PT - JOURNAL ARTICLE AU - R.W. Nuckols AU - T.J.M Dick AU - O.N. Beck AU - G.S. Sawicki TI - Ultrasound imaging links soleus muscle neuromechanics and energetics during human walking with elastic ankle exoskeletons AID - 10.1101/2020.01.20.909259 DP - 2020 Jan 01 TA - bioRxiv PG - 2020.01.20.909259 4099 - http://biorxiv.org/content/early/2020/01/20/2020.01.20.909259.short 4100 - http://biorxiv.org/content/early/2020/01/20/2020.01.20.909259.full AB - Unpowered exoskeletons with springs in parallel to human plantar flexor muscle-tendons can reduce the metabolic cost of walking. We used ultrasound imaging to look ‘under the skin’ and measure how exoskeleton stiffness alters soleus muscle contractile dynamics and shapes the user’s metabolic rate during walking. Eleven participants (4F, 7M; age: 27.7 ± 3.3 years) walked on a treadmill at 1.25 m s-1 and 0% grade with elastic ankle exoskeletons (rotational stiffness: 0-250 Nm rad-1) in one training and two testing days. Metabolic savings were maximized (4.2%) at a stiffness of 50 Nm rad-1. As exoskeleton stiffness increased, the soleus muscle operated at longer lengths and improved economy (force/activation) during early stance, but this benefit was offset by faster shortening velocity and poorer economy in late stance. Changes in soleus activation rate correlated with changes in users’ metabolic rate (p = 0.038, R2 = 0.44), highlighting a crucial link between muscle neuromechanics and exoskeleton performance; perhaps informing future ‘muscle-in-the loop’ exoskeleton controllers designed to steer contractile dynamics toward more economical force production.