Academic Appointments

All Publications

  • An Acute Randomized Controlled Trial of Noninvasive Peripheral Nerve Stimulation in Essential Tremor NEUROMODULATION Pahwa, R., Dhall, R., Ostrem, J., Gwinn, R., Lyons, K., Ro, S., Dietiker, C., Luthra, N., Chidester, P., Hamner, S., Ross, E., Delp, S. 2019; 22 (5): 537–45

    View details for DOI 10.1111/ner.12930

    View details for Web of Science ID 000475988900004

  • OpenSim: Simulating musculoskeletal dynamics and neuromuscular control to study human and animal movement PLOS COMPUTATIONAL BIOLOGY Seth, A., Hicks, J. L., Uchida, T. K., Habib, A., Dembia, C. L., Dunne, J. J., Ong, C. F., DeMers, M. S., Rajagopal, A., Millard, M., Hamner, S. R., Arnold, E. M., Yong, J. R., Lakshmikanth, S. K., Sherman, M. A., Ku, J. P., Delp, S. L. 2018; 14 (7)
  • Optimizing Locomotion Controllers Using Biologically-Based Actuators and Objectives ACM TRANSACTIONS ON GRAPHICS Wang, J. M., Hamner, S. R., Delp, S. L., Koltun, V. 2012; 31 (4)
  • Muscle contributions to propulsion and support during running JOURNAL OF BIOMECHANICS Hamner, S. R., Seth, A., Delp, S. L. 2010; 43 (14): 2709-2716


    Muscles actuate running by developing forces that propel the body forward while supporting the body's weight. To understand how muscles contribute to propulsion (i.e., forward acceleration of the mass center) and support (i.e., upward acceleration of the mass center) during running we developed a three-dimensional muscle-actuated simulation of the running gait cycle. The simulation is driven by 92 musculotendon actuators of the lower extremities and torso and includes the dynamics of arm motion. We analyzed the simulation to determine how each muscle contributed to the acceleration of the body mass center. During the early part of the stance phase, the quadriceps muscle group was the largest contributor to braking (i.e., backward acceleration of the mass center) and support. During the second half of the stance phase, the soleus and gastrocnemius muscles were the greatest contributors to propulsion and support. The arms did not contribute substantially to either propulsion or support, generating less than 1% of the peak mass center acceleration. However, the arms effectively counterbalanced the vertical angular momentum of the lower extremities. Our analysis reveals that the quadriceps and plantarflexors are the major contributors to acceleration of the body mass center during running.

    View details for DOI 10.1016/j.jbiomech.2010.06.025

    View details for Web of Science ID 000284343700009

    View details for PubMedID 20691972

    View details for PubMedCentralID PMC2973845

  • A rolling constraint reproduces ground reaction forces and moments in dynamic simulations of walking, running, and crouch gait JOURNAL OF BIOMECHANICS Hamner, S. R., Seth, A., Steele, K. M., Delp, S. L. 2013; 46 (10): 1772-1776


    Recent advances in computational technology have dramatically increased the use of muscle-driven simulation to study accelerations produced by muscles during gait. Accelerations computed from muscle-driven simulations are sensitive to the model used to represent contact between the foot and ground. A foot-ground contact model must be able to calculate ground reaction forces and moments that are consistent with experimentally measured ground reaction forces and moments. We show here that a rolling constraint can model foot-ground contact and reproduce measured ground reaction forces and moments in an induced acceleration analysis of muscle-driven simulations of walking, running, and crouch gait. We also illustrate that a point constraint and a weld constraint used to model foot-ground contact in previous studies produce inaccurate reaction moments and lead to contradictory interpretations of muscle function. To enable others to use and test these different constraint types (i.e., rolling, point, and weld constraints) we have included them as part of an induced acceleration analysis in OpenSim, a freely-available biomechanics simulation package.

    View details for DOI 10.1016/j.jbiomech.2013.03.030

    View details for Web of Science ID 000321422400025

    View details for PubMedID 23702045

  • How muscle fiber lengths and velocities affect muscle force generation as humans walk and run at different speeds. journal of experimental biology Arnold, E. M., Hamner, S. R., Seth, A., Millard, M., Delp, S. L. 2013; 216: 2150-2160


    The lengths and velocities of muscle fibers have a dramatic effect on muscle force generation. It is unknown, however, whether the lengths and velocities of lower limb muscle fibers substantially affect the ability of muscles to generate force during walking and running. We examined this issue by developing simulations of muscle-tendon dynamics to calculate the lengths and velocities of muscle fibers from electromyographic recordings of 11 lower limb muscles and kinematic measurements of the hip, knee and ankle made as five subjects walked at speeds of 1.0-1.75 m s(-1) and ran at speeds of 2.0-5.0 m s(-1). We analyzed the simulated fiber lengths, fiber velocities and forces to evaluate the influence of force-length and force-velocity properties on force generation at different walking and running speeds. The simulations revealed that force generation ability (i.e. the force generated per unit of activation) of eight of the 11 muscles was significantly affected by walking or running speed. Soleus force generation ability decreased with increasing walking speed, but the transition from walking to running increased the force generation ability by reducing fiber velocities. Our results demonstrate the influence of soleus muscle architecture on the walk-to-run transition and the effects of muscle-tendon compliance on the plantarflexors' ability to generate ankle moment and power. The study presents data that permit lower limb muscles to be studied in unprecedented detail by relating muscle fiber dynamics and force generation to the mechanical demands of walking and running.

    View details for DOI 10.1242/jeb.075697

    View details for PubMedID 23470656

  • Muscle contributions to fore-aft and vertical body mass center accelerations over a range of running speeds JOURNAL OF BIOMECHANICS Hamner, S. R., Delp, S. L. 2013; 46 (4): 780-787


    Running is a bouncing gait in which the body mass center slows and lowers during the first half of the stance phase; the mass center is then accelerated forward and upward into flight during the second half of the stance phase. Muscle-driven simulations can be analyzed to determine how muscle forces accelerate the body mass center. However, muscle-driven simulations of running at different speeds have not been previously developed, and it remains unclear how muscle forces modulate mass center accelerations at different running speeds. Thus, to examine how muscles generate accelerations of the body mass center, we created three-dimensional muscle-driven simulations of ten subjects running at 2.0, 3.0, 4.0, and 5.0m/s. An induced acceleration analysis determined the contribution of each muscle to mass center accelerations. Our simulations included arms, allowing us to investigate the contributions of arm motion to running dynamics. Analysis of the simulations revealed that soleus provides the greatest upward mass center acceleration at all running speeds; soleus generates a peak upward acceleration of 19.8m/s(2) (i.e., the equivalent of approximately 2.0 bodyweights of ground reaction force) at 5.0m/s. Soleus also provided the greatest contribution to forward mass center acceleration, which increased from 2.5m/s(2) at 2.0m/s to 4.0m/s(2) at 5.0m/s. At faster running speeds, greater velocity of the legs produced larger angular momentum about the vertical axis passing through the body mass center; angular momentum about this vertical axis from arm swing simultaneously increased to counterbalance the legs. We provide open-access to data and simulations from this study for further analysis in OpenSim at, enabling muscle actions during running to be studied in unprecedented detail.

    View details for DOI 10.1016/j.jbiomech.2012.11.024

    View details for Web of Science ID 000315973700022

    View details for PubMedID 23246045

  • Passive and Dynamic Shoulder Rotation Range in Uninjured and Previously Injured Overhead Throwing Athletes and the Effect of Shoulder Taping PM&R McConnell, J., Donnelly, C., Hamner, S., Dunne, J., Besier, T. 2012; 4 (2): 111-116


    To investigate: (1) the passive and dynamic shoulder internal (IR) and external (ER) rotation range of motion (ROM) of 2 groups of asymptomatic overhead throwing athletes: one group who had never experienced shoulder symptoms and another who had shoulder symptoms >12 months ago, (2) the effect of taping on the passive and dynamic IR-ER ROM in both these groups.A within-subject repeated measures analysis of variance design to determine the differences in passive and dynamic shoulder rotation range and the effect of shoulder taping on the rotation range in a group of uninjured and previously injured overhead throwing athletes.Academic institution sports medicine setting.Twenty-six overhead throwing collegiate athletes: 17 with no history of shoulder injury and 9 with previous shoulder injury.Passive shoulder ROM was measured with a goniometer with the subject in the supine position. To measure dynamic ROM, the subjects sat on a chair and threw a handball into a net. An 8-camera Vicon Motion Capture system recorded markers placed on the upper limb and trunk. Dynamic ROM was calculated with inverse kinematics by using OpenSim.Shoulder IR-ER ROM.Dynamic IR-ER ROM was significantly greater than passive IR-ER ROM (P < .0001). There was no difference in passive IR-ER ROM between the uninjured and previously injured overhead throwing athletes. However, there was a significant difference in the total dynamic IR-ER ROM, whereby the overhead throwing athletes who had never experienced shoulder symptoms had less IR-ER ROM than the previously injured group (173.9° versus 196.9°, respectively; P = .049). Taping the shoulder increased the passive ROM in both groups of subjects (P < .001), increased the dynamic IR-ER ROM in the uninjured subjects, but decreased the dynamic IR-ER ROM in the previously injured subjects, although this was not statistically significant (P = .07).Passive IR-ER ROM is a poor indication of dynamic shoulder function. Athletes who have had a previous shoulder injury demonstrate a greater dynamic IR-ER ROM than athletes who have never had a shoulder injury. Shoulder taping decreased the dynamic range of the previously injured athlete, so that it was nearer the dynamic range of the uninjured athlete. Shoulder taping might provide increased protection for the injured athlete by decreasing the dynamic IR-ER ROM and by facilitating better shoulder and scapular muscle control. Further studies are necessary to demonstrate whether this finding is clinically significant.

    View details for DOI 10.1016/j.pmrj.2011.11.010

    View details for Web of Science ID 000305438300004

    View details for PubMedID 22373460

  • Effect of Shoulder Taping on Maximum Shoulder External and Internal Rotation Range in Uninjured and Previously Injured Overhead Athletes during a Seated Throw JOURNAL OF ORTHOPAEDIC RESEARCH McConnell, J., Donnelly, C., Hamner, S., Dunne, J., Besier, T. 2011; 29 (9): 1406-1411


    The purpose of our study was to investigate whether shoulder taping affects shoulder kinematics in injured and previously injured overhead athletes during a seated throw. Twenty-six overhead college athletes threw a handball three times with and without tape, while seated on a chair. An 8-camera Vicon Motion Capture system recorded markers placed on the upper limb and trunk during each of the throwing conditions. Scaled musculoskeletal models of the upper limb were created using OpenSim and inverse kinematics used to obtain relevant joint angles. Shoulder taping had no main effect on external (ER) and internal (IR) rotation range (ROM) of the shoulder, but a significant interaction effect was found (p = 0.003 and 0.02, respectively), depending on previous injury status, whereby both the ER and IR ROM of the shoulder in the group of previously injured athletes decreased when taped (143-138° and 54-51°, respectively), but increased in the group who had never been injured (131-135° and 42-44°, respectively). Maximum abduction range and ball velocity were not affected by the application of shoulder taping, regardless of previous injury status. Thus, application of shoulder taping has a differential effect on maximum shoulder ER and IR ROM during throwing depending on previous injury status. These findings have implications for returning athletes to sport after injury and for screening athletes at risk of injury.

    View details for DOI 10.1002/jor.21399

    View details for Web of Science ID 000293735800014

    View details for PubMedID 21437968