Question

Increased muscle fiber endurance can be affected by __________.

Answer 1

...the type of activity performed.

To get better  muscle endurance you need to exercise those muscles.
There are two types of increase: increase of the size of the fibers or a increase in the number of fibers (it's when the muscle fibers divide).  Either way, both this methods need training.

Answer 2

eating a diet rich in fibers, such as whole grain foods and oats. It can also be "affected" consistent daily exercises that strengthen and tone muscles.