A study of fatigue during repetitive static work performed in two different segmental positions

Sixteen subjects, nine women and seven men, aged between 19 and 35 years, performed three series of isometric contractions of the flexor muscles of the forearm at 40% MVC. Each series consisted of four isometric contractions sustained until exhaustion, and separated from each other by an incomplete rest interval of 5 min. The position of the arm (either the lower or the upper position) was variable according to the series. Under these conditions, study of heart rate (HR), systolic blood pressure (SBP) and limit-time showed that: HR and SBP were not affected by the arm position even when fatigue was important. For a given load with a progressive appearance of fatigue, the limit-time in female subjects was higher. In both groups, in spite of the fatigue appearing after several isometric contractions performed in the upper position, movement of the arm to the lower position was sufficient to significantly increase the time-limit of another isometric contraction at 40% MVC without concomitant increase in cardiac work.     

A simple mathematical model of the interaction between intracranial pressure and cerebral hemodynamics

Ursino, Mauro, and Carlo Alberto Lodi. A simplemathematical model of the interaction between intracranial pressure andcerebral hemodynamics. J. Appl.Physiol. 82(4): 1256-1269, 1997.A simplemathematical model of intracranial pressure (ICP) dynamics oriented toclinical practice is presented. It includes the hemodynamics of thearterial-arteriolar cerebrovascular bed, cerebrospinal fluid (CSF)production and reabsorption processes, the nonlinear pressure-volumerelationship of the craniospinal compartment, and a Starling resistormechanism for the cerebral veins. Moreover, arterioles are controlledby cerebral autoregulation mechanisms, which are simulated by means ofa time constant and a sigmoidal static characteristic. The model isused to simulate interactions between ICP, cerebral blood volume, andautoregulation. Three different related phenomena are analyzed: thegeneration of plateau waves, the effect of acute arterial hypotensionon ICP, and the role of cerebral hemodynamics during pressure-volume index (PVI) tests. Simulation results suggest the following:1) ICP dynamics may become unstablein patients with elevated CSF outflow resistance and decreasedintracranial compliance, provided cerebral autoregulation is efficient.Instability manifests itself with the occurrence of self-sustainedplateau waves. 2) Moderate acutearterial hypotension may have completely different effects on ICP,depending on the value of model parameters. If physiological compensatory mechanisms (CSF circulation and intracranial storage capacity) are efficient, acute hypotension has only negligible effectson ICP and cerebral blood flow (CBF). If these compensatory mechanismsare poor, even modest hypotension may induce a large transient increasein ICP and a significant transient reduction in CBF, with risks ofsecondary brain damage. 3) The ICPresponse to a bolus injection (PVI test) is sharply affected, viacerebral blood volume changes, by cerebral hemodynamics andautoregulation. We suggest that PVI tests may be used to extractinformation not only on intracranial compliance and CSF circulation,but also on the status of mechanisms controlling CBF.

     

Sex differences in static muscular endurance

The relationship between the force level of sustained isometric contraction and the time until two indices of muscular fatigue was compared for the right elbow flexors between 6 young women and 6 men. The strength of the maximal voluntary contraction (MVC) measured at the wrist with the elbow at right angle was significantly greater in men (267 +/- 35 N) than in women (163 +/- 23 N). Endurance tasks were performed at randomly assigned contraction levels of 5, 10, 15, 20, 30, 40, and 50% MVC. The relative force (F% MVC) or the absolute force (F N) versus the endurance time or the time of pain ("itai") appearance (T min) curves were fitted to the following equation: log T = a + b.log (F--k). The most fitted equations were determined for each individual and each sex group by the value in the coefficient of determination. When comparing the curves in terms of the relative force, the endurance time and pain appearance time were longer in women than in men, particularly at lower contraction levels. In terms of the absolute force, however, they were longer in men than in women, particularly at stronger contractions. At lower contraction levels, less than 70 N, some women were similar to some men in fatigability.     

Is rotating between static and dynamic work beneficial for our fatigue state?

Shoulder disorders comprise a large part of work-related musculoskeletal disorders. Risk factors, such as repetitiveness and monotony, may cause muscle fatigue and be attenuated by task rotation. We investigated rotation between a dynamic box-lifting task and a relatively static pick-and-place task and aimed to determine whether (1) a high rotation frequency leads to less fatigue development than a low rotation frequency, and (2) a self-selected rotation frequency leads to less fatigue development than imposed rotation frequencies. Ten participants performed four one-hour rotation schedules: two low frequency rotation schedules rotating at 30 min, one high frequency rotation schedule rotating every sixth minute, and a self-selected rotation schedule. Borg, SOFI and electromyography of Trapezius and Deltoid subparts served as fatigue indicators. We found significant signs of fatigue for most schedules regarding the Borg and SOFI ratings and the M. Trapezius pars Descendens. Task rotation frequency had no significant effect on any of the outcome parameters, whereas the self-selected rotation schedule clearly resulted in less development of perceived fatigue than imposed schedules. In conclusion, we think that freedom of rotation has the greatest potential to attenuate potential development of musculoskeletal disorders and we require due caution with the use and interpretation of EMG indicators of fatigue.     

Spin-trappers and vitamin E prolong endurance to muscle fatigue in mice

The involvement of free radicals in endurance to muscle effort is suggested by experimental and clinical data. Therefore, experiments have been performed to observe the effect of trapping free radicals on endurance to swimming in mice. Animals were injected intraperitoneally with each of three spin-trappers [N-tert-Butyl-alpha-Phenyl-Nitrone (PBN),alpha-4-Pyridyil-1-Oxide-N-tert-Butyl-Nitrone (POBN) and 5,5-Dimethyl-1-Pirrolyn-N-Oxide (DMPO): 0.2 ml of 10(-1) molar solution]. Each mouse was submitted to a swimming test to control resistance to exhaustion a) without any treatment, b) after administration of each spin-trapper in a random order c) after saline. Control experiments were performed with saline and with vitamin E. Endurance to swimming was greatly prolonged by pretreatment with all the spin-trappers (DMPO less than 0.0001; POBN less than 0.0001; PBN less than 0.001) and with Vitamin E. Experiments state that compared to treatment with spin-trappers or Vitamin E, administration of saline alone did not enhance time to exhaustion so that the increase in time to exhaustion with the various free radical scavengers was not the effect of training. Therefore, free radicals could be considered as one of the factors terminating muscle effort in mice.     

A three-compartment muscle fatigue model accurately predicts joint-specific maximum endurance times for sustained isometric tasks

The development of localized muscle fatigue has classically been described by the nonlinear intensity-endurance time (ET) curve (Rohmert, 1960; El Ahrache et al., 2006). These empirical intensity-ET relationships have been well-documented and vary between joint regions. We previously proposed a three-compartment biophysical fatigue model, consisting of compartments (i.e. states) for active (M(A)), fatigued (M(F)), and resting (M(R)) muscles, to predict the decay and recovery of muscle force (Xia and Frey Law, 2008). The purpose of this investigation was to determine optimal model parameter values, fatigue (F) and recovery (R), which define the "flow rate" between muscle states and to evaluate the model's accuracy for estimating expected intensity-ET curves. Using a grid-search approach with modified Monte Carlo simulations, over 1 million F and R permutations were used to predict the maximum ET for sustained isometric tasks at 9 intensities ranging from 10% to 90% of maximum in 10% increments (over 9 million simulations total). Optimal F and R values ranged from 0.00589 (F(ankle)) and 0.0182 (R(ankle)) to 0.00058 (F(shoulder)) and 0.00168 (R(shoulder)), reproducing the intensity-ET curves with low mean RMS errors: shoulder (2.7s), hand/grip (5.6s), knee (6.7s), trunk (9.3s), elbow (9.9s), and ankle (11.2s). Testing the model at different task intensities (15-95% maximum in 10% increments) produced slightly higher errors, but largely within the 95% prediction intervals expected for the intensity-ET curves. We conclude that this three-compartment fatigue model can be used to accurately represent joint-specific intensity-ET curves, which may be useful for ergonomic analyses and/or digital human modeling applications.     

A simple work flow for biologically inspired model reduction - application to early JAK-STAT signaling

Background  

Modeling of biological pathways is a key issue in systems biology. When constructing a model, it is tempting to incorporate all known interactions of pathway species, which results in models with a large number of unknown parameters. Fortunately, unknown parameters need not necessarily be measured directly, but some parameter values can be estimated indirectly by fitting the model to experimental data. However, parameter fitting, or, more precisely, maximum likelihood parameter estimation, only provides valid results, if the complexity of the model is in balance with the amount and quality of the experimental data. If this is the case the model is said to be identifiable for the given data. If a model turns out to be unidentifiable, two steps can be taken. Either additional experiments need to be conducted, or the model has to be simplified.     

Electromyography and fatigue during prolonged, low-level static contractions

Findings from five separate studies of EMG changes and muscle fatigue during prolonged low-level static contractions are summarized, and the possible mechanisms behind the changes are briefly discussed. Sustained static contractions (10%, 7% and 5% MVC) of up to 1 h duration were performed by finger flexors, elbow flexors and extensors, and knee extensors. In one experiment, intermittent static arm pulling (triceps) (10 s contraction and 5 s rest, average work load 14% and 10% MVC) was performed for 7 h. The endurance time for the sustained contractions was around one hour for 10% MVC, and it was shown--all in all--that the concept of "indefinite" endurance times at contractions below 15-20% MVC cannot be maintained. After 5% MVC sustained contractions for one hour a 12% reduction in MVC was seen, and significant increases in EMG amplitude and decreases in the mean spectral frequency of the EMG-power spectrum were found. Marked differences were also seen in the EMG changes in the elbow flexors and extensors, and transcutaneous electrical stimulation of the knee extensors showed that low frequency fatigue was present after the contraction. With intermittent contractions similar changes in the EMG parameters were seen after 2-3 h of contractions at 14% MVC. On average, during contractions of 10% MVC no EMG changes were detected. Increased extracellular potassium concentration in the contracting muscles is suggested as a possible explanation of these findings.     

Pressure-time product, work rate, and endurance during resistive breathing in humans

We examined the effect of increasing work rate, without a corresponding increase in the pressure-time product, on energy cost and inspiratory muscle endurance (Tlim) in five normal subjects during inspiratory resistive breathing. Tidal volume, mean inspiratory mouth pressure, duty cycle, and hence the pressure-time product were kept constant, whereas work rate was varied by changing the frequency of breathing. There was a linear decrease in Tlim of -2.1 +/- 0.5 s.J-1.min-1 (r = 0.87 +/- 0.06) with increasing work rate. The data satisfied a model of energy balance during fatiguing runs (Monod and Scherrer. Ergonomics 8: 329-337, 1965) and were consistent with the hypothesis that the rate of energy supply, or respiratory muscle blood flow, is fixed when the pressure-time product is constant. Our results indicate that during inspiratory resistive breathing against fatiguing loads, work rate determines endurance independently of the pressure-time product. On the basis of the model, our results lead to estimates of respiratory muscle blood flow and available energy stores under the conditions of our experiment.     

A dynamical model of muscle activation, fatigue, and recovery

A dynamical model is presented as a framework for muscle activation, fatigue, and recovery. By describing the effects of muscle fatigue and recovery in terms of two phenomenological parameters (F, R), we develop a set of dynamical equations to describe the behavior of muscles as a group of motor units activated by voluntary effort. This model provides a macroscopic view for understanding biophysical mechanisms of voluntary drive, fatigue effect, and recovery in stimulating, limiting, and modulating the force output from muscles. The model is investigated under the condition in which brain effort is assumed to be constant. Experimental validation of the model is performed by fitting force data measured from healthy human subjects during a 3-min sustained maximal voluntary handgrip contraction. The experimental results confirm a theoretical inference from the model regarding the possibility of maximal muscle force production, and suggest that only 97% of the true maximal force can be reached under maximal voluntary effort, assuming that all motor units can be recruited voluntarily. The effects of different motor unit types, time-dependent brain effort, sources of artifacts, and other factors that could affect the model are discussed. The applications of the model are also discussed.     

One-handed load carrying — Cardiovascular, muscular and subjective indices of endurance and fatigue

A group of 5 women and 11 men walked on a treadmill, each carrying a weight in the right hand. In separate experiments, the mass was varied to give total exhaustion within 3 min, 5 min, 9 min, and 13 min. In additional experiments 50% and 25% of the masses leading to exhaustion after 5 min were used, and these were stopped after 16 min. Heart rate (fc) and systolic blood pressure (BPs) were measured noninvasively. There was a consistent increase in fc x BPs during the experiments leading to exhaustion, while steady-states were obtained in the nonexhausting trials. An electromyogram (EMG) was recorded with cutaneous electrodes over the flexor carpi ulnaris and flexor digitorum superficialis muscles and the number of zero crossings (ZC) of the EMG signal per time unit were analysed. As the subjects approached exhaustion, the number of ZC declined exponentially, reaching approximately 50% of their initial values. In the nonexhausting experiments, however, the decline was slower and less marked, and during the second half of the experiment the number of ZC increased again. Subjectively, endurance was underestimated by all the subjects. It was concluded that cardiovascular and muscle criteria of fatigue in carrying coincided. Prolonged carrying in one hand of more than 6 kg or 10 kg for young healthy women and men respectively should not be recommended, since it could lead to cardiovascular non steady-states and EMG signs of fatigue.     

A simple method for the preparation of fish chromosomes applicable to field work, teaching and banding

A method is described which has broad application for field acquisition of fish karyotypes for research and teaching. The benefits of the method include: low initial skill level, relatively inexpensive equipment and supplies, simplicity and speed, and the high quality of preparations. The technique was applied to a field study of Apogon maculatus which was shown to contain 34 chromosomes, 2 nucleolar organizing regions (NOR's), and resolvable G-bands.