mainiti sukosi zutu ude wo ugokasite iru uti ni, kata ga mae yori raku ni narimasita.

少し means a little.

ずつ adds the idea of in small increments, little by little, or bit by bit.

So 少しずつ means:

• little by little
• gradually
• a little at a time

In this sentence, it shows that the arm movement was not sudden or intense, but done in small amounts each day.

動かす is the plain dictionary form: to move something.

動かしている is the ている form, which here describes an ongoing or repeated action:

• moving
• keeping moving
• doing the action regularly

Because the sentence includes 毎日 and うちに, 動かしている suggests a continuing habit or repeated action over a period of time.

So the nuance is closer to:

• while I was moving my arms every day
• as I kept moving my arms every day

not just a one-time action.

In this sentence, うちに means something like:

• while
• during the time that
• in the course of
• as I kept doing that

So:

腕を動かしているうちに
= while I was moving my arms / as I kept moving my arms

A very common pattern is:

• verbている + うちに

This often means that during an ongoing activity or situation, some change happened naturally.

Here, the change is:

肩が前より楽になりました
= the shoulders came to feel better than before.

Because うちに often works very naturally with an ongoing state or repeated action, and ている expresses exactly that.

Compare:

• 動かす = move (general action)
• 動かしている = be moving / keep moving / be in the habit of moving

In this sentence, the speaker is talking about a process over time: moving the arms every day, and then noticing a change. So 動かしているうちに fits better because it emphasizes the continuing activity during which the change occurred.

Because the two body parts play different roles in the sentence.

• 腕を動かして: the arms are the thing being moved, so they take を as the direct object.
• 肩が楽になりました: the shoulders are the thing that became comfortable/easier, so they take が as the subject of the change.

So:

• 腕を動かす = move one’s arms
• 肩が楽になる = one’s shoulders feel better / become more comfortable

This is very normal Japanese grammar.

前 means before, and より means than.

So 前より means:

• than before
• more than previously
• compared with before

In this sentence:

肩が前より楽になりました
= my shoulders became more comfortable than before

It is a comparative expression, even though English might not always translate it with more directly.

楽 can mean:

• comfortable
• easy
• relieved
• less strained

With body parts or physical condition, 楽になる often means:

• to feel better
• to become less tense
• to become less painful or less burdensome

So here 肩が楽になりました does not mean the shoulders became easy in a strange literal sense. It means something like:

• my shoulders felt better
• the tension in my shoulders eased
• my shoulders became more comfortable

This is because of the pattern:

• Aになる = to become A

For na-adjectives like 楽, you usually change them to the adverbial form with に before なる:

• 静か → 静かになる = become quiet
• 元気 → 元気になる = become well/energetic
• 楽 → 楽になる = become comfortable / feel better

So に is required here as part of the grammar for becoming.

Because the sentence is describing a change that has already happened.

The speaker is looking back and saying that, as a result of moving the arms every day, the shoulders became better.

• なります = become
• なりました = became

So the sentence reports the result after some time passed.

Japanese often leaves out the subject when it is obvious from context.

In English, you might say:

• I moved my arms every day, and my shoulders got better.

In Japanese, the I does not need to be said if it is already understood.

So the sentence naturally omits it. The listener understands that the speaker is talking about their own body unless the context says otherwise.

Japanese nouns usually do not mark singular vs. plural the way English does.

So 腕 can mean:

• an arm
• arms

In this sentence, it most likely means the arms in a general sense, but Japanese does not need to specify that explicitly.

The same is true for 肩:

• it can mean shoulder or shoulders
• the English translation depends on the context

Japanese often places time expressions and adverbial expressions before the main result.

Here the sentence builds up the situation first:

• 毎日 = every day
• 少しずつ = little by little
• 腕を動かしているうちに = while I was moving my arms / as I kept moving my arms

Then it gives the result:

• 肩が前より楽になりました

So the structure is roughly:

Every day, little by little, while moving my arms, my shoulders became better than before.

This kind of ordering is very natural in Japanese.

Yes. In normal Japanese writing, it is typically written as:

少しずつ

with no spaces at all, because Japanese normally does not use spaces between words.

It is best to think of 少しずつ as a very common set expression meaning:

• little by little
• gradually

So even though it is made from two parts, learners often memorize it as one useful phrase.

They are similar, but not exactly the same.

Both can relate to during a period of time, but うちに often has the nuance that:

• something happened in the course of that time
• a change occurred before the situation ended
• something happened almost naturally as time passed

In this sentence, 動かしているうちに feels very natural because the improvement in the shoulders happened gradually during the ongoing activity.

If you replaced it with あいだに, it would sound more like a simpler factual during the time that, and the nuance would shift.

So うちに is a good choice because it matches the idea of a gradual change emerging over time.