Gravity-Replacement Load Device Developed and Implemented for the Cleveland Clinic

This year, NASA Glenn Research Center and ZIN Technologies, Inc. developed a gravity-replacement load device for the Cleveland Clinic’s Department of Biomedical Engineering for use with their Zero Gravity Locomotion Simulator (ZLS). The gravity-replacement device was critically needed for a bedrest campaign at the Cleveland Clinic to validate exercise prescriptions for mitigating the bone and muscle loss that astronauts experience during long-duration space missions.

Conceptual view of the ZLS.

Using the ZLS, researchers can mimic microgravity loading conditions on the musculoskeletal system by suspending a human subject in a horizontal position while the subject runs on a vertically mounted treadmill (see the preceding photograph). To accomplish this, a gravity-replacement load was required to keep a constant force on the subject during exercise.

The gravity-replacement load device had to satisfy the following requirements:

1. Protect subjects from higher than prescribed loads, especially impact loads
2. Require minimal training for operators and subjects
3. Provide fairly constant loading on the subject throughout locomotion
4. Provide high operational reliability

ZIN Technologies, Inc., developed the engineering concept and prototyped the highly compliant pneumatic Subject Load Device (pSLD) (see the following photograph). They then teamed with Cleveland Clinic’s prototype shop to build the necessary interfaces for this unique application. The pSLD provides the following features:

1. Safety--The transient loading created by any pneumatic spring failure is a load decrease, which is an inherent safety feature to prevent inadvertent overload to the test subject. The pneumatic spring limits the amount of created force, which is easily controllable by a pressure relief valve, preventing an overload to a subject.
2. Simplicity--Training is minimal. Operators just learn the sequence of the three on-off steps. Subjects just learn to bend their knees to release the load. The desired subject load is achieved by setting the corresponding pressure in the pneumatic spring.
3. Accuracy--A custom-designed force reducer-displacement multiplier is used to minimize the inertial load. The measured load variation was equal to or less than ±8 percent of the nominal subject load, depending on the running speed and the nominal load.
4. Reliability--out of 140 hr total of ZLS testing, two 1-hr tests had to be conducted with a backup device. The root causes of both malfunctions were addressed, and the pSLD was back in operation within 4 hr.

Pneumatic Subject Load Device installed at the Cleveland Clinic.

The pSLD has proven to be a safe and reliable device over 7 months of operation and will continue to be used in the future. Implementation of this device enabled the Cleveland Clinic to begin an extensive bedrest study in a timely manner.

Research Testing

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Last updated: December 15, 2007

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