If my understanding of muscular endurance training is correct,
it’s designed such that you mainly work and stress very localized
muscle groups (say legs) while heart rate, breathing rate
are not impacted as severely (ie, HR is relatively low).

However, if one uses uphill (or some sort of resisted) double-poling,
legs contribute a significant amount. The steeper the hill the larger the
percentage of leg usage (can be over 50%). So, in this case, you may be
working on upper body muscular endurance, but legs are also stressed (but not
necessarily in the way we want).

There’s no question here, just thinking aloud…