Aging of the U.S. population has become a major health challenge during the 21st century. According to The State of Aging and Health in America 2007 [1], there will be 71 million American older adults, accounting for roughly 20% of the U.S. population by 2030. In general, aging is associated with profound changes in body composition and muscle strength, often resulting in reduced functional capacity. As a result, a large number of elderly individuals suffer from impaired mobility due to strength and endurance weakness in the lower extremity. With the impaired mobility, such individuals are more likely to live in a sedentary lifestyle, and suffer from the problems associated with the sedentary lifestyle, e.g. high blood pressure, depression, obesity, etc.

Currently, the dominant tool to restore the mobility of elderly persons is the wheelchair, which presents multiple problems in the daily use. Wheelchairs are difficult to carry and...

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