Energetics of Walking With a Robotic Knee Exoskeleton

Abstract

The authors tested 4 young healthy subjects walking with a powered knee exoskeleton to determine if it could reduce the metabolic cost of locomotion. Subjects walked with a backpack loaded and unloaded, on a treadmill with inclinations of 0° and 15°, and outdoors with varied natural terrain. Participants walked at a self-selected speed (average 1.0 m/s) for all conditions, except incline treadmill walking (average 0.5 m/s). The authors hypothesized that the knee exoskeleton would reduce the metabolic cost of walking uphill and with a load compared with walking without the exoskeleton. The knee exoskeleton reduced metabolic cost by 4.2% in the 15° incline with the backpack load. All other conditions had an increase in metabolic cost when using the knee exoskeleton compared with not using the exoskeleton. There was more variation in metabolic cost over the outdoor walking course with the knee exoskeleton than without it. Our findings indicate that powered assistance at the knee is more likely to decrease the metabolic cost of walking in uphill conditions and during loaded walking rather than in level conditions without a backpack load. Differences in positive mechanical work demand at the knee for varying conditions may explain the differences in metabolic benefit from the exoskeleton.

The exoskeleton expansion: improving walking and running economy

“…We excluded studies that did not experimentally compare exoskeleton assisted walking or running to a no device condition, choosing to focus on devices that have been shown to break the metabolic cost barrier in the strictest sense. In total, 23 publications satisfied our criteria, and six of these articles improved walking economy during "special" conditions: load carriage [19][20][21], inclined slope [21,22], stair ascent [23], and with enforced long steps [24] (Fig. 2 and Table 1).…”

“…We labeled six studies as "special" due to an added metabolic penalty placed on the user such as load carriage [19][20][21], enforced unnaturally long steps [24], inclined ground slope [21,22], and/or stair ascent [23] ( Fig. 1).…”

“…Each of these exoskeletons mitigated the negative penalty by reducing metabolic cost. Yet, in some cases [21,24], the authors also performed a comparison at level ground walking without an added "special" penalty. In these cases, the exoskeleton did not significantly mitigate (and may have increased) metabolic cost.…”

“…Perhaps that is because researchers initially targeted the ankles because they yield the greatest positive mechanical power output of any joint [37]. Notably, only one knee exoskeleton has improved walking economy [21] (Fig. 2).…”

Mechanics of walking and running up and downhill: A joint-level perspective to guide design of lower-limb exoskeletons

“…The first mode of device operation entails adding positive mechanical work at a joint(s) when the joint is producing positive power. This is the most prevalent strategy used in exoskeletons targeting the hip, knee, and ankle with the common desired goal being the reduction of metabolic demand in healthy individuals [4,7,8,15,18,25,62,63]. The common expectation is the outcome where the addition of mechanical power causes a concomitant reduction of biological power while total power mostly remains constant (O1: replacement).…”