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PubMed2001

Influence of errors in sampling time and in activity measurement on the single sample clearance determination.

INTRODUCTION: Plasma clearance rate of 51Cr-EDTA estimated by using one blood sample is commonly used for the calculation of glomerular filtration rate. AIM: To estimate the error on single-sample clearance determination induced by errors in sampling time and activity measurement, and to compare it with the error observed on the clearance determination obtained using the slope-intercept method. METHODS: Forty-five adult patients were chosen from a data base of 51Cr-EDTA plasma clearance values determined by using two blood samples taken around 2 and 4 h. Patients were selected in such a way as to include clearances from 30 ml.min-1 to 155 ml.min-1, with steps of 3 ml.min-1. Based on the slope and the intercept of the slope with the y-axis, the plasma concentration at exactly 2 and 4 h was determined. Normally distributed random errors were then introduced in the sampling time (SD of 0, 1 and 2 min) as well as in the activity measurement (SD of 0, 1, 2 and 5%). Then, clearance was calculated using two single-sample methods (i.e. the algorithms of Groth and Tauxe), and the slope-intercept method, which requires two blood samples. For each setting, the simulation was repeated 200 times. The effects on clearance of a random error on the time sampling and/or the activity measurement were then evaluated. RESULTS: The error on single-sample clearance induced by a 2 min error in sampling time associated with a 5% error in activity measurement was negligible. For all clearance levels, the SD of the error on the calculated clearance was less than 3.8 ml.min-1. Whatever algorithm was chosen, the errors on the single-sample clearance were systematically lower than those observed with the slope-intercept method, for the whole clearance range. CONCLUSION: Errors in sampling time and in activity measurement induced only a very small error on the single-sample EDTA clearance, which is systematically lower compared to that observed on the slope-intercept method using two blood samples.

Adult↗
PubMed1978

Distribution of membrane thickness determined by lineal analysis.

The expected statistical distributions of intercept length are derived in terms of geometrical probability density functions pertaining to plates with known thickness penetrated by lines with random orientation. These expressions provide arithmetic and graphical solutions for obtaining distributions of membrane thickness and reciprocal membrane thickness from empirical distributions of intercept lengths. Furthermore, general relationships between probability density functions of distributions of intercept length and membrane thickness are derived as well as those between their moments. Examples of the application of the method to biological samples are given, and estimated distributions of glomerular basement membrane thickness are compared to those obtained by an independent, direct method. Various sources of bias, which in practice may occur due to departures from the sample model, are discussed and the influence of some of them is estimated. The knowledge of the probability density function of reciprocal intercepts makes it possible to perform a correction of the distribution of measured intercept length, which to some extent eliminates bias.

Animals↗
PubMed1998

Characterisation of coronary atherosclerotic morphology by spectral analysis of radiofrequency signal: in vitro intravascular ultrasound study with histological and radiological validation.

OBJECTIVE: To determine whether spectral analysis of unprocessed radiofrequency (RF) signal offers advantages over standard videodensitometric analysis in identifying the morphology of coronary atherosclerotic plaques. METHODS: 97 regions of interest (ROI) were imaged at 30 MHz from postmortem, pressure perfused (80 mm Hg) coronary arteries in saline baths. RF data were digitised at 250 MHz. Two different sizes of ROI were identified from scan converted images, and relative amplitudes of different frequency components were analysed from raw data. Normalised spectra was used to calculate spectral slope (dB/MHz), y-axis intercept (dB), mean power (dB), and maximum power (dB) over a given bandwidth (17-42 MHz). RF images were constructed and compared with comparative histology derived from microscopy and radiological techniques in three dimensions. RESULTS: Mean power was similar from dense fibrotic tissue and heavy calcium, but spectral slope was steeper in heavy calcium (-0.45 (0.1)) than in dense fibrotic tissue (-0.31 (0.1)), and maximum power was higher for heavy calcium (-7.7 (2.0)) than for dense fibrotic tissue (-10.2 (3.9)). Maximum power was significantly higher in heavy calcium (-7.7 (2.0) dB) and dense fibrotic tissue (-10.2 (3.9) dB) than in microcalcification (-13.9 (3.8) dB). Y-axis intercept was higher in microcalcification (-5.8 (1.1) dB) than in moderately fibrotic tissue (-11.9 (2.0) dB). Moderate and dense fibrotic tissue were discriminated with mean power: moderate -20.2 (1.1) dB, dense -14.7 (3.7) dB; and y-axis intercept: moderate -11.9 (2.0) dB, dense -5.5 (5.4) dB. Different densities of fibrosis, loose, moderate, and dense, were discriminated with both y-axis intercept, spectral slope, and mean power. Lipid could be differentiated from other types of plaque tissue on the basis of spectral slope, lipid -0.17 (0.08). Also y-axis intercept from lipid (-17.6 (3.9)) differed significantly from moderately fibrotic tissue, dense fibrotic tissue, microcalcification, and heavy calcium. No significant differences in any of the measured parameters were seen between the results obtained from small and large ROIs. CONCLUSION: Frequency based spectral analysis of unprocessed ultrasound signal may lead to accurate identification of atherosclerotic plaque morphology.

Calcinosis↗
PubMed2005

Nonlinear isochrones in murine left ventricular pressure-volume loops: how well does the time-varying elastance concept hold?

The linear time-varying elastance theory is frequently used to describe the change in ventricular stiffness during the cardiac cycle. The concept assumes that all isochrones (i.e., curves that connect pressure-volume data occurring at the same time) are linear and have a common volume intercept. Of specific interest is the steepest isochrone, the end-systolic pressure-volume relationship (ESPVR), of which the slope serves as an index for cardiac contractile function. Pressure-volume measurements, achieved with a combined pressure-conductance catheter in the left ventricle of 13 open-chest anesthetized mice, showed a marked curvilinearity of the isochrones. We therefore analyzed the shape of the isochrones by using six regression algorithms (two linear, two quadratic, and two logarithmic, each with a fixed or time-varying intercept) and discussed the consequences for the elastance concept. Our main observations were 1) the volume intercept varies considerably with time; 2) isochrones are equally well described by using quadratic or logarithmic regression; 3) linear regression with a fixed intercept shows poor correlation (R(2) < 0.75) during isovolumic relaxation and early filling; and 4) logarithmic regression is superior in estimating the fixed volume intercept of the ESPVR. In conclusion, the linear time-varying elastance fails to provide a sufficiently robust model to account for changes in pressure and volume during the cardiac cycle in the mouse ventricle. A new framework accounting for the nonlinear shape of the isochrones needs to be developed.

Algorithms↗
PubMed2003

Segregation and linkage analysis for longitudinal measurements of a quantitative trait.

We present a method for using slopes and intercepts from a linear regression of a quantitative trait as outcomes in segregation and linkage analyses. We apply the method to the analysis of longitudinal systolic blood pressure (SBP) data from the Framingham Heart Study. A first-stage linear model was fit to each subject's SBP measurements to estimate both their slope over time and an intercept, the latter scaled to represent the mean SBP at the average observed age (53.7 years). The subject-specific intercepts and slopes were then analyzed using segregation and linkage analysis. We describe a method for using the standard errors of the first-stage intercepts and slopes as weights in the genetic analyses. For the intercepts, we found significant evidence of a Mendelian gene in segregation analysis and suggestive linkage results (with LOD scores >or= 1.5) for specific markers on chromosomes 1, 3, 5, 9, 10, and 17. For the slopes, however, the data did not support a Mendelian model, and thus no formal linkage analyses were conducted.

Adult Children↗
PubMed2006

Impact of computerized physician order entry on medication prescription errors in the intensive care unit: a controlled cross-sectional trial.

INTRODUCTION: Medication errors in the intensive care unit (ICU) are frequent and lead to attributable patient morbidity and mortality, increased length of ICU stay and substantial extra costs. We investigated if the introduction of a computerized ICU system (Centricity Critical Care Clinisoft, GE Healthcare) reduced the incidence and severity of medication prescription errors (MPEs). METHODS: A prospective trial was conducted in a paper-based unit (PB-U) versus a computerized unit (C-U) in a 22-bed ICU of a tertiary university hospital. Every medication order and medication prescription error was validated by a clinical pharmacist. The registration of different classes of MPE was done according to the National Coordinating Council for Medication Error Reporting and Prevention guidelines. An independent panel evaluated the severity of MPEs. We identified three groups: minor MPEs (no potential to cause harm); intercepted MPEs (potential to cause harm but intercepted on time); and serious MPEs (non-intercepted potential adverse drug events (ADE) or ADEs, being MPEs with potential to cause, or actually causing, patient harm). RESULTS: The C-U and the PB-U each contained 80 patient-days, and a total of 2,510 medication prescriptions were evaluated. The clinical pharmacist identified 375 MPEs. The incidence of MPEs was significantly lower in the C-U compared with the PB-U (44/1286 (3.4%) versus 331/1224 (27.0%); P < 0.001). There were significantly less minor MPEs in the C-U than in the PB-U (9 versus 225; P < 0.001). Intercepted MPEs were also lower in the C-U (12 versus 46; P < 0.001), as well as the non-intercepted potential ADEs (21 versus 48; P < 0.001). There was also a reduction of ADEs (2 in the C-U versus 12 in the PB-U; P < 0.01). No fatal errors occurred. The most frequent drug classes involved were cardiovascular medication and antibiotics in both groups. Patients with renal failure experienced less dosing errors in the C-U versus the PB-U (12 versus 35 serious MPEs; P < 0.001). CONCLUSION: The ICU computerization, including the medication order entry, resulted in a significant decrease in the occurrence and severity of medication errors in the ICU.

Adult↗
PubMed1997

EEG-based communication: evaluation of alternative signal prediction methods.

Individuals can learn to control the amplitude of EEG activity in specific frequency bands over sensorimotor cortex and use it to move a cursor to a target on a computer screen. For one-dimensional (i.e., vertical) cursor movement, a linear equation translates the EEG activity into cursor movement. To translate an individual's EEG control into cursor control as effectively as possible, the intercept in this equation, which determines whether upward or downward movement occurs, should be set so that top and bottom targets are equally accessible. The present study compares alternative methods for using an individual's previous performance to select the intercept for subsequent trials. In offline analyses, five different intercept selection methods were applied to EEG data collected while trained subjects were moving the cursor to targets at the top or bottom edge of the screen. In the first two methods-moving average, and weighted sum-a single intercept was selected for the entire 1-2 sec period of each trial. In the other three methods-blocked moving average, blocked weighted sum, and blocked recursive sum (a variation of the weighted sum)-an intercept was selected for each 200-ms segment of the trial. The results from these methods were compared in regard to their balance between upward and downward movements and their consistency of performance across trials. For all subjects combined, the five methods performed similarly. However, performance across subjects was more consistent for the moving average, blocked moving average, and blocked recursive sum methods than for the weighted sum and blocked weighted sum methods. Due to its consistent performance and its computational simplicity, the moving average method, using the five most recent pairs of top and bottom trials, appears to be the method of choice.

Adult↗
PubMed2007

Prevention of pediatric medication errors by hospital pharmacists and the potential benefit of computerized physician order entry.

OBJECTIVES: The purpose of this work was to characterize medication errors and adverse drug events intercepted by a system of pediatric clinical pharmacists and to determine whether the addition of a computerized physician order entry system would improve medication safety. METHODS: The study included 16,938 medication orders for 678 admissions to the pediatric units of a large academic community hospital. Pediatric clinical pharmacists reviewed medication orders and monitored subsequent medication use. Medication errors and adverse drug events were identified by daily review of documentation, voluntary reporting, and solicitation. Each potentially harmful medication error was judged whether or not it was intercepted and, if not, whether it would have been captured by a computerized physician order entry system. RESULTS: Overall, 865 medication errors occurred, corresponding with a rate of 5.2 per 100 medication orders. A near-miss rate of 0.96% and a preventable adverse drug event rate of 0.09% were observed. Overall, 78% of potentially harmful prescribing errors were intercepted; however, none of the potentially harmful errors occurring at administration was intercepted and accounted for 50% of preventable adverse drug events. A computerized physician order entry system could capture additional potentially harmful prescribing and transcription errors (54%-73%) but not administration errors (0% vs 6%). CONCLUSIONS: A system of pediatric clinical pharmacists effectively intercepted inpatient prescribing errors but did not capture potentially harmful medication administration errors. The addition of a computerized physician order entry system to pharmacists is unlikely to prevent administration errors, which pose the highest risk of patient injury.

Child↗
PubMed2002

Radiation Use Efficiency and Biomass Production in Soybean at Different Plant Population Densities.

As population density (POP) increases in a soybean [Glycine max (L.) Merr.] crop, maximum light interception (LI) occurs earlier in the season. Earlier canopy closure would be expected to increase the cumulative radiation intercepted. We hypothesized that if radiation use efficiency (RUE) was constant across a range of population densities in a nonstressful environment, then increasing POP would increase biomass at the end of the season. To test this hypothesis, we evaluated the response of total biomass produced during the season to cumulative intercepted photosynthetically active radiation (PAR) in field experiments at Fayetteville, AR, with soybean cultivars selected from Maturity Groups (MGs) 00 to IV. Additionally, from field experiments at Keiser, AR, with MG IV soybean cultivars, we assessed the response of RUE to POP. At both locations with MG IV cultivars, a late sowing date shortened the life cycle of the crop by 13 to 25 d compared with an early sowing date, resulting in less PAR accumulated. Similarly, early maturing cultivars had less time for PAR and biomass accumulation relative to later maturing cultivars. At Keiser, in three of the four environments, RUE decreased linearly by 26 to 30% as the POP increased from 7 to 135 plants m(-2). Final biomass at the end of the season, as a function of PAR accumulated from emergence to the full-seed-size stage of development, responded linearly to intercepted PAR up to approximately 400 MJ m(-2). Above 400 MJ m(-2), the response was curvilinear with little increases in biomass >700 MJ m(-2). Our data clearly indicate that RUE decreased as POP increased and that maximum biomass production in these environments was not limited by intercepted PAR.

Journal Article↗
PubMed2000

A meta-analytic approach to growth curve analysis.

A meta-analytic approach to growth curve analysis is described and illustrated by applying it to the evaluation of the Arizona Pilot Project, an experimental project for financing the treatment of the severely mentally ill. In this approach to longitudinal data analysis, each individual subject for which repeated measures are obtained is initially treated as a separate case study for analysis. This approach has at least two distinct advantages. First, it does not assume a balanced design (equal numbers of repeated observations) across all subjects; to accommodate a variable number of observations for each subject, individual growth curve parameters are differentially weighted by the number of repeated measures on which they are based. Second, it does not assume homogeneity of treatment effects (equal slopes) across all subjects. Individual differences in growth curve parameters representing potentially unequal developmental rates through time are explicitly modeled. A meta-analytic approach to growth curve analysis may be the optimal analytical strategy for longitudinal studies where either (1) a balanced design is not feasible or (2) an assumption of homogeneity of treatment effects across all individuals is theoretically indefensible. In our evaluation of the Arizona Pilot Project, individual growth curve parameters were obtained for each of the 13 rationally derived subscales of the New York Functional Assessment Survey, over time, by linear regression analysis. The slopes, intercepts, and residuals obtained for each individual were then subjected to meta-analytic causal modeling. Using factor analytic models and then general linear models for the latent constructs, the growth curve parameters of all individuals were systematically related to each other via common factors and predicted based on hypothesized exogenous causal factors. The same two highly correlated common factors were found for all three growth curve parameters analyzed, a general psychological factor and a general functional factor. The factor patterns were found to be nearly identical across the separate analyses of individual intercepts, slopes, and residuals. Direct effects on the unique factors of each subscale of the New York Functional Assessment Survey were tested for each growth curve parameter by including the common factors as hierarchically prior predictors in the structural model for each of the indicator variables, thus statistically controlling for any indirect effect produced on the indicator through the common factors. The exogenous predictors modeled were theoretically specified orthogonal contrasts for Method of Payment (comparing Arizona Pilot Project treatment or "capitation" to traditional or "fee-for-service" care as a control), Treatment Administration Site (comparing various locations within treatment or control groups), Pretreatment Assessment (comparing general functional level at intake as assigned by an Outside Assessment Team), and various interactions among these main effects. The intercepts, representing the initial status of individual subjects on both the two common factors and the 13 unique factors of the subscales of the New York Functional Assessment Survey, were found to vary significantly across many of the various different treatment conditions, treatment administration sites, and pretreatment functional levels. This indicated a severe threat to the validity of the originally intended design of the Arizona Pilot Project as a randomized experiment. When the systematic variations were statistically controlled by including intercepts as hierarchically prior predictors in the structural models for slopes, recasting the experiment as a nonequivalent groups design, the effects of the intercepts on the slopes were found to be both statistically significant and substantial in magnitude. (ABSTRACT TRUNCATED)

Capitation Fee↗
PubMed1993

Circadian rhythm in heat production of limit-fed growing pigs of several breeds kept at and below thermal neutrality.

Circadian rhythm in total and activity-free heat production (H and Hacf, respectively) was studied in Norwegian Landrace (N), Finnish Landrace (F), Dutch Landrace (D), and Great Yorkshire (Y) barrows. Animals, weighing 26 kg at the start of the study, were kept in groups for 18 2-d periods in climate respiration chambers at environmental temperatures (Tenv) between 11 and 26 degrees C. Measurements of H and physical activity of the pigs were done in 12-min intervals for 36 h per breed per Tenv. Feeding level of animals in a group was 93 g.kg-.75 x d-1 (2.5 times maintenance) and based on mean BW. Circadian rhythm in H and Hacf with data excluding the feeding periods was assessed using a sine wave with a 24-h periodicity per breed and per Tenv. With respect to H, the intercepts and amplitudes for the three Landrace breeds were similar, but the Y pigs had a lower intercept and a lower amplitude. The intercepts increased with decreasing Tenv. In N pigs, but not in F, D, or Y pigs, the amplitude increased with decreasing Tenv. With respect to Hacf, all breeds had similar intercepts and amplitudes. The Hacf varied less within a day than did H. In all breeds, the intercepts were increased with decreasing Tenv. The amplitudes were not affected by Tenv.

Animals↗
PubMed1994

Preweaning growth of Yorkshire, Hampshire, and Landrace pigs.

Individual pig growth curves were derived on 445 Yorkshire, 323 Hampshire, and 316 Landrace boars and gilts. The range in age was from 1 to 35 d and these growth curves were linear. The regression parameters (x- and y-intercepts, and slopes) were different among the three breeds, and differences between boars and gilts were found in the Yorkshire breed for the y-intercept and slope and in the Landrace breed for the y-intercept and slope and in the Landrace breed for the y-intercept. An additional 130 Yorkshire, 126 Hampshire, and 96 Landrace boars and gilts were used to evaluate the effectiveness of adjustments that were additive, multiplicative, additive-multiplicative, or based on the x- and y-intercepts. Effectiveness was evaluated in terms of accuracy, equality of variance, and practical application. The multiplicative adjustments were the most effective in the Hampshire breed and the average of the additive and multiplicative adjustments was the most effective in the Yorkshire breed. In the Landrace breed none of the four methods evaluated produced a credible adjusted 21-d weight. From an industry standpoint this study suggests the need for breed-specific specific adjustments, and in some breeds this may have to be extended to sex-specific adjustments.

Analysis of Variance↗
PubMedDate not supplied

Prevalence of errors in a pediatric hospital medication system: implications for error proofing.

The purpose of this study was to determine the prevalence of errors in the medication system of a pediatric teaching hospital. Error was defined broadly to capture all deviations in the process from medication order through administration of the dose. The long-term goal was to provide direction to efforts to error-proof the system. The sample was 3,312 medication orders written during 669 patient-days for which a total of 11,978 doses were passed. Errors were categorized as intercepted errors (intercepted through the normal processes of the medication system) or administration errors (errors that involve the patient with or without adverse sequelae). Errors were also categorized as errors in primary activities (e.g., prescribing or preparing the medication for administration) or supporting activity (e.g., transferring the order to another record). A total of 784 errors were identified; 98% were intercepted and 2% were administration errors. More errors (71%) occurred in supporting activities than in primary activities. Medication systems are complex processes. Errors are imbedded in the medication system and are typically intercepted before patients are involved. Intercepting errors involves additional work that adds to an already cumbersome process. Error proofing will be different for errors in primary activities and for supporting activities.

Child↗
PubMed1991

Effects of ligustrazine on the pressure/flow relationship in isolated perfused rat lungs.

Ligustrazine, the synthesized principle of a Chinese herbal remedy shown previously to be a pulmonary vasodilator, was tested in chronically hypoxic and normal rats. Pressure/flow, (P/Q), relations were measured in isolated perfused lungs during normoxia, hypoxia and after reversal of hypoxic vasoconstriction by increasing doses of ligustrazine. P/Q lines were linear over a wide range and extrapolation to the pressure axis gave an intercept which was the effective downstream pressure for flow. In chronically hypoxic rats the slope of the line was steeper and the intercept greater than in control rats, which we attributed to newly muscularized arterioles with tone. Hypoxia caused an increase in slope and intercept in both groups but the intercept increase was greater in chronically hypoxic rats. In both groups of rats increasing doses of ligustrazine given during continued hypoxia caused a fan of lines which moved progressively towards the control normoxic line. In chronically hypoxic rats it required only 2 mg of ligustrazine to bring the line back to the normoxic position, whereas in controls it required 4 mg. In chronically hypoxic rats the change in intercept with every dose was greater than in control rats; this suggests that ligustrazine mainly relaxes the muscle of small collapsible vessels. The action of ligustrazine remained in both control and chronically hypoxic rats after administration of an arginine analogue which blocks synthesis of the endothelial relaxant factor nitric oxide. This and previous evidence suggest that ligustrazine is a non-endothelial-dependent pulmonary vasodilator.

Animals↗
PubMed1992

An indirect method of measuring widths suitable for automated bone histomorphometry.

Orthogonal intercepts from random test lines (OIr), uniformly distributed intercepts (OIu), and areas and lengths (Ar/Le) were measured manually and by computer to determine individual profile and sample widths of artificial profiles and human osteoid seams. Individual widths were equal by Ar/Le and OIu methods. The means of individual profile widths (all methods), of all orthogonal intercepts (OIu and OIr), or of profile widths weighted in proportion to their lengths (Ar/Le) were also equal. Ar/Le and OIu had smaller variance than OIr. Discrepant individual OIu and Ar/Le widths in digitized images were corrected by thresholding and did not significantly affect sample means. Unweighted Ar/Le sample means were 15-44% lower than weighted means. Distributions of osteoid seam widths were not normal, and all but one had more than one mode. We conclude that the Ar/Le method is comparable to direct orthogonal intercept methods and suitable for automated histomorphometry.

Analysis of Variance↗
PubMed2000

An Improved Method of Determining the zeta-Potential and Surface Conductance.

In the classical "slope-intercept" method of determining the zeta potential and the surface conductance, the relationship between DeltaP and E(s) is measured experimentally at a number of different channel sizes (e.g., the height of a slit channel, h). The parameter (epsilon(r)epsilon(0)DeltaP/µE(s)lambda(b)) is then plotted as a function of 1/h and linear regression is performed. The y-intercept of the regressed line is then related to the zeta-potential and its slope to the surface conductance. However, in this classical method, the electrical double layer effect or the electrokinetic effects on the liquid flow are not considered. Consequently, this technique is valid or accurate only when the following conditions are met: (1) relatively large channels are used; (2) the electrical double layer is sufficiently thin; and (3) the streaming potential is sufficiently small that the electroosmotic body force on the mobile ions in the double layer region can be ignored. In this paper a more general or improved slope-intercept method is developed to account for cases where the above three conditions are not met. Additionally a general least-squares analysis is described which accounts for uncertainty in the measured channel height as well as unequal variance in the streaming potential measurements. In this paper, both the classical and the improved slope-intercept techniques have been applied to streaming potential data measured with slit glass channels, ranging in height from 3 µm to 66 µm, for several aqueous electrolyte solutions. The comparison shows that the classical method will always overestimate both the zeta-potential and the surface conductance. Significant errors will occur when the classical method is applied to systems with small channel heights and low ionic concentrations. Furthermore, it is demonstrated that traditional regression techniques where the uncertainty is confined only to the dependent variable and each measurement is given equal weight may produce physically inconsistent results. Copyright 2000 Academic Press.

Journal Article↗
PubMed1993

Kinetics of regeneration of cross-bridge power stroke in shortening muscle.

The force developed by a muscle during steady shortening is due to cyclic interactions between the cross-bridges extending from the thick myosin filament to the thin actin filament. Each interaction consists of a power stroke of the myosin molecule that accounts for a limited amount of sliding between the two sets of filaments (about 12 nm according to quick release experiments), and is widely believed to be coupled to the hydrolysis of one ATP molecule. On the other hand both energetics studies in muscle and in vitro motility assays, indicating that shortening per ATP split is much larger than 12 nm, postulate that during shortening cross-bridges interact at a rate much faster than the ATP splitting rate. In the experiments reported here, made on intact fibres from frog skeletal muscle, the rate of regeneration of the power stroke was determined. Tension transients were elicited by imposing test step releases at different times (2-20 ms) after a conditioning release of about 5 nm. When the test step was imposed at 2 ms after the conditioning step, the tension attained at the end of the quick phase of recovery (T2, due to the force generating stroke of the attached cross-bridges) was depressed and the T2 curve (the plot of T2 tension versus size of the test step) intercepted the length axis to the right, with respect to the intercept of the control T2 curve, by an amount similar to the size of the conditioning step. By increasing the interval between conditioning and test step the T2 tension increased progressively and the T2 curve intercept approached the intercept of the control curve with a time constant of 6-7 ms. These results indicate that the force generating stroke elicited by a shortening step is followed by a relatively rapid process of detachment and reattachment by most of the cross-bridges, allowing for the generation of another power stroke. The rate of this process, 150/s, is one order of magnitude higher than that expected from the ATPase rate, suggesting that several actomyosin interactions occur in shortening muscle by the time one ATP is split. The results are stimulated with a mechanical kinetic model of contraction, in which, for a critical amount of shortening, cross-bridges can detach, rapidly reattach and generate force before the completion of the "normal" isometric cycle.

Animals↗
PubMed1995

Effect of end-point cadence on the maximal work-time relationship.

This study examined the effect of end-point cadence on the parameters of the work-time relationship determined for cycle ergometry. Eight male subjects completed four maximal tests on an electrically-braked cycle ergometer that regulated a constant power output independent of cadence. The power outputs imposed ranged between an average of 259 W and 403 W, whereas the corresponding durations ranged between 139 s and 1691 s. During each test subjects were required to maintain a cadence of 80-90 rpm. Accumulated time to end-point cadences of 70, 60 and 50 rpm were recorded. The four work-time determinations for each of three end-point cadences were used to determine linear relationships between work and time, yielding both a y-intercept, which represents anaerobic work capacity, and a slope, which is termed critical power (CP), for each end-point cadence. There was a significant increase in the y-intercept as end-point cadence decreased from 70 to 60 rpm (F[1,7] = 36.7, p < 0.001) or 70 to 50 rpm (F[1,7] = 80.1, p < 0.001), but not from 60 rpm to 50 rpm (F[1,7] = 3.28, p > 0.05). In contrast, there was no effect of end-point cadence on CP (F[2,14] = 1.89, p < 0.05). These results demonstrate that the end-point cadence selected to terminate tests only affects the y-intercept of the work-time relationship. To control for this effect, the cadence at which each test is terminated should be standardised if determination of anaerobic work capacity, as represented by the y-intercept, is required.

Adult↗