Outcome measures enable objective evaluation of functional changes and can inform clinical decisions, predict future ability, and fulfill healthcare documentation requirements . Kennedy and colleagues called for core gait and functional ambulation outcome measures for use in pediatric clinical and research settings . This study is a first step towards a possible core set of performance-based functional outcome measures for use in adolescents and adults born with AMC. Good-to-excellent between-days, test–retest reliability was found for the 10mWT, F8WT, 360TT, and mFSST, which are currently used in other patient populations. ICCs ≥ 0.90 suggest gait speed per the 10mWT, as well as evaluation of curved path walking and dynamic balance per the timed F8WT and 360TT, may be reliably evaluated among individuals with AMC on two separate occasions. Provided MDCs may enable clinicians to determine whether changes surpass measurement error and indicate ‘true change’ in their patients with AMC, i.e., pre-to-post rehabilitation (using MDC90) and pre-to-post surgery (using MDC95). Additionally, based on ICCs ≥ 0.70, evaluation of quality of movement, i.e., number of steps, as well as dynamic balance per the mFSST may have clinical trials utility in evaluating changes among groups of ambulatory individuals with AMC, although a potential floor effect should be considered when using the average of 2 mFSST trials, due to ‘invalid’ trials.
While our study did not include a control group, it appears our individuals with AMC are presenting with worse gait speed and dynamic balance when compared to controls and peers with other lower-extremity pathologies. Among typically-developing children and young adults, ‘self-selected’ gait speeds for 11–30 year olds are, on average, 1.28–1.36 m/sec,  which is significantly faster than speeds obtained in our participants with AMC (i.e., 1.01–1.02 m/sec). Further, Scott et al. found gait speeds of 1.2 ± 0.2 m/sec among adolescents and young adults with hip dysplasia (n = 24), suggesting gait speeds with AMC, where multiple lower-limb regions are typically involved, are worse . Collectively, results highlight the importance of evaluating and addressing reduced gait speed among individuals with AMC, particularly since ‘self-selected’ gait speed is better correlated to perceived gait quality when compared to other performance-based measures, like the 6-Minute Walk Test, in young adults with congenital, mobility-limiting conditions . Scott et al. also reported adolescents and young adults with hip dysplasia (n = 24) had FSST times of 6.6 ± 2.5 s, as compared to controls (n = 21; 4.0 ± 0.7 s) . Individuals with AMC in our study had mFSST times that were double that of individuals with single-joint involvement and triple that of controls. Hence, with AMC, dynamic balance appears considerably compromised.
Mean 360TT times among young healthy adults (n = 34) have been reported to be 2.2 s,  which is about 20% faster than timed 360TT among our participants with AMC. Among children and young adults who are typically developing, peak turn velocity during 180 degree turns are, on average, 221–289 degrees/sec . Among our participants with AMC, as evaluated with a ‘quick’ 360-degree turn, mean turn velocity was about 129 degrees/sec. Combined, data suggests impaired turning with AMC. As community-ambulation requires frequent turning,  and impaired turning has been associated with recurrent falls,  it may be imperative to incorporate turning into gait training among individuals with AMC.
Our participants with AMC had better FSST performance (median: 10.52–12.01 s) than children, aged 5–12 years, with Down syndrome and cerebral palsy, (i.e., mean: 18.7 ± 5.7 s),  which might be due to use of the mFSST without canes in our study and/or impaired cognition or inattention in the aforementioned pediatric study. Conversely, our participants had worse dynamic balance performance as compared to adults with unilateral lower-limb amputation  despite being younger and our use of the mFSST;  differences might be attributed to multi-region, lower-limb involvement with AMC.
Our between-days, test–retest reliability findings for performance-based tests are generally similar to reports among other patient populations [23, 24, 28, 29, 34,35,36,37, 39, 40, 51, 52]. For example, among adolescents and young adults with hip dysplasia and controls (n = 34), Scott et al. reported self-selected gait speed test–retest reliability over 10 timed meters of a 14-m course (ICC2,1 = 0.93; 95%CI: 0.87-0.96),  similar to our 10mWT reliability results (ICC3,k = 0.95; 95%CI: 0.90-0.97). Among children with neurological conditions, Graser et al. also reported similar between-days reliability using 10-timed meters over a 14-m course (ICC2,1 = 0.90; 95%CI: 0.80-0.95).  A lower MDC95, i.e., 0.13 m/s in our study and 0.18 m/sec in another study,  as compared to 0.35 m/sec in the Scott et al. study,  may be due to trial averaging.
Between-days test–retest reliability for the F8WT, performed at a given individual’s self-selected speed, has been reported by Hess et al. for older adults (n = 18; time: ICC = 0.84; 95%CI: 0.62-0.94; number of steps: ICC = 0.82; 95%CI: 0.59-0.93)  and among individuals post-stroke (n = 35; time: ICC2,1 = 0.98; 95%CI: 0.96-0.99) . Better F8WT reliability among our participants with AMC (time: ICC3,k = 0.96; 95%CI: 0.92-0.98; steps: ICC = 0.91; 95%CI: 0.82-0.95) compared to the Hess et al. study  may be secondary to our larger sample size and averaging two trials. For F8WT at fast speed using 2 loops, among older women, Jarnlo and Nordell reported test–retest reliability comparable to our study (n = 30; ICC3,1 = 0.93; 95%CI: 0.85-0.97) . To our knowledge, comparative MDC values are unavailable.
The timed 360TT has published between-days test–retest reliability among individuals with Multiple Sclerosis (n = 61; ICC2,2 = 0.91-0.96; 95%CI: 0.86-0.97; MDC95 = 1.5 s), . Parkinson’s Disease (n = 14; ICC = 0.80; lower bound of 95%CI: 0.66),  and post-stroke (n = 37; ICC3,2 = 0.82-0.95; 95%CI: 0.66-0.98; MDC95 = 0.8–1.2 s) . Between-days test–retest reliability for number of steps is reported in Parkinson’s Disease (n = 14; ICC = 0.77, lower bound of 95%CI: 0.61) (34) and among older adults (ICC = 0.92) . We report similarly good-to-excellent reliability for 360TT time (ICC3,k = 0.97, 95%CI: 0.93-0.98) and number of steps (ICC3,k = 0.85; 95%CI: 0.70-0.92), but a lower MDC95 for 360TT time (i.e., 0.5 s), suggesting changes in turning speed may be more easily identified among individuals with AMC as compared those with other neurological conditions.
While many studies report FSST or mFSST within-day test–retest reliability, only a few report between-days test–retest reliability, [23, 39, 40, 51] which better parallels ‘evaluations’ and ‘re-evaluations’ in clinical practice, upon which patient improvements are determined. Among children with neurological conditions (n = 30), FSST between-days, test–retest reliability (ICC1,1 = 0.54-0.89; 95%CI: 0.24-0.95) is reported . Among adolescents and young adults with hip dysplasia and controls (n = 34) and adults post-stroke (n = 17), between-days, test–retest reliability for average FSST performance (ICC2,1 = 0.93; 95%CI: 0.87-0.96; MDC95 = 1.66 s) and best mFSST performance (ICC3,1 = 0.90; 95%CI: 0.68-0.97) are reported [23, 39]. Between-days, test–retest reliability for best FSST performance is also reported among adults with unilateral lower-limb amputation (n = 60; ICC2,1 = 0.97; 95%CI: 0.94-0.98; MDC90 = 2.0 s) . Our mFSST reliability (ICC = 0.87-0.94; 95%CI: 0.71-0.98) was comparable to aforementioned adult studies; (23, 39, 51). MDCs, i.e., 2.70–4.46 s, were less than those reported among children with neurological conditions, i.e., 5.29 s. .