Measurement of lower extremity muscle strength and the energy expenditure during walking was taken in 36 spinal cord injury patients to assess functional mobility. Patients were categorized according to the type of orthotic prescription (knee-ankle-foot orthosis [KAFO] or ankle-foot orthosis [AFO]) or upper extremity assistive device (cane, crutches, or walker) used during gait. The rates of O2 consumption per minute, O2 cost per meter, heart rate, respiratory quotient, velocity, cadence, and peak axial load exerted by the arms on upper extremity assistive devices were measured. The Ambulatory Motor Index (AMI), derived from the manual muscle grades of both lower limbs, was used as the indicator of the degree of paralysis. The AMI was strongly correlated with the percentage increase in the rate of O2 consumption above normal (p less than .0001), O2 cost per meter (p less than .0001), peak axial load (p less than .0001), velocity (p less than .0001), and cadence (p less than .0001). Differences in these parameters among patient groups categorized according to the type of orthotic prescription (no KAFO, one KAFO, two KAFOs) or upper extremity assistive device (no device, cane or one crutch, two crutches, or walker) were attributable to differences in the AMI. The AMI, therefore, could be used as a reliable clinical indicator of functional mobility after spinal cord injury.