When your shoulders are stiff, stretching can help
Stretching is good for stiff shoulders
It is good to stretch when you have stiff shoulders
「背伸び」= stretch, stretch your (back, arms, shoulders, etc)
「肩がこったときは背伸びをするといいよ」= When your shoulders are stiff, stretching can help / Stretching is good for stiff shoulders / It is good to stretch when you have stiff shoulders
「肩こり」= stiff shoulders/stiff neck