During high-intensity exercise, the buildup of protons affects muscle force production more strongly in knee extension than in plantar flexion, suggesting that different types of exercise rely on...
Mechanism
Synthesis from 1 study
In all-out knee extensions, acid builds up fast and gets stuck in the muscle's force-producing parts, making them less effective. In slower plantar flexion, a different waste product does more damage. That’s why the muscles tire differently depending on the exercise.
Most probable mechanism
During all-out knee extensions, muscles produce a lot of acid quickly, and this acid sticks to the parts of muscle fibers that generate force, making it harder for them to lock into place and push. This happens more than in slower exercises like plantar flexion, where other waste products matter more, so the muscles lose strength faster in knee extensions because of the acid.
High-intensity all-out knee extension exercise causes rapid ATP hydrolysis, leading to significant accumulation of protons (H+) in muscle cells.
Accumulated protons remain bound to the actin-myosin cross-bridge complex during the weakly bound (A1) state and inhibit the conformational change required to transition to the strongly bound (A2) state.
This inhibition reduces the number of myosin heads that can form strong, force-generating attachments to actin filaments.
The sensitivity of force generation to proton concentration is greater during all-out knee extension than during constant-power plantar flexion, where inorganic phosphate dominates fatigue mechanisms.
Less supported by current evidence, but not ruled out
During slower, steady plantar flexion exercises, muscle fatigue comes more from a buildup of phosphate waste, which pulls the force-generating parts of muscle fibers apart before they can fully engage.
Constant-power plantar flexion exercise leads to sustained ATP hydrolysis and accumulation of inorganic phosphate (Pi) in muscle cells.
Inorganic phosphate binds to the actin-myosin cross-bridge in the weakly bound state, promoting detachment and reducing the number of force-generating units.
Force reduction during constant-power plantar flexion is more strongly correlated with Pi accumulation than with proton accumulation.
Evidence from Studies
Supporting (1)
Community contributions welcome
Cross-bridge model-based quantification of muscle metabolite alterations leading to fatigue during all-out knee extension exercise
Contradicting (0)
Community contributions welcome
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