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Introduction/aims: Despite disease-modifying therapies, fatigability persists in spinal muscular atrophy (SMA). Performance fatigability (PF) during the six-minute walk test (6MWT) is mostly unchanged in treated SMA. This cross-sectional study characterized PF using instrumented insoles. Methods: Ambulatory individuals with SMA (n = 14) and controls (HC) (n = 10) were included. Spatiotemporal and kinetic parameters were collected with custom-engineered instrumented insoles during the 6MWT. Linear mixed models analyzed parameter trends, with trend slope representing PF. Changes in mean velocity (Vavg) and stride-by-stride parameters were compared between minute 1 and 6 and between groups. Results: Decreases in Vavg were greatest for severe SMA (p < 0.001). Changes were found in stride length (SL) (p = 0.048) and stride velocity (SV) (p = 0.030) for severe SMA, and in stance phase (%St) (p = 0.012) and percent terminal double support (%DS) (p = 0.02) for mild SMA. SMA subgroups showed downward trends from minute 1 to 6 in SL, SV, and anterior-posterior center of pressure (AP-COP) (p < 0.001), and increases in Absolute COP-Cyclogram Asymmetry Index (|ASI|) (p < 0.05). Trends differed between severe SMA and other groups for SL, SV, %St, and %DS (p < 0.001), and for AP-COP and |ASI| (p < 0.05). Trends for SL (p < 0.001), SV and AP-COP (p < 0.01) differed between HC and mild SMA. Discussion: PF in SMA manifests as changes in gait parameters. Instrumented insoles revealed fatigue-related changes not captured with the conventional method of comparing the first and last minutes of the 6MWT. Spatiotemporal and kinetic parameters contribute to understanding of impairments and inform therapeutic development. Trial registration: ClinicalTrials.gov: NCT04193085.