Nem sempre uma intensidade alta melhora o condicionamento físico.

In Portuguese, nem sempre is the normal way to say not always.

• nem sempre = not always
• não sempre sounds unnatural in standard Portuguese

Even though nem often means nor / neither, in the fixed expression nem sempre it works like not always.

So:

• Nem sempre uma intensidade alta melhora o condicionamento físico. = A high intensity does not always improve physical conditioning.

This is a very common structure in Portuguese:

• Nem sempre é fácil. = It’s not always easy.
• Nem sempre funciona. = It doesn’t always work.

Here, nem is part of the expression nem sempre.

By itself, nem often appears in negative combinations like:

• nem... nem... = neither... nor...

Example:

• Nem café nem chá. = Neither coffee nor tea.

But in nem sempre, you should learn it as a set phrase meaning not always. It does not mean nor always in natural English, even if that looks more literal.

So in this sentence, it is best to understand:

• nem sempre = not always

Both are possible, but they are used a little differently.

uma intensidade alta

This is a very natural way to describe the noun:

• intensidade = intensity
• alta = high

It is a straightforward noun + adjective structure:

• uma intensidade alta = a high intensity

alta intensidade

This is also common, especially in technical or set expressions:

• treino de alta intensidade = high-intensity training

So the difference is often about style and context:

• uma intensidade alta = more descriptive, sentence-based
• alta intensidade = often more fixed, more compact, sometimes more technical

For example:

• Um exercício de alta intensidade = a high-intensity exercise
• Uma intensidade alta pode cansar mais. = A high intensity can make you more tired.

Because in Portuguese, adjectives often come after the noun.

So:

• intensidade alta = high intensity
• condicionamento físico = physical conditioning
• casa grande = big house

English usually puts adjectives before the noun, but Portuguese very often puts them after.

That said, adjective position in Portuguese can sometimes change:

• for style
• for emphasis
• for meaning

But in this sentence, intensidade alta is completely normal and natural.

Not naturally in this sentence.

Alta intensidade by itself usually works better as part of a larger expression, such as:

• treino de alta intensidade
• exercício de alta intensidade

If you say:

• Alta intensidade melhora o condicionamento físico

it sounds more abstract or headline-like, and less natural in everyday phrasing.

The original sentence,

• Nem sempre uma intensidade alta melhora o condicionamento físico,
  sounds smoother because uma intensidade alta functions clearly as the subject.

Because the subject is singular:

• uma intensidade alta = singular

So the verb must also be singular:

• uma intensidade alta melhora

If the subject were plural, then the verb would be plural:

• Intensidades altas nem sempre melhoram o condicionamento físico.

So the agreement is:

• uma intensidade alta → melhora
• intensidades altas → melhoram

Here, melhora is a verb.

It comes from melhorar = to improve.

In this sentence:

• uma intensidade alta melhora o condicionamento físico
• improves is the meaning

Portuguese melhora can also be a noun in other contexts:

• Houve uma melhora. = There was an improvement.

So melhora can be:

• verb: ela melhora = it improves
• noun: uma melhora = an improvement

In your sentence, it is clearly the verb, because it connects the subject to the object.

Portuguese often uses definite articles more than English does.

Here:

• o condicionamento físico literally = the physical conditioning
• but in natural English, this is often just physical conditioning or fitness

Portuguese commonly uses the article with general nouns where English may omit it.

So this is normal:

• melhora o condicionamento físico
• melhora a saúde
• aumenta a resistência

The article does not necessarily mean a very specific, already-known conditioning in the way English the often does. It is just idiomatic Portuguese.

Condicionamento físico means something like:

• physical conditioning
• fitness
• physical fitness

It refers to the body’s trained condition, especially in exercise, sport, or health contexts.

Nuance

• condicionamento físico sounds a bit more like training-conditioned physical capacity
• forma física is another common expression and often means physical shape / fitness level

Examples:

• Ele quer melhorar o condicionamento físico. = He wants to improve his fitness.
• Ela está em boa forma física. = She is in good physical shape.

So in your sentence, condicionamento físico is a very natural choice for an exercise or training context.

Because físico is an adjective modifying condicionamento.

So:

• condicionamento físico = physical conditioning

Again, Portuguese commonly places adjectives after the noun.

Compare:

• saúde mental = mental health
• atividade física = physical activity
• preparo físico = physical preparation / fitness

So this word order is normal Portuguese grammar.

Because melhorar can be used as a transitive verb, meaning it can take a direct object.

So:

• melhorar algo = to improve something

In the sentence:

• uma intensidade alta melhora o condicionamento físico
• o condicionamento físico is the direct object

This is perfectly normal.

You may also see related structures like:

• há melhora no condicionamento físico = there is improvement in physical conditioning

But that is a different structure:

• melhora as a noun
• no condicionamento físico = in physical conditioning

So:

• melhorar o condicionamento físico = to improve physical conditioning
• haver melhora no condicionamento físico = to be improvement in physical conditioning

Yes, sometimes you can, but the original placement is very natural.

Original:

• Nem sempre uma intensidade alta melhora o condicionamento físico.

You could also say:

• Uma intensidade alta nem sempre melhora o condicionamento físico.

That also sounds natural and means basically the same thing.

The original version puts early emphasis on not always.
The second version starts with the subject first and may feel slightly more neutral.

Both are acceptable in Brazilian Portuguese.

Yes. The present tense in Portuguese is often used for general truths, habits, and broad statements, just like in English.

So:

• melhora here means improves in a general sense

It is not necessarily talking about one single moment. It means something like:

• in general
• as a rule
• under many circumstances

This use is very common:

• Exercício ajuda a saúde. = Exercise helps health.
• Dormir bem melhora o desempenho. = Sleeping well improves performance.
• Nem sempre mais treino traz melhores resultados. = More training does not always bring better results.

Yes, elevada can work:

• uma intensidade elevada

This can sound a bit more formal or technical than alta.

Comparison

• alta = very common, everyday, natural
• elevada = slightly more formal, scientific, or technical

So both are correct, but the original sentence with alta sounds very natural and accessible.

In this sentence, uma is natural and important because it introduces the subject as a high intensity.

• uma intensidade alta = a high intensity

If you remove uma:

• Nem sempre intensidade alta melhora o condicionamento físico

that sounds unnatural in standard Portuguese.

Portuguese usually needs an article here. So the sentence works best with:

• uma intensidade alta

This is one of those places where Portuguese is less likely than English to omit the article.