Effect Of Pre-stretching Research Articles

Optimizing working space in laparoscopy: CT measurement of the effect of pre-stretching of the abdominal wall in a porcine model

Determinants of working space in minimal access surgery have not been well studied. Using computed tomography (CT) to measure volumes and linear dimensions, we are studying the effect of a number of determinants of CO2 working space in a porcine laparoscopy model. Here we report the effects of pre-stretching of the abdominal wall. Earlier we had noted an increase in CO2 pneumoperitoneum volume at repeat insufflation with an intra-abdominal pressure (IAP) of 5 mmHg after previous stepwise insufflation up to an IAP of 15 mmHg. We reviewed the data of this serendipity group; data of 16 pigs were available. In a new group of eight pigs, we also explored this effect at repeat IAPs of 10 and 15 mmHg. Volumes and linear dimensions of the CO2 pneumoperitoneum were measured on reconstructed CT images and compared between the initial and repeat insufflation runs. Previous stepwise insufflation of the abdomen with CO2 up to 15 mmHg significantly (p < 0.01) increased subsequent working-space volume at a repeat IAP of 5 mmHg by 21 %, 7 % at a repeat IAP of 10 mmHg and 3 % at a repeat IAP of 15 mmHg. The external anteroposterior diameter significantly (p < 0.01) increased by 0.5 cm (14 %) at repeat 5 mmHg. Other linear dimensions showed a much smaller change. There was no statistically significant correlation between the duration of the insufflation run and the volume increase after pre-stretching at all IAP levels. Pre-stretching of the abdominal wall allows for the same surgical-field exposure at lower IAPs, reducing the negative effects of prolonged high-pressure CO2 pneumoperitoneum on the cardiorespiratory system and microcirculation. Pre-stretching has important scientific consequences in studies addressing ways of increasing working space in that its effect may confound the possible effects of other interventions aimed at increasing working space.

Research on the Effect of Pre-Stress on Bending Stiffness of the Ball Screw Based on ANSYS

In this paper, the effect of pre-stress of ball screws on bending stiffness is studied. By using ANSYS APDL parameterized method and adding stress stiffness effect, the relationship between the maximum deflection of ball screws under gravity load and pre-stress is given. The research result shows that with the increasing of pre-stress, the maximum deflection of the ball screw under gravity load gets smaller nonlinearly. The effects of pre-stretching of the ball screw on bending stiffness vary with different cross sections.

Timing and Magnitude of Prestretch Strongly Affect Cardiac Muscle Tension Development

Ectopic ventricular pacing leads to regional myocardial prestretch. In-vivo studies suggest that the timing of prestretch determines the effect on pump function and can lead to regional wall remodeling. Here we measure the direct effects of prestretch timing on the magnitude of tension development in isolated cardiac muscle. Prestretch strongly affected tension development but primarily during a brief interval of the twitch. The effects of prestretch were simulated using excised murine right ventricular papillary muscles. A high-speed computer-controlled motor was used to impose precisely timed stretches, while a force transducer measured force output and strain was monitored using a CCD camera and topical markers. The timing of the stretches greatly influenced tension production. A critical stretch timing interval was observed to begin 25 ms after stimulation and extending until 100 ms after stimulation, where stretches occurring in this interval exhibited statistically significant increases in peak tension of as much as 500%. Muscle shortening occurring in the critical interval (from stretches initiated prior to stimulation) showed the opposite effect of stretching in the interval, with peak tension inhibited significantly by as much as 40%. A simple model showed that the varying impact of stretch timing could not be explained by time-varying elastance alone. Therefore, a more detailed and mechanistic myofilament model that included cooperative activation (Campbell et al 2010) was refined to include length dependence and strain-dependent cross-bridge kinetics. By comparing model results and experimental measurements, the strain history dependent mechanisms of these observations was evaluated. These evaluations have led us to the conclusion that even prestretches performed at slow physiologic velocities cause alterations in twitch kinetics that cannot be explained by time varying elastance.

Effects of Pre-stretching on the Tensile Properties of Knitted Glass Fiber Fabric Reinforced Polypropylene Composite

Weft knitted glass fiber (GF) fabric reinforced polypropylene (PP) composite sheets were fabricated by hot pressing of stacked rib 1: 1 knitted fabric preform layers. The knitted preforms were made from GF/PP commingled yarn containing 50 vol% (≈73 wt%)GF. Before hot pressing, the preforms were stretched in the wale direction at various ratios. The tensile properties of the pre-stretched GF-PP composites were studied. It was found that there is a critical value of pre-stretch ratio as far as tensile stiffness and tensile strength are concerned. On increasing the pre-stretch ratio within the range of this critical value, both tensile stiffness and tensile strength increase. If the pre-stretch ratio goes beyond this critical value and continues to increase, both tensile stiffness and tensile strength display a tendency of slight decrease because of the break of more glass fibers. The failure strain of the composites decreases with the increase of the pre-stretch ratio because of the decrease of the stretchability of the knitted reinforcements.

Tensile Properties of Glass Fiber Knitted Fabric Reinforced Polypropylene Composite Made from GF/PP Commingled Yarn Affected by Prestretching

The effects of prestretching of rib 1: 1 knitted fabric preform on the tensile properties of GF/PP composites are investigated in this study. The preform is made from GF/PP commingled yarn. As far as tensile stiffness and tensile strength are concerned, there is a critical value of prestretch ratio. An increase in the prestretch ratio within the range of this critical value increases both the tensile stiffness and the tensile strength. If the prestretch ratio goes beyond this critical value and continues to increase, both tensile stiffness and tensile strength display a tendency of slight decrease because of the break of more glass fibers. The failure strain of the composites decreases with the increase of the prestretch ratio because of the decrease of the stretchability of the knitted reinforcements.

Effects of prestretching on force degradation of synthetic elastomeric chains

The purpose of this study was to evaluate the effects of prestretching on time-dependant force decay of synthetic elastomeric chains. Five-unit (12.5 mm) and 6-unit (15.5 mm) modules (Generation II, Ormco, Glendora, Calif) were prestretched 100% for 1 hour (n = 12), 24 hours (n = 12), 2 weeks (n = 12), and 4 weeks (n = 12) in 37 degrees C distilled water. The prestretched and unprestretched (control) modules were then stretched to 30 mm in 37 degrees C water, and their forces were measured at 0 hour, 1 hour, 24 hours, and weekly for 4 weeks with a digital force gauge. The prestretched 5- and 6-unit modules yielded significantly lower initial force than the controls. All 5- and 6-unit prestretched and control groups showed substantial force decay during the first hour. However, at 1 hour, similar remaining forces were found in the 5-unit prestretched and control groups (P > .05), and small differences were seen in the 6-unit groups. The rates and patterns of force decay from 1 hour to 4 weeks were quite similar between the control and the prestretched modules of both the 5- and 6-unit groups. The effects of prestretching on force decay of elastomeric chains were noted mainly in the first hour. Thus, the clinical value of prestretching a synthetic elastomeric chain is questionable.

Development of a novel flexible composite material

The primary objective of this research work is to develop a thin and flexible composite material suitable for biomedical and electronic packing applications. A flexible composite of 0.2 mm thickness was developed using knitted polyester fiber interlock fabric as the reinforcement and polyurethane elastomer as the matrix. Details regarding composite fabrication and mechanical characterization are presented. The failure strain of this new flexible composite material is as high as 85–320%, depending on the fiber architecture. Another objective of this study is to identify the effect of pre-stretching of the knitted fabric on the composite tensile properties. For this purpose different composite specimens were made by reinforcing polyurethane with knitted fabrics that were pre-stretched differently prior to composite fabrication. Both the stiffness and strength of composite improved in the direction of fabric pre-stretch and they deteriorated in the direction normal to pre-stretch. However, the failure strain followed the opposite trend. These trends are attributed to the changes in the failure mechanisms with the pre-stretched condition of the fabric.

The Effect of Prestretch on Skeletal Muscle Behavior

The Effect of Prestretch on Skeletal Muscle Behavior

Effects of Pre-Stretching and Aging Process on Tensile Properties of 2195 Al-Cu-Li Alloy

Effects of Pre-Stretching and Aging Process on Tensile Properties of 2195 Al-Cu-Li Alloy

The Potentiating Effect of Prestretch on the Contractile Performance of Rat Gastrocnemius Medialis Muscle During Subsequent Shortening and Isometric Contractions

The aim of the present study was to investigate the effect of an active stretch during the onset of a muscle contraction on subsequent active behaviour of the contractile machinery within an intact mammalian muscle-tendon complex. Muscle length and shortening velocity were studied because they may be important variables affecting this so-called prestretch effect. Seven gastrocnemius medialis (GM) muscles of the rat were examined. Tetanic, isovelocity shortening contractions from 3 mm above muscle optimum length (l0) to l0 - 2 mm, at velocities of 10-50 mm s-1 (dynamic experiments), were preceded by either an isometric contraction (PI) or an active stretch (PS). By imposing quick length decreases between the prephase and the concentric phase, all excess force generated in the prephase was instantaneously eliminated. This procedure only allowed small force changes during subsequent shortening (caused by the intrinsic properties of the contractile machinery). In this way, the influence of series elastic structures on subsequent muscle performance was minimized. Experiments were also performed at lengths ranging from l0 + 2.5 mm to l0 - 1.5 mm, keeping the length constant after the initial quick length changes (isometric experiments). For the dynamic experiments, enhancement of the performance of the contractile machinery (potentiation) was calculated as the ratio of the average force level over each millimetre of shortening during PS to that during PI conditions (PS/PI). For the isometric experiments, the PS/PI force ratio after 300 ms of stimulation was used. The main result of the present study confirmed results reported in the literature and experiments on isolated muscle fibres. For all conditions, a potentiation effect was found, ranging from about 2 to 16%. Muscle length appeared to have a large positive effect on the degree of potentiation. At the greatest lengths potentiation was largest, but at lengths below optimum a small effect was also found. A negative influence of shortening velocity was mainly present at increased muscle lengths (l0 + 2.5 mm and l0 + 1.5 mm). For the dynamic experiments, no interaction was found between the effects of muscle length and shortening velocity on potentiation. However, there was a clear difference between the isometric and dynamic responses: the dependence of potentiation on muscle length was significantly greater for the isometric contractions than for the dynamic ones. These isometric-dynamic differences indicate that the processes underlying prestretch effects operate differently under isometric and dynamic conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of prestretch at the onset of stimulation on mechanical work output of rat medial gastrocnemius muscle-tendon complex

Work output of rat gastrocnemius medialis (GM) muscle (N = 5) was measured for stretch-shortening contractions, in which initiation of stretch occurred prior to the onset of activation, and for contractions with an isometric prephase. Duration of the active prephase (prestretch and pre-isometric) varied from 20 to 200 ms. Subsequent shortening (from optimum length + 4 mm to optimum length -2mm) lasted 150 ms. Stretch velocities of 5, 10 and 20 mm s-1 were used, and the shortening velocity was 40 mm s-1. The effects of several combinations of active stretch duration and active stretch amplitude were compared. Using force-compliance characteristics, the work of the contractile element (CE), elastic energy storage and release of the undamped series elastic component (SEC) were distinguished. During shortening, an extra amount of work output was produced, induced by active stretch, of which the largest contribution (70-80%) was due to higher elastic energy release. Enhancement of the storage and utilization of elastic energy during the stretch-shortening cycle, caused by higher transition-point forces (i.e. force at onset of shortening), increased with active stretch amplitude and was associated with a net loss of work, probably due to cross-bridge detachment during active stretch. Net work over the stretch-shortening cycle remained positive for all prestretch contractions, indicating that when a muscle performs this type of contraction, it is able to contribute to work performance on body segments. It is concluded that, in stretch-shortening movements of rat GM muscle, maximal positive work output is incompatible with maximal net work output. Consequences for complex movements in vivo are discussed.

Relationship between the efficiency of muscular work during jumping and the energetics of running.

The running economy of seventeen athletes was studied during running at a low speed (3.3 m X s-1) on a motor-driven treadmill. The net energetic cost during running expressed in kJ X kg-1 X km-1 was on average 4.06. As expected, a positive relationship was found between the energetic cost and the percentage of fast twitch fibres (r = 0.60, n = 17, p less than 0.01). In addition, the mechanical efficiency during two different series of jumps performed with and without prestretch was measured in thirteen subjects. The effect of prestretch on muscle economy was represented by the ratio between the efficiency of muscular work performed during prestretch jumps and the corresponding value calculated in no prestretch conditions. This ratio demonstrated a statistically significant relationship with energy expenditure during running (r = -0.66, n = 13, P less than 0.01), suggesting that the elastic behaviour of leg extensor muscles is similar in running and jumping if the speeds of muscular contraction during eccentric and concentric work are of similar magnitudes.

Pressurising a prestretched membrane to form a paraboloid

A flat rubber membrane of variable thickness is stretched in its plane and then inflated over a circular frame. A theory is presented which enables the thickness profile and inflation pressure to be determined in order that the inflated membrane is a paraboloid of revolution. The problem is reduced to solving one ordinary non-linear differential equation. A series solution is presented, valid for any amount of inplane prestretch but for moderately shallow bowls. It is shown that a certain amount of prestretch is essential and that the initial thickness profile cannot be constant. Graphical results illustrating the effects of prestretch, bowl depth and material constants on inflation pressure and thickness profile are obtained from the analytic solution.