Tensegrity and mechanoregulation: from skeleton to cytoskeleton.
(1/95)
OBJECTIVE: To elucidate how mechanical stresses that are applied to the whole organism are transmitted to individual cells and transduced into a biochemical response. DESIGN: In this article, we describe fundamental design principles that are used to stabilize the musculoskeletal system at many different size scales and show that these design features are embodied in one particular form of architecture that is known as tensegrity. RESULTS: Tensegrity structures are characterized by use of continuous tension and local compression; architecture, prestress (internal stress prior to application of external force), and triangulation play the most critical roles in terms of determining their mechanical stability. In living organisms, use of a hierarchy of tensegrity networks both optimizes structural efficiency and provides a mechanism to mechanically couple the parts with the whole: mechanical stresses applied at the macroscale result in structural rearrangements at the cell and molecular level. CONCLUSION: Due to use of tensegrity architecture, mechanical stress is concentrated and focused on signal transducing molecules that physically associate with cell surface molecules that anchor cells to extracellular matrix, such as integrins, and with load-bearing elements within the internal cytoskeleton and nucleus. Mechanochemical transduction may then proceed through local stress-dependent changes in molecular mechanics, thermodynamics, and kinetics within the cell. In this manner, the entire cellular response to stress may be orchestrated and tuned by altering the prestress in the cell, just as changing muscular tone can alter mechanical stability and structural coordination throughout the whole musculoskeletal system. (+info)
Equal opportunities, equal risks? Overuse injuries in female military recruits.
(2/95)
BACKGROUND: The rate of female personnel medically discharged from service in the British Army has been rising steadily since 1992 from around 3 per 1,000 per year to over 35 per 1,000 in 1996, although there has been only a minor increase in medical discharges for males over the same period. This paper examines the increasing rate of medical discharge in young female members of the British Army from an aetiological perspective and reviews the literature to identify risk factors that may be relevant. METHODS: Data from published military medical statistical reports were reviewed and the clinical records of a 10 per cent sample of females medically discharged for relevant conditions were examined. RESULTS: The majority of the excess medical discharges had occurred in females under the age of 22 and were due to musculoskeletal disorders and injuries caused by military training. Data from the clinical records showed that 75.5 per cent (37/49) of those medically discharged for these conditions were recruits. Stress fractures and other overuse syndromes accounted for 70.2 per cent of medical discharges among the recruits in the sample. CONCLUSION: Females undertaking strenuous exercise alongside males are at increased risk of injury. Risk factors include smoking, short stature, restricted dietary intake and menstrual disturbance. Equal opportunities legislation has been interpreted to require identical training for males and females, but some segregation of training may be acceptable provided the outcome of training is no less favourable to either gender, and this may reduce the excess risk of injury to females. (+info)
Jumping in frogs: assessing the design of the skeletal system by anatomically realistic modeling and forward dynamic simulation.
(3/95)
Comparative musculoskeletal modeling represents a tool to understand better how motor system parameters are fine-tuned for specific behaviors. Frog jumping is a behavior in which the physical properties of the body and musculotendon actuators may have evolved specifically to extend the limits of performance. Little is known about how the joints of the frog contribute to and limit jumping performance. To address these issues, we developed a skeletal model of the frog Rana pipiens that contained realistic bones, joints and body-segment properties. We performed forward dynamic simulations of jumping to determine the minimal number of joint degrees of freedom required to produce maximal-distance jumps and to produce jumps of varied take-off angles. The forward dynamics of the models was driven with joint torque patterns determined from inverse dynamic analysis of jumping in experimental frogs. When the joints were constrained to rotate in the extension-flexion plane, the simulations produced short jumps with a fixed angle of take-off. We found that, to produce maximal-distance jumping, the skeletal system of the frog must minimally include a gimbal joint at the hip (three rotational degrees of freedom), a universal Hooke's joint at the knee (two rotational degrees of freedom) and pin joints at the ankle, tarsometatarsal, metatarsophalangeal and iliosacral joints (one rotational degree of freedom). One of the knee degrees of freedom represented a unique kinematic mechanism (internal rotation about the long axis of the tibiofibula) and played a crucial role in bringing the feet under the body so that maximal jump distances could be attained. Finally, the out-of-plane degrees of freedom were found to be essential to enable the frog to alter the angle of take-off and thereby permit flexible neuromotor control. The results of this study form a foundation upon which additional model subsystems (e.g. musculotendon and neural) can be added to test the integrative action of the neuromusculoskeletal system during frog jumping. (+info)
Mutual associations among microstructural, physical and mechanical properties of human cancellous bone.
(4/95)
Previous studies have shown that low-density, rod-like trabecular structures develop in regions of low stress, whereas high-density, plate-like trabecular structures are found in regions of high stress. This phenomenon suggests that there may be a close relationship between the type of trabecular structure and mechanical properties. In this study, 160 cancellous bone specimens were produced from 40 normal human tibiae aged from 16 to 85 years at post-mortem. The specimens underwent micro-CT and the microstructural properties were calculated using unbiased three-dimensional methods. The specimens were tested to determine the mechanical properties and the physical/compositional properties were evaluated. The type of structure together with anisotropy correlated well with Young's modulus of human tibial cancellous bone. The plate-like structure reflected high mechanical stress and the rod-like structure low mechanical stress. There was a strong correlation between the type of trabecular structure and the bone-volume fraction. The most effective microstructural properties for predicting the mechanical properties of cancellous bone seem to differ with age. (+info)
Treatment of IgM antibody associated polyneuropathies using rituximab.
(5/95)
OBJECTIVES: Polyneuropathies with associated serum IgM antibodies are often difficult to treat. Rituximab is a monoclonal antibody directed against the B cell surface membrane marker CD20. Rituximab eliminates B cells from the circulation, and, over time, could reduce cells producing autoantibodies. This study tested the ability of rituximab to produce changes in serum antibody titres, and improvement in strength, in patients with neuromuscular disorders and IgM autoantibodies. METHODS: Over a period of two years, the authors evaluated changes in strength, measured by quantitative dynamometry, and concentrations of several types of serum antibodies in patients with polyneuropathies and serum IgM autoantibodies. Twenty one patients treated with rituximab were compared with 13 untreated controls. RESULTS: Treatment with rituximab was followed by improved strength (an increase of mean (SEM) 23% (2%)of normal levels of strength), a reduction in serum IgM autoantibodies (to 43% (4%) of initial values), and a reduction in total levels of IgM (to 55% (4%) of initial values). There was no change in levels of serum IgG antibodies. There were no major side effects, even though B cells were virtually eliminated from the circulation for periods up to two years. CONCLUSIONS: In patients with IgM autoantibody associated peripheral neuropathies, rituximab treatment is followed by reduced serum concentrations of IgM, but not IgG, antibodies, and by improvement in strength. Additional studies, with placebo controls and blinded outcome measures, are warranted to further test the efficacy of rituximab treatment of IgM associated polyneuropathies. (+info)
Noninvasive determination of venomuscular efficiency.
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PURPOSE: The purpose of this study was to express quantitatively and noninvasively the efficiency of the venomuscular pump by using photoplethysmography (PPG). METHODS AND PATIENTS PPG was used to record volume changes induced by dorsiflexion resulting in exercise displacement volume (EDV) and subsequently by passive limb elevation resulting in tilt displacement volume (TDV). Dividing EDV by TDV yields efficiency of the venomuscular pump. Twenty-four control subjects, 21 patients with venous valvular insufficiency (VVI), and 10 additional patients with both VVI and deep venous thrombosis (DVT) were studied. Data for one leg in each subject are reported. Though calculation of efficiency does not require calibrated or quantitative PPG, in this case because we used calibrated PPG we recorded the respective values for EDV after dorsiflexion and TDV after passive leg elevation. RESULTS: EDV decreased from a normal value of 3.15% optical reflectance (% OR) to 1.68% OR in patients with VVI and 1.09% OR in patients with VVI plus DVT. TDV response was quite opposite, increasing from 6.55% OR in the control group to 10.30% OR in the VVI group and 16.66% OR in the VVI plus DVT group. Finally, efficiency decreased from the average normal value of 50.5% to 24.4% in the VVI group and 8.8% in the VVI plus DVT group. CONCLUSION: Dividing EDV by TDV, venomuscular efficiency can be determined and may be considered a useful index of venous hemodynamics. (+info)
Why rest stimulates bone formation: a hypothesis based on complex adaptive phenomenon.
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Moderate exercise is an ineffective strategy to build bone mass. The authors present data demonstrating that allowing bone to rest between each load cycle transforms low- and moderate-magnitude mechanical loading into a signal that potently induces bone accretion. They hypothesize that the osteogenic nature of rest-inserted loading arises by enabling osteocytes to communicate as a small world network. (+info)
In the absence of external forces, the largest contributor to intervertebral disc (IVD) loads and stresses is trunk muscular activity. The relationship between trunk posture, spine geometry, extensor muscle activity, and the loads and stresses acting on the IVD is not well understood. The objective of this study was to characterize changes in thoracolumbar disc loads and extensor muscle forces following anterior translation of the thoracic spine in the upright posture. Vertebral body geometries (C2 to S1) and the location of the femoral head and acetabulum centroids were obtained by digitizing lateral, full-spine radiographs of 13 men and five women volunteers without previous history of back pain. Two standing, lateral, full-spine radiographic views were obtained for each subject: a neutral-posture lateral radiograph and a radiograph during anterior translation of the thorax relative to the pelvis (while keeping T1 aligned over T12). Extensor muscle loads, and compression and shear stresses acting on the IVDs, were calculated for each posture using a previously validated biomechanical model. Comparing vertebral centroids for the neutral posture to the anterior posture, subjects were able to anterior translate +101.5 mm+/-33.0 mm (C7-hip axis), +81.5 mm+/-39.2 mm (C7-S1) (vertebral centroid of C7 compared with a vertical line through the vertebral centroid of S1), and +58.9 mm+/-19.1 mm (T12-S1). In the anterior translated posture, disc loads and stresses were significantly increased for all levels below T9. Increases in IVD compressive loads and shear loads, and the corresponding stresses, were most marked at the L5-S1 level and L3-L4 level, respectively. The extensor muscle loads required to maintain static equilibrium in the upright posture increased from 147.2 N (mean, neutral posture) to 667.1 N (mean, translated posture) at L5-S1. Compressive loads on the anterior and posterior L5-S1 disc nearly doubled in the anterior translated posture. Anterior translation of the thorax resulted in significantly increased loads and stresses acting on the thoracolumbar spine. This posture is common in lumbar spinal disorders and could contribute to lumbar disc pathologies, progression of L5-S1 spondylolisthesis deformities, and poor outcomes after lumbar spine surgery. In conclusion, anterior trunk translation in the standing subject increases extensor muscle activity and loads and stresses acting on the intervertebral disc in the lower thoracic and lumbar regions. (+info)