Gaining muscular endurance (ME) means improving the amount of time a given amount of force can be sustained and/or how frequently it can be produced. As such, ME is load/force specific and is only possible if the required force can even be produced; if you can't lift 100lbs, how can you sustain/repeat its suspension?
Furthermore, the force-specific nature of ME render assertions like "competitive lifters are strong but don't have endurance" misleading. While an Olympic lifter might not hold a paint brush up as long as a distance-swimmer, she/he will hold a heavy sheet of drywall up longer!
So now that we know that ME requires at least enough strength to lift the load to be endured, would a strength surplus beyond the endurance-specific-load be even more helpful? ....
The answer is a resounding YES!. In fact, increasing muscular strength is the fastest way to improve ME. At a certain point, however, higher-rep endurance work is necessary to increase ME.
Question now is when to utilize high-force/low-rep strength work and when low-force/high-rep fuel work is warranted.
Well to answer the aforementioned, a few things need to be known; those being the endurance-load, strength relative to the endurance load, and whether more strength or more endurance is needed.
If strength capacity far exceeds the endurance load, it's certain that structural integrity is more than adequate to tolerate extended time under the endurance load. Additionally, a large strength surplus indicates that adequate fuel is possessed to move sub-maximal loads repetitively (if you can lift 350*1, you will be able to lift half of that more than once!) .
Note, however, that while the fuel sufficient for heavy lifting allows for more/longer lighter lifting, peaking endurance capacity requires training muscles to store more fuel and lift lighter loads more fuel-efficiently. Training to increase fuel-storage/efficiency entails expending a lot of fuel which means lots more reps than that typical of true strength training.
If strength surplus is modest relative to the endurance load, increasing that surplus is first priority; as any structures, muscles or otherwise, designed to support a given load repetitively/long-periods, must possess a substantial integrity surplus beyond said load (ie a latter designed for 300lb workers better have a breaking point far beyond that!).
As such, high-rep fuel work is of lesser value if strength surplus is deficient. High-rep fuel work for strength deficient muscles is akin to increasing the gas tank of a Prius in order to tow a truck!
A great example scenario to analyze is the NFL combine 225lb bench-presse test for reps. As the endurance load is known (225lbs). With the endurance load known, we'll examine 2 separate scenarios with one entailing a substantial strength surplus (350lbs) and another far less (245lbs).
In the first scenario the surplus is more than adequate indicating that there's enough tissue integrity and fuel to press 225 multiple times. To maximize the 225lb rep-max, however, fuel work will likely be required as it's likely such a high strength level indicates a low-rep/low-fuel training history.
An effective strategy in this situation would be to simply train by repping 225 as many times as possible in a 3-5 minute period. Such a strategy would allow enough reps to challenge energy storage and encourage fuel-efficient recruitment.
In the second scenario, the strength surplus is too low for the nervous system to allow many reps at 225 regardless of how much fuel happens to be in the muscles. In order for the nervous system to allow a lot of reps at 225, tissue capacity of sufficient surplus must be established. Such tissue capacity is best forged with high-force/low-rep training.
As can be seen, muscular endurance training is not straightforward. ME is force/load specific; which means strength is prerequisite to endurance. Endurance also depends on fuel/fuel-efficiency. As such, ME training should endeavor to address whichever traits that are deficient.