This study aimed to evaluate the lumbar intervertebral discs and the maximum isometric strength and size of the trunk muscles of middle-aged healthy volunteers (60 persons) and low back pain patients (48 persons). Disc degeneration was more frequently seen in the patients than in the healthy volunteers. The psoas and back muscles (erector spinae and multifidus) of the patients were smaller than those of the volunteers. Patients had also more fat deposits in the back muscles than controls. The maximum isometric strength of trunk muscles of the patients was on average weaker than that of the volunteers. However, the size of the back muscles was not related to the maximum isometric extension strength of the trunk.