Twist-to-Bend Ratios and Cross-Sectional Shapes of Petioles and Stems

Two structural properties, resistance to twisting (torsionalrigidity or torsional stiffness, GJ) and resistance to bending(flexural rigidity or flexural stiffness, El), were measuredon a variety of herbaceous stems and petioles. Specimens withnon-circular cross-sections had higher values of the ratio ofEl to GJ that is, such specimens were relatively more flexiblein twisting than in bending. But both kinds had higher ratiosthan those that characterize simple, isotropic materials, andthus both structural and material factors contribute to achievinga high twist-to-bend ratio. The composite property expressedas the dimensionless variable EI/GJ appears to be a functionallyrelevant parameter in many biological situations. Key words: Stems, petioles, flexural rigidity, torsional rigidity, biomechanics     

The Mechanical Significance of Clasping Leaf Sheaths in Grasses: Evidence from Two Cultivars of Avena sativa

The elastic (Young's) modulus and flexural rigidity of internodeswith and without their clasping leaf sheaths were determinedfor culms from two cultivars (‘Astro’) and (‘Garry’)of Avena sativa L. Data indicate that early in the developmentof culms, leaf sheaths can have a higher elastic modulus thanthe internodes they envelope, and by virtue of their location,leaf sheaths contribute significantly to the flexural rigidity(hence, resistance to bending) of internodal segments. As culmsmature, the elastic modulus of leaf sheath and internodal tissuesreach parity. However, because of the acropetal pattern by whichnew internodes are produced by shoot apices, sheaths continueto provide mechanical support to distal internodes, particularlythe peduncle. Data for the two cultivars indicate that the elasticmodulus and flexural rigidity of culms can vary significantlywithin the species. Comparisons between the flexural rigidityof the two cultures and the resistance of stems to lodging indicatethat flexural rigidity is not significant to lodging. The engineeringprinciples relevant to the mechanical advantages conferred byclasping leaf sheaths are discussed within the context of grassshoot morphology. Biomechanics, leaf sheath, Avena, elastic modulus     

Mechanical loading of orthodontic miniscrews - significance and problems: an experimental study

Orthodontic miniscrews are exposed to three mechanical loading phases during clinical use: torsional loading upon insertion, flexural loading during anchorage function, and torsional loading upon removal. The aim of this study was to simulate clinical loading conditions for different types of orthodontic miniscrews in vitro to quantify the effects of combined torsional and bending stress. Various orthodontic miniscrew systems (Lomas, Dual-top, Aarhus anchorage, Tomas-pin and T.I.T.A.N.-pin) comprising 10 samples each were subjected to the following loading sequences in vitro: a torsional load corresponding to manual insertion with limited torque; and flexural loading at two different insertion depths. For all screw systems with torsional pre-loading (simulating insertion), subsequent flexural loading (simulating anchorage) yielded permanent deformations of approximately 0.15-0.25 mm, depending on the insertion depth. Since EDX analysis revealed comparable elemental compositions for the different screw systems, the differences in mechanical properties are attributed to screw design. Torsional loading during screw insertion may cause premature mechanical weakening and needs to be minimized. Unless fully inserted, screws show pronounced plastic deformation and hence fracture risk under subsequent flexural loading.     

The Role of Host Vigour in the Susceptibility of Pines to Fomes annosus

The resistance of suppressed pines to F. annosus and in particularthe role of such trees as infection foci in disease-free standsis a matter of potential significance for the spread of thepathogen. Mycelial inoculations showed that, in contrast todominant trees, the roots of very heavily suppressed trees possessno active resistance to the spread of the pathogen. However,at this extreme stage of suppression root-surface-fungi canalso invade the roots and to a considerable extent replace thepathogen. Spore inoculations of small root wounds gave similarresults except that the rate of infection was very much slower. Mycelial inoculations with F. annosus, Leptographium lundbergii,Peniophora gigantea, and Trichoderma viride were carried outin the trunks of pines suffering from different degrees of suppression.The fungi differed in their adaptation to the host, L. lundbergiibeing the most specialized to the biological, chemical, andphysical characteristics of pine wood and T. viride the least.This adaptation was shown to be inversely related to the competitivesaprophytic ability of each fungus, including the ability toproduce and tolerate antibiotics.     

Hydraulic Conductivity and Anatomy for Lateral Roots of Agave deserti During Root Growth and Drought-induced Abscission

Hydraulic conductivity (L_p), radial conductivity (L_R), axialconductance (K_h), and related anatomical characteristics forlateral roots of Agave deserti were investigated during rootgrowth and drought-induced abscission. The elongation rate oflateral roots averaged 5 mm d^–1 under wet conditions andwas reduced 95% by 17 d of drought (     

Biomechanical and Morphometric Differences in Triticum aestivum Seedlings Differing in Rht Gene-Dosage

Biomechanical and morphometric comparisons among coleoptilesfrom wheat seedlings differing in Rht gene-dosage (Rht = 0,2, 4 doses) are presented in an effort to evaluate the influenceof Rht on the mechanics of soil penetration by this organ. Rhtis known to reduce seedling establishment compared to the wildtype. Data from 3–7-day-old seedlings indicate that Rhtreduces tissue elastic modulus E, increases the second momentof area I, and decreases the slenderness ratio (l/r) of coleoptiles.Rht-relatedchanges in E and I are such that the flexural stiffness of coleoptilesfrom Rht plants does not differ significantly from the wildtype-hence the growing coleoptiles of all three genotypes haveequivalent biomechanical capacity to penetrate the soil. Rhtreduction of coleoptile slenderness ratios confers a capacityto safely sustain higher axial compressive loads compared tocoleoptiles with equivalent flexural stiffness but higher ratios.However, wild type seedlings produce longer coleoptiles andlonger subcrown internodes than Rht seedlings. Longer coleoptilesdeliver the crown node closer to the top of the soil beforethe crown node extends beyond the lateral confinement of thecoleoptile. This reduces the potential for buckling of the subcrowninternode and leaves due to the compressive loading of soil.Rht affects a variety of mechanical features whose influenceis dependent upon the stage of seedling growth and the degreeof soil compaction. However, at equivalent depths of burialwhich exceed the maximum length of coleoptiles and moderatesoil compaction, Rht is biomechanically disadvantageous to seedlingestablishment. Wheat, germination, biomechanics, Rht-gene     

Mechanics and Growth Form of the MossHylocomium splendens

The mossHylocomium splendenshas two different growth forms.Sympodial growth occurs where the apical meristem ceases activityannually and growth is continued by a lateral bud. Sympodialplants are vertical and self-supporting. Monopodial growth occurswhen the apex continues growth. Monopodial plants are prostrate.The aims of the study were to examine stem mechanics of thedifferent growth forms and to compare mechanical propertiesalong stems. Stems of annual segments were subjected to threepoint bending tests. In sympodial plants the stiffness of thestem material increased significantly with segment age. Flexuralrigidity increased significantly with age in segments from 1to 4 years old, and then declined. Segment diameter decreasedsignificantly with age in sympodial plants. Monopodial plantsshowed no significant effects of segment age on the diameter,material stiffness or flexural rigidity of stems. Sympodialsegments were significantly wider, stronger, more rigid andcomposed of stiffer material with a higher stress at yield thanmonopodial segments, but did not have a larger proportion ofstrengthening material. Sympodial stems had significantly morecellulose than monopodial stems. The mossHylocomium splendensshowsa range of mechanical adaptations, as a self-supporting or aprostrate plant, which suit it to life in very different environments.Copyright1998 Annals of Botany Company. Hylocomium splendens, growth form, mechanics, cellulose, stiffness, flexural rigidity, bending, monopodial, sympodial, adaptation, anatomy, stems, plant.     

Using "Mighty Mouse" to understand masticatory plasticity: myostatin-deficient mice and musculoskeletal function

Knockout mice lacking myostatin (Mstn), a negative regulatorof the growth of skeletal muscle, develop significant increasesin the relative mass of masticatory muscles as well as the abilityto generate higher maximal muscle forces. Wild-type and Mstn-deficientmice were compared to investigate the postnatal influence ofelevated masticatory loads due to increased jaw-adductor andbite forces on the biomineralization of mandibular articularand cortical bone, the internal structure of the jaw joints,and the composition of temporomandibular joint (TMJ) articularcartilage. To provide an interspecific perspective on the long-termresponses of mammalian jaw joints to altered loading conditions,the findings on mice were compared to similar data for growingrabbits subjected to long-term dietary manipulation. Statisticallysignificant differences in joint proportions and bone mineraldensity between normal and Mstn-deficient mice, which are similarto those observed between rabbit loading cohorts, underscorethe need for a comprehensive analysis of masticatory tissueplasticity vis-à-vis altered mechanical loads, one inwhich variation in external and internal structure are considered.Differences in the expression of proteoglycans and type-II collagenin TMJ articular cartilage between the mouse and rabbit comparisonssuggest that the duration and magnitude of the loading stimuluswill significantly affect patterns of adaptive and degradativeresponses. These data on mammals subjected to long-term loadingconditions offer novel insights regarding variation in ontogeny,life history, and the ecomorphology of the feeding apparatus.     

Variations of the mechanical properties of Acer saccharum roots

Longitudinal and transverse variations in the compressive strength c and stiffness measured in bending and torsion (E and G, respectively) were determined for wood samples removed from three old lateral roots of a large sugar maple Acer saccharum L. whose canopy had been heavily pruned on one side such that the three roots had loading conditions assumed to be dominated by different stresses: tensile stresses along the axis of root I, compressive stresses along root III, and torsional shear stresses in root II.Values of c, E and G tended to decrease from the base toward the tip of each root, but maximized at approximately 1 m from the base (perhaps as a consequence of fibre fatigue near the base of the root system). Comparisons between the mechanical properties of wood sampled from the upper and lower root surfaces showed that the wood along the lower surface of roots I and II was stiffer and stronger; the reverse was found to root III.Longitudinal variations in the geometry and size of root transactions resulted in a curvilinear pattern of root taper; maximum taper occurred at 1 m from each root base. These variations, which resulted in longitudinal changes in the second moment of area I and the polar second moment of area J, were sufficient to mask the effects of longitudinal variations in E and G on the flexural and torsional rigidity of roots (El and GJ, respectively) both of which decreased exponentially away from the base of the tree.Based on the correlations observed between the stresses presumed to dominate the loading conditions of each of the three roots and the mechanical properties of wood samples, it appears that the 'biomechanical plasticity' of woody roots holds the potential to maximize the individual ability of roots to cope with the dominant stresses attending their growth and development.     

Transpiration Stream of Desert Species: Resistances and Capacitances for a C3, a C4, and a CAM Plant

Transpiration rates and water potentials of three sympatricdesert perennials, a C3 subshrub (Encelia farinosa), a C₄ bunchgrass(Hilaria rigida), and a CAM succulent (Agave deserti), wereanalysed using an electrical circuit analogue that includedresistances and capacitances for the leaves, stems, and roots.The water storage capability of the organs differed considerably,capacitance ranging over 1000-fold from the thin leaves of H,rigida to the massive leaves of A. deserti, although the capacitanceper unit volume varied only 1.9-fold. The diurnal changes inwater storage could support maximum transpiration rates of H.rigida for 4 min, E. farinosa for 7 min, and A. deserti for16 h. The time constant for equilibration of water from storageto the xylem ranged from 29 s for roots of H. rigida to 52 minfor leaves of A. deserti. Resistances for such movement wererelatively low for the succulent leaves of A. deserti and wereup to about 50-fold higher for the three organs of E. farinosa.Xylem resistances calculated using the Hagen-Poiseuille lawand measured xylem dimensions were 2.1- to 2.1-fold lower thanresistances estimated from observed water potential drops, adiscrepancy which is in agreement with other published data.Contrary to data on other plants, the xylem resistances in theroots and leaves of E. farinosa and H. rigida averaged only15% of the stem xylem resistance. Key words: Capacitance, Xylem resistance, Transpiration stream, Desert     

Effects of soil mechanical impedance on root and shoot growth of Lolium perenne L., Agrostis capillaris and Trifolium repens L.

A method was used for applying a uniform mechanical impedanceto plant roots using sand packed at different bulk densitieswith depth within each growth cylinder. In a growth experiment,replicate cylinders were packed with sand to give the followingmechanical impedances: 0.25 MPa penetration resistance (negligibleimpedance), 1.40 MPa (moderate impedance) and 2.30 MPa (severeimpedance). Seedlings of Lolium perenne L., Trifolium repensL. and Agrostis capillaris were grown for 23 d in each impedancetreatment and effects on both roots and shoots were studied.Severe mechanical impedance affected both root and shoot growthrates for all three species resulting in smaller leaves andshorter roots. For the grasses, the root-to-shoot ratio at harvestwas the same for all the treatments, but a delay in the initiationof both shoots and roots was observed in the severe mechanicalimpedance treatment. The results are discussed in relation tothe possibility that roots penetrating the impeding treatmentsmay have caused signalling that kept shoot and root growth insynchrony. Differences observed in the response to mechanicalimpedance of T. repens compared to the other two species maybe a function of differences in the physiology between mono-and dicotyledonous species. Key words: Mechanical impedance, Lolium perenne, Trifolium repens, roots, shoots     

Cross sectional geometry of the forelimb skeleton and flight mode in pelecaniform birds

Avian wing elements have been shown to experience both dorsoventral bending and torsional loads during flapping flight. However, not all birds use continuous flapping as a primary flight strategy. The pelecaniforms exhibit extraordinary diversity in flight mode, utilizing flapping, flap‐gliding, and soaring. Here we (1) characterize the cross‐sectional geometry of the three main wing bone (humerus, ulna, carpometacarpus), (2) use elements of beam theory to estimate resistance to loading, and (3) examine patterns of variation in hypothesized loading resistance relative to flight and diving mode in 16 species of pelecaniform birds. Patterns emerge that are common to all species, as well as some characteristics that are flight‐ and diving‐mode specific. In all birds examined, the distal most wing segment (carpometacarpus) is the most elliptical (relatively high I_max/I_min) at mid‐shaft, suggesting a shape optimized to resist bending loads in a dorsoventral direction. As primary flight feathers attach at an oblique angle relative to the long axis of the carpometacarpus, they are likely responsible for inducing bending of this element during flight. Moreover, among flight modes examined the flapping group (cormorants) exhibits more elliptical humeri and carpometacarpi than other flight modes, perhaps pertaining to the higher frequency of bending loads in these elements. The soaring birds (pelicans and gannets) exhibit wing elements with near‐circular cross‐sections and higher polar moments of area than in the flap and flap‐gliding birds, suggesting shapes optimized to offer increased resistance to torsional loads. This analysis of cross‐sectional geometry has enhanced our interpretation of how the wing elements are being loaded and ultimately how they are being used during normal activities. J. Morphol., 2011. © 2011 Wiley‐Liss,Inc.     

A Test for the Resource Remobilization Hypothesis: Tree Sprouting using Carbohydrates from Above-ground Parts

To test the resource remobilization hypothesis, i.e. the hypothesisthat some trees sprout from root-collars or from the lower partof trunks using resources obtained from above-ground parts ratherthan from resources reserved in their roots, we conducted cuttingexperiments forEuptelea polyandra, a frequently sprouting treespecies with little carbohydrate reserves in its roots,Quercusserrata,a frequently sprouting tree species with large reservesin the roots, andMallotus japonicus, a rarely sprouting treespecies. Trees of each species were cut down in winter leavingtwo kinds of stumps, those approx. 1.5 m in height and thosecut off near the ground. The number and total dry weight ofnewly sprouted shoots per stump were compared between the twotreatments and among the three species at the end of the followinggrowing season. InE. polyandra,both the number and total dryweight of sprouts per stump were very small for both treatmentsand were similar to, or less than, those ofM. japonicus. Onthe other hand,Q. serratasprouted abundantly in both treatments.These results indicate thatE. polyandracannot sprout sufficientlywithout a considerably large volume of above-ground parts orthat additional structures such as foliage and branches maybe necessary for sprouting. We conclude that the resource remobilizationhypothesis is supported for this species.Copyright 1998 Annalsof Botany Company Euptelea polyandraSieb. et Zacc,Quercus serrataThunb,Mallotus japonicus(Thunb.) Muell. Arg., tree sprouting, cutting experiment, resprouter, resource movement, carbohydrate allocation, ground-surface disturbance, root stock, resource remobilization hypothesis.     

Change in Size and Cell Number during the Development of Lateral Root Primordia in Zea mays L.

Increase in cell number, anlage volume and length have beeninvestigated during the overall period of lateral root primordiumdevelopment in excised primaries and in attached roots of Zeamays L. Each of these aspects of anlage growth was found toincrease more or less exponentially during the interval betweenprimordium initiation and subsequent emergence as a lateralin both batches of roots. Values were then determined for celldoubling time (Td), the size of the proliferative fraction (Pf),and for anlage volume (Tv) and length (Tt) doubling times duringthe overall period of primordium development and at intervalsduring this period in both the excised and attached roots. Thepattern of change which took place in Td, Tv, Tl and Pf duringlateral primordium development was found to be similar in bothbatches of roots. However, the overall period of anlage developmentwas shorter in the excised roots than in the attached ones.Moreover, when laterals grew out of the excised roots they didso with fewer cells than comparable laterals emerging from theattached roots. Zea mays L., maize, root primordia, lateral emergence, cell doubling time     

褐飞虱侵害对不同抗性水平水稻根部吸收氮、磷、钾的影响

为了解褐飞虱Nilaparvata lugens侵害后水稻抗性水平与根系吸收氮（N）、磷（P）、钾（K）的关系,对不同抗性水平的水稻受褐飞虱侵害后的平均受害水平、植株功能损失指数及水稻吸收水培液营养元素N,P,K情况进行了研究。结果表明：不同水稻品种（TN1、协优63、协优963和超级培矮64S/E32）接种褐飞虱后,TN1受害最严重,协优63和超级培矮64S/E32 次之;受害最轻的是协优963,30头/株侵害后植株功能损失指数仅为0.661。40,60和80头/株侵害水稻后根系对K吸收下降程度最显著,其次为P,最后为N;且随褐飞虱侵害时间的延长（6 d, 9 d）影响愈显著。60头/株侵害TN1、协优63、协优963和超级培矮64S/E32后9 d,根系对K吸收下降率分别为164.11%,74.61%,55.16%和46.60%。由此可见,随水稻品种抗性水平的下降,接种褐飞虱后,水稻根部对水培液营养元素吸收下降程度愈显著。本研究结果可对深入阐明不同抗性水平水稻抗（耐）虫机制与根系吸收能力关系提供参考。     

An ELISA Procedure for Quantification of Relative Amounts of Intercellular Glycoprotein in Legume Nodules

An enzyme-linked immunosorbant assay (ELISA) method based ona monoclonal antibody (MAC236) is described in which relativeamounts of an intercellular glycoprotein were quantified inextracts of whole legume nodules. This glycoprotein has recentlybeen shown to be an important component of the cortical oxygendiffusion barrier. The ELISA method is demonstrated on threeexamples of soybean (Glycine max L. Merr.) nodule systems whichhave been the subject of previously published investigations:(a), cv. Clarke inoculated with Bradyrhizobium japonicum RCR3442,nodulated root systems of which were subject to 10, 21 or 40%oxygen continuously for 28 d; (b), cv. Bragg and its supernodulatingmutant derivative (nts382) inoculated with Bradyrhizobium japonicumUSDA110; (c), cv. Clarke inoculated with Bradyrhizobium japonicumRCR3442 or RCR3407. ELISA results are related to oxygen diffusioncharacteristics defined in previous publications and show thatincreases in the amount of glycoprotein present correlated withincreases in supra-ambient (40%) levels of rhizosphere pO₂,in minimum gas diffusion resistance and in speed of diffusionbarrier response. Area data of component parts of nodule inner cortices suggestthat diffusion resistance control under sub-ambient (10%) oxygenlevels also involves cell expansion. The amount of MAC236 antigen in nodules is affected by bothhost plant genotype and rhizobial strain and the latter alsoappears to be involved in determining the morphological developmentof the nodule inner cortex.Copyright 1993, 1999 Academic Press Oxygen diffusion resistance, glycoprotein, nodules, Glycine max, ELISA     

Uptake of 15N by Kiwifruit Vines from Applications of Nitrogen Fertilizer Prior to Budbreak

Mature field-grown kiwifruit vines (Actinidia deliciosa var.deliciosa cv. Hayward) were fertilized with ¹⁵N-labelled fertilizer(ammonium sulphate, 10 atom % ¹⁵N, 50 kgN ha^-1) to investigatethe timing of uptake of fertilizer nitrogen (N) and its availabilityfor new season's growth. Treatments were applied on four occasions,representing 2, 6, 10 and 14 weeks prior to budbreak. Samplesof root, stem, cordon, fruiting cane, vacuum-extracted xylemsap, and new season's growth were collected at fortnightly intervalfrom early winter until 2 months after budbreak. Two weeks after application of each treatment, ¹⁵N equivalentto an average of 7% of the applied label was recovered in rootmaterial. Although label was taken up by roots, there was nomovement of ¹⁵N within the plant until about 1 month prior tobudbreak when it was measured in the stem and cordon. Fertilizernitrogen was not detected at the distal end of fruiting canes,and in new season's growth until 3-4 weeks after budbreak. Beforebudbreak, all nitrogen in the xylem sap was in amino forms.Nitrate appeared 4 weeks after budbreak, and although more enrichedwith ¹⁵N than the amino nitrogen, accounted for only 19% ofthe label. Eight weeks after budbreak, nitrate nitrogen accountedfor 57% of the label. There were no major treatment effects of ¹⁵N on vines in eitherspring or at harvest, although enrichments in fruit and leavesfrom the earliest treatment tended to be less at the end ofthe season than those from the later applications.Copyright1993, 1999 Academic Press Actinidia deliciosa, kiwifruit, nitrogen, ¹⁵N, nutrient uptake     

Effects of Water Stress on Fruit Quality Attributes of Kiwifruit

Four-year-old kiwifruit vines (Actinidia deliciosa(A. Chev.)C. F. Liang et A. R. Ferguson var.deliciosacv. Hayward) werestudied to determine response of the plant and effects on fruitquality when irrigation water was withheld either early or latein the growing season. The greatest effect on fruit growth occurredwhen water was withheld early in the season. Harvest weightof fruit from early-stressed vines was approx. 25% less thanthe weight of fruit on control vines. Early season water stressresulted in a transient increase in concentrations of solublecarbohydrates in both leaves and fruit. This was accompaniedby a reduction in stomatal conductance of the leaves. Starchlevels in leaves but not fruit were reduced by both stress treatments.Concentrations of sucrose at harvest in fruit from vines stressedlate in the season were markedly higher than in other fruit,and softness of the fruit was unaffected. These differenceswere maintained through the 12 weeks in cool storage after harvest.Withholding irrigation water to kiwifruit vines late in theseason may prove a useful management tool to manipulate somequality attributes of the fruit.Copyright 1998 Annals of BotanyCompany Kiwifruit;Actinidia deliciosa; water stress; fruit quality; soluble solids.     

Seasonal Accumulation of Starch by Components of the Kiwifruit Vine

The accumulation of starch by various components of 6-year-oldkiwifruit vines (Actinidia deliciosa var dehciosa cv Hayward)was recorded over one season Twenty vines were harvested periodicallythroughout the year and separated into perennial components(fibrous roots, structural roots, stump, stem, cordon, laterals)and current season's growth (shoots, leaves, and fruit) The concentration of starch in the fibrous roots followed asinusoidal trend Minimum concentrations occurred 98 d afterbudbreak, while the maximum concentrations occurred 182 d laterCorresponding times in the structural roots were approximately42 d earlier In the above-ground perennial components, elevatedconcentrations of starch in the cordon, fruiting wood and barkof the stem were evident at budbreak and fruit harvest (approx220 d later) In the case of the stem, concentrations were greatestat fruit harvest Because the biomass of the perennial componentswas found to be relatively constant throughout the year, starchconcentrations and contents were directly proportional in thesetissues For current season's growth, peak concentrations and contentsin leaves and shoots were observed at fruitset and fruit harvest,respectively For fruit, starch increased continuously untilharvest Approximately 30% of the total starch content accumulated inthe perennial components by leaf abscission was lost duringwinter and early summer Quantitative losses were greatest forthe roots Regeneration of the starch pools in the perennialcomponents of the vine occurred from midseason until leaf abscissionAt the same time, approximately five times more starch was accumulatedby the current season's growth, in particular the fruit, thanby the perennial components As a result of the difference inthe rate of accumulation, the starch content of the currentseason's growth increased from less than 10% midseason to nearly60% of the total starch content of the vine by fruit harvest The results were discussed in relation to the carbon economyof the kiwifruit vine, and compared with seasonal trends instarch concentrations found for other deciduous crops Actinidia deliciosa, kiwifruit, seasonal changes, starch content, whole plant     

Estimation of torsional rigidity in primate long bones

Comparative studies of long bone biomechanics in primates frequently use the polar moment of inertia (J ) as a variable reflecting overall mechanical rigidity, average bending rigidity, or resistance to torsional shear stresses. While the use of this variable for characterizing the first two properties is appropriate, it is potentially a highly misleading measure of torsional resistance. Errors result from violations of assumptions required for the use of the polar moment of inertia; in particular, the predictive utility of J diminishes with departures from axial symmetry (i.e., a cylindrical cross-sectional shape). The magnitude of these errors is estimated both theoretically and experimentally. It is argued that the use of the polar moment of inertia for estimating long bone torsional rigidity should be restricted to samples of relatively invariant and/or cylindrical geometry. Alternative measures for torsional resistance are evaluated and reviewed.