Learn Hardcore Swahili: Babu amekuwa akitembea kila asubuhi kwa matembezi mafupi ili kuimarisha misuli yake ya miguu. - Grandfather has been walking every morning for short walks to strengthen his leg muscles.

Amekuwa akitembea literally combines amekuwa (has been / has become) and akitembea (walking in a continuous sense). Together they give the idea:

“has been walking / has gotten into the habit of walking”

It usually implies:

the activity started some time ago,
it has been going on regularly up to now,
and it may still be continuing.

Compare:

Babu anatembea kila asubuhi.
= Grandpa walks every morning. (plain statement of habit)
Babu amekuwa akitembea kila asubuhi.
= Grandpa has been walking every morning. (suggests a newer or changed habit, or emphasizes the ongoing nature up to now)

Why are there two verb forms (amekuwa and akitembea) instead of just one?

The pattern [amekuwa] + [aki-verb] is a common way in Swahili to express something like the English present perfect continuous (“has been doing”).

Breakdown:

amekuwa
- a- = he/she (subject prefix, class 1)
- -me- = perfect aspect (“has”)
- -kuwa = to be/become
akitembea
- a- = he/she (subject prefix)
- -ki- = continuous / progressive marker (“while, in the process of”)
- -tembea = walk

So amekuwa akitembea literally feels like “he has been (in the state of) walking”.

You could say Babu anatembea kila asubuhi (simple habit) or Babu amekuwa akitembea kila asubuhi (has been in this new habit recently). Both are correct, but the nuance is different.

Could we say Babu anatembea kila asubuhi kwa matembezi mafupi instead of amekuwa akitembea?

Yes, Babu anatembea kila asubuhi kwa matembezi mafupi is grammatically correct and understandable. The difference is nuance:

anatembea = walks (general, timeless habit)
amekuwa akitembea = has been walking (emphasizes that this routine has developed or been going on for some period up to now)

If you want to highlight that this is a relatively new or consciously adopted routine—maybe for health reasons—amekuwa akitembea fits better.

What is the role of kwa in kwa matembezi mafupi?

In kwa matembezi mafupi, kwa functions like:

“by / through / by means of”, or
“for (the purpose of doing X as an activity)”.

So kwa matembezi mafupi can be understood as:

“by (doing) short walks”
“for short walks”

It links the general idea of walking (akitembea) with the more specific form that walking takes (short walks as an activity).

What exactly does matembezi mean, and how is it related to the verb tembea?

Matembezi is a noun derived from the verb tembea (to walk).

tembea = to walk
ma- + tembezi → matembezi = walk(s), outing(s), stroll(s)

Typical uses:

Tulikwenda matembezini. = We went for a walk / outing.
Napenda matembezi ya kutazama mandhari. = I like sightseeing walks.

In the sentence, kwa matembezi mafupi = “for/by means of short walks (as an activity)” rather than just the bare action “walking”.

Why is it matembezi mafupi and not matembezi fupi?

The adjective must agree with the noun class of the noun it describes.

matembezi is in the ma- (class 6) noun class.
The adjective fupi (short) takes the ma- agreement prefix for class 6: mafupi.

So:

mtu mfupi (short person, m-/wa- class)
kiti kifupi (short chair, ki-/vi- class)
matembezi mafupi (short walks, ma- class)

Matembezi fupi would be ungrammatical in standard Swahili because the agreement prefix is missing.

What does ili kuimarisha express, and how does it differ from using kwa?

Ili kuimarisha introduces a purpose:

ili = in order that / so that
kuimarisha = to strengthen

So ili kuimarisha misuli yake ya miguu = “in order to strengthen the muscles of his legs”.

Difference from kwa:

kwa matembezi mafupi → shows means / manner (“by means of short walks”).
ili kuimarisha … → shows goal / purpose (“in order to strengthen …”).

Roughly:

kwa answers “how / by what means?”
ili answers “why / for what purpose?”

What is the structure of misuli yake ya miguu, and why do we see both yake and ya?

Misuli yake ya miguu breaks down like this:

misuli = muscles (class 4, plural of msuli)
yake = his/her (possessive agreeing with misuli, class 4)
ya miguu = of the legs
- ya = connector “of” agreeing with misuli (class 4)
- miguu = legs

So literally: “the muscles his of-legs” → “his leg muscles / the muscles of his legs”.

Roles:

yake = whose muscles? (his)
ya miguu = what kind of muscles / which part? (of the legs)

You could also say misuli ya miguu yake. That is also correct; it tends to put a bit more emphasis on his legs specifically, but the meaning is very close.

Why is it ya miguu and not za miguu after misuli?

The connector (sometimes called “of-” agreement) agrees with the head noun, not with the following noun.

Head noun: misuli (class 4, mi-) → connector is ya.
So we say: misuli ya miguu = muscles of the legs.

Examples with the same pattern:

miili ya wanariadha = bodies of the athletes
misitu ya Afrika = forests of Africa

Za is used when the head noun is in a class that takes za, for example:

nguo za watoto = clothes of the children (nguo: class 10)
sheria za nchi = laws of the country (sheria: class 10)

Here the head noun is misuli, which needs ya, so misuli ya miguu is correct.

Could we just say kuimarisha miguu yake instead of kuimarisha misuli yake ya miguu?

You could say kuimarisha miguu yake (“to strengthen his legs”), and it would be grammatically fine. The nuance is:

kuimarisha miguu yake = to strengthen his legs in general.
kuimarisha misuli yake ya miguu = to strengthen the muscles of his legs (more specific and more anatomical/fitness-focused).

So the original phrase is more precise about what exactly is being strengthened: not the bones or joints in general, but the muscles.

Can kila asubuhi be placed elsewhere in the sentence, and does that change the meaning?

Yes, kila asubuhi (every morning) is fairly flexible in position. For example:

Babu amekuwa akitembea kila asubuhi kwa matembezi mafupi …
Kila asubuhi, babu amekuwa akitembea kwa matembezi mafupi …

Both are natural and mean the same thing: the walking happens every morning.

Usually:

Putting kila asubuhi near the verb (amekuwa akitembea) links the time directly to the action.
Putting it at the beginning adds slight emphasis to when this routine happens, but does not change the basic meaning.