Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for “central” fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal cha...

Spinal and Supraspinal Factors in Human Muscle Fatigue

Repeated, intense use of muscles leads to a decline in performance known as muscle fatigue. Many muscle properties change during fatigue including the action potential, extracellular and intracellular ions, and many intracellular metabolites. A range of mechanisms have been identified that contribute to the decline of performance. The traditional explanation, accumulation of intracellular lactate and hydrogen ions causing impaired function of the contractile proteins, is probably of limited importance in mammals. Alternative explanations that will be considered are the effects of ionic changes on the action potential, failure of SR Ca2+ release by various mechanisms, and the effects of reactive oxygen species. Many different activities lead to fatigue, and an important challenge is to identify the various mechanisms that contribute under different circumstances. Most of the mechanistic studies of fatigue are on isolated animal tissues, and another major challenge is to use the knowledge generated in these studies to identify the mechanisms of fatigue in intact animals and particularly in human diseases.

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Skeletal Muscle Fatigue: Cellular Mechanisms

Photoinduced motions in azo-containing polymers.

Muscle fatigue encompasses a class of acute effects that impair motor performance. The mechanisms that can produce fatigue involve all elements of the motor system, from a failure of the formulation of the descending drive provided by suprasegmental centers to a reduction in the activity of the contractile proteins. We propose four themes that provide a basis for the systematic evaluation of the neural and neuromuscular fatigue mechanisms: 1) task dependency to identify the conditions that activate the various mechanisms; 2) force-fatigability relationship to explore the interaction between the mechanisms that results in a hyperbolic relationship between force and endurance time; 3) muscle wisdom to examine the association among a concurrent decline in force, relaxation rate, and motor neuron discharge that results in an optimization of force; and 4) sense of effort to determine the role of effort in the impairment of performance. On the basis of this perspective with an emphasis on neural mechanisms, we suggest a number of experiments to advance our understanding of the neurobiology of muscle fatigue.

Neurobiology of muscle fatigue.

Cyanines during the 1990s: A Review

The Maxwell displacement current generated from single fatty acid monolayers containing azobenzene by photoisomerization was investigated in the context of the phase diagram of monolayers on a water surface. Transient displacement‐current pulses were found to be produced when successive isomerizations were induced in monolayers by irradiation with ultraviolet and visible light. In contrast, the displacement current was never generated in the so‐called solid phase by the application of surface pressure or photoirradiation, probably because of the lack of free volume for the isomerizing chromophore in single monolayers and as well as the aggregation of molecules in the monolayer. The displacement current generated during monolayer compression was also investigated in connection with phase transitions.

Generation of Maxwell displacement current from spread monolayers containing azobenzene

Simultaneous determination of uric and ascorbic acids in human serum by reversed-phase high-performance liquid chromatography with electrochemical detection.

Ultraviolet-visible (UV−vis), infrared transmission, reflection−absorption (RA), and fluorescence spectra have been measured for one-, three-, and nine-monolayer Langmuir−Blodgett (LB) films of an azobenzene-containing long-chain fatty acid (4-dodecyloxy-4‘-(3-carboxytrimethyleneoxy)azobenzene, abbreviated 12A3H) at various temperatures to investigate dependences of phase transitions and annealing effects on the number of monolayers. The UV−vis spectra of the LB films have revealed that the fatty acid investigated forms H-aggregates in the LB films irrespective of the number of monolayers at room temperature. With a temperature increase the H-aggregate in the one-monolayer LB film gradually breaks into the monomers, while that in the three- and nine-monolayer LB films abruptly changes into the J-aggregate near 90 °C. These observations show that the one-monolayer LB film does not have a clear order−disorder transition, while the three-and nine-monolayer films have a phase transition near 90 °C. The infrar...

Spectroscopic Studies on Phase Transitions in Langmuir−Blodgett Films of an Azobenzene-Containing Long-Chain Fatty Acid: Dependence of Phase Transitions on the Number of Monolayers and Transition Cycles among H-, J-, and J‘-Aggregates in Multilayer Films

Molecular Aggregation and Photoisomerization of Langmuir-Blodgett Films of Azobenzene-Containing Long-Chain Fatty Acids and Their Salts Studied by Ultraviolet-Visible and Infrared Spectroscopies

The effect of chromophore aggregation upon the photoconductivity in the dye-sensitized Langmuir films has been examined employing three derivatives of a surface-active merocyanine with a rhodanine nucleus. The values of the lateral component Δσt and the anisotropy ratio Δσ∥/Δσ⊥ for the case with a strongly developed aggregate are found to be more than a decade larger than those for no or incomplete aggregation.

Keiji Iriyama

Papers

Generation of Maxwell displacement current from spread monolayers containing azobenzene

Simultaneous determination of uric and ascorbic acids in human serum by reversed-phase high-performance liquid chromatography with electrochemical detection.

Spectroscopic Studies on Phase Transitions in Langmuir−Blodgett Films of an Azobenzene-Containing Long-Chain Fatty Acid: Dependence of Phase Transitions on the Number of Monolayers and Transition Cycles among H-, J-, and J‘-Aggregates in Multilayer Films

Molecular Aggregation and Photoisomerization of Langmuir-Blodgett Films of Azobenzene-Containing Long-Chain Fatty Acids and Their Salts Studied by Ultraviolet-Visible and Infrared Spectroscopies

Effect of Chromophore Aggregation in the Langmuir Multilayer Photoconductors