The genome sequences of the marine diatom Epithemia pelagica strain UHM3201 (Schvarcz, Stancheva & Steward, 2022) and its nitrogen-fixing, endosymbiotic cyanobacterium.

We present the genome assembly of the pennate diatom Epithemia pelagica strain UHM3201 (Ochrophyta; Bacillariophyceae; Rhopalodiales; Rhopalodiaceae) and that of its cyanobacterial endosymbiont (Chroococcales: Aphanothecaceae). The genome sequence of the diatom is 60.3 megabases in span, and the cyanobacterial genome has a length of 2.48 megabases. Most of the diatom nuclear genome assembly is scaffolded into 15 chromosomal pseudomolecules. The organelle genomes have also been assembled, with the mitochondrial genome 40.08 kilobases and the plastid genome 130.75 kilobases in length. A number of other prokaryote MAGs were also assembled.

Sinking rates, orientation, and behavior of pennate diatoms.

Phytoplankton cells are now recognized as dynamic entities rather than as passive and isolated particles because they can actively modulate impacts of selection factors (nutrients, light, turbidity, and mixing) through a wide range of adaptations. Cell shape and/or chain length modulation is one of these processes but has predominantly been studied as an adaptation or an acclimatation to a specific growth limitation (light, nutrients, predation, etc.). In this study we have demonstrated that cell shape and size may have greater roles than previously known in phytoplankton ecology and species adaptation by permitting cell-to-cell signaling and more complex ecological processes that result from it. By exploring microscale biophysical interactions that lead to specific cell reorientation processes, we demonstrated that cell geometry not only modulates cell sinking rates but can also provide fast sensor responses to the cells' environment. Although gyrotaxis has been described in detail for motile phytoplankton cells, our findings illustrate that the reorientation process described here can occur even in non-motile cells within their natural environment. An additional consistent behavior was also recently described for a diatom species (Pseudo-nitzschia delicatessima), and with this study, we extend this observation to Pseudo-nitzschia pungens and Pseudo-nitzschia fraudulenta. Our observations emphasize the generality of this process, which adds a new level of complexity to our understanding of cellular interactions and their network of sensors.

Emerging phylogeographic perspective on the toxigenic diatom genus Pseudo-nitzschia in coastal northern European waters and gateways to eastern Arctic seas: Causes, ecological consequences and socio-economic impacts.

The diatom Pseudo-nitzschia H. Peragallo is perhaps the most intensively researched genus of marine pennate diatoms, with respect to species diversity, life history strategies, toxigenicity, and biogeographical distribution. The global magnitude and consequences of harmful algal blooms (HABs) of Pseudo-nitzschia are particularly significant because of the high socioeconomic impacts and environmental and human health risks associated with the production of the neurotoxin domoic acid (DA) among populations of many (although not all) species. This has led to enhanced monitoring and mitigation strategies for toxigenic Pseudo-nitzschia blooms and their toxins in recent years. Nevertheless, human adaptive actions based on future scenarios of bloom dynamics and proposed shifts in biogeographical distribution under climate-change regimes have not been implemented on a regional scale. In the CoCliME (Co-development of climate services for adaptation to changing marine ecosystems) program these issues were addressed with respect to past, current and anticipated future status of key HAB genera such as Pseudo-nitzschia and expected benefits of enhanced monitoring. Data on the distribution and frequency of Pseudo-nitzschia blooms in relation to DA occurrence and associated amnesic shellfish toxin (AST) events were evaluated in a contemporary and historical context over the past several decades from key northern CoCliME Case Study areas. The regional studies comprised the greater North Sea and adjacent Kattegat-Skagerrak and Norwegian Sea, eastern North Atlantic marginal seas and Arctic gateways, and the Baltic Sea. The first evidence of possible biogeographical expansion of Pseudo-nitzschia taxa into frontier eastern Arctic gateways was provided from DNA barcoding signatures. Key climate change indicators, such as salinity, temperature, and water-column stratification were identified as drivers of upwelling and advection related to the distribution of regional Pseudo-nitzschia blooms. The possible influence of changing variables on bloom dynamics, magnitude, frequency and spatial and temporal distribution were interpreted in the context of regional ocean climate models. These climate change indicators may play key roles in selecting for the occurrence and diversity of Pseudo-nitzschia species within the broader microeukaryote communities. Shifts to higher temperature and lower salinity regimes predicted for the southern North Sea indicate the potential for high-magnitude Pseudo-nitzschia blooms, currently absent from this area. Ecological and socioeconomic impacts of Pseudo-nitzschia blooms are evaluated with reference to effects on fisheries and mariculture resources and coastal ecosystem function. Where feasible, effective adaptation strategies are proposed herein as emerging climate services for the northern CoCLiME region.

Phytoplankton community shift in response to experimental Cu addition at the elevated CO2 levels (Arabian Sea, winter monsoon).

Understanding phytoplankton community shifts under multiple stressors is becoming increasingly important. Among other combinations of stressors, the impact of trace metal toxicity on marine phytoplankton under the ocean acidification scenario is an important aspect to address. Such multiple stressor studies are rare from the Arabian Sea, one of the highest productive oceanic provinces within the North Indian Ocean. We studied the interactive impacts of copper (Cu) and CO2 enrichment on two natural phytoplankton communities from the eastern and central Arabian Sea. Low dissolved silicate (DSi < 2 µM) favoured smaller diatoms (e.g. Nitzschia sp.) and non-diatom (Phaeocystis). CO2 enrichment caused both positive (Nitzschia sp. and Phaeocystis sp.) and negative (Cylindrotheca closterium, Navicula sp., Pseudo-nitzschia sp., Alexandrium sp., and Gymnodinium sp.) growth impacts. The addition of Cu under the ambient CO2 level (A-CO2) hindered cell division in most of the species, whereas Chla contents were nearly unaffected. Interestingly, CO2 enrichment seemed to alleviate Cu toxicity in some species (Nitzschia sp., Cylindrotheca closterium, Guinardia flaccida, and Phaeocystis) and increased their growth rates. This could be related to the cellular Cu demand and energy budget at elevated CO2 levels. Dinoflagellates were more sensitive to Cu supply compared to diatoms and prymnesiophytes and could be related to the unavailability of prey. Such community shifts in response to the projected ocean acidification, oligotrophy, and Cu pollution may impact trophic transfer and carbon cycling in this region.

Comparison of Meat Quality and Muscle Fiber Characteristics between Porcine Skeletal Muscles with Different Architectures.

This study aimed to compare the similarities, physicochemical properties, and muscle fiber characteristics of porcine skeletal muscles. Fourteen types of muscles were collected from nine pig carcasses at 24 h post-mortem and classified by muscle architecture into two main groups, namely parallel and pennate. The muscles were further differentiated into three subtypes per group. These included fan-shaped, fusiform, and strap for the parallel group, and unipennate, bipennate, and multipennate for the pennate group. Parallel-fibered muscles, which were composed of larger I, IIA, IIX, and IIXB fibers and a lower density of IIA fibers, showed higher redness and yellowness values than pennate-fibered muscles (p<0.05). However, the relative fiber area was not significantly different between the parallel and pennate groups (p>0.05). In the subtypes of parallel architecture, the strap group showed lower moisture content and higher redness values than the other subtypes and had considerably higher amounts of oxidative fibers (I and IIA; 72.3%) than the fan-shaped and fusiform groups (p<0.05). In the pennate group, unipennate showed comparatively lower moisture content and higher lightness than other pennate subtypes and was composed of smaller I, IIA, and IIX fibers than the bipennate and multipennate groups (p<0.05). Finally, a different trend of muscle clustering by hierarchical cluster analysis was found between physicochemical properties and muscle fiber characteristics. These results suggest that the physicochemical properties and muscle fiber characteristics of porcine skeletal muscles are not significantly dependent on morphological properties but are rather related to the intrinsic properties of the individual muscles.

Numerical Analysis of the Light Modulation by the Frustule of Gomphonema parvulum: The Role of Integrated Optical Components.

Siliceous diatom frustules present a huge variety of shapes and nanometric pore patterns. A better understanding of the light modulation by these frustules is required to determine whether or not they might have photobiological roles besides their possible utilization as building blocks in photonic applications. In this study, we propose a novel approach for analyzing the near-field light modulation by small pennate diatom frustules, utilizing the frustule of Gomphonema parvulum as a model. Numerical analysis was carried out for the wave propagation across selected 2D cross-sections in a statistically representative 3D model for the valve based on the finite element frequency domain method. The influences of light wavelength (vacuum wavelengths from 300 to 800 nm) and refractive index changes, as well as structural parameters, on the light modulation were investigated and compared to theoretical predictions when possible. The results showed complex interference patterns resulting from the overlay of different optical phenomena, which can be explained by the presence of a few integrated optical components in the valve. Moreover, studies on the complete frustule in an aqueous medium allow the discussion of its possible photobiological relevance. Furthermore, our results may enable the simple screening of unstudied pennate frustules for photonic applications.

Simulated ocean acidification altered community composition and growth of a coastal phytoplankton assemblage (South West coast of India, eastern Arabian Sea).

Marine phytoplankton can be highly sensitive to ocean acidification; however, their responses are diverse and therefore, phytoplankton response study on the regional scale is of high research priority. The present study documented the community shift and growth responses of a natural phytoplankton assemblage from the South West coastal water of India (South Eastern Arabian Sea) under ambient CO2 (A-CO2 ≈ 400 µatm) and high CO2 (H-CO2 ≈ 830 µatm) levels in microcosms during the winter monsoon. A doubling of pCO2 resulted in increased cell density, particulate organic carbon and nitrogen (POC, PON) contents, and C:N ratios. The depleted values of δ13CPOC in the H-CO2-incubated cells indicated a higher diffusive CO2 influx. HPLC marker pigment analysis revealed that the community was microphytoplankton dominated (mostly diatoms); nanoplanktonic prymnesiophytic algae and picoplanktonic cyanobacteria showed insignificant response to the simulated ocean acidification. A high CO2-induced increased growth rate was noticed in 6 diatoms (Leptocylindrus danicus; Rhizosolenia setigera; Navicula sp., Asterionella glacialis, Dactyliosolen fragilissimus, and Thalassiosira sp.). The cell volumes of Thalassionema frauenfeldii, Asterionella glacialis, and Cylindrotheca closterium increased significantly, whereas Rhizosolenia setigera and Thalassiosira sp. showed decreased cell volume at the elevated CO2 levels. These changes in growth rate, cell volume, and elemental stoichiometry could be related to CO2 acquisition and the nutritional status of the cells. Some phytoplankton genera from this region are probably acclimatized to pCO2 fluctuations and are likely to benefit from the future increase in CO2 levels. Higher POC production and increased C:N ratio along with variable cell volume may impact the trophic transfer and cycling of organic carbon in this coastal water. However, a multi-stressor approach in a longer experimental exposure should be considered in future research.

Underwater Light Manipulation by the Benthic Diatom Ctenophora pulchella: From PAR Efficient Collection to UVR Screening.

Several species of diatoms, unicellular microalgae which constitute the main component of phytoplankton, are characterized by an impressive photosynthetic efficiency while presenting a noticeable tolerance versus exposure to detrimental UV radiation (UVR). In particular, the growth rate of the araphid diatom Ctenophora pulchella is not significantly affected by harsh treatments with UVR, even in absence of detectable, specific UV-absorbing pigments and even if it is not able to avoid high UV exposure by motility. In this work we applied a multi-disciplinary approach involving numerical computation, photonics, and biological parameters in order to investigate the possible role of the frustule, micro- and nano-patterned silica shell which encloses the cell, in the ability of C. pulchella to efficiently collect photosynthetic active radiation (PAR) and to simultaneously screen the protoplasm from UVR. The characterization of the photonic properties of the frustule has been accompanied by in vivo experiments conducted in water in order to investigate its function as optical coupler between light and plastids.

Ultrasonic characterization of localized passive elastic properties of human pennate muscle with a single-probe setup.

Quantitative evaluation of passive elastic properties of an individual skeletal muscle in vivo is among the major challenges of biomechanics, and its clinical application is severely limited. By combining shear-wave elastography (SWE) and B-mode imaging techniques, this study develops a novel non-invasive method to measure the local elastic modulus-fascicle strain curve of human pennation muscle during passive stretching using a single probe. Physiologically meaningful parameters are estimated and compared in subjects with different ages or pathological conditions. The in vivo experimental group comprised 12 healthy subjects (four children, four adults, and four seniors) and eight patients (four suffering from pseudohypertrophy, four from atrophy). Their gastrocnemius muscles were passively stretched using an ankle joint motion instrument. Local elastic moduli of the muscle were measured using SWE imaging frames and a built-in 'F-ROI' tool. The corresponding fascicle strains were simultaneously obtained using B-mode imaging frames and a gradient Radon transform. Three parameters (η, µ, G0) were estimated from a normalized elastic modulus-strain curve using the Gauss-Newton method. The measured elastic modulus-strain curves all agreed with models of the estimated parameters (0.910 < R2 < 0.999) and presented different patterns among normal and diseased subjects. η values were lower for pseudohypertrophies (1.93 ± 0.12), but higher for atrophies (63.40 ± 98.89), compared with normal ones (6.02 ± 2.53). In addition, µ values were higher for pseudohypertrophies (22.65 ± 16.40), but lower for atrophies (0.28 ± 0.41), compared with normal ones (1.07 ± 1.22). The proposed method may provide novel insight into the biomechanics of pennate muscle and has the potential to serve for clinical musculoskeletal medical diagnosis, as the single-probe scanning setup is broadly accepted.

Pheromone Mediated Sexual Reproduction of Pennate Diatom Cylindrotheca closterium.

Benthic diatoms dominate primary production in marine subtidal and intertidal environments. Their extraordinary species diversity and ecological success is thought to be linked with their predominantly heterothallic sexual reproduction. Little is known about pheromone involvement during mating of pennate diatoms. Here we describe pheromone guided mating in the coastal raphid diatom Cylindrotheca closterium. We show that the two mating types (mt+ and mt-) have distinct functions. Similar to other benthic diatoms, mt+ cells are searching for the mt- cells to pair. To enhance mating efficiency mt- exudes an attraction pheromone which we proved by establishing a novel capillary assay. Further, two more pheromones produced by mt- promote the sexual events. One arrests the cell cycle progression of mt+ while the other induces gametogenesis of mt+. We suggest that C. closterium shares a functionally similar pheromone system with other pennate diatoms like Seminavis robusta and Pseudostaurosira trainorii which synchronize sexual events and mate attraction. Remarkably, we found no evidence of mt+ producing pheromones, which differentiates C. closterium from other pennates and suggests a less complex pheromone system in C. closterium.

Diatomophthoraceae - a new family of olpidiopsis-like diatom parasitoids largely unrelated to Ectrogella.

The oomycete genus Ectrogella currently comprises a rather heterogeneous group of obligate endoparasitoids, mostly of diatoms and algae. Despite their widespread occurrence, little is known regarding the phylogenetic affinities of these bizarre organisms. Traditionally, the genus was included within the Saprolegniales, based on zoospore diplanetism and a saprolegnia/achlya-like zoospore discharge. The genus has undergone multiple re-definitions in the past, and has often been used largely indiscriminately for oomycetes forming sausage-like thalli in diatoms. While the phylogenetic affinity of the polyphyletic genus Olpidiopsis has recently been partially resolved, taxonomic placement of the genus Ectrogella remained unresolved, as no sequence data were available for species of this genus. In this study, we report the phylogenetic placement of Ectrogella bacillariacearum infecting the freshwater diatom Nitzschia sigmoidea. The phylogenetic reconstruction shows that Ectrogella bacillariacearum is grouped among the early diverging lineages of the Saprolegniomycetes with high support, and is unrelated to the monophyletic diatom-infecting olpidiopsis-like species. As these species are neither related to Ectrogella, nor to the early diverging lineages of Olpidiopsis s. str. and Miracula, they are placed in a new genus, Diatomophthora, in the present study.

Changes in human skeletal muscle architecture and function induced by extended spaceflight.

The aim of this study was to quantitatively describe the relationships between joint angles and muscle architecture (lengths (Lf) and angles (Θf) of fascicles) of human triceps surae [medial (MG) and lateral (LG) gastrocnemius and soleus (SOL) muscles] invivo for three men-cosmonaut after long-duration spaceflight. Sagittal sonographs of MG, LG, SOL were taken at ankle was positioned at 15° (dorsiflexion), 0° (neutral position), +15°, and +30° (plantarflexion), with the knee at 90° at rest and after a long-duration spaceflight. At each position, longitudinal ultrasonic images of the MG and LG and SOL were obtained while the cosmonauts was relaxed from which fascicle lengths and angles with respect to the aponeuroses were determined. After space flight plantarflexor force declined significantly (26%; p < 0.001). The internal architecture of the GM, and LG, and SOL muscle was significantly altered. In the passive condition, Lf changed from 45, 53, and 39 mm (knee, 0°, ankle, -15°) to 26, 33, and 28 mm (knee, 90° ankle, 30°) for MG, LG, and SOL, respectively. Different lengths and angles of fascicles, and their changes by contraction, might be related to differences in force-producing capabilities of the muscles and elastic characteristics of tendons and aponeuroses. The three heads of the triceps surae muscle substantially differ in architecture, which probably reflects their functional roles. Differences in fiber length and pennation angle that were observed among the muscles and could be associated with differences in force production and in elastic properties of musculo-tendinous complex and aponeuroses.

Effects of excess light energy on excitation-energy dynamics in a pennate diatom Phaeodactylum tricornutum.

Controlling excitation energy flow is a fundamental ability of photosynthetic organisms to keep a better performance of photosynthesis. Among the organisms, diatoms have unique light-harvesting complexes, fucoxanthin chlorophyll (Chl) a/c-binding proteins. We have recently investigated light-adaptation mechanisms of a marine centric diatom, Chaetoceros gracilis, by spectroscopic techniques. However, it remains unclear how pennate diatoms regulate excitation energy under different growth light conditions. Here, we studied light-adaptation mechanisms in a marine pennate diatom Phaeodactylum tricornutum grown at 30 µmol photons m-2 s-1 and further incubated for 24 h either in the dark, or at 30 or 300 µmol photons m-2 s-1 light intensity, by time-resolved fluorescence (TRF) spectroscopy. The high-light incubated cells showed no detectable oxygen-evolving activity of photosystem II, indicating the occurrence of a severe photodamage. The photodamaged cells showed alterations of steady-state absorption and fluorescence spectra and TRF spectra compared with the dark and low-light adapted cells. In particular, excitation-energy quenching is significantly accelerated in the photodamaged cells as shown by mean lifetime analysis of the Chl fluorescence. These spectral changes by the high-light treatment may result from arrangements of pigment-protein complexes to maintain the photosynthetic performance under excess light illumination. These growth-light dependent spectral properties in P. tricornutum are largely different from those in C. gracilis, thus providing insights into the different light-adaptation mechanisms between the pennate and centric diatoms.

Forefoot running requires shorter gastrocnemius fascicle length than rearfoot running.

This study aimed to investigate the influence of foot strike patterns on the behaviour of the triceps surae muscle-tendon unit, including the Achilles tendon whose length nearly corresponds to force of the triceps surae, and the medial gastrocnemius muscle (MG) during running. Seven male volunteers ran with forefoot and rearfoot strikes at 10, 14 and 18 km h-1 on a treadmill. The MG fascicle length was measured using ultrasonography. The in vivo length of the curved Achilles tendon was quantified by combining ultrasonography with optical motion capture of reflective markers on the right lower limb and an ultrasound probe. The forefoot strike resulted in a significantly shorter MG fascicle length at the initial contact, at Achilles tendon peak elongation, and at toe-off, than the rearfoot strike. The Achilles tendon length at initial contact was greater during the forefoot strike than during the rearfoot strike at 18 km h-1, while its peak elongation was not significantly different during forefoot and rearfoot running. These results indicate that the MG, with a shorter length during forefoot running, manages to address demands for a similar peak force of the triceps surae than during rearfoot running.

Complete chloroplast genome of the tiny marine diatom Nanofrustulum shiloi (Bacillariophyta) from the Adriatic Sea.

We report the chloroplast genome sequence of Nanofrustulum shiloi, a tiny araphid pennate diatom collected from the Adriatic Sea. The 160,994-bp N. shiloi genome displays a quadripartite structure and its gene repertoire resembles those of other diatom chloroplast genomes. Besides the genes located in the inverted repeat, psbY is duplicated. A gene-poor region in the large single-copy region contains multiple ORFs sharing sequence similarities with plasmids and chloroplast ORFs found in other diatom species. The genome features a single intron, a group II intron in petB. Phylogenomic analysis identified N. shiloi at a basal position within the araphid 2 clade.

Morphology and molecular phylogeny of epizoic araphid diatoms on marine zooplankton, including Pseudofalcula hyalina gen. & comb. nov. (Fragilariophyceae, Bacillariophyta).

Some diatoms are able to colonize as epibionts on their potential zooplankton predators. Here, we report Pseudohimantidium pacificum living on the copepod Corycaeus giesbrechti and as a new finding on Oithona nana, Protoraphis atlantica living on the copepod Pontellopsis brevis, Protoraphis hustedtiana on the cypris larvae of barnacles, and Falcula hyalina on the copepod Acartia lilljeborgii. The epizoic diatoms were able to grow as free-living forms under culture conditions. Pseudohimantidium pacificum and P. atlantica appeared as the most derived species from their benthic diatom ancestors. The mucilage pad or stalk of the strains of these species showed important morphological distinction when compared with their epizoic forms. Barnacle larvae explore benthic habitats before settlement, and epibiosis on them is an example where P. hustedtiana profits from the host behavior for dispersal of its benthic populations. Molecular phylogenies based on the SSU rRNA and RuBisCO large subunit (rbcL) gene sequences revealed F. hyalina as an independent lineage within the Fragilariales (Tabularia, Catacombas, and others), consistent with its morphological distinction in the low number of rows (≤6) in the ocellulimbus, among other features. We propose the transfer of F. hyalina to the genus Pseudofalcula gen. nov. Molecular phylogeny suggests a single order for the members of the Cyclophorales and the Protoraphidales, and that the epibioses of araphid diatoms on marine zooplankton have been independently acquired several times. These clades are constituted of both epizoic and epiphytic/epilithic forms that evidence a recent acquisition of the epizoic modus vivendi.

End-of-Fiber Signals Strongly Influence the First and Second Phases of the M Wave in the Vastus Lateralis: Implications for the Study of Muscle Excitability.

It has been recurrently observed that, for compound muscle action potentials (M wave) recorded over the innervation zone of the vastus lateralis, the descending portion of the first phase generally shows an "inflection" or "shoulder." We sought to clarify the electrical origin of this shoulder-like feature and examine its implications. M waves evoked by maximal single shocks to the femoral nerve were recorded in monopolar and bipolar configurations from 126 individuals using classical (10-mm recording diameter, 20-mm inter-electrode distance) electrodes and from eight individuals using small electrodes arranged in a linear array. The changes of the M-wave waveform at different positions along the muscle fibers' direction were examined. The shoulder was identified more frequently in monopolar (97%) than in bipolar (46%) M waves. The shoulder of M waves recorded at different distances from the innervation zone had the same latency. Furthermore, the shoulder of the M wave recorded over the innervation zone coincided in latency with the positive peak of that recorded beyond the muscle. The positive phase of the M wave detected 20 mm away from the innervation zone was essentially composed of non-propagating components. The shoulder-like feature in monopolar and bipolar M waves results from the termination of action potentials at the superficial aponeurosis of the vastus lateralis. We conclude that, only the amplitude of the first phase, and not the second, of M waves recorded monopolarly and/or bipolarly in close proximity to the innervation zone can be used reliably to monitor possible changes in muscle membrane excitability.

Stuck in gear: age-related loss of variable gearing in skeletal muscle.

Skeletal muscles power a broad diversity of animal movements, despite only being able to produce high forces over a limited range of velocities. Pennate muscles use a range of gear ratios, the ratio of muscle shortening velocity to fiber shortening velocity, to partially circumvent these force-velocity constraints. Muscles operate with a high gear ratio at low forces; fibers rotate to greater angles of pennation, enhancing velocity but compromising force. At higher forces, muscles operate with a lower gear ratio; fibers rotate little so limiting muscle shortening velocity, but helping to preserve force. This ability to shift gears is thought to be due to the interplay of contractile force and connective tissue constraints. In order to test this hypothesis, gear ratios were determined in the medial gastrocnemius muscles of both healthy young rats, and old rats where the interaction between contractile and connective tissue properties was assumed to be disrupted. Muscle fiber and aponeurosis stiffness increased with age (P<0.05) from 19.1±5.0 kPa and 188.5±24.2 MPa, respectively, in young rats to 39.1±4.2 kPa and 328.0±48.3 MPa in old rats, indicating a mechanical change in the interaction between contractile and connective tissues. Gear ratio decreased with increasing force in young (P<0.001) but not old (P=0.72) muscles, indicating that variable gearing is lost in old muscle. These findings support the hypothesis that variable gearing results from the interaction between contractile and connective tissues and suggest novel explanations for the decline in muscle performance with age.

Comparison of electromyographic signals from monopolar current and potential amplifiers derived from a penniform muscle, the gastrocnemius medialis.

Electromyograms (EMGs) are measured by bipolar surface electrodes that quantify potential differences. Bipolar potentials over penniform muscles may be associated with errors. Our assumption was that muscle activity can be quantified more reliably and with a higher spatial resolution using current measurements. The purpose of this work is: (a) to introduce the concept of current measurements to detect muscle activity, (b) to show the coherences observed over a segment of a typical penniform muscle, the gastrocnemius medialis where one would expect a synchronicity of the activation, and (c) to show the amount of mixing that is caused by the finite inter electrode resistance. A current amplifier was developed. EMGs were recorded at 40% of maximum voluntary contraction during isometric contractions of the gastrocnemius medialis. EMGs of twelve persons were recorded with an array of four peripheral and one central electrode. Monopolar EMGs were recorded for "all-potential", "center at current" and "all-current" conditions. Coherence revealed the similarity of signals recorded from neighboring electrodes. Coherence was high for the "all-potential", significant for the "current at center" condition and disappeared in the "all-current" condition. It was concluded that EMG array recordings strongly depends on the measurement configuration. The proposed current amplifier significantly improves spatial resolution of EMG array recordings because the inter-electrode cross talk is reduced.