Home About us Contact
|
Home About us Contact | ||
|
|
||
Mechanical Advantage (mechanical + advantage)
Selected AbstractsRegional variation of intracortical porosity in the midshaft of the human femur: age and sex differencesJOURNAL OF ANATOMY, Issue 2 2005C. David L. Thomas Abstract This study investigated age and sex differences in patterns of porosity distribution in the midshaft of the human femur. Cross-sections were obtained from 168 individuals from a modern Australian population. The sample comprised 73 females and 95 males, aged between 20 and 97 years. Microradiographs were made of 100-µm sections and pore and bone areas were determined using image processing software. Initially the sample was divided by age: young (20,44 years), middle (45,64 years) and old (65+ years), but it was found that analysis on the basis of the ratio of medullary area to total subperiosteal area gave clearer results. The cortex was divided into three rings radially and into octants circumferentially and the porosity of each segment was calculated. Results showed that a pattern with raised porosity in the posterior and anterolateral regions, and with greater porosity in the inner parts of the cortex, becomes more pronounced with age. In males this pattern develops steadily; in females there are much greater differences between the middle and older groups than earlier in life. The patterns observed are consistent with progressive bone loss occurring along a neutral axis of the cortex where bending stress is lowest and the mechanical advantage of the bone is least. [source] Musculoskeletal underpinnings to differences in killing behavior between North American accipiters (Falconiformes: Accipitridae) and falcons (Falconidae)JOURNAL OF MORPHOLOGY, Issue 3 2008Diego Sustaita Abstract Accipiters (Accipiter spp.) and falcons (Falco spp.) both use their feet to seize prey, but falcons kill primarily with their beaks, whereas accipiters kill with their feet. This study examines the mechanistic basis to differences in their modes of dispatching prey, by focusing on the myology and biomechanics of the jaws, digits, and distal hindlimb. Bite, grip, and distal hindlimb flexion forces were estimated from measurements of physiological cross-sectional area (PCSA) and indices of mechanical advantage (MA) for the major jaw adductors, and digit and tarsometatarsal flexors. Estimated bite force, total jaw adductor PCSA, and jaw MA (averaged over adductors) tended to be relatively and absolutely greater in falcons, reflecting their emphasis on biting for dispatching their prey. Differences between genera in estimated grip force, total digit flexor PCSA, and digit MA (averaged over inter-phalangeal joints and digits) were not as clear-cut; each of these parameters scaled positively allometric in accipiters, which may reflect the scaling of both prey size, and the proportion of mammalian prey consumed by this lineage with increasing body size. Estimated tarsometatarsal force was greater in falcons than in accipiters, due to their greater MA, which may reflect selection for incurring greater forces during prey strikes. Conversely, the comparatively lower tarsometatarsal MA in accipiters reflects their capacity for greater foot speed potentially necessary for grasping elusive prey. Thus, this study elucidates how differences in jaw and hindlimb musculoskeletal morphology of accipiters and falcons are reflected in differences in their killing modes, and through differences in their force-generating capacities. J. Morphol., 2008. © 2007 Wiley-Liss, Inc. [source] The interplay between speed, kinetics, and hand postures during primate terrestrial locomotionAMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY, Issue 2 2010Biren A. Patel Abstract Nonprimate terrestrial mammals may use digitigrade postures to help moderate distal limb joint moments and metapodial stresses that may arise during high-speed locomotion with high-ground reaction forces (GRF). This study evaluates the relationships between speed, GRFs, and distal forelimb kinematics in order to evaluate if primates also adopt digitigrade hand postures during terrestrial locomotion for these same reasons. Three cercopithecine monkey species (Papio anubis, Macaca mulatta, Erythrocebus patas) were videotaped moving unrestrained along a horizontal runway instrumented with a force platform. Three-dimensional forelimb kinematics and GRFs were measured when the vertical force component reached its peak. Hand posture was measured as the angle between the metacarpal segment and the ground (MGA). As predicted, digitigrade hand postures (larger MGA) are associated with shorter GRF moment arms and lower wrist joint moments. Contrary to expectations, individuals used more palmigrade-like (i.e. less digitigrade) hand postures (smaller MGA) when the forelimb was subjected to higher forces (at faster speeds) resulting in potentially larger wrist joint moments. Accordingly, these primates may not use their ability to alter their hand postures to reduce rising joint moments at faster speeds. Digitigrady at slow speeds may improve the mechanical advantage of antigravity muscles crossing the wrist joint. At faster speeds, greater palmigrady is likely caused by joint collapse, but this posture may be suited to distribute higher GRFs over a larger surface area to lower stresses throughout the hand. Thus, a digitigrade hand posture is not a cursorial (i.e. high speed) adaptation in primates and differs from that of other mammals. Am J Phys Anthropol 2010. © 2009 Wiley-Liss, Inc. [source] Ontogeny and phyletic size change in living and fossil lemursAMERICAN JOURNAL OF PRIMATOLOGY, Issue 2 2010Matthew J. Ravosa Abstract Lemurs are notable for encompassing the range of body-size variation for all primates past and present,close to four orders of magnitude. Benefiting from the phylogenetic proximity of subfossil lemurs to smaller-bodied living forms, we employ allometric data from the skull to probe the ontogenetic bases of size differentiation and morphological diversity across these clades. Building upon prior pairwise comparisons between sister taxa, we performed the first clade-wide analyses of craniomandibular growth allometries in 359 specimens from 10 lemuroids and 176 specimens from 8 indrioids. Ontogenetic trajectories for extant forms were used as a criterion of subtraction to evaluate morphological variation, and putative adaptations among sister taxa. In other words, do species-level differences in skull form result from the differential extension of common patterns of relative growth? In lemuroids, a pervasive pattern of ontogenetic scaling is observed for facial dimensions in all genera, with three genera also sharing relative growth trajectories for jaw proportions (Lemur, Eulemur, Varecia). Differences in masticatory growth and form characterizing Hapalemur and fossil Pachylemur likely reflect dietary factors. Pervasive ontogenetic scaling characterizes the facial skull in extant Indri, Avahi, and Propithecus, as well as their larger, extinct sister taxa Mesopropithecus and Babakotia. Significant interspecific differences are observed in the allometry of indrioid masticatory proportions, with variation in the mechanical advantage of the jaw adductors and stress-resisting elements correlated with diet. As the growth series and adult data are largely coincidental in each clade, interspecific variation in facial form may result from selection for body-size differentiation among sister taxa. Those cases where trajectories are discordant identify potential dietary adaptations linked to variation in masticatory forces during chewing and biting. Although such dissociations highlight selection to uncouple shared ancestral growth patterns, they occur largely via transpositions and retention of primitive size-shape covariation patterns or relative growth coefficients. Am. J. Primatol. 72:161,172, 2010. © 2009 Wiley-Liss, Inc. [source] Clinical Use of Sirolimus-Eluting StentsCARDIOVASCULAR THERAPEUTICS, Issue 4 2007Ajay J. Kirtane ABSTRACT Drug-eluting stents, or intracoronary stents that combine the local delivery of antirestenotic pharmacologic therapies while maintaining the mechanical advantage of bare metal stents over balloon angioplasty alone, are a highly complex technology that have profoundly affected the practice of percutaneous coronary intervention over the last 5 years. These devices were designed specifically to treat the neointimal hyperplasia occurring after conventional bare metal stent placement, and have been remarkably successful in this regard. However, recent concerns have been raised regarding the long-term safety of these devices, particularly when used outside of the specific patient and lesion subsets studied in the pivotal randomized trials that led to device approval by regulatory bodies within the United States and abroad. This review aims to present a brief description of the sirolimus-eluting stent device platform and its mechanism of action, followed by an overview of current data regarding efficacy and safety regarding the clinical use of sirolimus-eluting stent technology. [source] A Unique Microcracking Process Associated with the Inelastic Deformation of Haversian BoneADVANCED FUNCTIONAL MATERIALS, Issue 1 2009Vincent Ebacher Abstract Since the discovery of the Haversian system in human bone over three hundred years ago, researchers have been wondering about its mechanical advantages. Despite positive experimental evidences on the intervention of Haversian systems in the fracture process, the contributions of Haversian systems to bone fracture have been obscure. Here a unique microcracking process accompanying the inelastic deformation of Haversian bone is reported that may shine light on its structural advantages over other bones. When compressed transversely, the concentric bone lamellae surrounding each Haversian canal allow multiple radial microcracks and arc-shaped cracks to develop intralamellarly. Groups of circumferential arc-shaped microcracks develop in high shear zones and radiate out in oblique directions from each Haversian canal. At the cortical bone level, where the Haversian systems are randomly distributed within the interstitial matrix, multiple nucleations and stable development of such arc-shaped cracks happen to most Haversian systems progressively. As a result, Haversian bone is not sensitive to the presence of Haversian canals and demonstrates high inelastic strains at macroscopic level. [source] Introduction of pH-sensitivity into mechanically strong nanoclay composite hydrogels based on N -isopropylacrylamideJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 19 2008Siddharthya K. Mujumdar Abstract pH-sensitive nanoclay composite hydrogels based on N -isopropylacrylamide (NIPA) were synthesized by copolymerization with cationic and anionic comonomers. Laponite nanoclay particles served as multifunctional crosslinkers, producing hydrogels with exceptionally high mechanical strengths, as measured by elongation at break. Cationic copolymer gels based on NIPA and dimethylaminoethylmethacrylate were prepared by aqueous free radical polymerization, adopting a procedure reported by Haraguchi (Adv Mater 2002, 14, 1120,1124). Without modification, this technique failed to produce anionic copolymer gels of NIPA and methacrylic acid (MAA), due to flocculation of clay particles. Three methods were conceived to incorporate acidic MAA into nanoclay hydrogels. First, NIPA was copolymerized with sodium methacrylate under dilute conditions, producing hydrogels with good pH-sensitivity but weak mechanical characteristics. Second, NIPA was copolymerized with methyl methacrylate, which was then hydrolyzed to generate acid sidegroups, yielding hydrogels that were much stronger but less pH sensitive. Third, NIPA was copolymerized with MAA following modification of the nanoclay surface with pyrophosphate ions. The resulting hydrogels exhibited both strong pH-sensitivities at 37 °C and excellent tensile properties. Optical transparency changed during polymerization, depending on hydrophobicity of the components. This work increases the diversity and functionality of nanoclay hydrogels, which display certain mechanical advantages over conventionally crosslinked hydrogels. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6630,6640, 2008 [source] | |||||||||||||