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 47,337 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Heart valve substitutes of biological origin often fail by degenerative mechanisms. Many authors have hypothesized that mechanical fatigue and structural degradation are instrumental to in vivo failure. Since the properties of the structural matrix at implantation may predetermine failure, we have examined the ultrastructure, fracture, mechanics, and uniaxial high-strain-rate viscoelastic properties of: (1) fresh, (2) cryopreserved, and (3) cellular extracted porcine aortic valve leaflets. The cellular extraction process is being developed in order to reduce immunological attack and calcification. Cryopreservation causes cellular disruption and necrotic changes throughout the tissue, whereas extraction removes all cells and lipid membranes. Both processes leave an intact collagen and elastin structural matrix and preserve the high-strain-rate viscoelastic characteristics of the fresh leaflets. Extraction does cause a 20% reduction in the fracture tension and increases tissue extensibility, with the percent strain at fracture rising to 45.3 +/- 4 (mean +/- SEM) from 31.5 +/- 3 for fresh leaflets. However, extraction does preserve matrix structure and mechanics over the physiological loading range. Glutaraldehyde fixation produces increased extensibility, increased elastic behavior, and, when applied to extracted leaflets, it causes a marked drop in fracture tension, to 50% of that for fresh leaflets. The combination of extraction and fixation may lead to early degenerative failure. The cellular extraction technique alone may be a useful alternative to glutaraldehyde fixation in preparing bioprosthetic heart valves.
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PMID:Biomechanical and ultrastructural comparison of cryopreservation and a novel cellular extraction of porcine aortic valve leaflets. 860 Jan 41

Recovery from prolonged exercise involves both rehydration and replenishment of endogenous carbohydrate stores. The present study examined the influence of ingesting a carbohydrate-electrolyte (CE) solution following prolonged running, on exercise capacity 4 hr later. Twelve men and 4 women were divided into two matched groups, which were randomly assigned to either a control (P) or a carbohydrate (CHO) condition. Both groups ran at 70% of maximal oxygen uptake (VO2max) on a level treadmill for 90 min or until volitional fatigue (R1), and they ran at the same % VO2max to exhaustion 4 hr later to assess endurance capacity (R2). The CHO group ingested a 6.9% CE solution providing 1.0 g CHO.kg body weight-1 immediately post-R1 and again 2 hr later. The P group ingested equal volumes of a placebo solution. Run times (mean +/- SEM) for R1 did not differ between the groups (P 86.3 +/- 3.8 min; CHO 87.5 +/- 2.5 min). The CHO group ran 22.2 (+/- 3.5) min longer than the P group during R2 (P 39.8 +/- 6.1 min; CHO 62.0 +/- 6.2 min) (p < .05). Thus, ingesting a 6.9% carbohydrate-electrolyte beverage following prolonged, constant-pace running improves endurance capacity 4 hr later.
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PMID:The influence of ingesting a carbohydrate-electrolyte beverage during 4 hours of recovery on subsequent endurance capacity. 860 16

This study examined the control of ventilation during repetitive bouts of isometric exercise in simulated sailing. Eight male sailors completed four successive 3-min bouts of similar isometric effort on a dinghy simulator; bouts were separated by 15-s rest intervals. Quadriceps muscle integrated electromyograph activity (iEMG) was recorded during each bout and expressed as a percentage of activity during maximal voluntary contraction (%iEMGmax). From the first to the fourth bout, the 3-min mean averages for ventilation and for %iEMGmax increased from 19.8 (SEM 1.1) to 37.5 (SEM 3.0) l.min-1 and from 31 (SEM 4) to 39 (SEM 4)% respectively; also, ventilation and %iEMGmax over each minute throughout the four bouts were significantly correlated (r = 0.85; P < 0.05). Progressive hyperventilation reduced the mean end-tidal partial pressure of carbon dioxide from 5.0 (SEM 0.3) kPa during bout 1 to 4.3 (SEM 0.4) kPa during bout 4 [37.7 (SEM 2.0) to 32.4 (SEM 3.0) mmHg]. From the first to the fourth bout the end-of-bout blood lactate concentration did not increase significantly although the concentration from the third bout onwards was significantly greater than at rest. The results suggested that the development of muscle fatigue, which was enhanced by the insufficiency of recovery during the 15-s intervals and mirrored in the progressive increase in iEMG, was linked with stimuli causing progressive hyperventilation. Though these changes in ventilation and iEMG could not be associated with changes in blood lactate concentration, they could both have been related to accumulating metabolites within the muscles themselves.
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PMID:Changes in ventilation related to changes in electromyograph activity during repetitive bouts of isometric exercise in simulated sailing. 882 Aug 85

The aim of this study was to determine some characteristics of the fracture surface of an acrylic resin denture. For this purpose, five complete upper dentures and two complete lower dentures that had fractured into two pieces were collected, the fracture surfaces were duplicated with the SEM-replica technique and duplicated surfaces were investigated with a scanning electron microscope. The reproducibility of the SEM-replica technique was tested by comparing one original sample surface and its replica surface of the same area. The SEM-photomicrographs revealed a series of fine striations in the fracture surfaces of both the upper and lower dentures perpendicularly orientated to the direction of the fracture propagation. A smooth area on the fracture surface was assumed to indicate a slow fatigue fracture process, and a rougher area fracturing with increased velocity.
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PMID:Fracture surface characteristics of damaged acrylic-resin-based dentures as analysed by SEM-replica technique. 886 64

Previous exercise training studies in patients with chronic congestive heart failure (CHF) were performed for periods lasting > 2 months, and effects of activity restriction on exercise induced-benefits were not systematically assessed. With one exception study, patients were not reported to be transplant candidates. In this random-order crossover study, effects of 3 weeks of exercise training and 3 weeks of activity restriction on functional capacity in 18 hospitalized patients with severe CHF [(mean +/- SEM) age 52 +/- 2 years; ejection fraction 21 +/- 1%; half of them on a transplant waiting list] were assessed. The training program consisted of interval exercise with bicycle ergometer (15 minutes) 5 times weekly, interval treadmill walking (10 minutes), and exercises (20 minutes), each 3 times weekly. With training, the onset of ventilatory threshold was delayed (p < 0.001), with increased work rate by 57% (p < 0.001) and oxygen uptake by 23.7% (p < 0.001). On average, there was a 14.6% decrease in slope of ventilation/carbon dioxide production before the onset of ventilatory threshold (p < 0.05), and ventilatory equivalent of carbon dioxide production by 10.3% (p < 0.01). At the highest comparable work rate (56 +/- 5 W) the following variables were decreased: heart rate (7.3%; p < 0.05), lactate (26.6%; p < 0.001), and ratings of perceived leg fatigue and dyspnea (14.5% and 16.5%; p < 0.001 each). At peak exercise, oxygen uptake was increased by 19.7% (p < 0.01) and oxygen pulse by 14.2% (p < 0.01). There was a correlation of baseline peak oxygen uptake and increase of peak oxygen uptake due to training (r = -0.75; p < 0.004). Independently of the random order, data after activity restriction did not differ significantly from data measured at baseline. Patients with stable, severe CHF can achieve significant improvements in aerobic and ventilatory capacity and symptomology by short-term exercise training using interval exercise methods. Impairments due to activity restriction suggest the need for long-term exercise training.
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PMID:Effects of short-term exercise training and activity restriction on functional capacity in patients with severe chronic congestive heart failure. 891 81

Electromechanical delay (EMD) of knee extensors in isometric contraction was investigated in six healthy men before and after four periods of 30-s allout sprint cycling exercise, conducted pre and post a 7-week sprint cycling training programme. The EMD was lengthened from 40.4 (SEM 3.46) ms at rest to 63.4 (SEM 7.80) ms after the fatiguing exercise (P < or = 0.05) in the pre-training test. During maximal voluntary contractions (MVC) conducted after the fatiguing exercise, the peak contraction force (Fpeak) and peak rate of force development (RFDpeak) were reduced by 51%-56% and 38%-50%, respectively (both P < or = 0.05). The mechanisms of EMD lengthening during fatigue could have been due to the deterioration in muscle conductive, contractile or elastic properties and require further study. The training programme increased the total work performed during the four periods of sprint exercise (P < or = 0.05). However, no significant training effects were found in the resting or postexercise EMD, Fpeak and RFDpeak during isometric MVC. These unchanged isometric contraction variables but enhanced dynamic performance suggest that isometric tests of muscle are insensitive to the neuromuscular adaptations to sprint training.
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PMID:Effects of fatigue and sprint training on electromechanical delay of knee extensor muscles. 892 10

This paper compares the effects of 7-day dry immersion and intermittent muscle contraction on electrical and mechanical failure during muscle fatigue in the human triceps surae muscle electrically stimulated at 50 impulses.s-1 via its motor nerve. Intermittent contractions of 60-s duration were separated by 1-s intervals for identical duration of tension development. The 60-s intermittent contractions decreased tetanic force to 57% (P < 0.05) of initial values, but force reduction was not significantly different in the two fatigue tests: the fatigue index was 36.2 (SEM 5.4)% versus 38.6 (SEM 2.8)%, respectively (P > 0.05). Whilst identical force reduction was present in the two fatigue tests, it would appear that concomitant electrical failure was considerably different. This electromechanical dissociation would suggest that a slowing of conduction along nerve and muscle membranes did not explain the observed mechanical failure. It is suggested that intracellular processes played major role in contractile failure during intermittent contractions after muscle disuse.
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PMID:Changes in the action potential and contractile properties of skeletal muscle in human's with repetitive stimulation after long-term dry immersion. 897 90

The importance of accurately reproducing isokinetic strength values is critical for the assessment of training induced changes in muscle function. The purpose of this study was to determine the test re-test reliability of the Biodex System 2 Isokinetic Dynamometer for concentric quadricep and hamstring strength and muscular endurance. Twenty-one healthy volunteers underwent isokinetic testing on 2 occasions separated by 7 days. Each subject performed 5 repetitions at 60 degrees/ second followed by a muscular endurance test which consisted of 30 repetitions at 180 degrees/second. The results demonstrated high ICC values ranging from r = 0.88 to r = 0.97 and r = 0.82 to r = 0.96 for variables measured at 60 degrees/second and 180 degrees/second, respectively, SEM values were found to range from low to moderate, 4.8% to 11.6% and 5.6% to 10.8%, at 60 and 180 degrees/second, respectively. ICC coefficients were found to be low, r = 0.52 to r = 0.74, and SEM values were found to be high, 9.8% to 20.8%, for the work fatigue index at 180 degrees/second. The results demonstrate that isokinetic values obtained at 60 and 180 degrees/second are highly reproducible with the Biodex System 2 Dynamometer provided that adequate calibration, gravity correction and patient positioning is recorded and standardized. However, the work fatigue index as a measure of muscular endurance has not been shown to be reliable or precise. Changes in muscular strength over time should be considered valid if these differences lie beyond the SEM values reported in this study.
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PMID:Reliability and precision of isokinetic strength and muscular endurance for the quadriceps and hamstrings. 908 Dec 67

Eighteen hospitalized patients with severe chronic heart failure (ejection fraction [mean +/- SEM] 21% +/- 1%) underwent 3 weeks of exercise training (interval bicycle ergometer and treadmill walking training exercises) and 3 weeks of activity restriction in a random-order crossover trial. Before and after exercise training and after activity restriction, a 6-minute walking test was performed to determine the maximum distance walked, hemodynamic and cardiopulmonary responses, norepinephrine levels, and ratings of leg fatigue and dyspnea while walking. A ramp test on bicycle ergometer (increments of 12.5 W/min) was performed before and after exercise training and activity restriction to determine peak oxygen uptake. After training, the maximum distance walked was increased by 65% (from 232 +/- 21 m at baseline to 382 +/- 20 m; p < 0.001), whereas after activity restriction (253 +/- 19 m) there was no significant difference compared with baseline. No significant differences in hemodynamic and cardiopulmonary parameters (with the exception of the ventilatory equivalent for carbon dioxide and perceived exertion) or norepinephrine levels were observed during walking tests. Improvement in maximum distance walked correlated significantly with training-induced increase in peak oxygen uptake measured during bicycle ergometry (r = 0.47, p < 0.05). The lower the maximum distance walked at baseline, the more pronounced the training-induced prolongation of maximum distance (r= -0.73; p < 0.001). These data support the value of exercise training in patients with severe chronic heart failure for improving maximum distance walked, as documented by the 6-minute walking test. The impairment of walking test performance during activity restriction suggests a need for long-term exercise training programs.
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PMID:Effects of exercise training and activity restriction on 6-minute walking test performance in patients with chronic heart failure. 912 67

Ultra high molecular weight polyethylene (UHMWPE) has been used in artificial joints for a few decades, and wear of UHMWPE has been one of the main problems. Though many other materials have been tested over the years, the best clinical results are still achieved with UHMWPE. This makes the study of UHMWPE, especially in relation to artificial joints, very important. Frequently, more severe wear can be observed in artificial knee joints than in artificial hip joints especially when the flaking-like wear occurs. This flaking-like wear can lead to significant destruction of the artificial knee joint. Macroscopically, artificial knee joints have combinational movements of rolling and sliding in order to simulate the motion of the normal knee joint. The components of motion are separated to make study easier. Fatigue tests of UHMWPE under the rolling contact condition were performed in this study. Three ceramic spheres were rolled over the UHMWPE specimen using 37 degrees C distilled water as a lubricant. The UHMWPE specimen was observed by the scanning acoustic tomography, microscopy, and SEM. Some subsurface defects could be observed by SAT even before experiments. Although the apparent wear is not observed on the surface, there was an increase in the number of observable subsurface cracks in the UHMWPE specimen. This shows that cracks occur under the surface after a 10(7) rolling contact loading, which is very close to the cyclic loading and unloading with very little friction compared to the sliding contact.
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PMID:An observation on subsurface defects of ultra high molecular weight polyethylene due to rolling contact. 913 54


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