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First published online December 16, 2008
Journal of Experimental Biology 212, 21-31 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.017269
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Powered ankle exoskeletons reveal the metabolic cost of plantar flexor mechanical work during walking with longer steps at constant step frequency

Gregory S. Sawicki* and Daniel P. Ferris

Human Neuromechanics Laboratory, University of Michigan at Ann Arbor, Ann Arbor, MI 48109, USA


Figure 1
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  		Fig. 1. Experimental set-up. Subjects walked on a motorized treadmill for 7 min with exoskeletons unpowered, then rested for 3 min, then walked for 7 min with exoskeletons powered, while a metronome enforced their preferred step frequency (from unpowered walking at 1.25 m s–1). Treadmill belt speed was set to achieve speed/step-length conditions of 0.8, 1.0, 1.2 and 1.4x the preferred step length at 1.25 m s–1 (L*; i.e. 1.00, 1.25, 1.50 and 1.75 m s–1). Conditions were presented in randomized order. The boxes indicate periods when data were collected (minutes 4–6) in both unpowered and powered conditions. We collected joint kinematics using motion capture and reflective markers, O2 consumption and CO2 production using a metabolic cart, ankle muscle activation patterns using surface electromyography and artificial muscle forces using series load transducers.

 

Figure 2
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  		Fig. 2. Joint kinematics. The thick lines show the mean ankle (left column), knee (middle column) and hip (right column) joint angles (degrees) over the stride from heel strike (0%) to heel strike (100%) of nine subjects. Data are averages of left and right legs. Each row is walking data for a single speed/step length (0.8 L* at top to 1.4 L* at bottom). In each subplot, curves are for unpowered (black circles), and powered walking (gray circles) and thin lines are +1 s.d. Stance is ~0–60% of the stride, swing 60–100%. Ankle joint plantarflexion, knee joint extension and hip joint extension are all positive. For all joints, 0 deg. is upright standing posture.

 

Figure 3
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  		Fig. 3. Ankle exoskeleton mechanics. Thick lines show the mean ankle joint angular velocity (left column), exoskeleton torque (middle column) and exoskeleton mechanical power (right column) over the stride from heel strike (0%) to heel strike (100%) of nine subjects. Data are average of left and right legs. Each row is walking data at a single speed/step length (0.8 L* at top to 1.4 L* at bottom). In each subplot, lines are for unpowered (black circles), and powered walking (dark gray circles). Thin lines are +1 s.d. Stance is ~0–60% of the stride, swing 60–100%. Ankle joint angular velocity (deg. s–1) is positive for ankle plantarflexion. Exoskeleton torque that acts to plantar flex the ankle is positive. Torque is the product of artificial muscle force and moment arm length and is normalized by subject mass (Nm kg–1). Exoskeleton mechanical power is the product of exoskeleton torque and ankle joint angular velocity and is normalized by subject mass (W kg–1). Positive exoskeleton mechanical power indicates transfer of energy from exoskeletons to the user's ankle muscle–tendon system. In the second and third columns, the ankle joint net muscle moment and the ankle joint mechanical power from unpowered walking overground (light gray circles) are overlaid.

 

Figure 4
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  		Fig. 4. Average mechanical power. Bars are the mean (N=9) average muscle–tendon (MT) positive mechanical power delivered by the sum of the ankle, knee and hip (black bars) and the ankle muscle–tendon system only (white bars) during unpowered overground walking. Gray bars are average exoskeleton positive mechanical power during powered walking on the treadmill. Error bars are ±1 s.e.m. All mechanical power values are normalized by subject mass (W kg–1). Speeds/step lengths increase from left 0.8 L* (1.00 m s–1) to right 1.4 L* (1.75 m s–1). Brackets indicate the percentage contribution of bars from right to left. For example, in the 0.8 L* condition, the exoskeleton average positive mechanical power was 70% of the ankle muscle–tendon average positive mechanical power, ankle muscle–tendon positive mechanical power was 34% of the ankle + knee + hip positive mechanical power and the exoskeleton average positive mechanical power was 24% of the ankle + knee + hip positive average positive mechanical power over the stride.

 

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  		Fig. 5. Exoskeleton performance. Bars indicate nine subject means. Error bars are ±1 s.e.m. (A) Change in net metabolic power (powered–unpowered; W kg–1) as a result of powered assistance from bilateral ankle exoskeletons. Values listed below bars indicate percentage difference in net metabolic power for powered versus unpowered walking in each condition. Asterisks indicate statistical significance for comparison of powered versus unpowered net metabolic power (P<0.05). (B) Exoskeleton average positive (black), negative (white) and net (dark gray) mechanical power (W kg–1) over a stride for powered walking. (C) Exoskeleton performance index. Performance index (unitless) indicates the fraction of ankle muscle–tendon positive work performed by plantar flexor muscle shortening rather than Achilles' tendon recoil (i.e. muscle work fraction). Numbers listed above bars are equivalent ankle muscle–tendon `apparent efficiency' values (see Material and methods for details). For all panels, speeds/step lengths increase from left 0.8 L* (1.00 m s–1) to right 1.4 L* (1.75 m s–1).

 

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  		Fig. 6. Ankle muscle root mean square electromyography. Subplots are soleus (Sol.; top), medial gastrocnemius (MG), lateral gastrocnemius (LG) and tibialis anterior (TA; bottom). In each subplot, bars are normalized mean stance phase root mean square (r.m.s.) average muscle activation of nine subjects. All r.m.s. values (unitless) are normalized to the unpowered 1.4 L* condition. Error bars are ±1 s.e.m. Speeds/step lengths increase from left (0.8 L*; 1.00 m s–1) to right (1.4 L*; 1.75 m s–1) with unpowered walking (minutes 4–6) shown as white bars and powered walking (minutes 4–6) shown as gray bars. Numbers listed above bars indicate percentage difference in powered compared with unpowered condition. Asterisks indicate a statistically significant difference between powered and unpowered walking (P<0.05).

 

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