We identified the strength cutpoints concerning mobility impairment, then identified the muscle mass cutpoints concerning weakness, and compared the results with other diagnostic criteria to develop the clinical diagnostic criteria associated with functional impairment. In 7583 elderly people, classification and regression tree (CART) and receiver operating characteristic curve (ROC) analyses were used for determining cutpoints for handgrip strength (HGS) and appendicular lean mass (ALM) indices associated with slowness or weakness. Logistic regressions were then used to quantify the strength of the association between muscle mass (or strength) categories and weakness (or slowness). The CART second cutpoints of muscle mass and strength indices were lower than those specified by the ROC method and were between those cutpoints determined by the 20th and Mean-2SD methods. After adjusting for covariates, the associations remained significant in handgrip strength categories defined by the CART and ROC cutpoints and HGS/BMI categories defined by the CART, ROC, and 20th cutpoints in men and women (P < 0.05), ALM, ALM/Ht2 categories defined by all four cutpoints (P < 0.05) and ALM/BMI categories defined by CART and ROC cutpoints in men (P < 0.05), and ALM and ALM/Ht2 categories defined by the CART cutpoints in women (P < 0.05). Our approaches resulted in a definition of weak strength as handgrip strength or HGS/BMI less than 26.55 kg or 1.114 in men and less than 16.45 kg or 0.697 in women and then defined ALM, ALM/Ht2, or ALM/BMI less than 18.92 kg, 7.08 kg/m², or 0.795 in men and less than 15.04 kg, 5.99 kg/m², or 0.517 in women as low lean mass.