I hate the stairs.Sometimes the lift isn't working and you h

I hate the stairs.Sometimes the lift isn't working and you have to use the stairs.I can't get up the stairs by myself.It's my back,you see.Jane,my friend,lived on the ground foor,that's much easier.Nearly every morning I stop for a cup of tea before I come back up from here.
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上面这一小段,其中的“It's my back”,这个“back”在此处表示什么意思啊?