Falling among older adults is a major public health concern. In fact, falls are the leading cause of accidental death in older adults both in Taiwan and in the United States. In order to provide effective preventative strategies, the investigators must understand the mechanisms that cause older adults to fall. Thus, the investigators research aim for three objectives: (1) using biomechanical model to identify the deficits in neuromuscular control for older adults (2) to apply research findings to the development of new clinical methods for assessing and recovering balance control in older adults, and (3) to predict fall risks in 1-2 year for older adults from the research findings.

Inclusion Criteria:

1. can stand for 5 minutes without support
2. with normal or corrected to normal vision
3. age between 20-35 years old for young adults group, and 65-80 years old for older adults group
4. had fall history in the past 3 years for the older adults group.

Exclusion Criteria:

1. pregnant women
2. with cancer diagnosis
3. with sensory-motor deficits
4. with osteoporosis for older adults group (T-score below -2.5)

community sample