words - Meaning and transitivity of ゴミ袋を手に立ち上がる

While doing some translating, I stumbled upon this usage of 立ち上がる。

わたしがゴミ袋を手に立ち上がると、イツカ君とお姉ちゃんは二人一緒にわたしの所へやって来ました。

It doesn't match up with the definitions I'm familiar with, nor any of the other ones I found in the dictionary.

 ①座ったり腰かけたりしていた人が，立つ。 「座席から－・る」
 ③まったく打ちひしがれていた者が，勢いを取り戻す。「破産の憂き目から－・る」 「廃墟の中から－・る」

A google search of ~を手に立ち上がる gets over a million hits suggests it's used often enough.

総大将は軍扇を手に立ち上がる

Could someone elaborate on what's going on and the precise meaning of the word?

Answer

A を B に is a common literary adverbial expression that means with A in/on B, literally or figuratively.

In most cases you have a part of body in B, as 小銭を手に with coins in hand, 期待を胸に with expectation in chest (= heart), ドアを背に with door in back (= with back against the door), リュックを(背/肩)に with backpack on shoulder etc.

But it's also frequently used with position/moment words in B, notably 心配をよそに with concern in elsewhere (= much to others' unease), 敵を前に with enemy in front (= confronting one's enemy), その手紙を最後に with the letter at the end (= no contact since the letter) etc.

snailboat's analysis isn't wrong, but you can't expect the ～に and ～にして to be used interchangeably in actual world, because:

ゴミ袋を手に立ち上がる to stand up with garbage bag in hand
ゴミ袋を手にして立ち上がる to stand up after grasping the garbage bag

～にして contains a verb te-form, thus always describes two motions occur in succession, but not concurrently.

Here is a Japanese thesis about the difference between AをBに with and without して. You can also find more examples of this construction, which I didn't cover here.

P.S.
I forgot the most crucial thing. So the 立ち上がる is but an ordinary intransitive verb "to stand up", thank you.