Debold, E. P., H. Dave, and R. H. Fitts. Fiber type and temperature dependence of inorganic phosphate: implications for fatigue. Am J Physiol Cell Physiol 287: C673-C681, 2004. First published May 5, 2004; 10.1152/ajpcell.00044.2004.-Elevated levels of P i are thought to cause a substantial proportion of the loss in muscular force and power output during fatigue from intense contractile activity. However, support for this hypothesis is based, in part, on data from skinned single fibers obtained at low temperatures (Յ15°C). The effect of high (30 mM) P i concentration on the contractile function of chemically skinned single fibers was examined at both low (15°C) and high (30°C) temperatures using fibers isolated from rat soleus (type I fibers) and gastrocnemius (type II fibers) muscles. Elevating P i from 0 to 30 mM at saturating free Ca 2ϩ levels depressed maximum isometric force (Po) by 54% at 15°C and by 19% at 30°C (P Ͻ 0.05; significant interaction) in type I fibers. Similarly, the P o of type II fibers was significantly more sensitive to high levels of P i at the lower (50% decrease) vs. higher temperature (5% decrease). The maximal shortening velocity of both type I and type II fibers was not significantly affected by elevated P i at either temperature. However, peak fiber power was depressed by 49% at 15°C but by only 16% at 30°C in type I fibers. Similarly, in type II fibers, peak power was depressed by 40 and 18% at 15 and 30°C, respectively. These data suggest that near physiological temperatures and at saturating levels of intracellular Ca 2ϩ , elevated levels of Pi contribute less to fatigue than might be inferred from data obtained at lower temperatures. skinned single fiber; force; power REPEATED HIGH-FREQUENCY STIMULATION of muscle results in a rapid decline in muscular force and power, with the degree of change dependent on the duration and intensity of activity as well as on the fiber type composition of the muscle (2, 17). The decline in force has been demonstrated to strongly correlate with an increase in muscle P i concentration (7,44,50). Although the strength of the correlation varies (1), in the later stages of fatigue, when maximal isometric force is depressed by Ͼ70%, the intracellular P i concentration can exceed 30 mM compared with the 1-5 mM level in resting fibers (44). These observations suggest that P i may play a causative role in fatigue.Skinned single muscle fiber studies provide convincing evidence that elevations in P i depress maximum isometric force (P o ) in a concentration-dependent manner (9 -11, 18, 28, 30 -32, 38). Notably, Potma et al. (38) observed a 55% decline in P o after increasing P i from 0 to 30 mM at 15°C in chemically skinned rabbit soleus fibers. Even larger P i -induced reductions in P o were observed in psoas myofibrils (43) and in single fibers when the initial P i concentration was reduced to ϳ5 m with the use of a P i mopping enzyme system (36). High levels of P i are thought to reduce force by reversing the P i release/force-generating step by mass act...

Purpose The purpose of this paper to study the tensile strength of the fused deposition modelling (FDM) printed PLA part. In recent times, FDM has been evolving from rapid prototyping to rapid manufacturing where parts fabricated by FDM process can be directly used for application. However, application of FDM fabricated part is significantly affected by poor and anisotropic mechanical properties. Mechanical properties of FDM part can be improved by proper selection of process parameters. Design/methodology/approach In the present study, three process parameter, namely, raster angle, layer height and raster width, have been selected to study their effect on tensile properties. Parts are fabricated as per ASTM D638 Type I standard. Findings It has been observed that the highest tensile strength obtained at 0° raster angle. Lower value of layer height is observed to be good for higher tensile strength because of higher bonding area between the layers. At higher value of raster width, tensile strength is improved up to certain extent after which presence of void reduces the tensile strength. Originality/value In the present investigation, layer height and raster width have been also varied along with raster angle to study their effect on the tensile strength of FDM printed PLA part.

Historically, an increase in intracellular H + (decrease in cell pH) was thought to contribute to muscle fatigue by direct inhibition of the cross-bridge leading to a reduction in velocity and force. More recently, due to the observation that the effects were less at temperatures closer to those observed in vivo, the importance of H + as a fatigue agent has been questioned. The purpose of this work was to re-evaluate the role of H + in muscle fatigue by studying the effect of low pH (6.2) on force, velocity and peak power in rat fast-and slow-twitch muscle fibres at 15• C and 30 • C. Skinned fast type IIa and slow type I fibres were prepared from the gastrocnemius and soleus, respectively, mounted between a force transducer and position motor, and studied at 15• C and 30• C and pH 7.0 and 6.2, and fibre force (P 0 ), unloaded shortening velocity (V 0 ), force-velocity, and force-power relationships determined. Consistent with previous observations, low pH depressed the P 0 of both fast and slow fibres, less at 30• C (4-12%) than at 15 • C (30%). However, the low pH-induced depressions in slow type I fibre V 0 and peak power were both significantly greater at 30• C (25% versus 9% for V 0 and 34% versus 17% for peak power). For the fast type IIa fibre type, the inhibitory effect of low pH on V 0 was unaltered by temperature, while for peak power the inhibition was reduced at 30• C (37% versus 18%). The curvature of the force-velocity relationship was temperature sensitive, and showed a higher a/P 0 ratio (less curvature) at 30• C. Importantly, at 30• C low pH significantly depressed the ratio of the slow type I fibre, leading to less force and velocity at peak power. These data demonstrate that the direct effect of low pH on peak power in both slow-and fast-twitch fibres at near-in vivo temperatures (30 • C) is greater than would be predicted based on changes in P 0 , and that the fatigue-inducing effects of low pH on cross-bridge function are still substantial and important at temperatures approaching those observed in vivo.

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Fused deposition modeling is an additive manufacturing process in which successive layers of material are deposited to create a three-dimensional object. It is the most widely used additive manufacturing process because of its ability to make specimen having difficult geometrical shape. However, building end-user functional parts using fused deposition modeling proved to be a challenging task because of a wide variety of processing parameters. In the present paper, a detailed experimental study on open source 3D printer is reported to explore the effect of various fused deposition modeling process parameters viz. part orientation, infill density and infill pattern on tensile properties and modes of failure. Poly-lactic acid filament is used to make 3D specimens. The experimental values of tensile properties are measured and critically analysed. Failure modes under various tests are studied using scanning electron microscopy. Tensile test results indicate that part orientation, infill pattern and infill density significantly affect the tensile strength. It has been observed that parts printed with flat part orientation and concentric pattern exhibit maximum tensile strength. While tensile strength has been increasing with increment in infill density. From the SEM images, it has been found that one of the major causes of failure is weak strength within and between layers for the lower value of infill density.