Visible are the surface mucles of the back and the Erector spinae muscles (and leg muscles with the sciatic nerve).
The muscles that hold the spine (Erector spinae muscles) are important for upright posture and walking. If these muscles aren't trained, bad posture, chronic back pain and tension knots are the result. Bad posture leads to uneven stress on the intervertebral discs which can lead to disc herniation/prolapse. Pressure by the herniated material onto nerves causes pain and numness.