Hugh Herr

Contributions to Science

I. Science of biomechanics and biological movement control (See http://biomech.media.mit.edu/#organismal for a complete list of publications)

1. First to demonstrate that stiffness tuning of antagonist muscles is a primary determinant of the power delivered by muscle-actuated systems. Integrative approaches to studying the coupled dynamics of skeletal muscles with their loads while under neural control have focused largely on questions pertaining to the postural and dynamical stability of animals and humans. Prior studies have focused on how the central nervous system actively modulates muscle mechanical impedance to generate and stabilize motion and posture. However, the question of whether muscle impedance properties can be neurally modulated to create favorable mechanical energetics, particularly in the context of periodic tasks, remains open. Through muscle stiffness tuning, we hypothesize that a pair of antagonist muscles acting against a common load may produce significantly more power synergistically than individually when impedance matching conditions are met between muscle and load. Since neurally modulated muscle stiffness contributes to the coupled muscle-load stiffness, we further anticipate that power-optimal oscillation frequencies will occur at frequencies greater than the natural frequency of the load. We find a 7-fold increase in mechanical power when antagonist muscles act synergistically compared to individually at a frequency higher than the load natural frequency. These observed behaviors are interpreted in the context of resonance tuning and the engineering notion of impedance matching. These findings suggest that the central nervous system can adopt strategies to harness inherent muscle impedance in relation to external loads to attain favorable mechanical energetics. Role of Hugh Herr: PI

a. Farahat W., Herr H. Impedance matching as a means of enhancing mechanical energetics in muscle-actuated systems, Adaptive Motion in Animals and Machine, Cleveland, OH, Jun. 2008.

b. Farahat W., Herr H. Optimal Workloop Energetics of Muscle-Actuated Systems: An Impedance Matching View. PloS Computational Biology. 2010.

2. First to advance musculoskeletal leg models that predict kinematics, kinetics and metabolic economy of walking humans. Although joint biomechanics and whole-body energetics are well documented for human walking, the underlying mechanisms that govern individual muscle-tendon behaviors are not fully understood. We developed computational models of human walking that unify muscle and joint biomechanics with whole-body metabolism for level-ground walking. Following a forward dynamics optimization procedure, the walking models are shown to predict muscle and joint biomechanics, as well as whole-body metabolism, supporting the idea that the preponderance of leg muscles operate at low fascicle speeds, affording the relatively high metabolic walking economy of humans. Role of Hugh Herr: PI

a. Geyer H., Herr H. A Muscle-Reflex Model that Encodes Principles of Legged Mechanics Produces Human Walking Dynamics and Muscle Activities. IEEE Transactions on Neural Systems & Rehabilitation Engineering. 2010.

b. P . Krishnaswamy, E. N. Brown, and H. M. Herr. Human leg model predicts ankle muscle-tendon morphology, state, roles and energetics in walking, PLoS Computational Biology, vol. 7, no. 3, Mar. 2011.

c. Endo K., Herr H. A Model of Muscle-tendon Function in Human Walking at Self-selected Speed. IEEE Transactions on Neural Systems and Rehabilitation. 2013.

II. Wearable robotic technology for human rehabilitation and augmentation (See http://biomech.media.mit.edu/#organismal for a complete list of publications)

1. First powered bionic leg to normalize walking metabolism and speed in persons with transtibial leg amputation. Over time, leg prostheses have improved in design, but have been incapable of actively adapting to different walking velocities in a manner comparable to a biological limb. People with a leg amputation using such commercially available passive-elastic prostheses require significantly more metabolic energy to walk at the same velocities, prefer to walk slower and have abnormal biomechanics compared with non-amputees. A bionic prosthesis has been developed that emulates the function of a biological ankle during level-ground walking, specifically providing the net positive work required for a range of walking velocities. Using the bionic prosthesis resulted in metabolic energy costs, preferred walking velocities and biomechanical patterns that were not significantly different from people without an amputation. Role of Hugh Herr: PI

a. Au S., Weber J., Herr H. Powered Ankle-foot Prosthesis Improves Walking Metabolic Economy. IEEE Transactions on Robotics. 2009; 25 (1): 51-66.

b. Herr H. M. and Grabowski A. M. Bionic ankle-foot prosthesis normalizes walking gait for persons with leg amputation, Proceedings of the Royal Society B, vol. 279, no. 1728, pp. 457–464, Feb. 2012..

2. First autonomous exoskeletons to reduce metabolic cost of human hopping and walking. For over a century, technologists have strived to develop autonomous leg exoskeletons that reduce the metabolic energy consumed when humans hop, walk and run, but such technologies have traditionally remained unachievable. We developed autonomous exoskeletons that reduce the metabolic energy consumed during hopping and walking. In the design of leg exoskeletons, these studies underscore the importance of minimizing exoskeletal power dissipation and added limb mass, while providing substantial positive power to human movements. Role of Hugh Herr: PI.

a. Grabowski A., Herr H. Leg Exoskeleton Reduces the Metabolic Cost of Human Hopping. Journal of Applied Physiology. 2009, 107:670-678.

b. Mooney, L., Rouse, E., Herr, H., Autonomous Exoskeleton Reduces Metabolic Cost of Human Walking During Load Carriage. Journal of Neuroengineering and Rehabilitation. 2014.

3. First 3-D printed prosthetic socket from MRI data. Compliant features are seamlessly integrated into a three-dimensional printed socket to achieve lower interface peak pressures over bony protuberances by using biomechanical data acquired through surface scanning and magnetic resonance imaging techniques. An inverse linear mathematical transformation spatially maps quantitative measurements (bone tissue depth) of the human residual limb to the corresponding prosthetic socket impedance characteristics. These results underscore the possible benefits of spatially varying socket wall impedance based upon the soft tissue characteristics of the underlying residual limb anatomy. Role of Hugh Herr: PI.

a. Sengeh D., Herr H. A Variable Impedance Prosthetic Socket for a Transtibial Amputee Designed from MRI Data, J Prosthet Orthot. 2013.