Brachial plexus birth injury and cerebral palsy lead to a common contracture phenotype characterized by reduced functional muscle length and strength.

The study included only ten participants, making the results preliminary and subject to high variability. The study design is cross-sectional, measuring the phenotype after contractures had already formed, so it cannot establish cause and effect. Elbow extension strength was not analyzed because gravity confounded the measurements due to limited shoulder mobility in the subjects. The unaffected contralateral limb was used as a control, but subtle motor deficits in these limbs have been noted in previous literature, though they were not contractured in this study. Sarcomere length is an indirect measure of functional muscle length; the true number of sarcomeres in series cannot be measured directly in whole muscles in vivo.

The authors declare no competing interests. The study was supported by the National Institutes of Health.

Do not interpret these findings as proof that current treatments like botulinum toxin are ineffective. The study describes the underlying muscle phenotype but does not test any intervention. It suggests that treatments targeting muscle length may be important, but it does not demonstrate that changing the treatment focus improves patient outcomes.