Question about the maximum muscle contraction (tension) and strength of stimulus

[ColumbiaOrtho]

I just cannot figure out the specific mechanism that causes a muscle stops at certain tension strength when increasing the stimuli. For instance, let's say there is a voltage pin connected to a frog leg muscle. Increasing the stimulus will increase the contraction (tension) of the muscle to a certain point, but once it reaches a certain stimulus voltage, the tension goes down no matter how high the voltage is. I'm not talking about the "Frequency", but the "strength" of the stimuli. Can someone help me?

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[Zenabi90]

but once it reaches a certain stimulus voltage, the tension goes down

This part is confusing me. Goes down? Or does not increase?

AFAIK, any stimulus greater than the maximal stimulus will not result in a stronger contraction because at maximal stimulus, ALL of the muscle fibers have been recruited, and maximal tension is achieved. At that point, there are simply NO MORE fibers that can be activated, so the tension simply cannot increase. It is akin to the Vmax concept in enzyme kinetics. At a certain point, no matter how much substrate you add, the reaction cannot proceed any faster because all of the enzymes have already been occupied. In this case, all the fibers are activated, and there is no more increase in tension.

In terms of tension dropping, there is "tetanus" which results when stimuli are applied so rapidly that the muscle fibers are not allowed to relax, and thus provide continual tension. At a certain point however, muscle fatigue sets in due to a combination of acidic waste products, relative--not absolute!--lack of ATP, and ionic imbalances. These combined cause muscle fatigue and the inability of a muscle to respond to any further stimulation, resulting in a loss of tension.