Natural history of 10-meter walk/run test performance in spinal muscular atrophy: A longitudinal analysis

•10MWRT times are slower for those with SMA compared to typically developing peers.•Direction and magnitude of 12-month change in 10MWRT varies by age in those with SMA.•Age, SMA type, SMN2 copy number and BMI predict 10MWRT progression over time.•SMA type 3b, four SMN2 copies, and lower BMI were associated with faster 10MWRT time.•10 MWRT times are 74 percent slower for SMA type 3a versus 3b across ages.•In SMA a 1.0 kg/m2 increase in BMI was associated with a mean 3% slower 10MWRT time. As trials and treatments for spinal muscular atrophy (SMA) rapidly evolve, understanding the natural history and potential utility of the 10-meter walk/run test (10MWRT) in ambulant individuals is critical. Study aims were to: 1) establish change over time and across age for 10MWRT time in an untreated natural history cohort of young, ambulatory participants with SMA and 2) identify relations between 10MWRT time and age, SMA type, SMN2 copy number and anthropometrics. Untreated individuals (n = 56) age 2 to 21 years who were enrolled in a long-term natural history study between 2005 and 2014 and met inclusion criteria were included. Linear mixed effects models were used to assess changes in 10MWRT time with age and associations with SMA type, SMN2 copy number, and body mass. SMA type 3b (versus 3a), SMN2 copy number 4 (versus 3) and lower body mass were associated with faster 10MWRT. 10MWRT performance improved between 3 and 8 years of age, was stable between 9 and 10, and gradually declined from 11 to 18. Findings provide the first longitudinal natural history report of 10MWRT time in young individuals with SMA and offer a critical foundation for interpreting childhood change in short distance walking speed with pharmacologic treatment.