Samuel R. Hamner

Muscle contributions to propulsion and support during running

Muscles actuate running by developing forces that propel the body forward while supporting the bodys 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.

Optimizing locomotion controllers using biologically-based actuators and objectives

We present a technique for automatically synthesizing walking and running controllers for physically-simulated 3D humanoid characters. The sagittal hip, knee, and ankle degrees-of-freedom are actuated using a set of eight Hill-type musculotendon models in each leg, with biologically-motivated control laws. The parameters of these control laws are set by an optimization procedure that satisfies a number of locomotion task terms while minimizing a biological model of metabolic energy expenditure. We show that the use of biologically-based actuators and objectives measurably increases the realism of gaits generated by locomotion controllers that operate without the use of motion capture data, and that metabolic energy expenditure provides a simple and unifying measurement of effort that can be used for both walking and running control optimization.

Muscle contributions to fore-aft and vertical body mass center accelerations over a range of running speeds

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 simtk.org/home/nmbl_running, enabling muscle actions during running to be studied in unprecedented detail.

Passive and Dynamic Shoulder Rotation Range in Uninjured and Previously Injured Overhead Throwing Athletes and the Effect of Shoulder Taping

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 rolling constraint reproduces ground reaction forces and moments in dynamic simulations of walking, running, and crouch gait

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.

Effect of shoulder taping on maximum shoulder external and internal rotation range in uninjured and previously injured overhead athletes during a seated throw.

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.

Noninvasive neuromodulation in essential tremor demonstrates relief in a sham-controlled pilot trial: Neuromodulation for Essential Tremor

Although the precise mechanisms are uncertain, essential tremor (ET) is thought to be caused by tremulous activity within a central tremor neural network, which involves the ventral intermediate nucleus (VIM) of the thalamus. Clinical evidence supports targeting the VIM to treat tremor symptoms in ET with various methods. Previous studies have shown that electrical median nerve stimulation evokes activity within the VIM and other regions of the central tremor network. Based on these reports, we hypothesized that median and radial nerve stimulation at the wrist could reduce hand tremor. The objective of this study was to evaluate the efficacy of median and radial nerve stimulation as a noninvasive, nonpharmacological treatment to aid in the symptomatic relief of hand tremor in individuals with ET. Twenty-three blinded subjects were examined at a single site under an institutional review board-approved protocol (Fig. S1, Table S1). Subjects were randomized to treatment or sham groups. For stimulation, hydrogel electrodes were positioned on the wrist over the median and radial nerves (Fig. 1A; see Supporting Information). Efficacy was measured as the change in the Tremor Research Group’s Essential Tremor Rating Assessment Scale (TETRAS) Archimedes spiral drawing task following stimulation compared with prestimulation (Fig. 1B,C). The response in the treatment group was significant compared with both baseline and sham. In the treatment group, blinded rater scores significantly improved following stimulation (1.77 6 0.21) compared with prestimulation (2.77 6 0.22; P 5 0.01; Fig. 1D). This response was achieved without the risks of surgical or pharmacological intervention, such as the risk of hemorrhage or infection with DBS implantation, or side effects of ET medications, including the first-line therapies propranolol and primidone. In the sham group, scores did not change significantly following stimulation (2.37 6 0.22) compared with prestimulation (2.62 6 0.14; P 5 0.37; Fig. 1E). The response to treatment corresponded to an estimated hand tremor amplitude reduction of 60% 6 8.4% and was significantly greater in the treatment than in the sham group (P 5 0.02; Fig. 1F). Three subjects experienced transient redness and/or itchiness under the hydrogel electrodes that resolved without intervention. No unanticipated device effects occurred during the study. This was a pilot study with too few subjects for subanalyses of the effects of age, medication status, or medical history. Future studies should expand the subject count, investigate the response rate, repeatability, durability, and effects of chronic use, and add assessments of quality of life. This therapeutic approach was inspired by the idea that peripheral stimulation evokes central activity in brain regions such as the VIM, a thalamic target widely accepted to improve tremor with DBS. Although our data support this idea, other potential mechanisms are possible, including circuitry modulated in previous studies demonstrating tremor reduction by manipulation of peripheral sensory input. Future studies that are able to better characterize the precise mechanism may facilitate improvements to therapy. Nonetheless, this randomized, sham-controlled pilot study suggests that noninvasive neuroperipheral therapy may offer clinically meaningful symptomatic relief from hand tremor in ET with a favorable side effect profile compared with other available therapies.

OpenSim: Simulating musculoskeletal dynamics and neuromuscular control to study human and animal movement

Movement is fundamental to human and animal life, emerging through interaction of complex neural, muscular, and skeletal systems. Study of movement draws from and contributes to diverse fields, including biology, neuroscience, mechanics, and robotics. OpenSim unites methods from these fields to create fast and accurate simulations of movement, enabling two fundamental tasks. First, the software can calculate variables that are difficult to measure experimentally, such as the forces generated by muscles and the stretch and recoil of tendons during movement. Second, OpenSim can predict novel movements from models of motor control, such as kinematic adaptations of human gait during loaded or inclined walking. Changes in musculoskeletal dynamics following surgery or due to human–device interaction can also be simulated; these simulations have played a vital role in several applications, including the design of implantable mechanical devices to improve human grasping in individuals with paralysis. OpenSim is an extensible and user-friendly software package built on decades of knowledge about computational modeling and simulation of biomechanical systems. OpenSim’s design enables computational scientists to create new state-of-the-art software tools and empowers others to use these tools in research and clinical applications. OpenSim supports a large and growing community of biomechanics and rehabilitation researchers, facilitating exchange of models and simulations for reproducing and extending discoveries. Examples, tutorials, documentation, and an active user forum support this community. The OpenSim software is covered by the Apache License 2.0, which permits its use for any purpose including both nonprofit and commercial applications. The source code is freely and anonymously accessible on GitHub, where the community is welcomed to make contributions. Platform-specific installers of OpenSim include a GUI and are available on simtk.org.