Learn Hardcore Japanese: undou wo suru to, atama ga sukkirisimasu.

In Japanese, を marks the direct object of a verb. Here, 運動 is what you “do,” so you attach を to show that 運動 is the object of する, yielding 運動をする (“to do exercise”).

Can を be omitted in 運動をする?

Yes. Many Noun + する verbs allow you to drop を and say 運動する instead of 運動をする. Both are correct. 運動をする is a bit more explicit or formal; 運動する is more concise and common in everyday speech.

What grammatical function does 〜すると have in this sentence?

When you attach と to the dictionary form of a verb, you create a simple conditional meaning “when” or “if”. So 運動をする + と → 運動をすると means “when/if you exercise…”, signaling that the following result will occur.

Why is が used in 頭がすっきりします instead of は?

Here が marks the subject of the intransitive verb すっきりします. It introduces 頭 as the thing that becomes refreshed. Using は (the topic marker) would shift the emphasis to 頭 as a topic and might imply contrast or a broader statement (“As for my head…”), which isn’t intended here.

What kind of word is すっきり and what nuance does it add?

すっきり is an onomatopoeic/adverbial noun expressing a feeling of clarity or refreshment. When you say 頭がすっきりします, you’re describing that your mind becomes clear and free of drowsiness or clutter. It’s more vivid than simply saying 頭がいい or 気分がいい.

Why is する used with 運動? Isn’t 運動 itself a noun?

Correct—運動 is a noun. Japanese often uses the pattern [Noun] + する to turn many nouns into verbs. In this case, 運動する or 運動をする means “to do exercise.” It’s similar to English using “take a walk” or “take a trip.”

Why is the sentence in the polite form します? Can I make it more casual?

The use of します is the polite present/future tense. To be casual, switch to the plain form する:
Casual: 運動をすると、頭がすっきりする。
Polite: 運動をすると、頭がすっきりします。

What’s the purpose of the comma after すると?

The comma (、) indicates a slight pause, making the sentence easier to process. In 運動をすると、頭がすっきりします, it visually separates the conditional clause (運動をすると) from the main clause. It’s optional in very short sentences but common in writing for clarity.