Dynamic Balance in Spinal and Bulbar Muscular Atrophy: Relationship between Strength and Performance of Forward Lunge, Step Up and Over, and Step Quick Turn

Abstract

Introduction. Spinal and bulbar muscular atrophy (SBMA) is a neuromuscular disorder that leads to progressive weakness of bulbar and extremity muscles. Dynamic balance during functional tasks has not been reported in people with SBMA. Objectives. (1) To evaluate the ability to safely complete a forward lunge (FL), step quick turn (SQT), and step up and over (SUO), (2) to determine the presence and severity of dynamic balance impairments by comparing performance to normative data, and (3) to investigate the relationship between lower extremity strength and ability to complete each task. Design. Cross-sectional analysis. Participants. Fifty-three people with SBMA were included in a cross-sectional analysis. Normative datasets provided by the NeuroCom manufacturer and isometric strength literature facilitated patient comparisons. Outcome Measures. Force plate-based dynamic balance measures included FL (distance, impact index, contact time, and force impulse), SQT (turn time and turn sway), and SUO (lift up index, movement time, and impact index). Maximal isometric contractions of knee extensors, ankle dorsiflexors, ankle plantar flexors, and hip extensors were measured with fixed frame dynamometry. Results. The most difficult test, per completion rate, was SUO (52%), followed by FL (57%) and SQT (65%). $t$ -tests revealed significant abnormalities in eight of nine balance variables ( $p<0.05$ ) accompanied by large $\text{Cohe}{\text{n}}^{\text{’}}\text{s}D\text{effect}\text{sizes}\ge 0.8$ . Receiver operating characteristics analysis showed knee extensor (SUO 95% CI =0.78–1.00, SQT 95% CI =0.64-0.92) and ankle plantar flexor strength (SUO $95\%\text{CI}=0.75–0.99$ , SQT $95\%\text{CI}=0.64-0.92$ ) significantly discriminated the ability to perform SUO and SQT tests with acceptable to excellent areas under the curve. Conclusions. Considerable dynamic balance abnormalities were observed. Lower extremity strength helps explain low test completion rates. Patients modified task movement patterns, enabling safe task performance. Study results can help direct patient care and future protocol design for people with SBMA.