This is the 17850th most frequent Russian word.
усилятся
"Усиляться" means "to strengthen" or "to intensify."
The word 'усилились' here is used in its reflexive form to indicate that the sanctions became stronger or more impactful.
Экономические санкции усилились за последние месяцы.
Economic sanctions have intensified over the past months.
In this sentence, 'усилилась' refers to the improvement or increase in the athletes' physical endurance or capacity.
Спортсмены усердно тренируются, чтобы их выносливость усилилась.
Athletes train diligently so that their endurance intensifies.
The usage of 'усилилась' in this context implies that the level of competition has grown stronger or more challenging.