Energy costs of walking and standing with functional neuromuscular stimulation and long leg braces

Two complete paraplegic subjects (T4 and T8), implanted with intramuscular electrodes, walked via functional neuromuscular stimulation (FNS) using a rolling walker and a reciprocal gait. The subjects also ambulated with long leg braces (LLB), using a rolling walker, and a drag/swing-to gait; and they stood using LLB and FNS separately. Oxygen consumption (VO2), carbon dioxide production (VCO2), respiratory exchange ratio (VCO2/VO2), kcal/kg/meter, kcal/kg/minute, heart rate, blood pressure, and O2 debt with recovery time were recorded. During FNS walking, energy costs (kcal/kg/min) were 59% to 75% of maximal aerobic power (MAP), as determined during the combined activities of maximal arm ergometry and FNS leg stimulation. As the speed of FNS walking increased, there was no increase in energy costs; energy efficiency (kcal/kg/meter) equaled that of LLB ambulation. One subject showed decreased O2 debt with increased speed of FNS walking. At speeds approaching .4m/sec, FNS walking energy costs were similar to those of LLB ambulation. FNS energy costs relative to working muscle mass were less than those of LLB. When maximal stimulus parameters were used for FNS standing (20mA amplitude, 25 pps frequency, and 150 microseconds pulse width), energy costs were 100% more than those of normal or LLB standing. However, when the minimal stimulus necessary to maintain a standing position was used, FNS energy costs decreased by 35% to 47%. At speeds between .4 and .6m/sec, FNS walking is a viable alternative to LLB ambulation. Major energy cost reduction should be possible in the future.