This is the 13339th most frequent Russian word.
мышечную
"Muscular" (accusative singular feminine form in Russian).
The word 'мышечную' is used here as an adjective modifying the noun 'боль' to describe the type of pain (muscular pain).
Я почувствовал мышечную боль после тренировки.
I felt muscular pain after the workout.
In this sentence, 'мышечную' functions as an adjective describing 'активность', indicating that it involves muscles.
Они исследовали мышечную активность во время эксперимента.
They studied muscular activity during the experiment.
Here, 'мышечную' acts as an adjective modifying 'нагрузку', specifying that the load pertains to muscle-related exercises.