This is the 17990th most frequent Russian word.
физическим
"Physical."
The word 'физическим' here is an adjective meaning 'physical,' describing the development which pertains to the body and physique.
Он занимается физическим развитием детей.
He is involved in physical development of children.
In this context, 'физическим' describes 'difficulties' as pertaining to physical capabilities or conditions.
У него возникли физические трудности в выполнении задачи.
He encountered physical difficulties in completing the task.
The word 'физическим' pertains to preparation related to physical fitness or body conditioning needed for sports.