The objective of this randomized single blind trial is to compare the effects of strengthening and dynamic balance exercise on dynamic balance in healthy young-old people. in this study, 90 healthy old people (55 to 75 years old), randomly assigned into three groups. 1) strengthening exercise group, 2) dynamic balance exercise group, 3) control group. Subjects in strengthening group, do resistance training (6 exercises, 3 times a week, for 6 weeks) in lower extremity with elastic tube in three sets (10 repetitions). subjects in balance group, do jumping training on mini trampoline for 5 minutes (3 times a week, for 6 weeks). Control group wont receive any interventions. changes of the average actual score, standard deviation of postural sway and falling risk, will be measured before intervention, after intervention and after one month follow up and compared between groups.