The effect of plantar flexor muscle fatigue on postural control

Objective

Previous studies have demonstrated that ankle muscle fatigue alters postural sway. Our aim was to better understand postural control mechanisms during upright stance following plantar flexor fatigue.

Method

Ten healthy young volunteers, 25.7 ± 2.2 years old, were recruited. Foot center-of-pressure (CoP) displacement data were collected during narrow base upright stance and eyes closed (i.e. blindfolded) conditions. Subjects were instructed to stand upright and as still as possible on a force platform under five test conditions: (1) non-fatigue standing on firm surface; (2) non-fatigue standing on foam; (3) ankle plantar flexor fatigue, standing on firm surface; (4) ankle plantar flexor fatigue, standing on foam; and (5) upper limb fatigue, standing on firm surface. An average of the ten 30-s trials in each of five test conditions was calculated to assess the mean differences between the trials. Traditional measures of postural stability and stabilogram-diffusion analysis (SDA) parameters were analyzed.

Results

Traditional center of pressure parameters were affected by plantar flexor fatigue, especially in the AP direction. For the SDA parameters, plantar flexor fatigue caused significantly higher short-term diffusion coefficients, and critical displacement in both mediolateral (ML) and anteroposterior (AP) directions. Long-term postural sway was different only in the AP direction.

Conclusions

Localized plantar flexor fatigue caused impairment to postural control mainly in the Sagittal plane. The findings indicate that postural corrections, on average, occurred at a higher threshold of sway during plantar flexor fatigue compared to non-fatigue conditions.     

DNA单分子近场光学成像与荧光探测

介绍了扫描近场光学(SNOM-Scanning Near-Field Optical Microscope)／原子力显微镜（AFM－Atomic Force Microscope)系统（SNO／AM）的工作原理。在AFM模式和SNOM模式下对DNA分子进行成像和荧光探测,得到了清晰的DNA单分子的形貌像和荧光像。由形貌圆像得到的DNA分子尺寸横向为20nm,高度为2nm,其中包含了探针形貌的影响。实验中采Tapping模式的AFM成像,样品经多次搜索扫描无明显损坏。AFM模式的分辨率优于1nm。SNOM模式下DNA分子形貌像和荧光像清晰,由近场荧光分布可以确定分子取向和浓度。用YOYO－1染料对λDNA分子进行染色和荧光探测。通过对DNA分子多个截面进行测量,分析染料 与DNA结合状态。     

生物元件库国内外研究进展

生物元件库是对元件数据和实物进行收集、整理和共享的重要平台,对合成生物学的研究和应用具有非常重要的支撑作用。本文对国内外生物元件库的研究进展进行了介绍和综述。国外先后出现了标准生物元件登记库等多个元件库,通过制定Open MTA协议实现了元件实物的免费分发共享,并通过制定"合成生物学开放语言"(SBOL)实现了多个元件库之间的数据交换;通过bionet项目对元件数据和实物的去中心化管理作了进一步尝试。近年来我们与国内多家科研单位合作建设了合成生物学元件与数据库(RDBSB,https://www.biosino.org/rdbsb/or https://www.biosino.org/npbiosys/)。在建设过程中,我们首先建立了与SBOL语言兼容的《催化元件数据标准》、多种重要元件的标准功能测试方法以及安全高效的元件提交系统,并在此基础上完成了元件数量最多且信息最为全面的催化元件数据库的构建。目前,收集了30多万个催化元件,包括7万多个具有文献支持功能表征信息的催化元件,并保藏了1万个以上的实物元件和5000个底盘菌株,通过网站实现了数据和实物的公开与共享。上述元件库已经对国内合...     

Adhesive and conformational behaviour of mycolic acid monolayers

We have studied the pH-dependent interaction between mycolic acid (MA) monolayers and hydrophobic and hydrophilic surfaces using molecular (colloidal probe) force spectroscopy. In both cases, hydrophobic and hydrophilic monolayers (prepared by Langmuir-Blodgett and Langmuir-Schaefer deposition on silicon or hydrophobized silicon substrates, respectively) were studied. The force spectroscopy data, fitted with classical DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory to examine the contribution of electrostatic and van der Waals forces, revealed that electrostatic forces are the dominant contribution to the repulsive force between the approaching colloidal probe and MA monolayers. The good agreement between data and the DLVO model suggest that beyond a few nm away from the surface, hydrophobic, hydration, and specific chemical bonding are unlikely to contribute to any significant extent to the interaction energy between the probe and the surface. The pH-dependent conformation of MA molecules in the monolayer at the solid-liquid interface was studied by ellipsometry, neutron reflectometry, and with a quartz crystal microbalance. Monolayers prepared by the Langmuir-Blodgett method demonstrated a distinct pH-responsive behaviour, while monolayers prepared by the Langmuir-Schaefer method were less sensitive to pH variation. It was found that the attachment of water molecules plays a vital role in determining the conformation of the MA monolayers.     

Foot centre of pressure and ground reaction force during quadriceps resistance exercises; a comparison between force plates and a pressure insole system

The study compared the centre of pressure measurements (COP) and vertical ground reaction forces (vGRF) from a pressure insole system to that from force plates (FP) during two flywheel quadriceps resistance exercises: leg press and squat. The comparison was performed using a motion capture system and simultaneous measurements of COP and vGRF from FP and insoles. At lower insole-vGRF (<250 N/insole) COP accuracy deteriorated and those data were excluded from further analysis. The insoles systematically displaced the COP slightly posteriorly and medially compared to the FP measurements. Pearson’s coefficient of correlation (r) between insole- and FP-COP showed good agreement in both the anteroposterior (squat: r = 0.96, leg press: r = 0.97) and mediolateral direction (squat: r = 0.84, leg press: r = 0.90), whereas the root-mean-square errors (RMSE) were lower in the mediolateral (squat: 3.9 mm, leg press: 4.5 mm) than the anteroposterior (squat and leg press: 11.8 mm) direction. Vertical GRF was slightly overestimated by the insoles in leg press and RMSE were greater in leg press (8% of peak force) than in squat (6%). Overall, results were within the range of previous studies performed on gait. The strong agreement between insole and FP measurements indicates that insoles may replace FPs in field applications and biomechanical computations during resistance exercise, provided that the applied force is sufficient.     

Structural transitions of concanavalin A adsorbed onto bare mica plates: surface force measurements

The structure of the -sheet rich molecule, concanavalin A (Con A), shows complicated behaviour when mechanical stress is imposed on adsorbed Con A layers. In the pH range 3.9 to 7.4, the structural changes are dependent on the pH and ionic strength of the solution at the liquid/solid interface. Several molecular shapes are observed ranging from a highly unfolded state characterized by a 1.25 nm thickness of each expelled layer at pH 3.9 to other states with various layer thicknesses at pH 6–7.4 and at a low ionic strength (< 10^–4). In the pH range 6 to 7.4, different molecular species appear to be present. Correspondence to: J P. Gallinet     

Evaluation of carbohydrate molecular mechanical force fields by quantum mechanical calculations

A quantitative evaluation of 20 second-generation carbohydrate force fields was carried out using ab initio and density functional methods. Geometry-optimized structures (B3LYP/6-31G(d)) and relative energies using augmented correlation consistent basis sets were calculated in gas phase for monosaccharide carbohydrate benchmark systems. Selected results are: (i). The interaction energy of the alpha-d-glucopyranose.H(2)O heterodimer is estimated to be 4.9 kcal/mol, using a composite method including terms at highly correlated (CCSD(T)) level. Most molecular mechanics force fields are in error in this respect; (ii). The (3)E envelope (south) pseudorotational conformer of methyl 5-deoxy-beta-d-xylofuranoside is 0.66 kcal/mol more stable than the (3)E envelope (north) conformer and the alpha-anomer of methyl d-glucopyranoside is 0.82 kcal/mol more stable than the beta-anomer; (iii). The relative energies of the (gg, gt and tg) rotamers of methyl alpha-d-glucopyranoside and methyl alpha-d-galactopyranoside are (0.13, 0.00, 0.15) and (0.64, 0.00, 0.77) kcal/mol, respectively. The results of the quantum mechanical calculations are compared with the results of calculations using the 20 second-generation carbohydrate force fields. No single force field is consistently better than the others for all the test cases. A statistical assessment of the performance of the force fields indicates that CHEAT(95), CFF, certain versions of Amber and of MM3 have the best overall performance, for these gas phase monosaccharide systems.     

Variability of motoneuron activation and the modulation of force production in a postural reflex of the hermit crab abdomen

The tri-phasic reflex in hermit crab (Pagurus pollicarus) abdomen is triggered by local mechanoreceptors and is essential for postural control. The reflex consists of three stereotypical phases: a brief, high-frequency burst, a transient cessation of firing, and a late-discharge that is much lower in frequency than the initial burst. To better understand the reflex generation of force, variability of motoneuron discharge in each of five parameters of reflex activation was assessed. An intracellular current injection routine was used to correlate each of these parameters with force production. Phase 3 motoneuron firing frequency showed the greatest correlation with force production. Phase 3 spike rate increased as a function of phase 2 duration, but the relationship between phase 2 duration and force produced by the reflex was weak. Junction potential amplitude decreased as phase 2 duration increased, and we hypothesize that this trend counteracts the increased phase 3 frequency, explaining the weak relationship of phase 2 duration and force production. Surprisingly, when phase 3 frequency was held constant and phase 2 was increased in duration, the concurrent decrease in junction potential amplitude did not reduce force production.