Efter träningen känns armen och benet bättre.

Because träningen is the definite form: the training / the workout.

• träning = training, exercise, practice
• träningen = the training, the workout, the practice session

In this sentence, it usually refers to a specific workout or training session, so the definite form is natural.

Also, Swedish often puts the definite article at the end of the noun:

• en träning = a workout
• träningen = the workout

This is because of the Swedish V2 rule: in a main clause, the finite verb usually comes in second position.

Here, Efter träningen is placed first. That means the verb must come next:

• Efter träningen
  • känns
    • armen och benet
      • bättre

If you start with the subject instead, the order changes:

• Armen och benet känns bättre efter träningen.

So the meaning stays basically the same, but the word order follows Swedish sentence structure.

Känns is the present tense of kännas, which means to feel in the sense of seem / feel like / feel to someone.

Compare:

• känna = to feel, to touch, to know
• känna sig = to feel oneself, to feel
• kännas = to feel, to seem

Examples:

• Jag känner armen. = I feel the arm / I can feel the arm.
• Jag känner mig bättre. = I feel better.
• Armen känns bättre. = The arm feels better.

So in your sentence, känns is the natural verb because the arm and leg are feeling better.

Swedish often uses the definite form for body parts when it is already clear whose body part is meant.

So instead of saying:

• min arm
• mitt ben

Swedish very often says:

• armen
• benet

This is especially common when the owner is obvious from context.

So armen och benet känns bättre often means something like:

• my arm and leg feel better
• or the arm and the leg feel better, depending on context

English usually prefers a possessive here, but Swedish often does not.

Because the nouns have different grammatical genders.

• arm is an en-word
• ben is an ett-word

That affects the definite ending:

• en arm → armen
• ett ben → benet

This is a very common pattern in Swedish:

• en bok → boken
• ett hus → huset

So the different endings are not random; they come from the noun gender.

Bättre is the comparative form of bra.

• bra = good / well
• bättre = better
• bäst = best

This is an irregular pattern, so it does not become something predictable like brare.

In the sentence, bättre means better:

• armen känns bättre = the arm feels better

It is used after känns as a complement describing the condition of the arm and leg.

No. In this sentence, bättre stays the same.

That is very common with comparatives in Swedish. Whether the noun is an en-word, an ett-word, or plural, bättre does not change:

• armen känns bättre
• benet känns bättre
• armarna känns bättre

So even though the sentence has both armen and benet, the form bättre stays exactly the same.

Because Swedish verbs do not change for person or number in the present tense.

So the same present-tense verb form is used with:

• I
• you
• he/she/it
• we
• they
• singular subjects
• plural subjects

For example:

• Armen känns bättre.
• Armen och benet känns bättre.
• Armarna känns bättre.

The verb stays känns in all of these.

Yes, normally it refers to the specific arm and leg relevant in the situation.

Because both nouns are definite:

• armen = the arm
• benet = the leg

In real context, this often means:

• the speaker’s arm and leg
• or someone else’s arm and leg already known from context

It does not usually mean arms and legs in general.

Yes, but the meaning shifts a little.

• Efter träningen = after the workout / after the training session
  → a specific session

• Efter träning = after training / after exercise
  → more general

So if you are talking about one particular workout, Efter träningen is the more natural choice.

Sometimes, but the nuance is different.

• känns bättre = feels better
• är bättre = is better

Känns bättre focuses on sensation or perceived improvement.
Är bättre sounds more objective, as if the condition itself has improved.

With body parts, känns bättre is very natural when talking about pain, soreness, stiffness, or recovery:

• Efter träningen känns armen bättre.

So in this sentence, känns bättre is the best choice if you mean that the arm and leg feel better.