El corredor viene mejorando su resistencia y su zancada.

Viene mejorando is a very useful structure in Spanish: venir + gerund.

Here it suggests that the improvement has been happening gradually over a period of time and is still in progress now.

• mejora = he improves / he is improving (more neutral, less emphasis on the process)
• está mejorando = he is improving (focuses on what is happening now)
• viene mejorando = he has been improving / he’s been gradually improving

So this version highlights ongoing progress up to the present.

Venir + gerund often expresses an action or change that has been developing little by little over time.

In this sentence:

• viene = from venir
• mejorando = gerund of mejorar

Together, viene mejorando means something like:

• has been improving
• has gradually been improving
• has been getting better

It often gives a sense of continuity and progression, not just a single moment of action.

Because after venir in this structure, Spanish uses the gerund.

So:

• venir + gerund → correct
• venir + infinitive → not correct here

That is why you get:

• viene mejorando not
• viene mejorar

The gerund in Spanish often corresponds to English -ing forms in structures like is improving or has been improving.

In Spanish, singular countable nouns usually need an article or another determiner.

So:

• el corredor = the runner
• un corredor = a runner

Just corredor by itself would normally be incomplete in a sentence like this.

Using el suggests we are talking about:

• a specific runner already known from context, or
• a general type in some contexts, though here it most naturally sounds specific.

Zancada means stride.

In running, it refers to the length or quality of the runner’s steps. So mejorar su zancada means improving how they stride while running.

It can be understood as:

• stride
• running stride
• step length / stride pattern, depending on context

So the sentence is about improving both:

• su resistencia = his endurance/stamina
• su zancada = his stride

Spanish often repeats the possessive before each noun when talking about two separate things, especially when that sounds clearer or more natural.

So:

• su resistencia y su zancada = natural and clear

You might sometimes see a possessive used only once with two nouns, but repeating it is very common and often sounds better here.

It makes it explicit that both things belong to or relate to the runner.

It might be understood, but it sounds less natural.

In this sentence, su resistencia y su zancada is better because both nouns are being presented as separate areas of improvement. Repeating su makes the sentence smoother and clearer.

So for a learner, the safest natural version is:

• su resistencia y su zancada

Yes. Su can mean several things depending on context:

• his
• her
• its
• your (formal singular)
• their

In this sentence, because el corredor is singular and masculine, the natural interpretation is his:

• his endurance and his stride

But grammatically, su itself does not tell you the gender of the owner. Context does that.

Because each noun is singular here.

The runner is improving:

• his endurance → usually treated as a singular quality
• his stride → also singular here as a general aspect of running technique

So Spanish uses:

• su resistencia y su zancada

The possessive su agrees with the fact that each noun is singular. If the nouns were plural, then you would use sus.

Not usually in this context.

Although resistencia can look like English resistance, with exercise and running it more often means:

• endurance
• stamina

So in a sports sentence like this, su resistencia is best understood as his endurance or his stamina, not his resistance.

Yes, that is a very good translation.

Because of viene mejorando, a natural English rendering is:

• The runner has been improving his endurance and his stride.
• The runner has been gradually improving his endurance and stride.

That captures the sense of improvement happening over time and continuing into the present.

They are related, but not the same.

• ha mejorado = he has improved

  • focuses more on the result or fact of improvement

• viene mejorando = he has been improving

  • focuses more on the ongoing process over time

So if you want to stress development, viene mejorando is a better choice. If you want to stress the achieved improvement, ha mejorado may fit better.

Yes, it is understood and used in Spain, although learners may hear está mejorando or ha ido mejorando in similar contexts too.

Venir + gerund is perfectly grammatical and useful when the speaker wants to emphasise that something has been happening progressively.

So the sentence sounds natural in Spain Spanish, especially in a descriptive or slightly formal register.