spacer gif spacer gif spacer gif spacer gif spacer gif
QUICK SEARCH:   [advanced]


spacer gif
     Home     Help     Feedback     Subscriptions     Archive     Search     Table of Contents    

First published online April 18, 2008
Journal of Experimental Biology 211, 1402-1413 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.009241
This Article
Right arrow Summary Freely available
Right arrow Full Text
Right arrow Full Text (PDF)
Right arrow Supplementary Material
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Related articles in JEB
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Sawicki, G. S.
Right arrow Articles by Ferris, D. P.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Sawicki, G. S.
Right arrow Articles by Ferris, D. P.
Social Bookmarking
Add to CiteULike   Add to Complore   Add to Connotea   Add to Del.icio.us   Add to Digg   Add to Reddit   Add to Technorati   Add to Twitter  
What's this?

Mechanics and energetics of level walking with powered ankle exoskeletons

Gregory S. Sawicki1,2,3,* and Daniel P. Ferris1,2,4,5

1 Human Neuromechanics Laboratory, University of Michigan-Ann Arbor, Ann Arbor, MI 48109, USA
2 Department of Movement Science, University of Michigan-Ann Arbor, Ann Arbor, MI 48109, USA
3 Department of Mechanical Engineering, University of Michigan-Ann Arbor, Ann Arbor, MI 48109, USA
4 Department of Biomedical Engineering, University of Michigan-Ann Arbor, Ann Arbor, MI 48109, USA
5 Department of Physical Medicine and Rehabilitation, University of Michigan-Ann Arbor, Ann Arbor, MI 48109, USA


Figure 1
View larger version (38K):
[in this window]
[in a new window]

  		Fig. 1. Experimental set-up. (A) Subjects completed three practice sessions over a 7 day period. In each session, subjects walked on a motorized treadmill for 10 min with exoskeletons unpowered, 30 min with exoskeletons powered, and 15 min with exoskeletons unpowered. Outlined boxes indicate periods during which data were analyzed: unpowered beginning (minutes 7–9), powered beginning (minutes 3–5), powered end (minutes 27–29) and unpowered end (minutes 12–14). (B) During powered walking, bilateral ankle–foot orthoses (i.e. exoskeletons) drove ankle extension with artificial pneumatic muscles controlled using the subject's own soleus surface electromyography (EMG; i.e. under proportional myoelectric control). We collected joint kinematics using reflective markers and motion capture, O2 and CO2 flow rates using open-circuit spirometry, and artificial muscle forces using compression force transducers.

 

Figure 2
View larger version (18K):
[in this window]
[in a new window]

  		Fig. 2. Joint kinematics. Thick curves are nine subject mean ankle, knee and hip joint kinematics for unpowered walking from the beginning of practice session 3 (black) and powered walking at the beginning of practice session 1 (light gray) and end of practice session 3 (dark gray). Thin curves are +1 s.d. and match colors for means. Curves are the stride average of left and right legs and are plotted from heel-strike (0%) to heel-strike (100%). Stance is ~0% to 60% of the stride, swing 60% to 100%. For all joints, zero degrees is upright standing posture. Ankle plantarflexion, knee extension and hip extension are positive.

 

Figure 3
View larger version (18K):
[in this window]
[in a new window]

  		Fig. 3. Ankle exoskeleton mechanical power. (A) Nine subject mean (thick curves) + 1 s.d. (thin curves) of exoskeleton mechanical power delivered over the stride from heel-strike (0%) to heel-strike (100%; left and right exoskeletons are averaged for each subject). Curves are three session average for unpowered walking (black) and powered walking at the beginning of practice session 1 (light gray) and end of practice session 3 (dark gray). Mechanical power is computed as the product of exoskeleton torque and ankle joint angular velocity and is normalized by subject mass. Positive power indicates energy transferred to the user and negative power indicates energy absorbed from the user. (B) Bars show the nine subject mean exoskeleton average positive and negative mechanical power over a stride for powered walking. Error bars are ±1 s.e.m. Practice sessions (1–3) are shown left to right with beginning period (minutes 3–5) in light gray and end period (minutes 27–29) in dark gray. All mechanical power values are normalized by subject mass.

 

Figure 4
View larger version (16K):
[in this window]
[in a new window]

  		Fig. 4 Lower limb joint kinetics. (A) Nine subject mean (thick black curve) + 1 s.d. (thin black curve) mechanical power delivered by each of the lower limb joints over the stride from heel-strike (0%) to heel-strike (100%). Left and right legs are averaged for each subject. Curves are for unpowered walking overground at 1.25 m s–1. The mean exoskeleton mechanical power from the end of practice session 3 (thick dark gray curve) + 1 s.d. (thin dark gray curve) is overlaid on the bottom subplot for the ankle joint mechanical power. Mechanical power is computed as the product of exoskeleton torque and ankle joint angular velocity and is normalized by subject mass. Positive power indicates energy transferred to the user and negative power indicates energy absorbed from the user. (B) Bars showing the nine subject mean positive mechanical power delivered by the sum of the ankle, knee and hip joints (black) and ankle joint (white) during unpowered walking and the exoskeletons (gray) during powered walking. Error bars are ±1 s.e.m. All mechanical power values are normalized by subject mass. Braces indicate the percentage contribution of bars from right to left. For example, the exoskeleton average positive mechanical power was 63% of the ankle joint average positive mechanical power over the stride.

 

Figure 5
View larger version (9K):
[in this window]
[in a new window]

  		Fig. 5 Exoskeleton performance. (A) Bars showing the nine subject mean change in net metabolic power (powered – unpowered) due to powered assistance from bilateral exoskeletons. Error bars are ±1 s.e.m. All metabolic power values are normalized by subject mass. Right axis indicates the change in net metabolic power as a percentage difference from unpowered walking during each session. (B) Bars indicating nine subject mean ± 1 s.e.m. exoskeleton performance index (unitless). Performance index indicates the fraction of average exoskeleton positive mechanical power that results in a reduction in net metabolic power, assuming that artificial muscle work directly replaces biological muscle work. A performance index of 1.0 would indicate that all of the exoskeleton pneumatic muscle work replaced underlying biological muscle performing positive mechanical work with {eta}+muscle. For both panels, practice sessions (1–3) are shown left to right with beginning period (minutes 3–5) in light gray and end period (minutes 27–29) in dark gray.

 

Figure 6
View larger version (15K):
[in this window]
[in a new window]

  		Fig. 6 Soleus EMG. (A) Nine subject mean (thick curves) + 1 s.d. (thin curves) of soleus normalized linear enveloped (high-pass cutoff frequency 20 Hz and low-pass cutoff frequency 10 Hz) muscle activity over the stride from heel-strike (0%) to heel-strike (100%). Left and right legs are averaged for each subject. Stance phase is ~0% to 60% and swing ~60% to 100% of the stride. Thick curves are three session average for unpowered walking (black) and powered walking at the beginning of practice session 1 (light gray) and the end of practice session 3 (dark gray). Thin curves are +1 s.d. and follow the same color scheme as means. Curves are normalized to the peak value during unpowered walking at the beginning of each session (unitless). (B) Bars showing the nine subject mean of stance phase r.m.s. average soleus muscle activation (unitless). Error bars are ±1 s.e.m. Practice sessions (1–3) are shown left to right with unpowered walking periods (minutes 7–9 and minutes 12–14 at beginning and end, respectively) in white and powered beginning periods (minutes 3–5) in light gray and powered end periods (minutes 27–29) in dark gray. Percentages listed above bars for powered walking indicate the difference from unpowered beginning in each session. Asterisks indicate a statistically significant difference between powered and unpowered walking (ANOVA, P<0.05).

 

Figure 7
View larger version (15K):
[in this window]
[in a new window]

  		Fig. 7 Tibialis anterior EMG. (A) Nine subject mean (thick curves) + 1 s.d. (thin curves) of tibialis anterior normalized linear enveloped (high-pass cutoff frequency 20 Hz and low-pass cutoff frequency 10 Hz) muscle activity over the stride from heel-strike (0%) to heel-strike (100%). Left and right legs are averaged for each subject. Stance phase is ~0% to 60% and swing ~60% to 100% of the stride. Thick curves are three session average for unpowered walking (black) and powered walking at the beginning of practice session 1 (light gray) and the end of practice session 3 (dark gray). Thin curves are +1 s.d. and follow the same color scheme as means. Curves are normalized to the peak value during unpowered walking at the beginning of each session (unitless). (B) Bars showing the nine subject mean of stance phase r.m.s. average tibialis anterior muscle activation (unitless). Error bars are ±1 s.e.m. Practice sessions (1–3) are shown left to right with unpowered walking periods (minutes 7–9 and minutes 12–14 at the beginning and end, respectively) in white and powered beginning periods (minutes 3–5) in light gray and powered end periods (minutes 27–29) in dark gray. Percentages listed above bars for powered walking indicate the difference from unpowered beginning in each session. Asterisks indicate a statistically significant difference between powered and unpowered walking (ANOVA, P<0.05).

 

Add to CiteULike CiteULike   Add to Complore Complore   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us   Add to Digg Digg   Add to Reddit Reddit   Add to Technorati Technorati   Add to Twitter Twitter    What's this?



© The Company of Biologists Ltd 2008