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Query: UMLS:C0015672 (fatigue)
 51,768 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

1. The fatigue in rat anterior tibial (a.t.) motor units was studied and related to microphotometric determinations of succinate dehydrogenase (SDH) activity of the motor unit muscle fibres. 2. Anterior tibial contains fast-twitch type II fibre units with an average contraction time of 11 msec and about 5% slow-twitch type I fibre units with an average contraction time of 20 msec. 3. In type II fibres stained for SDH, absorbance varied continuously from 0.046 to 0.569 and inversely to fibre size, except for the largest fibres. 4. Resistance to fatigue of fast motor units to 100 Hz intermittent stimulation varied continuously within a wide range in near linear relations to absorbance for SDH of unit fibres and inversely to tetanic tension, except for motor units with the largest fibres and the largest tetanic tension. 5. Neither resistance to fatigue nor SDH activity lent itself to any categorization of motor units or fibres into well demarcated functional or histochemical types, since both parameters varied continuously in the unit and fibre population of the muscle. 6. The direct relation between resistance to fatigue of fast-twitch motor units and SDH activity of unit fibres appeared valid for fatigue resistance of: (a) neuromuscular transmission, tested with 100 Hz intermittent stimulation which gave concomitant failure of electrical and mechanical response, (b) excitation--contraction coupling, demonstrated by post-stimulatory depression of twitch tension with preserved maximum tetanus tension and action potential, and (c) contractile mechanism; excitation--contraction coupling?, tested with low frequency stimulation which gave decline of twitch and maximum tetanus tension with preserved action potential. 7. It is suggested that the endurance of each link in the chain of events leading to contraction, including neuromuscular junction and the excitation--contraction coupling system, is under aerobic conditions matched to the contractile capacity of the fibre expressed by its oxidative enzyme activity.
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PMID:Transmission and contraction fatigue of rat motor units in relation to succinate dehydrogenase activity of motor unit fibres. 22 67

Anterior displacement in spondylolisthesis is a slow process. The forces contributing to anterior shear arise from the vertical load on the spine above the lesion, from activity in the muscles of the spine and trunk, and from the effects of movements. The shear is normally resisted by the annulus fibrosus, compression of the facets at the apophyseal joints, and by tension in the anterolateral layers of cortical bone in the pars interarticularis. The anterior shear-strength of the disk is not known though Cyron and Hutton have shown that it is a major contributor to the total stiffness; in the early stages of spondylolysis it provides the only resistance. The capacity of the pars for resisting tensile forces anteriorly has been studied in tests of single vertebrae in which posterior forces were applied to the inferior articular processes. Forces at failure at L4 and L5 have been reported at up to 2500 N by Cyron et al. and up to 5800 N by Lamy et al. with means of 1800 N and 3200 N respectively. The mean posterior displacement of the inferior facets reported by Cyron et al. was 6.5 mm at failure. Too little is known of the mechanical factors in the etiology of spondylolysis for a definition of the postures and movements of the body which put the lumbar neural arch at greatest risk. While a combination of fatigue-failure and high strain-rate of forces induced in extended postures seems likely, further investigations are needed to elucidate the pathogenesis in many cases.
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PMID:Mechanical factors in spondylolisthesis and spondylolysis. 127 87

Recent reports of total knee prosthesis fractures have raised concerns over the long-term structural integrity of metal-backed tibial components. Both the development of a fibrous tissue membrane under the tibial plateau of a total knee prosthesis and loading conditions may seriously alter the fatigue life of the metal tibial tray. The effects of the cement and fibrous tissue at the bone-prosthesis interface were studied. Using the method of three-dimensional finite element analysis, peak loads of normal gait were simulated at several locations on the plateau of a generic, single-stemmed, porous-coated, CoCrMo tibial component model, providing information on the effect of abnormal loading patterns. According to the analysis, stresses below the material endurance limit are predicted throughout the prosthesis prior to the development of the fibrous membrane. However, stresses exceeding the yield strength of the material are predicted in a prosthesis that is supported by a fully developed 1 mm membrane, meaning that it has a markedly increased risk of low-cycle fatigue failure. Lateral displacement of the loading is detrimental to prosthesis life because maximum stress increases 100% while posterior displacement of the loading increases maximum stress by only 30%. Anterior loading creates stresses similar to those created by central loading. Because of their susceptibility to low-cycle fatigue failure, simple, single-stemmed prostheses are not recommended in cases of questionable bone stock unless modified. Several design alternatives are proposed.
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PMID:Prediction of fatigue failure of a total knee replacement tibial plateau using finite element analysis. 342 40

Laryngeal fatigue affects the physical sensations, effort, and perceptual quality of voice production. The underlying physiology of fatigue is not well understood. Acoustic, aerodynamic, and videostroboscopic data were measured in 10 normal speakers before and after prolonged voice use. Significant changes were found in the fundamental frequency of connected speech. Anterior glottal chinks were induced in a majority of subjects. Implications are discussed.
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PMID:Objective measures of voice production in normal subjects following prolonged voice use. 762 May 34

Telemeterized internal spinal fixation devices were implanted in a patient with degenerative instability and a narrow spinal canal in order to measure the fixator loads during daily activities. Anterior interbody fusion was performed three weeks later. During walking, the typical maximum flexion bending moments were 10 N m in the left and 5 N m in the right fixator. On removal of the implants three months later, a fatigue fracture was found not on the high loaded left side but in the upper right pedicle screw. The crack started on the caudal side of the cross-sectional area and progressed cranially. Upper vertebral tilting in the sagittal plane must have caused the screw breakage. This would probably have been prevented by a more posteriorly placed bone graft.
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PMID:Placing a bone graft more posteriorly may reduce the risk of pedicle screw breakage: analysis of an unexpected case of pedicle screw breakage. 979 78

Anterior lumbar interbody fusion has several clinical advantages over posterior or posterolateral lumbar fusion. Interbody fusion procedures place bone grafts within the disc space at the center of rotation of the vertebral motion segment. The intervertebral area is highly vascular, and the grafts have a wide contact area in the weight-bearing axis of the spinal motion segment. The high rates of fusion associated with the use of the threaded intervertebral fusion cages may be attributed, in part, to the following: (1) removal of the cartilagenous end plates and exposure of bleeding cancellous bony surfaces, (2) reestablishment of anatomic intradiscal height and tensioning of the annulus and ligamentous structures around the disc space, (3) use of appropriately sized implants to engage the peripheral apophysis of the vertebral end plates, and (4) use of autogenous grafts. Threaded interbody constructs provide adequate strength to ensure that no plastic deformation occurs within the maximum physiologic range. Dynamic testing of these implants also has shown that these implants are able to resist cyclic fatigue within typical normal daily physiologic loading. Stability testing has shown that when inserted anteriorly, these devices reduce intervertebral motion and increase spinal stiffness.
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PMID:Intervertebral fixation: clinical results with anterior cages. 1238 80

After L1 corpectomy in T11-L3 human cadaveric spine, anterior thoracolumbar instrumentation with strut grafting restores spinal stability. T12-L2 angular rotation was measured in response to moments of 0.0, 1.5, 3.0, 4.5, and 6.0 Nm in flexion, extension, lateral bending, and axial rotation, respectively. The spines were tested: 1) intact; 2) after partial L1 corpectomy, grafting, and instrumentation (Profile plate, DePuy-AcroMed, Raynham, MA), with the wooden dowel graft screwed to the plate; 3) without graft screw fixation; and 4) after flexion-extension cyclic fatiguing for 5000 cycles at a load of +/-3.0 Nm. Before and after fatiguing, the instrumented spine was significantly (p <or= 0.05) stiffer than the intact spine in flexion, extension, and right and left lateral bending but not in axial rotation. There were no significant differences between the constructs with or without graft-to-plate fixation before or after fatigue. The instrumented spines were more rigid in bending away from the implant than bending toward the implant. Anterior spinal instrumentation with the Profile implant augments stiffness in the sagittal and coronal planes but not in the axial plane. Although graft-to-plate fixation may prevent graft migration into the canal, it does not contribute to spinal rigidity.
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PMID:In vitro biomechanical studies of an anterior thoracolumbar implant. 1239 57

A loading protocol approximating forces, torques and motions at the knee during stair descent was developed from previously published data for input into a force-controlled knee simulator. A set of total knee replacements (TKRs) was subjected to standard walking cycles and stair descent cycles at a ratio of 70: 1 for 5 million cycles. Another set of implants with similar articular geometry and the same ultra-high molecular weight polyethylene (UHMWPE) resin (GUR 415), sterilization and packaging was tested with standard walking cycles only. Implant kinematics, gravimetric wear and surface roughness of the UHMWPE inserts were analysed for both sets of implants. Contact stresses were calculated for both loading protocols using a Hertzian line contact model. Significantly greater weight loss (p < 0.05) and more severe surface damage of UHMWPE inserts resulted with the walking + stair descent loading protocol compared to walking cycles only. Anterior-posterior (AP) tibiofemoral contact point displacements were lower during stair descent than walking, but not significantly different (p = 0.05). Contact stresses were significantly higher during stair descent than walking, owing to higher axial loads and the smaller radius of curvature of the femoral components at higher flexion angles. High contact stresses on UHMWPE components are likely to accelerate the fatigue of the material, resulting in more severe wear, similar to what is observed in retrieved implants. Thus the inclusion of loading protocols for activities of daily living in addition to walking is warranted for more realistic in vitro testing of TKRs.
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PMID:Effect of stair descent loading on ultra-high molecular weight polyethylene wear in a force-controlled knee simulator. 1250 5

Adhesion, spreading, proliferation, and collagen matrix production of human bone marrow stromal cells (BMSCs) on an RGD-modified silk matrix was studied. Anterior cruciate ligament fibroblasts (ACLFs) were used as a control cell source. Scanning electron microscopy (SEM) and MTT analyses demonstrated that the modified silk matrices support improved BMSC and ACLF attachment and show higher cell density over 14 days in culture when compared with the non-RGD-modified matrices. Collagen type I transcript levels (at day 7) and content (at day 14) was significantly higher on the RGD-modified substrate than on the nonmodified group. The ability of RGD-coupled silk matrices to support BMSC attachment, which leads to higher cell density and collagen matrix production in vitro, combined with mechanical, fatigue, and biocompatibility properties of the silk protein matrix, suggest potential for use of this biomaterial for tissue engineering.
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PMID:Human bone marrow stromal cell and ligament fibroblast responses on RGD-modified silk fibers. 1456 98

Control of standing requires the continuous activity of the leg muscles. In single leg standing the system is less redundant and muscular activity is more intensive. The objective of this study was to examine the effect of force imbalance of the shank muscles, evoked by their selective fatiguing, on postural control in single-leg standing. Five healthy subjects performed two single-leg standing trials, lasting as long as the subject could maintain steady balance, and separated by a 240s quasi-isotonic sustained effort to induce fatigue of the Tibialis Anterior and Peroneus muscles. The following were on-line monitored: sway-related parameters, e.g., ground reaction force and center of pressure in the standing trials; and electromyogram of the Tibialis Anterior, Peroneus and Gastrocnemius muscles in all experiments. Simple and multiple linear regressions served to study the fatigue effects on the relationship between muscle activity and postural sway. The results indicate that the evoked muscle imbalance leads to (a) increased postural sway; (b) increased correlation between muscle activity, and sway-related parameters. Thus, with the reduction of the level of redundancy the system becomes more synchronized. These results have potential relevance for cases of muscle impairment, in which electrical stimulation is required to augment muscle activity.
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PMID:Effect of selective fatiguing of the shank muscles on single-leg-standing sway. 1735 Feb 88


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