a discontinuity of slope occurred at zero velocity. Isolated, unstimulated whole frog sartorius muscles were subjected to constant-velocity stretches and releases using a servo-controlled lever. The force due to damping (delta T) was calculated by subtracting the tension measured at a very low speed (1.0 mm s-1) from the tension measured at the same length while the muscle was shortening or lengthening at a particular test speed. The experiments were performed over a range of speeds at each of several lengths and at two temperatures. For comparison, the same experiments were performed using a strip of pure latex rubber and a steel spring. Curves showing the magnitude of delta T vs velocity were nearly symmetric about the zero-velocity axis for the steel spring and the rubber strip, but were markedly asymmetric for passive muscle, showing a positive delta T for lengthening at all speeds that was between four and 11 times the negative delta T for shortening at the same speed, depending on the temperature and initial stretch length. The force due to damping at a given speed increased with extension above the rest length in passive muscle but decreased with increasing length in experiments using the latex strip. Predictions obtained from a mathematical model based on a damping element in series with a lightly damped spring were fitted to the experimental measurements of delta T vs velocity. The damping parameter provisionally representing interfilamentary sliding was between six and 12 times larger for lengthening than for shortening.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynamic models for sideways falls from standing height.

Despite our growing understanding of the importance of fall mechanics in the etiology of hip fracture, previous studies have largely ignored the kinematics and dynamics of falls from standing height. Beginning from basic principles, we estimated peak impact force on the greater trochanter in a sideways fall from standing height. Using a one degree-of-freedom impact model, this force is determined by the impact velocity of the hip, the effective mass of that part of the body that is moving prior to impact, and the overall stiffness of the soft tissue overlying the hip. To determine impact velocity and effective mass, three different paradigms of increasing complexity were used: 1) a falling point mass or a rigid bar pivoting at its base; 2) two-link models consisting of a leg segment and a torso; and 3) three-link models including a knee. The total mechanical energy of each model before falling was equated to the total mechanical energy just prior to impact in order to estimate the hip impact velocity. In addition, the configuration of the model just before impact was used to estimate the effective mass. Our model predictions were compared with the results of an earlier experimental study with young subjects falling on a 10-inch thick mattress. Values from literature were used to estimate the soft tissue stiffness. For the models, predicted values for hip impact velocity and effective mass ranged from 2.47 to 4.34 m/s and from 15.9 to 70.0 kg, respectively. Predicted values for the peak force applied to the greater trochanter ranged from 2.90k to 9.99k N. Based on comparisons to the experimental falls, impact velocity and impact force were best predicted by a simple two-link model with the trunk at 45 degrees to the vertical at impact. A three-link model with a quadratic spring incorporated in the knee of the model was the best predictor of effective mass. Using our most accurate model, the peak impact force was 2.90k N for a 5th percentile female and 4.26k N for a 95th percentile female, thereby confirming the widely held perception that "the bigger they are, the harder they fall".