The purpose of the present study was to investigate the effect of the progressive walking program on lower limb muscle size and strength and evaluated whether the stair-climbing exercise provided additional training effects when combined with the walking program. Fifteen elderly subjects (age 69 ± 1 years, height 1.63 ± 0.02 m, body weight 64.5 ± 2.0 kg) were randomly assigned to a walking group or a walking and stair-climbing group. The progressive walking program comprised continuous (week 1-8) and interval (week 9-17) exercises. The walking and stair-climbing group also performed stair climbing. Muscle thickness, strength, and walking performance were evaluated before and 8 and 17 weeks after the start of the program. The muscle thickness of the anterior and posterior parts of the thigh significantly (p < 0.05) increased in both groups. There was also a significant (p < 0.01) main effect of time in isometric maximal strength and the values expressed relative to body mass for both knee extension and flexion. However, no group × time interactions were noted. Furthermore, the percentage change of knee flexion strength after the training period was significantly (p < 0.01) correlated with the pre-intervention value. Seventeen weeks of the progressive walking program can increase thigh muscle size and strength for older adults; however, an added stair-climbing exercise may not provide additional training effects. Furthermore, the magnitude of improvement in knee flexion strength would depend on the pre-intervention value.

• Progressive walking program including both moderate-intensity continuous and high-intensity interval exercise can increase thigh muscle size and strength for older adults.

• The magnitude of improvement in knee flexion strength after the walking program would depend on the pre-intervention value.

• Stair-climbing exercise may not provide additional training effects when combined with high-intensity walking exercise.