This is the 6339th most frequent Russian word.
физическую
The Russian word "физическую" translates to "physical" (feminine, singular, accusative case) in English.
Here, "физическую" describes the type of "активность" referring to activities involving physical effort or exercise.
Он предпочитает физическую активность дома, а не в тренажёрном зале.
He prefers physical activity at home rather than in the gym.
"Физическую" defines "природу" to indicate that it pertains to tangible, material properties rather than abstract or theoretical ones.
Наука изучает физическую природу вещей и явлений.
Science studies the physical nature of objects and phenomena.
In this sentence, "физическую" modifies "тренировку" to specify that the training pertains to the physical aspect, not mental or academic.