A lesser 6-min walk distance (6MWD) and timed up-and-go (TUG) in old compared with young adults was previously linked to slowing of muscle contractile properties. The purpose of the present study was to determine whether any further reductions in 6MWD and TUG over a 5-year period in septuagenarians are associated with further slowing of muscle contractile properties. We measured muscle function by a countermovement jump, isometric maximal knee extensor strength (MVC) on a dynamometer and quadriceps muscle size by magnetic resonance imaging (MRI) in 17 older women (71.1 ± 2.8 y) and 17 older men (71.3 ± 4.1y). Performance in TUG and 6MWD were reduced over the 5-year period, irrespective of sex (P< 0.001), and both were correlated with power at both baseline and follow-up (R≥ 0.53;P≤ 0.001). Jump take-off velocity (VCMJ) was slower at follow-up (P< 0.01) and correlated with 6MWD and TUG at both baseline and follow-up in both sexes (R≥ 0.54;P≤ 0.001). However, the relationship between ‘body mass: maximal muscle force ratio’ with VCMJwas not significantly changed, indicating that the lower VCMJwas attributable to muscles working at a higher relative load, hence a lower part of the force–velocity relationship, due to a reduction in MVC (body mass had not changed significantly), rather than slowing of the muscle. The lower VCMJin women than men (P< 0.001) was likewise attributable to a lower MVC rather than slower contractile properties in women. In conclusion, the decrement in 6MWD and TUG in septuagenarians is due to a loss of muscle mass, rather than further loss of muscle quality.