Significance: The one repetition maximum test (1RM) is commonly used for testing maximal muscle strength. Traditionally, when determining subject 1RM strength, resistance is systematically added to an exercise until the exercise can no longer be performed pain free, through the full range of motion, with proper form. In strength testing of older adults, the Rating of Perceived Exertion Scale (RPE) is often used as an alternative for determining 1RM. We hypothesize that the use of RPE to determine 1 RM will significantly underestimate true 1RM. Methods: Six older adults (73.3 ± 3.4 yrs) participated in 1RM strength testing on a Nautilus leg extension machine. Resistance was systematically added until subjects reported maximal effort (Perceived 1RM) while completing the exercise. The value was recorded and subjects were instructed to try heavier weight until they could no longer perform the exercise through the full range of motion, without pain, using proper form (True 1RM). The True 1 RM value was then recorded. Mean values were analyzed using a one-tailed, paired Student T-test, level of significance was set at p<0.05. Results: Perceived 1RM values were 37% lower than True 1RM values. Significant differences between the mean values recorded for Perceived 1RM (90.8 ± 9.87 lbs) and True 1RM (123 ± 11.95 lbs) were found (p=0.0003). Conclusion: Using perceived 1RM does not appear to be a valid alternative to traditional 1RM testing procedures. Older adults with little or no resistance training history significantly underestimate lower body maximal muscle effort using the RPE scale. Discussion: Underestimate of initial 1RM will result in inappropriate exercise prescription and overestimate of exercise protocol strength gain. Such an error can lead to difficulties in comparing the relative effectiveness of different strength training protocols. Keyword(s): adult fitness, older adult/aging issues, research