An efficient postural control needs functional motor skills, such as mobility over stability, i.e., the ability to move parts of the body while maintaining a stable base of support. Spinocerebellar ataxia (SCA) is a condition in which postural instability is usually the first and most frequent symptom with a huge impact of independence. The instruments currently available to assess postural control are focused on tasks with static or dynamic postures. There are rare tests for mobility over stability such as the Functional Reach Test and some items of the Berg Balance Scale (BBS).

This study proposes the Mobility Over Stability Test (MOST) to fill the gap in literature with a specific test for this functional motor skill.

MOST was composed by four tasks performed in standing position, with feet together, in this sequence: (1) looking up and down, (2) right and left, (3) upper limbs flexion, and (4) turning the trunk to look back at an object 1.5m behind the participant, both sides. In tasks 1, 2 and 4, the arms remain crossed over the trunk. Each block of tasks was repeated 3 times. Stability during the task’s execution was assessed considering the level of postural displacement, recorded by an accelerometer attached to a smartphone fixed at L4-L5. The extent of instability was measured by counting how often the person lost balance, which was reflected in the number of times they required assistance from the evaluator to regain stability. This test was carried out with 31 individuals with SCA and 14 healthy individuals matched for gender, age, and body mass. Individuals with SCA were assessed using the Scale for the Assessment and Rating of Ataxia (SARA), Inventory of Non-Ataxia Signs (INAS) and BBS. The association between the data generated by MOST and the other instruments was evaluated.

31 participants with SCA were assessed. Of these, 21 did not need any additional support to complete the test, while 10 needed one or more touches. The maximum duration of the test was 35 seconds. There was no significative association between performance on the MOST and scores on the SARA, INAS and BBS instruments. Despite this, the correlations between the scores of the instruments and those of the MOST were strong. In the accelerometry analysis, individuals with SCA had a shorter test duration compared to controls, suggesting greater postural instability.

The MOST proved to be a quick-to-apply, easy-to-access and easy-to-use test. The data indicate that, although there is a difference in MOST performance between healthy people and people with SCA, the clinical and postural displacement variables did not significantly explain these differences. Studies with a larger sample size and greater variability in clinical presentations will be necessary to confirm the usefulness of the MOST.

These results indicate that MOST can be an important tool in assessing and monitoring the evolution of stability and the degree of imbalance in individuals with SCA, even in mild levels of ataxia severity, and can also be used to prescribe exercises.