Student or Learner
I'm sorry, probably a sports scientist can explain the two lines' meanings. I wish I could see a video explaining this. These two lines' movement can't be pictured. If you happened to know the meanings, please be benevolent to help me.
ex)A muscle does not have to be maximally stretched to be torn. Muscle tears are the result of a special combination of a sudden stretch and a contraction at the same time. Great differences in strength between two opposing muscle groups, as well as a strength imbalance of 10% between these same muscle groups on both sides of the body, are the main causes of injuries. Improving the strength of weaker muscles is the best prevention of injuries. A careful analysis of the form of movement may also hold the key to injury prevention....Eliminate those moments in your technique in which you use the maximal tension of already streched muscle to counter the fast movement of a relatively big mass. This may lead to tears in the muscles of a supporting leg in kicking, for example...
If, say, you have huge biceps, but weak triceps, you may tear a muscle. (Not a sports scientist)