Muscle fatigue or neuromuscular fatigue is defined as a decrease in maximal voluntary contraction (MVC) torque/force/power that can be produced by a muscle group. To identify if an experimental manipulation induces muscle fatigue, you will need to measure MVCs pre-post intervention. If MVC decreases, you demonstrates presence of muscle fatigue. However you do not know the origin of muscle fatigue.

Muscle fatigue can be caused by a central (fatigue) or peripheral (fatigue) component. Central fatigue corresponds to a decrease in maximal voluntary activation level (inhibition of the central motor drive, measured by the twitch interpolation tehnique) and peripheral fatigue to all changes at or distal to the neuromuscular junction. Briefly, to measure these central and peripheral components you can use various stimulation techniques such as peripheral nerve stimulation, transcranial magnetic stimulation and the twitch interpolation technique.

Also, muscle fatigue and mental fatigue are two different phenomena. Mental fatigue does not induce a decrease in MVC but increase your perception of effort and will impair your performance (during endurance exercise) independently of any impairment of neuromuscular function. When you are mentally fatigued, your neuromuscular system is still able to produce a maximal force, but effort is perceived higher.

http://www.ncbi.nlm.nih.gov/pubmed/11581501

http://www.ncbi.nlm.nih.gov/pubmed/18195089

http://www.ncbi.nlm.nih.gov/pubmed/19131473

http://www.ncbi.nlm.nih.gov/pubmed/23698244

Testing fatigue can be challenging. Especially at the muscle level where so many factors can contribute to the process of (muscular) fatigue. That said, a reduction in action potentials at muscle membrane can lead to a reduced activation of specific gates within t-tubules affecting calcium release. This is just one neural factor contributing to muscular fatigue.

In addition to the increase of lactic acid and the reduction of muscular glycogen, the MAIN cause of muscular fatigue is the change of ph in the microenvironment. If we measured the ph level inside the muscle cells, where the myosin head interacts with the actin filament, we would note a physiological ph level. But when the exercise is extreme, that is when too many times the ATP has been hydrolyzed, H+ ions increases and the ph level may low from 7 to 4. Because the myosin head has an optimal conformation at ph 7-7.2, ph 4 levels brakes the contractile event, because each enzyme needs for an optimal ph to act. This is a transitory condition, but it’s sufficient to prevent the exercise continuation.

Muscle fatigue or neuromuscular fatigue is defined as a decrease in maximal voluntary contraction (MVC) torque/force/power that can be produced by a muscle group. To identify if an experimental manipulation induces muscle fatigue, you will need to measure MVCs pre-post intervention. If MVC decreases, you demonstrates presence of muscle fatigue. However you do not know the origin of muscle fatigue.

Muscle fatigue can be caused by a central (fatigue) or peripheral (fatigue) component. Central fatigue corresponds to a decrease in maximal voluntary activation level (inhibition of the central motor drive, measured by the twitch interpolation tehnique) and peripheral fatigue to all changes at or distal to the neuromuscular junction. Briefly, to measure these central and peripheral components you can use various stimulation techniques such as peripheral nerve stimulation, transcranial magnetic stimulation and the twitch interpolation technique.

Also, muscle fatigue and mental fatigue are two different phenomena. Mental fatigue does not induce a decrease in MVC but increase your perception of effort and will impair your performance (during endurance exercise) independently of any impairment of neuromuscular function. When you are mentally fatigued, your neuromuscular system is still able to produce a maximal force, but effort is perceived higher.

http://www.ncbi.nlm.nih.gov/pubmed/11581501

http://www.ncbi.nlm.nih.gov/pubmed/18195089

http://www.ncbi.nlm.nih.gov/pubmed/19131473

http://www.ncbi.nlm.nih.gov/pubmed/23698244

You can measure it by a dynamometer or by a surface EMG or by the combination of both.You may also use a digital inclinometer to measure the pre motor time in order to estimate central fatigue.