Cuando corro rápido, siento que cada músculo trabaja.

I

Breakdown of Cuando corro rápido, siento que cada músculo trabaja.

• cada: each, every
• trabajar: to work
• que: that

Questions & Answers about Cuando corro rápido, siento que cada músculo trabaja.

Why is it corro and not yo corro?

In Spanish the subject pronoun (yo, tú, él, ella, etc.) is usually dropped because the verb ending already shows who the subject is.

• 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.