Constraints on muscular performance: trade-offs between power output and fatigue resistance

An important functional and evolutionary constraint on the physical performance of vertebrates is believed to be the trade-off between speed and endurance capacity. However, despite the pervasiveness of physiological arguments, most studies have found no evidence of the trade-off when tested at the whole-animal level. We investigated the existence of this trade-off at the whole-muscle level, the presumed site of this physiological conflict, by examining inter-individual variation in both maximum power output and fatigue resistance for mouse extensor digitorum longus (EDL) muscle using the work-loop technique. We found negative correlations between several measures of in vitro maximum power output and force production with fatigue resistance for individual mouse EDL muscles, indicating functional trade-offs between these performance parameters. We suggest that this trade-off detected at the whole-muscle level has imposed an important constraint on the evolution of vertebrate physical performance.

Adult mouse intrinsic laryngeal muscles express high levels of the myogenic regulatory factor, MYF-5

The intrinsic laryngeal muscles display unique structural and functional characteristics that distinguish them from the skeletal muscle of the trunk and limbs. These features include relatively small muscle fibers, super-fast contraction speed, and fatigue resistance. The molecular basis of tissue-specific functions and other characteristics is differential gene expression. Accordingly, we have investigated the molecular basis of the functional specialization of the intrinsic laryngeal muscles by examining the expression of two key genes in the larynx, known to be important for skeletal muscle development and function: (a) the muscle regulatory factor, Myf-5, and (b) the superfast-contracting myosin heavy chain (EO-MyHC). We have found that the adult thyroarytenoid muscles express much higher levels of both Myf-5 and EO-MyHC messenger ribonucleic acid (mRNA), compared to lower hindlimb skeletal muscle where Myf-5 mRNA levels are very low and EO-MyHC is not detectable. These findings suggest that the unique functional characteristics of the intrinsic laryngeal muscles may be based in laryngeal muscle-specific gene expression directed by a unique combination of muscle regulatory factors. Such laryngeal muscle-specific genes may allow the future development of new treatments for laryngeal muscle dysfunction.

Designing biostable polyurethane elastomers for biomedical implants

The chemical structure, synthesis, morphology, and properties of polyurethane elastomers are briefly discussed. The current understanding of the effect of chemical structure and the associated morphology on the stability of polyurethanes in the biological environments is reviewed. The degradation of conventional polyurethanes appears as surface or deep cracking, stiffening, and deterioration of mechanical properties, such as flex-fatigue resistance. Polyester and poly( tetramethylene oxide) based polyurethanes degrade by hydrolytic and oxidative degradation of ester and ether functional groups, respectively. The recent approaches to develop polyurethanes with improved long-term biostability are based on developing novel polyether, hydrocarbon, polycarbonate, and siloxane macrodiols to replace degradation-prone polyester and polyether macrodiols in polyurethane formulations. The new approaches are discussed with respect to synthesis, properties and biostability based on reported in vivo studies. Among the newly developed materials, siloxane-based polyurethanes have exhibited excellent biostability and are expected to find many applications in biomedical implants.

Interindividual variation of isolated muscle performance and fibre-type composition in the toad Bufo viridus

Interindividual analyses of physiological performance represent one of the most powerful tools for identifying functional positive and negative linkages between various performance traits. In this study we investigated functional linkages in the whole-gastrocnemius performance of juvenile Bufo viridis by examining interindividual variation in in vitro muscle performance and muscle fibre-type composition. We used the work-loop technique to investigate the maximum in vitro power output and fatigue resistance of the gastrocnemius muscle during repeated sets of three cycles at the cycle frequency of 5 Hz, simulating an intermittent style of locomotion. We found several significant correlations between different measures of in vitro muscle performance, including a negative correlation between maximum net power output and fatigue resistance of power, indicating functional trade-offs between these performance traits. We also investigated the extent of individual variation in the proportions of different fibre types, and tested for correlations between individual variation in muscle fibre-type composition and the previously measured isolated muscle performance. Fast glycolytic fibres represented 84.0+/-3.4% of the muscle, while the combined slow oxidative and fast oxidative-glycolytic fibres represented 16+/-3.4%. We found no significant correlations between measures of in vitro muscle performance and the proportion of different fibre types in the gastrocnemius muscle. However, despite this lack of correlation between whole-muscle performance and muscle fibre-type composition data, we suggest the functional linkages detected between different measures of in vitro muscular performance have important ecological and evolutionary consequences.

Reduced inspiratory muscle endurance following successful weaning from prolonged mechanical ventilation

Study objectives: Respiratory muscle weakness and decreased endurance have been demonstrated following mechanical ventilation. However, its relationship to the duration of mechanical ventilation is not known. The aim of this study was to assess respiratory muscle endurance and its relationship to the duration of mechanical ventilation. Design: Prospective study. Setting: Tertiary teaching hospital ICU. Patients: Twenty subjects were recruited for the study who had received mechanical ventilation for a 48 h and had been discharged from the ICU. Measurements: FEV1 FVC, and maximal inspiratory pressure (Pimax) at functional residual capacity were recorded. The Pimax attained following resisted inspiration at 30% of the initial Pimax for 2 min was recorded, and the fatigue resistance index (FRI) [Pimax final/Pimax initial] was calculated. The duration of ICU length of stay (ICULOS), duration of mechanical ventilation (MVD), duration of weaning (WD), and Charlson comorbidities score (CCS) were also recorded. Relationships between fatigue and other parameters were analyzed using the Spearman correlations (p). Results: Subjects were admitted to the ICU for a mean duration of 7.7 days (SD, 3.7 days) and required mechanical ventilation for a mean duration of 4.6 days (SD, 2.5 days). The mean FRI was 0.88 (SD, 0.13), indicating a 12% fall in Pimax, and was negatively correlated with MVD (r = -0.65; p = 0.007). No correlations were found between the FRI and FEV1, FVC, ICULOS, WD, or CCS. Conclusions: Patients who had received mechanical ventilation for > 48 h have reduced inspiratory muscle endurance that worsens with the duration of mechanical ventilation and is present following successful weaning. These data suggest that patients needing prolonged mechanical ventilation are at risk of respiratory muscle fatigue and may benefit from respiratory muscle training.

Association between chronic fatigue syndrome and the corticosteroid-binding globulin gene ALA SER224 polymorphism

Chronic fatigue syndrome (CFS) is characterized by idiopathic fatigue of greater than 6 months' duration with postexertional exacerbation and many other symptoms. A trend toward relative hypocortisolism is described in CFS. Twin and family studies indicate a substantial genetic etiologic component to CFS. Recently, severe corticosteroid-binding globulin (CBG) gene mutations have been associated with CFS in isolated kindreds. Human leukocyte elastase, an enzyme important in CBG catabolism at inflammatory sites, is reported to be elevated in CFS. We hypothesized that CBG gene polymorphisms may act as a genetic risk factor for CFS. A total of 248 patients with CFS defined by Centers for Disease Control criteria, and 248 controls were recruited. Sequencing and restriction enzyme testing of the CBG gene coding region allowed detection of severe CBG gene mutations and a common exon 3 polymorphism (c.825G --> T, Ala-Ser(224)). Plasma CBG levels were measured in 125 CFS patients and 198 controls by radioimmunoassay. Total and free (calculated and measured) cortisol levels were ascertained in single samples between 8-10 a.m. The age of onset (mid 30s) and gender ratio (2.2:1, female:male) of the patients were similar to those reported in U.S. epidemiologic studies. A trend toward a preponderance of serine(224) homozygosity among the CFS patients was noted, compared with controls (chi(2) = 5.31, P = 0.07). Immunoreactive-CBG (IR-CBG) levels were higher in Serine/Alanine (Ser/Ala) than Ala/Ala subjects and higher again in Ser/Ser subjects, this effect was strongest in controls; Ser/Ser: 46.1 +/- 1.8 (n = 31, P = 0.03) vs. Ser/Ala: 42.4 +/- 1.0 (n = 56, P = 0.05) vs. Ala/Ala: 40.8 +/- 1.7 mug/mL (n = 21). Despite higher CBG levels, there was a nonsignificant trend toward lower total and free plasma cortisol in serine allele positive patients, total cortisol: Ser/Ser: 13.3 +/- 1.4 (n = 34) vs. Ser/Ala: 14.0 +/- 0.7 (n = 66) vs. Ala/Ala: 15.4 +/- 1.0 (n = 23). Homozygosity for the serine allele of the CBG gene may predispose to CFS, perhaps due to an effect on hypothalamic-pituitary-adrenal axis function related to altered CBG-cortisol transport function or immune-cortisol interactions.

Anodizing treatments for magnesium alloys and their effecton corrosion resistance in various environments

This paper reviews various aspects of anodizing of magnesium alloys, such as the basics, processes, properties and applications. It systematically summarises the existing fundamental studies and technical developments of anodizing of magnesium alloys, and concludes that new anodizing processes based on electrolytic plasma anodizing that convert the surface of a magnesium alloy into a hard ceramic coating in an electrolytic bath using high energy electric discharges can offer improved wear and corrosion resistance. These new anodized coatings are often claimed to perform better than the traditional ones obtained through older anodizing processes, such as DOW17 or HAE. The new anodizing techniques are chromate free and hence environment friendly. It is expected that more cost-effective, environment-friendly and non-toxic anodizing techniques will be developed and applied to magnesium alloy components in the future.

The sites of neural adaptation induced by resistance training in humans

Although it has long been supposed that resistance training causes adaptive changes in the CNS, the sites and nature of these adaptations have not previously been identified. In order to determine whether the neural adaptations to resistance training occur to a greater extent at cortical or subcortical sites in the CNS, we compared the effects of resistance training on the electromyographic (EMG) responses to transcranial magnetic (TMS) and electrical (TES) stimulation. Motor evoked potentials (MEPs) were recorded from the first dorsal interosseous muscle of 16 individuals before and after 4 weeks of resistance training for the index finger abductors (n = 8), or training involving finger abduction-adduction without external resistance (n = 8). TMS was delivered at rest at intensities from 5 % below the passive threshold to the maximal output of the stimulator. TMS and TES were also delivered at the active threshold intensity while the participants exerted torques ranging from 5 to 60 % of their maximum voluntary contraction (MVC) torque. The average latency of MEPs elicited by TES was significantly shorter than that of TMS MEPs (TES latency = 21.5 ± 1.4 ms; TMS latency = 23.4 ± 1.4 ms; P < 0.05), which indicates that the site of activation differed between the two forms of stimulation. Training resulted in a significant increase in MVC torque for the resistance-training group, but not the control group. There were no statistically significant changes in the corticospinal properties measured at rest for either group. For the active trials involving both TMS and TES, however, the slope of the relationship between MEP size and the torque exerted was significantly lower after training for the resistance-training group (P < 0.05). Thus, for a specific level of muscle activity, the magnitude of the EMG responses to both forms of transcranial stimulation were smaller following resistance training. These results suggest that resistance training changes the functional properties of spinal cord circuitry in humans, but does not substantially affect the organisation of the motor cortex.

Demotivation: Understanding Resistance to English Language Learning - The Case of Vietnamese Students

Demotivation in English language learning was investigated, using Vietnam as a case study, with three main foci: (i) the reasons (i.e., the demotives) underlying demotivation; (ii) the degree of influence of different demotives; and (iii) students’ experiences in overcoming demotivation. Using stimulated recall essays from 100 university students of their foreign language learning experiences, the findings indicated that demotivation was a significant issue for EFL learning, and a framework for discussing the different sources of demotives was developed. While some categories of demotives occurred more frequent than others, no category appeared to be more or less difficult to overcome. Rather, students’ awareness of the role of English language and their determination to succeed were critical factors in overcoming demotivation.

Acetylcholinesterase cDNA of the cattle tick, Boophilus microplus: characterisation and role in organophosphate resistance

Acetylcholinesterase is the target of organophosphate and carbamate pesticides. Organophosphate resistance is widespread in the cattle tick, Boophilus microplus, in Australia. We have isolated a cDNA of acetylcholinesterase from B. microplus and show that it would encode a protein 62 kDa in size. The predicted amino acid sequence contains all the residues characteristic of an acetylcholinesterase. Alternative splicing of the transcript was detected at both the 5' and 3' ends. Alternative splicing at the 5' end would result in two proteins differing by six amino acids. This is the first report of alternative splicing of the N-terminal coding region in a cholinesterase. No point mutations were detected in the acetylcholinesterase gene from organophosphate resistant strains of B. microplus. Alternative explanations for resistance to organophosphates in B. microplus are discussed. (C) 1998 Elsevier Science Ltd. All rights reserved.

A gene (Cargl) that regulates tissue resistance to Candida albicans maps to chromosome 14 of the mouse

Tissue susceptibility and resistance to infection with the yeast Candida albicans is genetically regulated. Analysis of the strain distribution pattern of the C. albicans resistance gene (Carg1) and additional gene and DNA segment markers in the AKXL recombinant inbred (RI) set showed that 13/15 RI strains were concordant for Carg1, Tcra and Rib1. Therefore, Carg1 is probably located within a 17 cM segment of chromosome 14, within approximately 4 cM of the other two genes. (C) 1998 Academic Press.

A second Candida albicans resistance gene (Carg2) regulates tissue damage, but not fungal clearance, in sub-lethal murine systemic infection

The severity of systemic infection with the yeast Candida albicans has been shown to be under complex genetic control. C57/L mice carry an allele that is associated with an increase in tissue destruction when compared with C57BI/6 mice; however, the gene affects only the severity of tissue lesions, and does not influence the magnitude of the fungal burden in either kidney or brain. Studies in [C57/L x C57BI/6]F1 hybrid mice, and [C57/L x C57BI/6]F1 x C57/L backcross mice, demonstrated that the gene behaves as a simple Mendelian co-dominant. (C) 1998 Academic Press.

m144, a murine cytomegalovirus (MCMV)-encoded major histocompatibility complex class I homologue, confers tumor resistance to natural killer cell-mediated rejection

Until now, it has been unclear whether murine cytomegalovirus (MCMV)-encoded protein m144 directly regulates natural killer (NK) cell effector function and whether the effects of m144 are only strictly evident in the context of MCMV infection. We have generated clones of the transporter associated with antigen processing (TAP)-2-deficient RMA-S T lymphoma cell line and its parent cell line, RMA, that stably express significant and equivalent levels of m144. In vivo NK cell-mediated rejection of RMA-S-m144 lymphomas was reduced compared with rejection of parental or mock-transfected RMA-S clones, indicating the ability of m144 to regulate NK cell-mediated responses in vivo. Significantly, the accumulation of NK cells in the peritoneum was reduced in mice challenged with RMA-S-m144, as was the lytic activity of NK cells recovered from the peritoneum. Expression of m144 on RMA-S cells also conferred resistance to cytotoxicity mediated in vitro by interleukin 2-activated adherent spleen NK cells. In summary, the data demonstrate that m144 confers some protection from NK cell effector function mediated in the absence of target cell class I expression, but that in vivo the major effect of m144 is to regulate NK cell accumulation and activation at the site of immune challenge.

Engineered detoxification confers resistance against a pathogenic bacterium

We generated transgenic sugarcane plants that express an albicidin detoxifying gene (albD), which was cloned from a bacterium that provides biocontrol against leaf scald disease. Plants with albicidin detoxification capacity equivalent to 1-10 ng of AlbD enzyme per mg of leaf protein did not develop chlorotic disease symptoms in inoculated leaves, whereas all untransformed control plants developed severe symptoms. Transgenic lines with high AlbD activity in young stems were also protected against systemic multiplication of the pathogen, which is the precursor to economic disease. We have shown that genetic modification to express a toxin-resistance gene can confer resistance to both disease symptoms and multiplication of a toxigenic pathogen in its host.

Monooxygenases play only a minor role in resistance to synthetic pyrethroids in the cattle tick, Boophilus microplus

We investigated the role of monooxygenases in resistance to synthetic pyrethroids (SPs) in the cattle tick, Boophilus microplus. We found that monooxygenases play only a minor role in resistance to SPs in both resistant and susceptible strains of B. microplus. We blocked the monooxygenases with piperonyl butoxide (PBO) and simultaneously applied the SPs, flumethrin and cypermethrin to larval B. microplus. PBO increased the effect of flumethrin (synergism ratios 2.7-8.9) more than it increased the effect of cypermethrin (synergism ratios 1.9-3.1). Of the four strains tested, Parkhurst, which is resistant to SPs, was the least affected by the addition of PBO (synergism ratios after cypermethrin was applied 1.9; after flumethrin 2.7) whereas N.R.F.S., the strain susceptible to SPs, was the most affected by synergism between PBO and SPs (synergism ratio after cypermethrin was applied 3.1; after flumethrin 8.9). We hypothesize that B. microplus lacks monooxygenases capable of conferring resistance to SPs because it and its recent ancestors were blood-feeders rather than herbivores.