Muscular power is important for maintaining physical functioning with aging. Proper quantification of the reliability of muscular power tests is crucial to inform monitoring of individuals and sample size planning for interventional studies. This study evaluated short- and long-term reliability of leg extensor power measurement in 72 adults (age 62.7 ± 8.6 years). Participants completed four repeat trials on the Nottingham leg extensor power rig, with a further trial twelve weeks later. Mean change, typical error, and intraclass correlation coefficients (ICC) were calculated. For short-term reliability, mean change in power output was trivial after two trials (1.2%-4.8%). Typical errors were small following four trials in the dominant leg of males (10.9% to 5.8%), three in the non-dominant leg of males (9.9% to 6.2%) and the dominant leg of females (10.0% to 9.6%) and two in the non-dominant leg in females (8.3%). Intraclass correlation coefficients (ICCs) were very high (0.88-0.96). For long-term reliability, mean change remained trivial (1%-2.5%), typical errors remained small (5.8-8.6%), and ICCs very high (0.94-0.96). The leg extensor power rig is a reliable method for assessing lower body muscular power, both short- and long-term, with only minimal habituation effects.