Relationships between Lower Limb Muscle Strength Impairments and Physical Limitations in DM1

Although adult and late-onset DM1 phenotypes DM1 present distinct lower limb weaknesses portraits, resulting physical limitations have never been described separately for each phenotype.

To characterize the lower limb weaknesses and physical limitations among the DM1 adult and late-onset phenotypes separately and to document the contribution of weaknesses on mobility to optimize the management of this population.

The strength of four muscle groups among 198 participants was quantified. Participants were categorized according to the severity of their muscular involvement using the Muscular Impairment Rating Scale (MIRS). Physical limitations were assessed using the Timed up-and-go (TUG), Berg Balance Scale (BBS) and 10 meters comfortable walking speed (10MWT). Multiple linear regressions were performed to identify the contribution of each muscle group to the mobility tests scores.

Late-onset demonstrated less weakness and physical limitations (p < 0.001 - 0.002) than the adult phenotype, but 21.9-47.5% of participants with this phenotype showed mobility scores below reference values. Physical limitations were observed in the first two MIRS grades (37.5-42.1% of the participants) for the TUG and 10MWT. Ankle dorsiflexors and knee extensors were the two muscle groups that showed the strongest relationships with mobility scores.

Although less impaired, the late-onset phenotype shows significant lower limb muscle weakness associated with physical limitations. The surprising presence of quantitative lower limb muscle weakness in the first two MIRS grades needs to be considered when using this scale. Both ankle dorsiflexors and knee extensors appear to be good indicators of physical limitations in DM1.