Research Questions

Our laboratory focuses on revealing the neuromuscular coordination that allows humans to retain a stable upright posture while standing and walking. That decades-long endeavor has required a variety of novel approaches and custom instrumentation to dissect the complex multiple coordinative strategies that humans employ. We have discovered the preferred coordination patterns that humans use to control the angular motion of their body, and thus, prevent falling. We have further revealed a lower limb coordination deficit caused by a stroke which explains a wide range of observed behaviors such as walking difficulty, adopted behaviors, and the failure of therapeutic approaches. The conclusions drawn from this research are being used to develop treatments to restore walking and reduce fall risk. This revolutionary advance in the understanding of human stability has implications for treating and enhancing human function and performance across the lifespan, and for a wide array of human endeavors.

Isolation of control strategies: Postural tasks require the coordination of a large number of muscles to meet the physical demands of retaining upright posture and not falling. That coordination involves multiple simultaneously active control strategies that vary in the degree to which they respond to ongoing feedback of the system state. A challenge that has limited the understanding of human postural control is the lack of a means to isolate and characterize the multiple control strategies that are active simultaneously during walking. We have taken novel approaches with tasks conducted in carefully designed mechanical environments to provide that isolation.

Foot force characterizes coordination: Of particular importance to inter-muscle coordination is the relative activation of each muscle throughout a task. That relative activation controls the direction of motion of the limb endpoint or the direction of the force produced against the environment by the limb endpoint. The overall muscle activation level controls the rate of movement or magnitude of force. While muscle activity can be estimated with electromyogram, muscle coordination can be understood with greater precision by sensing the final common output of the coordination which is the force at the foot-ground interface (foot force). That force is also responsible for driving the motion of the body as a whole. Thus, foot force is a powerful tool to characterize the coordination and how it accomplishes the task of whole body motion. The following research topics illustrate how our systematic and comprehensive series of studies contribute to advancing our understanding of human postural control.


Preferred Interjoint Coordination – Linear Force Paths

We have characterized the lower limb muscle coordination that humans produce when performing a task that does not constrain relative muscle coordination. That lack of constraint reveals the motor system preferred strategy which is hypothesized to be central to more complex behaviors. That hypothesis is supported by our subsequent work on walking and standing.

Foot Force Directional Control Across the Workspace

In non-balancing tasks humans systematically alter muscle coordination across the lower limb workspace to produce a foot force field that has a common divergent point (DP). That divergent point is located on the torso near the location of the center-of-mass (CM) if the person were in anatomical posture.

Foot Roll & Force Direction

Foot force direction varies systematically with ankle torque such that the foot force vectors intersect at a divergent point located near the knee joint.

Walking Stability & the Divergent Point

A systematic relationship between foot force direction and center-of-pressure was characterized as having a common intersection point we call the divergent point (DP). That point is located above the CM approximately at the level of the shoulder joints.

Walking Mechanics & Control – Xi Model

We have developed a model of walking that describes how humans combine a CM-centric coordination with body mechanics to achieve passive stability, reducing the cognitive burden of control.

Postural Control During Standing

The ability to stand requires control of angular motion of the whole body. We analyze center-of-pressure and foot force direction to understand the coordination responsible for that control.

Foot Force Control Deficit After Stroke

Despite the diffuse and person-specific neurological damage caused by stroke we discovered foot force control to be disrupted in a stereotypical manner that provides insight into the structure of the nervous system as well as a unified understanding of the behavioral deficits observed after stroke.

Disrupted Walking After Stroke

The disruption of walking caused by stroke and the poor performance of advanced therapies to restore walking are predicted by the presence of the single specific coordination deficit that manifests as misdirection of foot force.

Walking Restoration Following Stroke

We have applied our research findings to develop a device for retraining walking based on the fundamental source of impairment.