Ipsilateral corticomotor excitability is associated with increased gait variability in unilateral transtibial amputees

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Abstract

Ipsilateral primary motor cortex (M1) reorganisation after unilateral lower-limb amputation may degrade function of the amputated limb. We hypothesised unilateral lower-limb amputees would have a bilateral increase in corticomotor excitability, and increased excitability of ipsilateral M1 would be associated with increased step-time variability during gait. Twenty transtibial amputees (16 male) aged 60.1 years (range 45–80 years), and 20 age- and gender-matched healthy adult controls were recruited. Single-pulse transcranial magnetic stimulation assessed corticomotor excitability. Two indices of corticomotor excitability were calculated. An index of corticospinal excitability (ICE) determined relative excitability of ipsilateral and contralateral corticomotor projections to alpha-motoneurons innervating the quadriceps muscle (QM) of the amputated limb. A laterality index (LI) assessed relative excitability of contralateral projections from each hemisphere. Spatial-temporal gait analysis was performed to calculate step-time variability. Amputees had lower ICE values, indicating relatively greater excitability of ipsilateral corticomotor projections than controls (P = 0.04). A lower ICE value was associated with increased step-time variability for amputated (P = 0.04) and non-amputated limbs (P = 0.02). This association suggests corticomotor projections from ipsilateral M1 to alpha-motoneurons innervating the amputated limb QM may interfere with gait. Cortical excitability in amputees was not increased bilaterally, contrary to our hypothesis. There was no difference in excitability of contralateral M1 between amputees and controls (P = 0.10), and no difference in LI (P = 0.71). It appears both hemispheres control one QM, with predominance of contralateral corticomotor excitability in healthy adults. Following lower-limb amputation, putative ipsilateral corticomotor excitability is relatively increased in some amputees and may negatively impact on function.

There is suggestion of bilateral primary motor cortex reorganisation following lower-limb amputation. Increased corticomotor excitability of the ipsilateral primary motor cortex may influence function. The purpose of this study was to determine if the pattern of primary motor cortex excitability in lower-limb amputees was associated with gait function.

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