등에는

등에는

"On the back" or "at the back".

Here, '등에는' specifies the back ('등') of the person and indicates the burden was located 'on' it, serving as a locative marker.

그는 등에는 커다란 짐을 짊어지고 있었다.

He was carrying a large burden on his back.

In this sentence, '등에는' points to the back being the location of the sweat and emphasizes the physical condition.

나는 등에는 땀이 흠뻑 젖은 상태로 운동을 끝냈다.

I finished exercising with my back completely soaked in sweat.

Here, '등에는' describes the specific position ('on their back') of the child, denoting physical location.

어린 아이가 엄마의 등에는 타고 있었다.

A small child was riding on their mother's back.