Corticospinal responses during passive shortening and lengthening of tibialis anterior and soleus in older compared to younger adults

Corticospinal responses have been shown to increase and decrease with passive muscleshortening and lengthening, respectively, as a result of changes in muscle spindle afferentfeedback. The ageing sensory system is accompanied by a number of alterations that mightinfluence the processing and integration of sensory information. Consequently, corticospinalexcitability might be modulated differently whilst changing muscle length. In 10 older adults(66 ± 4 years), corticospinal responses (MEP/Mmax) were evoked in a static position, and duringpassive shortening and lengthening of soleus (SOL) and tibialis anterior (TA), and these datawere compared to the re-analysed data pool of 18 younger adults (25 ± 4 years) published previously. Resting motor threshold was greater in SOL compared to TA (P < 0.001), but did not differbetween young and older (P = 0.405). No differences were observed in MEP/Mmax between thestatic position, passive shortening or lengthening in SOL (young: all 0.02±0.01; older: 0.05±0.04,0.03 ± 0.02 and 0.04 ± 0.01, respectively; P = 0.298), and responses were not dependent on age(P = 0.090). Conversely, corticospinal responses in TA were modulated differently between theage groups (P = 0.002), with greater MEP/Mmax during passive shortening (0.22 ± 0.12) comparedto passive lengthening (0.13 ± 0.10) and static position (0.10 ± 0.05) in young (P < 0.001), butunchanged in older adults (0.19 ± 0.11, 0.22 ± 0.11 and 0.18 ± 0.07, respectively; P ≥ 0.867).The present experiment shows that length-dependent changes in corticospinal excitability in TAof the young are not evident in older adults. This suggests impaired sensorimotor response duringmuscle length changes in older age that might only be present in ankle flexors, but not extensors.