One of the most common, and untreated, health problems among older adults is mobility disability, observed in nearly one in five (17.9%) of older adults. Mobility disability is typically the first disability to develop and increases future risk for additional disabilities and death. Though the most effective treatments for mobility disability are forms of exercise, especially resistance training (RT), they are rarely used, with fewer than 20% of older adults meeting RT guidelines and even fewer among those with mobility disability. The core problem of disseminating the benefits of RT to older adults is adherence. One potential opportunity to enhance adherence to RT, which has not been tested, is to make RT programs shorter. Studies show that most of the benefits of RT accrue with the first few sets per week, consistent with the law of diminishing utility. While traditional RT programs for older adults, like those offered by Silver Sneakers, are typically 45 minutes three times weekly, fewer than 5% of older adults with free access to these programs participate. Our qualitative work shows that older adults often feel that 45 minute sessions are too challenging (e.g., "I don't think I have the strength to do it for 45 minutes") and 75% of older adults we surveyed preferred a 5 minute RT to a 45 minute RT option, assuming they were equally effective. In 2020, we set out to design a brief, home-based RT program that would lead to both high levels of adherence and functional improvement. We called the program FAST (Functional Activity Strength Training) and, to overcome its brevity, FAST was augmented with several standard behavior change techniques (e.g., feedback, reminders, self-monitoring) and a novel form of goal- setting, rarely used in RT studies, for the number of additional repetitions participants should be able to do during the study. In FAST-1, 24 healthy older adults were prescribed 30 seconds of squats and push-ups each day and given no personal supervision. Over 6 months, they performed the exercises on 73% of days and showed large increases in squat and push-up performance (Cohen's d \> 1.0). In FAST-2, we randomly assigned 97 older adults with mobility disability, and those assigned to 30 seconds each of chair stands and steps onto a stepper each day completed exercises on 81% of days (5.7 days per week) and improved their 5 time sit-to-stand test (-2.8 seconds, Cohen's d=0.53), 30 second chair stand test (+4.2 repetitions, d=1.1) and One Leg Stand test (+3.7 seconds, d=0.40), versus controls. In this multicenter study, we will randomly assign 360 older adults with an SPPB score \< 8 to 6 months of daily FAST or to a delayed control group. We will test the impact of FAST on physical performance, walking ability, falls and functional limitations. We hypothesize that FAST will improve these measures more than controls, and that those with greater adherence and perceived effort will improve the most. By rigorously testing FAST, we hope to change the paradigm of RT prescription from "More is better" to "What will people do that works?" and, if FAST proves superior, a future study will test whether FAST leads more older adults to do RT and, thereby, improve the public's health.