La resistencia mejora poco a poco.

In Spanish, abstract nouns are often used with the definite article more often than in English. So la resistencia sounds natural when talking about resistance/endurance as a general quality or capacity.

Compare:

• La paciencia es importante. = Patience is important.
• La resistencia mejora poco a poco. = Resistance/endurance improves little by little.

Leaving out la is usually not natural here.

Here, resistencia usually means endurance, stamina, or physical resistance, especially in a fitness, sports, or health context.

Depending on context, resistencia can also mean:

• resistance in a general sense
• opposition
• electrical resistance

But in this sentence, a learner would usually understand it as stamina/endurance.

Here, mejora is the simple present form of mejorar and means improves.

The structure is:

• La resistencia = subject
• mejora = verb

So the sentence literally works like:

• Endurance improves little by little.

You do not need se because mejorar can work intransitively, meaning something improves by itself.

Compare:

• La situación mejora. = The situation improves.
• Tu español mejora. = Your Spanish improves.

Mejora is the third person singular of the present indicative of mejorar.

Conjugation:

• yo mejoro
• tú mejoras
• él / ella / usted mejora
• nosotros mejoramos
• vosotros mejoráis
• ellos / ellas / ustedes mejoran

It matches la resistencia, which is singular.

The ending of the noun does not control the verb ending directly. What matters is whether the subject is singular or plural.

• La resistencia = singular
  → mejora
• Las resistencias = plural
  → mejoran

So the verb agrees with the subject in number, not with the final letter of the noun.

Poco a poco is a fixed expression meaning little by little, gradually, or step by step.

It functions adverbially, describing how the improvement happens.

You will hear it very often:

• Voy aprendiendo poco a poco. = I’m learning little by little.
• La ciudad cambia poco a poco. = The city is changing gradually.

It is best learned as a chunk rather than translated word by word.

Because poco a poco is the standard Spanish expression. It is idiomatic. Even if a direct word-for-word explanation is not obvious, this is simply the natural form Spanish uses for little by little.

Many common Spanish expressions work this way and should be memorised as set phrases.

Yes, but the meaning changes slightly.

• La resistencia mejora poco a poco.
  More general statement: endurance improves gradually.

• La resistencia está mejorando poco a poco.
  Focuses more on the process happening right now or over the current period.

In many contexts, the simple present (mejora) is the more natural and neutral option in Spanish.

Yes. Since the verb ending already shows person and number, Spanish often omits subject pronouns. But here the subject is a noun phrase, la resistencia, and it is included because it tells us what is improving.

If the context were already clear, someone might simply say:

• Mejora poco a poco.

But on its own, that would be less specific because we would not know what is improving.

Yes, you could say Poco a poco mejora la resistencia. That word order is possible and still natural.

The original order:

• La resistencia mejora poco a poco

is the most neutral and straightforward.

Changing the order can shift emphasis:

• Poco a poco mejora la resistencia
  puts more focus on the gradual process.

Spanish word order is often more flexible than English, as long as the sentence remains clear.

In standard Peninsular Spanish, it is pronounced approximately:

• reh-sees-THEN-thyah

A few points:

• The r is a soft tap here, not a strong rolled rr.
• The c before i is pronounced like th in much of Spain.
• The stress falls on -ten-: resisTENcia

So in Spain:

• resistencia → reh-sees-TEN-thya

In Latin American pronunciation, the c before i is usually an s sound instead.

It can do both.

Intransitive: something improves

• La resistencia mejora. = Endurance improves.

Transitive: someone improves something

• El entrenamiento mejora la resistencia. = Training improves endurance.
• Quiero mejorar mi español. = I want to improve my Spanish.

So mejorar is a very useful verb because it can work in both ways depending on the sentence.