Knee extension isometric strength, rate of absolute and relative tension development, and vastus lateralis muscle fiber type composition was examined in 15 old (means = 64.5 years) and 9 young (means = 23.2 years) subjects. Strength was assessed using a 5-sec maximal voluntary contraction (MVC) and a fast maximal voluntary contraction (FMVC). MVC and FMVC were not significantly different in the Young group while in the Old group the FMVC was 19% greater than the MVC. These data suggested that usual strength testing methodology may considerably overestimate age-related strength loss. MVC, FMVC, and FT/ST muscle fiber area ratio were significantly lower in the Old group; rate of relative tension development was similar in Young and Old groups. FT/ST area ratio correlated with strength in the Young group and with rate of relative tension development in both groups. It was concluded that relative rate of tension development is unimpaired and related to FT fiber size while absolute tension level is impaired and unrelated to FT fiber size in the Old group.