This cross-sectional study proposes two relative strength indexes in order to evaluate the risks of lower mobility in healthy older adults: 1) handgrip strength on body mass index and 2) quadriceps strength on body weight. Nine hundred and four men and women aged between 67–84 years old were tested for body composition, muscle strength and mobility function. Individuals in the lowest and middle tertiles of relative handgrip strength were respectively 2.2 (1.3–3.7) and 4.4 (2.6–7.6) more likely to have a lower mobility score. As for relative quadriceps strength, odd ratios for lowest and middle tertiles were respectively 2.8 (1.6–4.9) and 6.9 (3.9–12.1). Relative strength indexes, either using handgrip strength or quadriceps strength, are convenient to use in large scale studies and clinical practice.