Despite a normal amount of force being generated at the start of activity, once muscle fatigue has set in and progressively worsens, if the individual persists in the exercise they will eventually lose their hand grip, or become unable to lift or push with their arms or legs, or become unable to maintain an isometric position (such as plank).
[4][5][6] Muscle cells work by detecting a flow of electrical impulses from the brain which signals them to contract through the release of calcium by the sarcoplasmic reticulum.
There is no sensation of pain or discomfort, the muscle appears to simply ‘stop listening’ and gradually cease to contract, often going backwards.
[citation needed] Part of the process of strength training is increasing the nerve's ability to generate sustained, high frequency signals which allow a muscle to contract with its greatest force.
This neural training can cause several weeks of rapid gains in strength, which level off once the nerve is generating maximum contractions and the muscle reaches its physiological limit.
Past this point, training effects increase muscular strength through myofibrillar or sarcoplasmic hypertrophy and metabolic fatigue becomes the factor limiting contractile force.
[citation needed] Though not universally used, ‘metabolic fatigue’ is a common term for the reduction in contractile force due to the direct or indirect effects of two main factors: Substrates within the muscle serve to power muscular contractions.
They include chloride, potassium, lactic acid, ADP, magnesium (Mg2+), reactive oxygen species, and inorganic phosphate.
This can lower the sensitivity of contractile apparatus to Ca2+ but also has the effect of increasing cytoplasmic Ca2+ concentration through an inhibition of the chemical pump that actively transports calcium out of the cell.
It has been found that the ryanodine receptor present in skeletal muscle undergoes a conformational change during exercise, resulting in "leaky" channels that are deficient in calcium release.
In research studies, participants were found to show reduced voluntary force production in fatigued muscles (measured with concentric, eccentric, and isometric contractions), vertical jump heights, other field tests of lower body power, reduced throwing velocities, reduced kicking power and velocity, less accuracy in throwing and shooting activities, endurance capacity, anaerobic capacity, anaerobic power, mental concentration, and many other performance parameters when sport specific skills are examined.
Using the median power frequency, raw EMG data is filtered to reduce noise and then relevant time windows are Fourier Transformed.