Learn Hardcore Spanish (Latin America): Cuando corro rápido, siento que cada músculo trabaja. - When I run fast, I feel that every muscle is working.

corro = I run (1st person singular)
yo corro is also correct, but you only say yo for emphasis or contrast:
- Yo corro rápido, pero ella camina despacio.
  I run fast, but she walks slowly.

In your sentence, corro alone is the most natural option.

Why is rápido used instead of rápidamente?

In Spanish, some adjectives are commonly used as adverbs without adding -mente, and rápido is one of them.

Corro rápido. = I run fast.
Corro rápidamente. = I run quickly.

Both are grammatically correct, but:

rápido sounds more natural and conversational.
rápidamente sounds more formal or emphatic.

In Latin American everyday speech, corro rápido is by far more common.

Is rápido an adjective or an adverb here?

Here rápido functions as an adverb, modifying the verb corro (how you run).

As an adjective: un coche rápido (a fast car).
As an adverb: corro rápido (I run fast).

Same word, different function depending on context.

Why is there a comma after rápido?

The comma separates two clauses:

Cuando corro rápido – a time clause: When I run fast
siento que cada músculo trabaja – the main clause: I feel that every muscle works

In Spanish it’s common (and recommended) to put a comma after a cuando-clause when it comes before the main clause:

Cuando hace frío, me pongo una chaqueta.
Cuando llego a casa, descanso.

What’s the difference between siento and me siento?

They’re related but used differently:

sentir (without me) = to feel / to sense / to perceive / to regret
- Siento que cada músculo trabaja.
  I feel/sense that every muscle is working.
- Siento no poder ir.
  I’m sorry I can’t go.
sentirse (with me, te, se, nos, se) = to feel (a certain way)
- Me siento cansado.
  I feel tired.
- Me siento feliz.
  I feel happy.

So you say:

Siento que cada músculo trabaja. (I feel that… / I sense that…)
not
Me siento que cada músculo trabaja. (this is incorrect)

Why is it siento que cada músculo trabaja and not trabaje (subjunctive)?

After siento que, Spanish can use either indicative or subjunctive, but the meaning changes:

Indicative (trabaja) = you perceive or sense something as a fact.
- Siento que cada músculo trabaja.
  I feel/sense that every muscle is working. (a perception)
Subjunctive (trabaje) often appears when sentir means to be sorry, to regret:
- Siento que trabajes tanto.
  I’m sorry that you work so much. (regret)

In your sentence, you are describing a physical sensation you perceive as real, so trabaja (indicative) is correct.

Why does trabaja stay singular when músculo is part of a group (all the muscles)?

The subject of the verb is the phrase cada músculo, and cada (each) is grammatically singular, even though it refers to many items one by one.

cada músculo trabaja → singular verb (each muscle works)
todos los músculos trabajan → plural verb (all the muscles work)

So:

cada músculo trabaja ✅
cada músculo trabajan ❌ (wrong agreement)

Could I say todos mis músculos trabajan instead of cada músculo trabaja?

Yes, but the nuance changes slightly:

cada músculo trabaja
Emphasizes each muscle individually: every single muscle works.
todos mis músculos trabajan
Emphasizes the group as a whole: all my muscles work.

Both are natural. If you want to keep the idea of “each one”, cada músculo is a bit stronger.

Why is it cada músculo and not cada los músculos or something like that?

With cada you don’t use an article:

cada músculo ✅
cada el músculo ❌
cada los músculos ❌

Patterns:

cada + singular noun
- cada día, cada persona, cada músculo

If you want an article, you normally switch to todos:

todos los músculos (all the muscles)

Why is it cuándo with an accent sometimes, but cuando without an accent here?

The accent mark distinguishes question/exclamation use from normal use:

cuando (no accent) = when in statements:
- Cuando corro rápido, siento que…
- Te llamo cuando llegue.
cuándo (with accent) = when? in direct or indirect questions:
- ¿Cuándo corres? – When do you run?
- No sé cuándo corres. – I don’t know when you run.

In your sentence, cuando introduces a time clause, not a question, so no accent.

Why doesn’t Spanish use the progressive está trabajando here?

Spanish uses the simple present much more than English for actions happening now or regularly:

Siento que cada músculo trabaja.
Literally: I feel that each muscle works, but it naturally covers is working too.

You could say:

Siento que cada músculo está trabajando.

This is grammatically fine, but it sounds more explicitly focused on the ongoing process right at that moment. For a general sensation when you run fast, trabaja is the more neutral, natural choice.

Can I drop que and say siento cada músculo trabaja like in English “I feel every muscle works”?

No. In Spanish, when a verb like sentir, pensar, creer introduces a full clause, you must include que:

Siento que cada músculo trabaja. ✅
Siento cada músculo trabaja. ❌

So:

I feel (that) every muscle works. → Siento que cada músculo trabaja.
The that can be omitted in English, but que cannot be omitted in Spanish.

Can I change the word order in cada músculo trabaja?

Yes, but with some change in emphasis:

Siento que cada músculo trabaja.
Neutral word order: each muscle works.
Siento que trabaja cada músculo.
Less common, and puts more emphasis on cada músculo (like “it’s each muscle that works”).

Both are grammatically correct, but the original order (cada músculo trabaja) is the most typical and neutral.