Objectives To explore the relationship between the extension force deficiency of the lower extremity and fall risks among the senior female,so as to reveal the potential factors contributing to fall.Methods Forty community-dwelling senior females aged 65 and older were assigned to the group of fallers or non-fallers according to their reported fall history and measured fall risk index (FRI).Extension force of the lower extremity was measured through 3-consecutive fast squats on a force platform.The time for 3m-time up and going (TUG),as well as static and dynamic balance were also measured.Results Compared to the non-fallers,the fallers showed lower peak extension force per body weight (PEF/BW) of the left leg and larger asymmetry of the peak extension force per body weight between the two legs in squats.Correlation analysis showed that FRI had a strong negative relation with PEF/BW of the left leg and a strong positive relation with extension force asymmetry/body weight.Also time for 3m-TUG was positively related to all the force variables standardized by the body weight,especially for the left leg.However,the analysis failed to find a relationship between the velocity of the center of pressure (COP) sway on static balance and any force variables.In addition,PEF/BW of the left leg decreased and force asymmetry/body weight between the two legs increased with aging.Conclusion Extension force measurement in squat is an effective way to assess the muscle strength deficiency related to the increased fall risk.The extension force of the lower extremity to support the body weight in squat had a strong relation to the function decline in the dynamic balance,which contributes greatly to fall risk.The extensive force asymmetry in squat between the two legs is another important fall risk factor for the senior females.