If you want to describe a stretched plastic cable, would you say that it was hardly stretched or strongly stretched?
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Actually you say neither.
Hardly stretched means that it is barely stretched at all. (Although I see where your confusion arises) and strongly stretched is not something a native speaker would say.
I have never heard either of these words used with stretched. In fact, the connotation of hardly in the phrase "hardly stretched" means "barely". See the fourth definition from merriam-webster.com:
While definitions 1 and 2 do mean what you are intending, the phrase is more typically used to mean "barely".
I would use "tightly stretched" to mean something that is stretched as far as it can go.