I have seen this question in an article:
Why do we stretch after waking up?
Consider the following sentences:
We stretch after waking up
I stretch after waking up
I (Subject) | stretch (Verb) | after waking up (Manner)
Do we need to add an "object" in the sentence above?
I stretch my body after waking up.
"I stretch after waking up." is a complete sentence and is perfectly understandable as is. Extending your sentence to "I stretch my body after waking up." is also correct, but adds length without detail.
If you only stretch one part of your body you could mention that to add more detail. For example:
I stretch my arms and legs after waking up.
The word stretch can be used transitively (with an object) or intransitively (without an object). So your question may be, "If I use it without an object, how does the reader know what is being stretched?"
An "object" of an intransitive verb can come from it's definition. Google Definition: "straighten or extend one's body or a part of one's body to its full length, typically so as to tighten one's muscles or in order to reach something". So, by substitution, "I stretched" could be reworded with the definition, "I straightened or extended my body or a part of my body to its full length."
Then your question could be, "But which is it? The body or some part? What type of stretching was done?" This line of questioning could go on forever. How intense is the stretching? How long is the stretch held? How many repetitions? Language like this is always ambiguous at some point. Ambiguity can be filled from context: additional language, the environment, or world knowledge. From multiple experiences, we all have a sense of what a "morning stretch" can mean, so in this case, the ambiguity could be resolved from the definition + "world knowledge".
Consider the following dialog:
Mary: "Last year, I read this article that a regular, morning routine of stretching and calm breathing is good for one's health. So I stretch after waking up every day - for 30 minutes. First I do my legs - you know, hamstrings, calves, ankles. Then upper body stretches. All the while, I focus on my yoga breathing.
Joe: "Oh that's nothing! I've been doing a morning stretching-and-breathing routine for 20 years."
Mary: "Wow that's great! What's your routine like?"
Joe: "After I wake up, I yawn and stretch for 10 seconds. Then I'm good to go!"
Credit to @DamkerngT for pointing out transitive/intransitive.
I'd say stretching as the act of flexing one's muscles quasi-contains its object semantically, but this as a whole is merely a question of transitivity. Not every verb demands an object.
Sleeping, dying, jogging and many more can be done without a direct (or any) object to follow, and so can stretching. These are called intransitive verbs and it can differ from language to language whether or not a certain activity has to be complemented by an object. For example, in German "to stretch" would be "sich strecken", always accompanied by a reflexive pronoun in accusative standing explicitly for one's body; same for French "s'étirer" and probably a lot of other languages.
