Effects of the availability of accurate proprioceptive information on older adults' postural sway and muscle co‐contraction

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 increase 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: 2 min), followed by 3 min on a sway‐referenced platform (adaptation) and a final 3 min on a fixed platform again (reintegration). Sensory reweighting was slower in older adults, as shown by a significantly larger and longer postural sway after‐effect once a stable platform was restored. Muscle co‐contraction showed similar after‐effects, whereby older adults showed a larger increase in co‐contraction once the stable platform had been restored, compared to young adults. This co‐contraction after‐effect did not return to baseline until after 1 min. 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 minimise postural sway, which is increased during this sensory transition. Future research should examine whether muscle co‐contraction is typically a reactive or anticipatory response. Similarly to postural sway measures, older adults demonstrate increased lower limb muscle co‐contraction in response to both sway‐referenced support conditions and the restoration of a stable support surface. This suggests that muscle co‐contraction is not a selective compensatory response to a specific sensory deficit (i.e. proprioceptive input) but rather, may be a general postural response to conditions of increased postural instability.