A pulled muscle is what you get from running too hard for too long after something that keeps moving. That is the first thing "strained" tells us, and it is a surprising word to attach to a heart. In poems, hearts usually break, burn or ache. Here the heart is simply overworked. The speaker doesn't announce a tragedy. They report an injury, flatly, the way an athlete might after a bad race. That plainness is what makes the line affecting.
The sentence works as a small piece of logic: cause ("chasing yours everywhere"), then effect ("is strained"). What it describes is love as unequal effort. One heart does all the running, and the other is only a destination, or perhaps only a direction. We learn nothing about the person being chased except a possessive pronoun, "yours." The word doing the work of "your heart" is left bare, so one heart pursues another without either being named. There is no accusation either. The speaker never says the other person fled, refused or lied. Maybe they never knew. The poem's restraint leaves the reader to wonder whether the exhaustion comes from rejection, from distance, or from a love that was never put into words.
The sound of the line carries some of this. It runs to about eleven syllables and begins on a steady beat, so that "My heart is strained" sounds almost like a pulse. Then "from chasing yours everywhere" gathers speed, and the final word is long and trailing. "Everywhere" doesn't stop where the beat wants it to. It spills over, which is exactly what the word means. The sibilance in "strained," "chasing" and "yours" gives the middle of the line a hiss, like breath taken by someone who has run out of it. And there is no rhyme. Rhyme is a kind of arrival, a sound clicking shut on its partner, and the poem withholds it, just as it withholds arrival from the speaker.
Then there is the title. "Harbor" promises shelter, a place where ships stop straining against weather and tide. Under it, set off in guillemets, stands "Strained," which reads like a second title or a label, and which names the opposite condition. The two words pull against each other before the line even begins. "Harbor" is also a verb, as in "to harbor" a feeling, and the double meaning suits the poem: a heart that harbors someone else's image is a heart that can't rest. Judging by the imagery, the word "strained" may carry a nautical echo as well, since a mooring rope under tension is a strained rope. That is an inference, because the line itself never mentions water. The harbor exists only in the title, as an absence. The speaker is out in the open chasing, and the shelter the title offers never appears in the line.
That gap between title and text is where the poem's meaning sits. Harbor is the thing wanted. The beloved's heart is presumably meant to be that harbor, and the pursuit of it is wearing out the one who pursues. The tension between longing for rest and being denied it by the very thing you long for is the central idea, packed into a single sentence.
What makes the poem worth reading is how much it does with so little. It takes a common word, "strained," and lets its physical meaning bring a worn-out body into a love poem. It avoids self-pity and does not ask the reader for sympathy. The verb is in the present tense ("is strained"), and no future follows. We don't know whether the speaker will keep running, slow down or stop. The line ends on "everywhere," a word with no edge, and leaves us at the point where the chase hasn't yet given way to rest. For students, it is a good example of how a poem can hold a whole situation in one breath, and how much can hang on a single choice of word.