Changes in contractile and elastic properties of human plantar flexor muscles in ageing, were investigated in 12 young (19-24 years, YG) and 11 old (61-74 year, OG) men. Maximal isometric and concentric voluntary torques, at several angular velocities, were measured to construct torque-angular velocity relationship. This led to the calculation of an index of maximal shorting velocity (VI_max) at low torque. Two methods were then used to calculate musculotendinous (MT, quick-release movements) and musculoarticular (MA, sinusoidal perturbations) stiffness. In both cases, stiffness was linearly related to torque, leading to the calculation of a stiffness index (SI) as the slope of the stiffness-torque relationship: SI_MT and SI _MA, respectively. MA stiffness under passive conditions (K _p) was also determined. Surface electromyograms were useful to control agonist and antagonist myoelectrical activities. As expected, maximal isometric (P<0.05) and concentric torques (P<0.05), as well as VI _max(P<0.05) were lower in OG compared to YG. SI_MT values were higher for OG compared to YG (P<0.05), leading to a mean difference of 55%, whereas SI_MA and K_p were not significantly different between the two groups. Thus, older men were weaker and exhibited higher SI_MT values. These impairments seem to be principally due to muscular atrophy and modifications in both muscle fibre-type distribution and fibre composition, in ageing. Invariance of SI_MA and K_p would suggest an adaptive mechanism in articular structures to avoid the continuous integration of the ankle joint stiffness by the central nervous system, what may simplify most daily motor tasks.