We report a 31P Magnetic Resonance Spectroscopy (31P-MRS) study on the changes of energy metabolism in human leg anterior and lateral compartment muscles with paresis due to compression from a herniated lumbar disc at the L4-L5 level and recovery to normality. A low phosphocreatine to inorganic phosphate ratio due to both decreased phosphocreatine and increased inorganic phosphate contents and a normal intracellular pH were the features of muscles with paresis. Changes of 31P-MRS parameters were followed during 18 weeks of treatment with physical therapy until complete recovery. Results show that 31P-MRS is a useful clinical tool for detecting even small biochemical changes that may occur in muscles and for checking the effects of therapy.