Abstract
This study assessed the metabolic energy consumption of walking with the external components of a “Muscle-First” Motor Assisted Hybrid Neuroprosthesis (MAHNP), which combines implanted neuromuscular stimulation with a motorized exoskeleton. The “Muscle-First” approach prioritizes generating motion with the wearer's own muscles via electrical stimulation with the actuators assisting on an as-needed basis. The motorized exoskeleton contributes passive resistance torques at both the hip and knee joints of 6Nm and constrains motions to the sagittal plane. For the muscle contractions elicited by neural stimulation to be most effective, the motorized joints need to move freely when not actively assisting the desired motion. This study isolated the effect of the passive resistance or “friction” added at the joints by the assistive motors and transmissions on the metabolic energy consumption of walking in the device. Oxygen consumption was measured on six able-bodied subjects performing 6 min walk tests at three different speeds (0.4, 0.8, and 1.2 m/s) under two different conditions: one with the motors producing no torque to compensate for friction, and the other having the motors injecting power to overcome passive friction based on a feedforward friction model. Average oxygen consumption in the uncompensated condition across all speeds, measured in Metabolic Equivalent of Task (METs), was statistically different than the friction compensated condition. There was an average decrease of 8.8% for METs and 1.9% for heart rate across all speeds. While oxygen consumption was reduced when the brace performed friction compensation, other factors may have a greater contribution to the metabolic energy consumption when using the device. Future studies will assess the effects of gravity compensation on the muscular effort required to lift the weight of the distal segments of the exoskeleton as well as the sagittal plane constraint on walking motions in individuals with spinal cord injuries (SCI).
Highlights
Spinal cord injury affects 291,000 people in the US with 17,000 new cases per year (NSCISC, 2019)
Where p is the percent difference, f is the heart rate or Metabolic Equivalent of Task (METs) under the friction compensation condition at one particular speed, and u is the uncompensated value at that same speed
The average metabolic consumption across all subjects and speeds decreased by 8.8 % when friction compensation was applied
Summary
Spinal cord injury affects 291,000 people in the US with 17,000 new cases per year (NSCISC, 2019). Injuries of the thoracic spinal cord result in paraplegia, which compromises muscular and/or sensory function in the trunk and lower extremities that can impair the ability to walk. Ambulatory motion can be restored to this population via several methods. Functional Neuromuscular Stimulation (FNS) technologies, often referred to as neuroprostheses, can restore some ambulatory motion to the affected population by electrically exciting the intact peripheral nerves below the injury to contract the otherwise paralyzed muscle groups at the appropriate time and intensity. Coordinated stimulation of the muscle groups used for gait can produce motions that allow users to stand and support their body weight on their legs, or ambulate short distances with crutches, walkers, or assisted by others to maintain balance (Shimada et al, 1996; Kobetic et al, 1997; Brissot et al, 2000; Uhlir et al, 2000; Agarwal et al, 2003)
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