После бега мой пульс быстро пришёл в норму.

Because после requires the genitive case.

The noun бег becomes бега in the genitive singular:

• бег = running
• после бега = after running

This is a very common pattern:

• после урока = after the lesson
• после работы = after work
• после дождя = after the rain

So the important rule is:

• после + genitive

Here бега comes from бег, which means running or a run.

In this sentence, после бега is best understood as after running or after the run. Russian often uses a noun where English might prefer a gerund or a full clause.

So:

• После бега = after running / after the run

A native speaker may understand it either as:

• the activity of running in general, or
• a specific running session

Both are natural in context.

Yes, Russian could say just пульс, and in many situations that would sound completely natural.

• После бега пульс быстро пришёл в норму.
• После бега мой пульс быстро пришёл в норму.

Adding мой makes it explicit that we are talking about my pulse, not someone else’s. Russian often omits possessives when ownership is obvious, but speakers also include them when they want clarity, emphasis, or a slightly more personal tone.

So both are possible:

• пульс = the pulse / pulse
• мой пульс = my pulse

This is an idiomatic use of прийти.

The expression прийти в норму means:

• to return to normal
• to go back to normal
• to recover normal condition

So in Russian, something like a pulse, temperature, breathing, or even a situation can прийти в норму.

Examples:

• Температура пришла в норму. = The temperature returned to normal.
• Давление пришло в норму. = The blood pressure returned to normal.

So пришёл here is not about physical walking. It is part of a very common expression meaning returned to a normal state.

This is a very important Russian pattern.

• в норме = in a normal state
• в норму = into a normal state

Russian often uses:

• в + accusative for movement/change into a state
• в + prepositional for location/state

Here the pulse changed from an elevated state into a normal state, so Russian uses в норму.

Compare:

• Пульс уже в норме. = The pulse is already normal.
• Пульс быстро пришёл в норму. = The pulse quickly returned to normal.

So:

• в норме = state
• в норму = transition into that state

Because it agrees with пульс, which is a masculine singular noun.

In Russian past tense, the verb changes by gender and number:

• masculine: пришёл
• feminine: пришла
• neuter: пришло
• plural: пришли

Since пульс is masculine, we get:

• пульс пришёл

Compare:

• Температура пришла в норму.
  (температура is feminine)
• Давление пришло в норму.
  (давление is neuter)

Yes, Russian word order is flexible, and быстро can often move.

The version here:

• После бега мой пульс быстро пришёл в норму.

is very natural and neutral.

Other possible word orders include:

• После бега мой пульс пришёл в норму быстро.
• Мой пульс после бега быстро пришёл в норму.

But they may sound slightly different in emphasis.

Very roughly:

• быстро пришёл focuses naturally on the speed of returning to normal
• putting быстро later can sound more marked or contrastive

So the original sentence has a smooth, standard word order.

Пришёл is the past tense of прийти, which is a perfective verb.

Perfective verbs in Russian usually emphasize:

• completion
• a result
• a single whole event

That fits this sentence well, because the pulse returned to normal as a completed change.

If you want the idea of process rather than completed result, Russian often uses a different verb or wording.

Here the key idea is:

• before: the pulse was elevated
• after: it reached a normal state

So perfective пришёл is exactly what Russian normally uses.

Yes, you can say:

• После бега мой пульс быстро нормализовался.

This means almost the same thing.

The difference is mostly stylistic:

• прийти в норму = very common, natural, everyday
• нормализоваться = a bit more formal, technical, or medical

So in ordinary speech, пришёл в норму is often preferred. In medical or scientific contexts, нормализовался may sound more appropriate.

Yes. A very natural alternative is:

• После пробежки мой пульс быстро пришёл в норму.

The nuance is slightly different:

• бег = running as an activity, more general
• пробежка = a jog / a run, often a specific running session

So:

• после бега = after running / after the running
• после пробежки = after the jog / after the run

Both are correct. Пробежка may sound a bit more like a specific casual run or workout.

Yes, very often. Russian commonly talks about bodily states with concise expressions like this.

Some similar examples:

• Сердце быстро успокоилось. = My heart calmed down quickly.
• Дыхание пришло в норму. = My breathing returned to normal.
• Температура пришла в норму. = The temperature returned to normal.

This is a normal and idiomatic way to describe recovery after exercise, illness, stress, and so on.

Пришёл is pronounced roughly like pri-SHYOL.

The letter ё is important because it shows both:

• the pronunciation yo
• the stress

So:

• пришёл = stress on the last syllable
• pronunciation: pri-SHYOL

In writing, Russians sometimes replace ё with е, so you may see пришел, but it still means пришёл and is pronounced the same way. For learners, it is very helpful to remember the real form with ё.

Yes, it is completely natural.

It sounds like normal, standard Russian and would be understood easily in everyday conversation. It is especially natural in contexts like:

• talking about exercise
• health
• recovery after physical activity

So this is a very useful model sentence, especially for learning:

• после + genitive
• прийти в норму
• past tense agreement
• change of state with в + accusative