Changing hose, without required assistance, from one tank to another. Unhooked hose & it started sliding off pipe, so EE had to catch it. When EE caught hose EE heard EE's back pop. Completed hose change with one hand, held EE's back with the other. Diagnosis: acute lumbar pain & radicular symptoms. Restrictions: may lift up to 10 lbs., no climbing, bending or stooping.