Hello and welcome to HCM,
A lump or knot on the middle of the back is probably due to
soft tissue swelling.
The appearance of lump while bending and disappearance while standing is suggestive of muscle swelling.
However, other soft tissue lesions like
lipoma,
fibroma also need to be ruled out.
I suggest you to get an aspiration cytology of the swelling wherein the cells will be aspirated from the swelling using a fine needle.
The cells will be spread on a slide and stained with appropriate stains.
The stained cells will be studied to look determine the nature and character of the lesion.
If the lesion turns out to be benign and you are not facing problem of pain or any other symptom, the swelling may be left as it is.
If the swelling turns out to be malignant, appropriate treatment like surgical removal and
chemotherapy may be required.
Thanks and take care
Dr Shailja Puri