Aging is associated with a reduction in gait velocity, which is due to a shortened step length. This study investigated the relationship between joint kinetics and step length. Three-dimensional gait kinematics and kinetics were measured during usual pace gait in 26 older subjects (average age 79) and in 32 young subjects (average age 26). Gait measures were obtained at maximal velocity in five older subjects. Lower extremity strength was measured in the older subjects on an isokinetic dynamometer. Older persons had a 10% shorter step length during usual gait, when corrected for leg length (.65 +/- .07, .74 +/- .04/leg length, respectively, p < .001). Older persons had reduced ankle plantarflexion during late stance (13 +/- 5 degrees, 17 +/- 5 degrees, p = .02) and lower ankle plantarflexor power (2.9 +/- 0.9 W kg-1, 3.5 +/- 0.9 W kg-1, respectively, p = .007). Ankle strength was associated with plantarflexor power developed during late stance (r = .49, p < .001). When gait kinetics were corrected for step length, the older subjects developed 16% greater hip flexor power during late stance than younger subjects (estimate of effect: .15 W kg-1, p = .002). Older subjects were unable to increase ankle plantarflexor power at maximal pace, but increased hip flexor power 72% (1.1 +/- 0.3 W kg-1 to 1.9 +/- 1.0 W kg-1, p = .02). Older subjects had lower ankle plantarflexor power during the late stance phase of gait and appeared to compensate for reductions in plantarflexor power by increasing hip flexor power. Appropriate training of ankle plantarflexor muscles may be important in maintaining step length in advanced age.