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Background  Individuals with premutation alleles of the fragile X mental retardation 1 ( FMR1 ) gene are at risk of developing fragile X-associated tremor/ataxia syndrome (FXTAS) during aging. Characterization of motor issues associated with aging in  FMR1  premutation carriers is needed to determine neurodegenerative processes and establish new biobehavioral indicators to help identify individuals at greatest risk of developing FXTAS.    Methods  We examined postural stability in 18 premutation carriers ages 46–77 years and 14 age-matched healthy controls. Participants completed a test of static stance and two tests of dynamic postural sway on a force platform to quantify postural variability and complexity. CGG repeat length was measured for each premutation carrier, and MRI and neurological evaluations were conducted to identify carriers who currently met criteria for FXTAS. Of the 18 premutation carriers, seven met criteria for definite/probable FXTAS (FXTAS+), seven showed no MRI or neurological signs of FXTAS (FXTAS−), and four were inconclusive due to insufficient data.    Results  Compared to controls, premutation carriers showed increased center of pressure (COP) variability in the mediolateral (COP ML ) direction during static stance and reduced COP variability in the anterior-posterior (COP AP ) direction during dynamic AP sway. They also showed reductions in COP ML  complexity during each postural condition. FXTAS+ individuals showed reduced COP AP  variability compared to FXTAS− carriers and healthy controls during dynamic AP sway. Across all carriers, increased sway variability during static stance and decreased sway variability in target directions during dynamic sways were associated with greater CGG repeat length and more severe neurologically rated posture and gait abnormalities.    Conclusion  Our findings indicate that aging  FMR1  premutation carriers show static and dynamic postural control deficits relative to healthy controls implicating degenerative processes of spinocerebellar and cerebellar-brainstem circuits that may be independent of or precede the onset of FXTAS. Our finding that FXTAS+ and FXTAS− premutation carriers differed on their level of intentional AP sway suggests that neural mechanisms of dynamic postural control may be differentially impacted in patients with FXTAS, and its measurement may be useful for rapidly and precisely identifying disease presence and onset.    Electronic supplementary material  The online version of this article (10.1186/s11689-018-9261-x) contains supplementary material, which is available to authorized users.

Static and dynamic postural control deficits in aging fragile X mental retardation 1 (FMR1) gene premutation carriers