During conditions of increased postural instability, older adults exhibit greater lower-limb muscle co- contraction. This response has been interpreted as a compensatory postural strategy, which may be used to i006Ecrease proprioceptive information from muscle spindles or to stiffen the lower limb as a general response to minimise postural sway. The current study aimed to test these two hypotheses by investigating use of muscle co-contraction during sensory transitions that manipulated proprioceptive input. Surface EMG was recorded from the bilateral tibialis anterior and gastrocnemius medialis muscles, in young (aged 18-30) and older adults (aged 68-80) during blind- folded postural assessment. This commenced on a fixed platform (baseline: 2mins), followed by 3- minutes on a sway-referenced platform (adaptation) and a final 3-minutes on a fixed platform again (reintegration). Sensory reweighting was slower in older adults, as shown by a significantly larger and longer postural sway aftereffect once a stable platform was restored. Muscle co-contraction showed similar aftereffects, whereby older adults showed a larger increase in co-contraction once the stable platform had been restored, compared to young adults. This co-contraction aftereffect did not return to baseline until after 1 minute. Our evidence for high muscle co-contraction during the reintroduction of veridical proprioceptive input suggests that increased co-contraction in older adults is not dependent on contemporaneous proprioceptive input. Rather, it is more likely that co- contraction is a general postural strategy used to minimize postural sway, which is increased during this sensory transition. Future research should examine whether muscle co-contraction is typically a reactive or anticipatory response.