During leg extensions performed with a consistent upward motion, muscle fatigue tends to become unavoidable when the weight used is around 31.7% of a person's maximum lifting capacity, with a range...
Mechanism
Synthesis from 1 study
At about one-third of your maximum strength, your muscles use up their quick energy so fast that waste chemicals build up faster than your body can remove them. These chemicals make your muscle fibers weaker and send signals to your brain to stop, so even if you try hard, you can't keep going.
Most probable mechanism
When lifting a weight that's about one-third of your maximum strength, your muscles work hard enough to use up their quick energy supply and build up waste chemicals faster than they can be cleared. These chemicals make it harder for muscle fibers to contract properly and send signals back to your brain to slow down. Eventually, the muscles can't keep producing force, even if you try your hardest, and you have to stop.
Sustained concentric contractions at loads near 30% of maximum strength increase intramuscular pressure, partially restricting blood flow and reducing oxygen delivery to active muscle fibers.
Reduced oxygen availability and high energy demand cause rapid depletion of phosphocreatine and accumulation of inorganic phosphate, hydrogen ions, and lactate within muscle cells.
Accumulated metabolites inhibit the activity of myosin ATPase and reduce calcium release from the sarcoplasmic reticulum, impairing the ability of muscle fibers to generate force.
Metabolic byproducts activate sensory nerves in the muscle, which send inhibitory signals to the spinal cord and brain, reducing voluntary motor drive and motor unit recruitment.
Even as additional motor units are recruited to compensate for fatigue in initially active fibers, their contractile efficiency is limited by the same metabolic inhibition, preventing force maintenance.
When the rate of metabolite accumulation exceeds clearance capacity, a metabolic threshold is crossed, leading to irreversible decline in force production and task failure.
Evidence from Studies
Supporting (1)
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Minimum load threshold in resistance training: insights into muscle metabolism, excitation, and fatigue across the repetition continuum
Contradicting (0)
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