Nem sempre a frequência cardíaca fica alta na esteira.

Nem sempre means not always.

In this sentence, nem is part of a fixed expression:

• sempre = always
• nem sempre = not always

Even though nem often means nor or not even in other contexts, in nem sempre you should learn it as a chunk meaning not always.

Examples:

• Nem sempre ele vem. = He doesn’t always come.
• Nem sempre é fácil. = It’s not always easy.

The a is the feminine singular definite article, meaning the.

• a frequência cardíaca = the heart rate

It is feminine because the noun frequência is feminine:

• a frequência
• a frequência cardíaca

In Portuguese, nouns usually need an article more often than in English, especially in general statements like this one.

In Portuguese, adjectives usually come after the noun.

So:

• frequência = frequency/rate
• cardíaca = cardiac

Together:

• frequência cardíaca = heart rate

This is the normal order in Portuguese. English often uses a noun or adjective before another noun, but Portuguese usually uses noun + adjective instead.

Other examples:

• pressão arterial = blood pressure
• atividade física = physical activity

Fica is from the verb ficar, which has several meanings depending on context. Here it means something like:

• becomes
• gets
• stays/remains

In this sentence, fica alta means gets high or stays high, depending on the context.

So ficar + adjective is a very common structure in Portuguese:

• ficar feliz = to become happy
• ficar triste = to become sad
• ficar pronto = to become ready / be ready
• ficar alto = to become high / stay high

In your sentence, a frequência cardíaca fica alta is naturally understood as the heart rate gets/stays high.

Because alta agrees with frequência, which is a feminine noun.

• a frequência → feminine singular
• so the adjective must also be feminine singular: alta

Compare:

• A frequência fica alta.
• O batimento fica alto.

The adjective changes to match the noun’s gender and number.

Fica is third person singular, present indicative of ficar.

Full present tense of ficar:

• eu fico
• você/ele/ela fica
• nós ficamos
• vocês/eles/elas ficam

It matches the subject a frequência cardíaca, which is singular:

• A frequência cardíaca fica...

Because Portuguese often does not use an explicit subject pronoun when the subject is already clear.

Here, the subject is stated directly:

• a frequência cardíaca

So Portuguese does not need to add it:

• A frequência cardíaca fica alta...

This is very normal in Portuguese. English often requires a subject pronoun, but Portuguese frequently does not.

Na esteira means on the treadmill.

It is a contraction:

• em
  • a = na

So:

• em a esteira → na esteira

This is a very common contraction pattern in Portuguese:

• no = em + o
• na = em + a
• nos = em + os
• nas = em + as

Examples:

• no carro = in the car
• na academia = at/in the gym
• na esteira = on the treadmill

Portuguese often uses em (contracted here as na) where English might use on, in, or at, depending on the situation.

With exercise equipment, na esteira is the normal, natural way to say on the treadmill in Brazilian Portuguese.

Even though sobre can literally mean on top of/about, it would sound less natural here.

So:

• na esteira = the usual way to say on the treadmill

Sometimes, yes, but the meaning changes a bit.

• fica alta suggests becomes high, ends up high, or tends to stay high
• está alta means is high at that moment

So:

• A frequência cardíaca fica alta na esteira.
  = The heart rate gets/stays high on the treadmill.

• A frequência cardíaca está alta na esteira.
  = The heart rate is high on the treadmill.

In your sentence, fica sounds more general and descriptive, not just about one specific moment.

Yes, somewhat, but the original order is the most natural.

Original:

• Nem sempre a frequência cardíaca fica alta na esteira.

This is a very normal order:

1. adverbial expression (nem sempre)
2. subject (a frequência cardíaca)
3. verb (fica)
4. adjective (alta)
5. location (na esteira)

You could also hear variations like:

• A frequência cardíaca nem sempre fica alta na esteira.

That also sounds natural and means essentially the same thing. The original version just emphasizes not always a little more at the beginning.

The sentence already sounds natural, but in casual speech Brazilians might also say things like:

• Nem sempre os batimentos ficam altos na esteira.
• Na esteira, a frequência cardíaca nem sempre fica alta.

A few notes:

• frequência cardíaca sounds a bit more technical or medical
• batimentos or batimentos cardíacos can sound a little more everyday, depending on context

Still, your original sentence is perfectly good Brazilian Portuguese.

A rough pronunciation guide is:

• frequência ≈ fre-KWEN-see-a
• cardíaca ≈ kar-JEE-a-ka

A few pronunciation notes:

• qu in frequência sounds like kw
• the stressed syllable in frequência is -quên-
• the stressed syllable in cardíaca is -dí-

Very roughly, the whole sentence sounds like:

Nem SEM-pre a fre-KWEN-see-a kar-JEE-a-ka FEE-ka AW-ta na es-TEI-ra.

This is only approximate, but it can help an English speaker get started.