The preferred walking speed is a common measure of mobility that declines with age and has been related to maximal oxygen uptake (VO(2,max)). The present study determined whether this decline is associated with a higher percentage of the ventilatory threshold in older adults walking at their preferred speed. We compared the preferred walking speed and VO2 at this speed in relation to both VO(2,max) and VO2 corresponding to the ventilatory threshold (TVE) in healthy, physically active sexagenarians (G65, n=10) and octogenarians (G80, n=10) walking on a treadmill. The preferred walking speed was lower in G80 (1.16+/-0.09 m.s(-1)) than in G65 (1.38+/-0.09 m.s(-1); P<0.001). Energy expenditure and the energy cost of walking at the preferred walking speed were not significantly different between the two groups. G80 subjects exhibited significantly higher fractions of VO(2,max) (60.8+/-8.0%) and TVE (74.2+/-7.9%) at the preferred walking speed than G65 (42.9+/-5.0 and 53.2+/-5.7% respectively; P<0.001). Multiple regression analysis showed that the fraction of TVE was the main determinant, with a small contribution of height, in the decline in the preferred walking speed in healthy and active elderly subjects (R2=64%; P<0.001). These findings show that with age, walking at the preferred speed requires a higher fraction of TVE. This increase in the relative physiological effort at preferred walking speed could explain the reduction in this gait speed in healthy older subjects.