What does that mean?
"How often do you stretch before working out?"
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"How often do you stretch before working out?"
Stretch before working out is not an idiom. It simply means stretch before you work out. (After before you can use Verbing and leave the doer of that verb to be inferred)
This girl is stretching. (Image source)
The OALD gives the definition of stretch in this sense as follows:
4 [intransitive, transitive] to put your arms or legs out straight and contract your muscles
This woman is working out, which means she is doing a physical exercise (Image source), just as the OALD says under work out:
1 to train the body by physical exercise
Therefore, the question "How often do you stretch before working out?" basically means the same as "How often do you stretch before you exercise?" It is asking for the frequency you stretch (the first picture) before you work out (the second picture).