Muscle fibers are recruited by the nervous system. A group of fibers that are controlled by a single nerve are called a 'motor unit' When a 'signal' from the CNS demands a contraction, all these fibers in a common motor unit or "MU", are called upon at the same time. All fibers in a motor unit are of the same 'type' ie. Type I, II, IIb or IIx. The intervating nerve determines the type of fibers in that motor unit.
Muscle fibers contract on the 'All or None' principle. In simple terms, what this means, is that when the nervous system sends a signal to contract, IF that signal is strong enough, an 'action potential' is reached. It's like a switch, all fibers in that MU are now on for that quick point in time. This is called a 'twitch'. As a muscle repeatedly contracts, fatigue sets in, both mechanical and metabolic. A motor unit and it's fibers can experience damage, loss of available fuel and a "backing up" of metabolic waste products. These things will lower the force a fiber is capable of creating. The fiber may still be contracting as hard as possible, even though the force displayed is lessening.
When a fiber is first recruited, depending on the required force, it will be recruited at some frequency of rate coding. For an example, a fiber might be recruited at it's lowest frequency and it will display it's minimum force. As the frequency increases, the fiber will be 'on' more often. This will allow a higher build up of calcium within the cell and more crossbridges will have time to attach. The fiber then will be creating more tension. If a fiber is contracting at a normal low force, it will be in "unfused tetany". This means, the pulses are occuring fast enough that it never relaxes, but not fast enough for it's 'fully on maximum force'. If rate coding increases until the fiber is 'always on', it is then said to be in 'fused tentany'. This is it's maximum force.
If you perform a sub-maximal task long enough to induce fatigue, the loss of force will be compensated by increased recruitment and increased rate coding of the recruited fibers. If a fiber is creating a specific required tension and is at a frequency which induces it to create 75% of it's maximum for example, as it fatigues, the frequency will increase to keep it at that tension level. Eventually, rate coding will be maximized and the fiber will not be able to continue to display that tension level.
This brings up a point that I had overlooked for years. If you perform a maximum contraction while 'fresh', since your fibers are not fatigued, they will contract with their absolute maximum possible output. The fibers will be in 'fused tetany' while fresh. Now, if you perform a maximum effort while fatigued, say the 10th rep of your 10RM, since the fibers are fatigued and have been losing force, even though they will be displaying their 'momentary maximum fused tetanic force', that force will be less than if they were fresh.
One way I like to look at fiber tension is like this. "A fiber doesn't feel tension it creates tension."
What this means: If a fiber can generate (arbitrary number here for illustrative purpose) .0001 lbs of force, and that is all the tension it can generate, then this is also the most tension it can 'feel'. You cannot 'put more tension' on that fiber, all you can do is cause more fibers to contract, or contract more often.