Muscle Activation and Angular Location Affect Soft Tissue Stiffness

In this work, we use an actuated indenter device to measure soft tissue stiffness around the forearm circumference at different levels of muscle activation. The results show that both angular location and muscle activation level have significant effects on forearm soft tissue stiffness.

Wearable device performance is dependent on physical attachment points where the viscoelastic properties of soft tissue influence the nature of interaction between the human and the robot. The stiffness of soft tissue is especially important since it determines the force-displacement response at the interface surface and affects user comfort and safety during physical human-robot interaction (pHRI). Although human soft tissue stiffness has been previously characterized with surface indentation, there remain confounding factors which affect the repeatability and accuracy of measurement. In this work, we address this gap using an actuated indenter device to measure soft tissue stiffness around the forearm circumference at different levels of muscle activation. The results show that both angular location and muscle activation level have significant effects on forearm soft tissue stiffness. Wearable robot performance may benefit from careful consideration of attachment points on the human body and expected interaction effects from the human user.

For wearable robot design, the question about where to place attachments and how user muscle activation affects interaction is tied to how these variables affect underlying soft tissue properties.

For wearable robot design, the question about where to place attachments and how user muscle activation affects interaction is tied to how these variables affect underlying soft tissue properties.

The indenter device uses a single-axis load cell mounted on a linear carriage to measure the forcedisplacement response at the skin surface for various angles around the human forearm. The hand dynamometer measures the grip force exerted by the test

The indenter device uses a single-axis load cell mounted on a linear carriage to measure the forcedisplacement response at the skin surface for various angles around the human forearm. The hand dynamometer measures the grip force exerted by the test subject.

Four sEMG sensors used in the experiment target (A) finger flexion, (B) finger extension, (C) wrist flexion, and (D) wrist extension. The indentation location along the forearm is at the midpoint between the elbow and the wrist.

Four sEMG sensors used in the experiment target (A) finger flexion, (B) finger extension, (C) wrist flexion, and (D) wrist extension. The indentation location along the forearm is at the midpoint between the elbow and the wrist.

The six angular locations measured in this experiment are shown in the forearm cross section of the right arm, as viewed from the elbow looking out towards the wrist.

The six angular locations measured in this experiment are shown in the forearm cross section of the right arm, as viewed from the elbow looking out towards the wrist.

Force-displacement response from the first indentation trial at the 180 degree angular location for one subject at four muscle activation levels along with the fitted linear regression curve. The four muscle activation levels are 0%, 15%, 30%, and 45

Force-displacement response from the first indentation trial at the 180 degree angular location for one subject at four muscle activation levels along with the fitted linear regression curve. The four muscle activation levels are 0%, 15%, 30%, and 45% of maximum grip force.

Summary of soft tissue stiffness results from six subjects based on an average stiffness calculation from the loading section of the force-displacement response. The results from four muscle activation levels are given within each of the six angular

Summary of soft tissue stiffness results from six subjects based on an average stiffness calculation from the loading section of the force-displacement response. The results from four muscle activation levels are given within each of the six angular locations. Each bar is the average of three trials and six subjects, and the error bars represent standard error.

Summary of sEMG measurements based on an average of the four sEMG sensors during the loading section of the force-displacement response. The results from four sEMG sensors are given within each grip force level. Each bar is the average of six angular

Summary of sEMG measurements based on an average of the four sEMG sensors during the loading section of the force-displacement response. The results from four sEMG sensors are given within each grip force level. Each bar is the average of six angular locations, three trials and six subjects, and the error bars represent standard error.