Purpose To quantitatively describe passive lower extremity range of motion in participants with spinal muscular atrophy (SMA) types 2 and 3, and to establish preliminary thresholds to identify individuals at risk for performing poorly on disease-specific motor function outcome measures. Methods Eighty participants with SMA types 2 and 3, enrolled in an international multicenter natural history study, were evaluated with lower extremity range of motion testing and the Hammersmith Functional Motor Scale-Expanded. Results A hip extension joint angle of -7.5° or less for SMA type 2 and 0° or less for SMA type 3 identified diminished motor ability with good sensitivity. For knee extension, a joint angle of -9.0° or less for SMA type 2 or 0° or less for SMA type 3 was similarly sensitive. Conclusions Minimal hip and knee joint contractures were associated with diminished motor ability. Clinical trial designs should consider the effect of contractures on motor function.
