Higher Contraction Levels Research Articles

The Effect of Streamtube Contraction on the Characteristics of a Streamwise Vortex

The ingestion of a vortex by an aero-engine is potentially an area of concern for current and future aircraft-engine configurations. However, there are very little experimental data on the characteristics of a streamwise vortex undergoing ingestion through a contracting streamtube. To address this dearth of knowledge, the ingestion of a streamwise vortex has been studied experimentally using stereoscopic particle image velocimetry (stereo PIV). A subscale model of an aircraft intake has been used to generate a contracting capture streamtube, and an isolated streamwise vortex has been generated upstream of the intake using semispan NACA 0012 and delta wing vortex generators (VGs). A range of contraction ratios, vortex Reynolds numbers, and vortex initial conditions have been examined. Measurements on planes perpendicular to the freestream flow show that the vortex undergoes notable levels of intensification through the contraction streamtube. The characteristics of the vortex are dependent on the streamtube contraction level, the initial aerodynamic characteristics of the vortex, and the trajectory that the vortex follows inside the capture streamtube. Results from inviscid, incompressible vortex filament theory have been compared with the experimental data. At relatively low streamtube contraction ratios this theory provides a good estimate of the vortex characteristics. However, at higher contraction levels, there are notable levels of diffusion, which render the vortex less intense than that anticipated from vortex filament theory.

Achilles tendon moment arms: The importance of measuring at constant tendon load when using the tendon excursion method

Achilles tendon moment arms are commonly measured using the tendon-excursion technique and ultrasound imaging of the muscle–tendon junction. The tendon-excursion technique relies on the assumption that the tendon load is constant and thus it does not stretch. However, previous studies have not enforced this constraint and thus it is not known how sensitive the estimated Achilles tendon moment arms are to varying load during the measurement process. The aim of this study was to compare estimates of Achilles tendon moment arms when calculated using the different constraints of constant force (and thus tendon stretch), constant joint torque, or contraction effort. Achilles tendon moment arms were measured for the medial and lateral gastrocnemii in 8 healthy male subjects across five different ankle angles (−5° dorsiflexion to 35° plantarflexion), and a range of contraction levels. Moment arms were calculated for three different constraints of constant force, torque, or effort. Moment arms were significantly greater for the lateral gastrocnemius than for the medial gastrocnemius. At low contraction levels, including the passive condition, the moment arms increased with plantarflexion, whereas the moment arms decreased with plantarflexion at higher contraction levels. There was no difference between the calculated moment arms using the constant force and the constant torque methods; however both these methods yielded significantly different moment arms when compared to the commonly used constant effort method.

Impaired neural drive in patients with low back pain.

Control of trunk movement relies on the integration between central neuronal circuits and peripheral skeletomuscular activities and it can be altered by pain. There is increasing evidence that there are deficits within the central nervous system controlling the trunk muscles in people with low back pain (LBP). However, it is unclear how LBP impacts upon neural drive to back muscles at different levels of voluntary contraction. Therefore, the purpose of this study was to investigate if neural drive is impaired in these patients. Seventeen patients with LBP and 11 healthy controls were recruited. Bilateral electromyographic (EMG) recordings were obtained from the erector spinae (ES) muscles at two vertebral levels (T12 and L4). Participants performed a series of brief isometric back extensions (50-100% maximum voluntary contraction - MVC), during which transcranial magnetic stimulation was delivered. The size of the evoked (superimposed) twitch was measured using dynamometry. The size of the superimposed twitch decreased linearly with increasing contraction strength in the controls; however, this linear relationship was not observed in the patients. Additionally, patients had larger superimposed twitches and longer time-to-peak amplitudes during MVCs than those observed in controls. Furthermore, patients had lower MVC and root-mean-square EMG activity of ES muscles during MVCs. A decline of central neural drive to the back muscles at high level of voluntary contraction was observed in patients with LBP. These results suggest that it might be pertinent to include neuromuscular facilitation programmes and therapeutic exercise utilizing high voluntary contractions for patients with LBP.

Facilitation and habituation of the startle reflex over the tonically active biceps brachii muscle contralateral to electrical stimuli

The aim of the present investigation was to explore the impact of muscle contraction on startle reflex responses after electrical stimuli (single or trains of 3) and to study startle reflex habituation. The electromyogram was recorded over the tonically active biceps brachii muscle in 19 healthy subjects contralateral to electrical stimuli (9–12mA) that were delivered at 1.0 and 0.4Hz over the superficial radial nerve. The muscle contraction level was varied by loading weight on the subject's bent arm (0.0, 1.0 or 1.5kg). Furthermore, short term reflex habituation was investigated using 30 blocks of 5 subsequent stimuli. Startle response amplitudes gained significantly (p<0.05) after (i) train stimuli as compared with single stimuli, during (ii) high versus low levels of muscle contraction, and at (iii) 0.4Hz versus 1.0Hz stimuli. Startle reflex amplitudes decreased significantly by the influence of preceding stimuli (p<0.05). This study provides evidence that the startle reflex can be significantly influenced by weight load, i.e. by volitional influences. Startle reflex investigation over a contracted limb muscle results in a high probability of startle release and thereby improved detection of SR habituation following preceding stimuli.

Effect of muscle contraction levels on the force–length relationship of the human Achilles tendon during lengthening of the triceps surae muscle–tendon unit

Findings from animal experiments are sometimes contradictory to the idea that the tendon structure is a simple elastic spring in series with muscle fibers, and suggest influence of muscle contraction on the tendon mechanical properties. The purpose of the present study was to investigate the influence of muscle contraction levels on the force–length relationship of the human Achilles tendon during lengthening of the triceps surae muscle–tendon unit. For seven subjects, ankle dorsiflexion was performed without (passive condition) and with contraction of plantar flexor muscles (eccentric conditions, at 3 contraction levels) on an isokinetic dynamometer. Deformation of the Achilles tendon during each trial was measured using ultrasonography. The Achilles tendon force corresponding to the tendon elongation of 10mm in the passive condition was significantly smaller than those in the eccentric conditions (p<0.05 or p<0.01). Within the eccentric conditions, the Achilles tendon force corresponding to the tendon elongation of 10mm was significantly greater in the maximal contraction level than those in submaximal eccentric conditions (p<0.05 or p<0.01). In addition, the tendon stiffness was greater in higher contraction levels (p<0.05 or p<0.01). Present results suggest that the human tendon structure is not a simple elastic spring in series with muscle fibers.

Changes in motor unit characteristics after eccentric elbow flexor exercise

Morphological evidence suggests that fast-twitch fibers are prone to disruption of their membrane structures by eccentric exercise. However, it is unclear how this is reflected in the discharge rate and action potential propagation of individual motor units, especially at high contraction levels. High-density surface electromyograms were recorded from biceps brachii muscle and decomposed to individual motor unit action potentials at isometric contraction levels between 10% and 75% of maximal voluntary contraction (MVC) before intermittent maximal elbow flexor eccentric exercise, and two hours (2H), two days (2D) and four days (4D) post-exercise. Maximal voluntary force decreased by 21.3±5.6% 2H and by 12.6±11.1% 2D post-exercise. Motor unit discharge rate increased and mean muscle fiber conduction velocity decreased, at the highest isometric contraction levels only (50% and 75% of MVC) at 2H post-exercise. These results indicate that eccentric exercise can disturb the function of motor units active at high contraction levels in the early stages after exercise, which seems to be compensated by the central nervous system with an increase in neural drive during submaximal isometric contractions.

Changes in motor unit characteristics after eccentric elbow flexor exercise

Morphological evidence suggests that fast-twitch fibers are prone to disruption of their membrane structures by eccentric exercise. However, it is unclear how this is reflected in the discharge rate and action potential propagation of individual motor units, especially at high contraction levels. High-density surface electromyograms were recorded from biceps brachii muscle and decomposed to individual motor unit action potentials at isometric contraction levels between 10% and 75% of maximal voluntary contraction (MVC) before intermittent maximal elbow flexor eccentric exercise, and two hours (2H), two days (2D) and four days (4D) post-exercise. Maximal voluntary force decreased by 21.3±5.6% 2H and by 12.6±11.1% 2D post-exercise. Motor unit discharge rate increased and mean muscle fiber conduction velocity decreased, at the highest isometric contraction levels only (50% and 75% of MVC) at 2H post-exercise. These results indicate that eccentric exercise can disturb the function of motor units active at high contraction levels in the early stages after exercise, which seems to be compensated by the central nervous system with an increase in neural drive during submaximal isometric contractions.

Branching Restrictions, Financial Market Integration, and Firm Growth: Evidence from U.S. Banking Deregulation

Branching Restrictions, Financial Market Integration, and Firm Growth: Evidence from U.S. Banking Deregulation Prasad Krishnamurthy UC Berkeley prasad@econ.berkeley.edu This version: August 17, 2009 Abstract I argue that legal restrictions on bank entry and expansion result in inadequate levels of nancial integration that leave rms and workers vulnerable to local credit supply shocks. Examining the history of intra and inter-state bank- branch deregulation in the United States, I show that branching restrictions limited nancial integration. This resulted in ine ciently high levels of expansion and contraction by rms in response to variations in local credit supply. My results suggest that, as a matter of industrial policy, countries should attempt to lower barriers to entry and expansion for commercial banks both within and across their borders. They also suggest that these restrictions on banks increase the employment uncertainty of workers in mid-sized rms and thus raise the cost of entrepreneurial activity. To substantiate these claims, I rst develop a model in which an increase in nancial integration|the ability of a market to draw on funds from other markets|insulates credit-constrained rms from the impact of local credit supply shocks. I then test the model's predictions by estimating the marginal reduction in the dependence of business growth on local credit growth after an increase in nancial integration. I nd that intra-state deregulation results in an 80% reduction in the e ect of local credit supply growth on mid-sized establishment growth (20-99 employees) in metropolitan statistical area (MSA) markets and that inter-state deregulation has a similar e ect for such establishments in non-MSA markets. I nd e ects of similar magnitude using employment growth and payroll growth as measures of rm growth. These results are robust to a variety of econometric speci cations. Preliminary and Incomplete. Please do not cite without the author's permission.

Impaired action potential conduction at high force levels after eccentric exercise

High-density surface electromyography was used to examine whether gross sarcolemmal function is impaired in m. biceps brachii after intensive eccentric elbow flexor exercise, when measured at wide range of isometric contraction levels. Root mean square (RMS), mean power frequency (MNF) and mean muscle fibre conduction velocity (CV) were calculated before and up to four days post-exercise. Maximal isometric voluntary (MVC) force decreased by 21.3 ± 5.6% two hours after exercise, and by 12.6 ± 11.1% two days post-exercise. CV and MNF decreased both during MVC (CV from 4.1 ± 0.3 m/s to 3.8 ± 0.4 m/s and MNF from 92.6 ± 10 Hz to 85.2 ± 11 Hz) and during electrically evoked maximal M-wave (CV from 4.1 ± 0.3 m/s to 3.0 ± 0.5 m/s and MNF from 97.1 ± 27.2 Hz to 78.0 ± 24.4 Hz) two hours post-exercise. Furthermore, at submaximal isometric force levels, CV and MNF decreased only at higher contraction levels (40%, 50% and 75% of MVC) two hour post-exercise. It can be concluded that intensive exercise can temporarily impair gross sarcolemmal function. In addition, since this only occurred at high force levels, based on Henneman’s size principle, it seems that higher threshold motor units were predominantly affected.

Peroxynitrite Increases Protein Phosphatase Activity and Promotes the Interaction of Phospholamban with Protein Phosphatase 2a in the Myocardium

Nitric oxide and superoxide react to form the potent oxidant peroxynitrite. The production of peroxynitrite increases during the pathogenesis of heart failure and is detrimental to myocardial function. We previously demonstrated that high levels of peroxynitrite decrease myocardial contraction by reducing phospholamban (PLB) phosphorylation through a protein phosphatase-dependent mechanism. However, we did not examine the direct effect of peroxynitrite on protein phosphatase activity in the myocardium or the specific protein phosphatase which is activated. Here we test: 1.) the effect of SIN-1 (peroxynitrite donor) on protein phosphatase activity in whole heart homogenates using a colorimetric assay, and 2.) the effect of SIN-1 on the interaction of PLB with protein phosphatase 1 (PP1) and protein phosphatase 2a (PP2a) using co-immunoprecipitation. SIN-1 induced a 63% increase in total protein phosphatase activity (1.6±0.2 vs. 2.6±0.3 nmol/min/mg, p<0.05 vs. Control), which was abolished with specific PP1/PP2a inhibition using okadaic acid (1.4±0.2 nmol/min/mg, p<0.05 vs. SIN-1). Since okadaic acid prevented the effects of SIN-1, we next examined the effect of SIN-1 on the interaction of PLB with PP1 and PP2a. SIN-1 increased the interaction of PLB with PP2a by 350% (0.6± 0.3 vs. 2.7±0.7 A.U., p<0.05 vs. Control), but had no effect on the interaction with PP1. The peroxynitrite scavenger, urate, prevented both the SIN-1-induced increase in protein phosphatase activity and the interaction of PLB with PP2a, thus implicating peroxynitrite as the causal species. The results of this study provide further insight into the mechanism through which high levels of peroxynitrite serve to decrease PLB phosphorylation and myocardial contraction. Therefore, increased peroxynitrite production may play a key role in heart failure where protein phosphatase activity is increased and PLB phosphorylation is decreased, ultimately leading to contractile dysfunction.

Effects of volitional contraction on intracortical inhibition and facilitation in the human motor cortex

Short-interval intracortical inhibition (SICI), intracortical facilitation (ICF) and short-interval intracortical facilitation (SICF) were assessed in the cortical motor area of the first dorsal interosseous muscle (FDI) of 16 healthy subjects. Paired-pulse TMS was delivered to the left hemisphere at the following interstimulus intervals (ISIs): 2 and 3 ms for SICI, 10 and 15 ms for ICF and 1-5 ms for SICF. Motor-evoked potentials were recorded from the resting and active right FDI. The effects exerted on SICI and ICF by four intensities (60-90% of active motor threshold, AMT) of the conditioning stimulus (S1) and by three levels of muscle contraction (10%, 25%, 50% of maximal voluntary contraction, MVC) were evaluated. The effects exerted on SICF were evaluated with two intensities (90% and 70% of AMT) of the test stimulus (S2) and with the same levels of muscle contraction. Results showed that: (i) during 10% MVC, maximum SICI was observed with S1 = 70% AMT; (ii) the amount of SICI obtained with S1 = 70% AMT was the same at rest as during 10% MVC, but decreased at higher contraction levels; (iii) ICF was observed only at rest with S1 = 90% AMT; (iv) SICF was facilitated at 10% and 25% MVC, but not at 50% MVC. We conclude that during muscle activation, intracortical excitability reflects a balance between activation of SICI and SICF systems. Part of the reduction in SICI during contraction is due to superimposed recruitment of SICF. Low intensity (70% AMT) conditioning stimuli can test SICI independently of effects on SICF at low contraction levels.

The mechanomyography of persons after stroke during isometric voluntary contractions

This study was to investigate the properties of mechanomyography (MMG), or muscle sound, of the paretic muscle in the affected side of hemiplegic subjects after stroke during isometric voluntary contractions, in comparison with those from the muscle in the unaffected side of the hemiplegic subjects and from the healthy muscle of unimpaired subjects. MMG and electromyography (EMG) signals were recorded simultaneously from the biceps brachii muscles of the dominant arm of unimpaired subjects ( n = 5) and the unaffected and affected arms of subjects after stroke ( n = 8), when performing a fatiguing maximal voluntary contraction (MVC) associated with the decrease in elbow flexion torque, and then submaximal elbow flexions at 20%, 40%, 60% and 80% MVCs. The root mean squared (RMS) values, the mean power frequencies (MPF, in the power density spectrum, PDS) of the EMG and MMG, and the high frequency rate (HF-rate, the ratio of the power above 15 Hz in the MMG PDS) were used for the analysis. The MMG RMS decreased more slowly during the MVC in the affected muscle compared to the healthy and unaffected muscles. A transient increase could be observed in the MMG MPFs from the unaffected and healthy muscles during the MVC, associated with the decrease in their simultaneous EMG MPFs due to the muscular fatigue. No significant variation could be seen in the EMG and MMG MPFs in the affected muscles during the MVC. The values in the MPF and HF-rate of MMG from the affected muscles were significantly lower than those from the healthy and unaffected muscles ( P < 0.05) at the high contraction level (80% MVC). Both the MMG and EMG RMS values in the healthy and unaffected groups were found to be significantly higher than the affected group ( P < 0.05) at 60% and 80% MVCs. These observations were related to an atrophy of the fast-twitch fibers and a reduction of the neural input in the affected muscles of the hemiplegic subjects. The results in this study suggested MMG could be used as a complementary to EMG for the analysis on muscular characteristics in subjects after stroke.

Effects of wrist position and contraction on wrist flexors H-reflex, and its functional implications

In order to determine whether joint position exerts a powerful influence on length–tension regulation in multiarticulate wrist flexors, three wrist positions (neutral, flexion and extension) and four levels of flexor contraction [0%, 10%, 20% and 30% maximum voluntary contraction (MVC)] were manipulated. There were significant differences in H-reflex amplitudes according to wrist positions and levels of flexor contraction. H-reflex increased linearly as a function of contraction in all three wrist positions. H-reflex was consistently larger in the wrist flexion than in the wrist extension position. The strength of the relationship ( ω 2) indicated that wrist position had a greater effect on H-reflex than force of muscle contraction. The interaction between wrist flexors contraction and joint position was significant only in the wrist flexion position. Trend analysis showed that, in the wrist flexion position, a low level of contraction was sufficient to maximally facilitate the H-reflex; however, a quadratic component was seen at higher contraction levels. The above findings may reflect the length–tension relationship of the multiarticulate wrist flexors. Therefore, this paper will discuss the functional implications related to the larger H-reflex in flexion position and the depressed H-reflex in the wrist extension position.

Repeatability of surface EMG parameters at various isometric contraction levels and during fatigue using bipolar and Laplacian electrode configurations

The objective of this work was to assess the repeatability of two surface electromyographic (sEMG) recording techniques, the classical bipolar configuration and a Laplacian configuration to document their ability to provide reliable information during follow-up studies. The signals were recorded on 10 healthy subjects during voluntary isometric contractions of the biceps brachii muscle at different constant contraction levels. Slopes, area ratios (at 60% of the maximal voluntary contraction (MVC)) and initial values (at 20%, 40%, 60%, 80% and 100% MVC) of the root mean square (RMS), the mean power frequency (MPF) and the muscle fibre conduction velocity (CV) were estimated. Experimental sessions were repeated on three different days with both electrode sets to evaluate the repeatability of sEMG parameter estimates. Classical results were observed, such as an increase in the RMS and the CV with the contraction level. Only initial values of RMS and MPF were shown to be dependent on electrode type. These two parameters presented intra-class correlation coefficient values higher than .80 for high contraction levels. On the whole, the repeatability of the measures was good; however it was better for all sEMG parameter estimates with bipolar electrodes than Laplacian electrodes. Because a bipolar configuration is less selective than a Laplacian one, it provides a global view of muscular activity, which is more repeatable, hence more suitable for follow-up studies.

Comparison of Bone Density in Competitive Male Cyclists and Recreationally Active Men

1081 PURPOSE: The purpose of this study was to examine the effects of cycling training on the total body, lumbar spine, and proximal femur bone mineral density (BMD) of men aged 20–45 years (n = 27). A sample (n = 10) of age-matched and body weight-matched recreationally active men served as the comparison group. METHODS: The athletes were competitive racing cyclists (USCF categories 1–5) with an average of 8.9 years of racing experience who trained 7–20 hours per week. Subjects completed calcium intake, health status, and training history questionnaires. BMD was measured using Dual Energy X-Ray Absorptiometry (GE Lunar, Prodigy, v. 6.70.021) at the AP lumbar spine (L2-L4), the dual femur (femoral neck, Ward's Area, trochanter, total hip) and the total body sites. Body composition variables were obtained from the total body scan. RESULTS: There were no significant (p>.05) differences between the cyclists (CYC) and controls (CON) for age, height, or body weight, however, the % body fat for the CYC group was significantly lower (p<.05) than the CON group. The calcium intake of the CON group was significantly lower (p<.05) than the CYC group (910 ± 113 vs1536 ± 144). There were no significant (p>.05) group differences for the total body and hip sites. The spine BMD was significantly (p<.01) lower for the CYC than CON. Based on the ISCD guidelines (2002) for osteoporosis diagnosis in men, 11% of the CYC had lumbar spine T-scores in the osteoporotic range(≥-2.5), whereas 19% and 20% of CYC and CON, respectively had osteopenia (T-score −1.1 to −2.4) at the spine. In addition, 22% of CYC fell into the osteopenia category for the total hip compared to 10% for the CON group. CONCLUSIONS: In conclusion, these findings suggest that competitive road cycling does not load the spine. However, mechanical loading may occur at the hip as a result of the high levels of muscular contraction by the leg and hip musculature during this type of training.Table

Comparison of Bone Density in Competitive Male Cyclists and Recreationally Active Men

1081 PURPOSE: The purpose of this study was to examine the effects of cycling training on the total body, lumbar spine, and proximal femur bone mineral density (BMD) of men aged 20–45 years (n = 27). A sample (n = 10) of age-matched and body weight-matched recreationally active men served as the comparison group. METHODS: The athletes were competitive racing cyclists (USCF categories 1–5) with an average of 8.9 years of racing experience who trained 7–20 hours per week. Subjects completed calcium intake, health status, and training history questionnaires. BMD was measured using Dual Energy X-Ray Absorptiometry (GE Lunar, Prodigy, v. 6.70.021) at the AP lumbar spine (L2-L4), the dual femur (femoral neck, Ward's Area, trochanter, total hip) and the total body sites. Body composition variables were obtained from the total body scan. RESULTS: There were no significant (p>.05) differences between the cyclists (CYC) and controls (CON) for age, height, or body weight, however, the % body fat for the CYC group was significantly lower (p<.05) than the CON group. The calcium intake of the CON group was significantly lower (p<.05) than the CYC group (910 ± 113 vs1536 ± 144). There were no significant (p>.05) group differences for the total body and hip sites. The spine BMD was significantly (p<.01) lower for the CYC than CON. Based on the ISCD guidelines (2002) for osteoporosis diagnosis in men, 11% of the CYC had lumbar spine T-scores in the osteoporotic range(≥-2.5), whereas 19% and 20% of CYC and CON, respectively had osteopenia (T-score −1.1 to −2.4) at the spine. In addition, 22% of CYC fell into the osteopenia category for the total hip compared to 10% for the CON group. CONCLUSIONS: In conclusion, these findings suggest that competitive road cycling does not load the spine. However, mechanical loading may occur at the hip as a result of the high levels of muscular contraction by the leg and hip musculature during this type of training.Table

Variation of magnitude and timing of wrist flexor stretch reflex across the full range of voluntary activation.

This paper reports an investigation of the magnitude and timing of the stretch reflex over the full range of activation of flexor carpi radialis. While it is well established that the magnitude of the reflex increases with the level of muscle activation, there have been few studies of reflex magnitude above 50% of maximum voluntary contraction (MVC) and virtually no study of the timing of the response in relation to activation level. Continuous small amplitude (approximately 2 degrees) perturbations were applied to the wrist of 12 normal subjects while they maintained contraction levels between 2.5-95% MVC, monitored via surface electromyography (EMG). Both narrow band (4-5 Hz) and broad band (0-10 Hz) stretch perturbations were employed. The gain (EMG output/stretch input) and phase advance of the reflex varied with the level of muscle activation in a similar manner for both types of stretch, but there were significant differences in the patterns of change due to stretch bandwidth. Consistent with previous studies, the group average reflex gain initially increased with muscle activation level and then saturated. Inspection of individual data, however, revealed that the gain reached a peak at about 60% MVC and then decreased at higher contraction levels, the pattern across the full range of activation being well described by quadratic functions (mean r2=0.82). This quadratic pattern has not been reported previously for the neural reflex response in any muscle but is consistent with the pattern that has been reliably observed in studies of the mechanical reflex response in lower limb muscles. In contrast to the pattern for reflex gain, the phase advance of the reflex (at a stretch frequency of 4.5 Hz) decreased linearly from approximately 130 degrees at the lowest contraction levels to approximately 50 degrees as maximum voluntary contraction was reached (mean r2=0.69). This decrease corresponds to a delay of 49 ms introduced centrally in reflex pathways. All subjects showed clearly defined quadratic functions relating reflex gain and linear functions relating reflex phase to activation level, but there were considerable individual differences in the slopes of these functions which point to systematic differences in synaptic behaviour of the motoneuron pool. Thus, there was wide inter-subject variation in both the contraction level at which the reflex gain reached a peak (31-69% MVC) and the highest target contraction level that could be sustained during reflex measurement (47-95% MVC). A high correlation between these variables (r2=0.78) suggests a linear relation between afferent support of contraction and muscle fatigability. The decline in reflex gain at high levels of muscle activation signals a failure of muscle afferent input and subjects in whom the gain reached a peak and declined early were unable to sustain higher target contraction levels. The results of the study show that both the timing and magnitude of the stretch reflex vary markedly over the full range of voluntary muscle activation. The pattern of variation may account for why the stretch reflex contributes most effectively to muscle mechanics over the lower half of the range of activation, while progressive reductions in both gain and phase advance at higher levels render the reflex mechanically less effective and make tremor more likely.

On-site weathering of sealants under tropical conditions

An on-site weathering rack was developed to simulate the tropical conditions combining the simultaneous effects of joint movement, temperature, humidity, rainfall, water retention and sunlight. The simulated movement of joints was achieved by utilizing the high level of extension and contraction of aluminium under temperature change. The 10-m long aluminium rack with concrete slabs bolted on it allows sealants to be tested on concrete surfaces. Test results have shown that the rack is efficient in subjecting test specimens to real level construction joint movements exposed to the tropical environment.

TNF-alpha acutely inhibits vascular effects of physiological but not high insulin or contraction.

TNF-alpha is elevated in many states of insulin resistance, and acutely administered TNF-alpha in vivo inhibits insulin-mediated hemodynamic effects and glucose uptake in muscle. In this study, we assess whether the inhibitory effects of TNF-alpha are affected by insulin dose or muscle contraction. Whole body glucose infusion rate (GIR), femoral blood flow (FBF), hindleg vascular resistance, hindleg glucose uptake (HGU), 2-deoxyglucose uptake into muscles of the lower leg (R'g) and hindleg metabolism of infused 1-methylxanthine (1-MX), a measure of capillary recruitment, were determined. Three groups were studied with and without infusion of TNF-alpha: euglycemic insulin-clamped, one-leg field-stimulated (2 Hz, 0.1 ms at 30 V), and saline-infused control anesthetized rats. Insulin infusions were 3, 10, or 30 mU x kg-1 x min-1 for 2 h x 1-MX metabolism was maximally increased by all three doses of insulin. GIR, HGU, and R'g were maximal at 10 mU and FBF was maximal at 30 mU of insulin. Contraction increased FBF, HGU, and 1-MX. TNF-alpha (0.5 microg x kg-1 x h-1) totally blocked the 3 and 10 mU insulin-mediated increases in FBF and 1-MX, and partly blocked GIR, HGU, and R'g. None of the increases due to twitch contraction was affected by TNF-alpha, and only the increase in FBF due to 30 mU of insulin was partly affected. We conclude that muscle capillary recruitment and glucose uptake due to high levels of insulin or muscle contraction under twitch stimuli at 2 Hz are resistant to TNF-alpha. These findings may have implications for ameliorating muscle insulin resistance resulting from increased plasma TNF-alpha and for the differing mechanisms by which contraction and insulin recruit capillary flow in muscle.

Non-invasive approach of motor unit recording during muscle contractions in humans.

Information about the structural and functional characteristics of the motor unit (MU) is highly relevant for the diagnosis of neuromuscular disorders. Electromyography (EMG) is a suitable method for obtaining the information needed. The problem is the separation of the activity of one MU from others which are simultaneously active. Such investigations of single MU activity have commonly used invasive methods, e.g. employing a needle or a wire. Conventional surface-EMG methods have limited resolution and detect, at high contraction levels, multiple MU superimposed one on the other. The separation of the activity of a single MU can be achieved in a non-invasive way when highly specialised acquisition techniques are used. One approach, called high spatial resolution EMG (HSR-EMG), is based on the use of multi-electrode arrays in combination with a two-dimensional Laplace filter. The HSR-EMG permits the completely non-invasive detection of single MU activity even during maximal voluntary contractions. First applications have shown that the method provides a deeper insight into the functional and structural characteristics of the MU. In this paper the application of HSR-EMG to the diagnosis of neuromuscular disorders will be presented, and the latest results will be given of its application in the evaluation of treatment of patients with plexus lesion.