Sarcomere length, fascicle length, and serial sarcomere number are preserved in paretic hindlimb muscles following chronic stroke in rats despite persistent motor impairment

Abstract

Stroke causes motor impairments that are commonly attributed to altered neural control, but chronic changes in neural activation and muscle use may also influence skeletal muscle structure. This study examined whether chronic stroke alters sarcomere length and dispersion, fascicle length, or serial sarcomere number in adult skeletal muscle. Twenty-four female Sprague-Dawley rats underwent photothrombotic stroke or sham surgery. Limb-specific motor impairment was assessed longitudinally using a beam traversal task. At 4.5 months post-surgery, the lateral gastrocnemius and extensor digitorum longus muscles were harvested from paretic and non-paretic limbs. We measured fascicle length directly from isolated fascicle bundles and quantified sarcomere length and dispersion using laser diffraction. Stroke animals exhibited persistent impairment of the paretic limb during beam traversal with elevated misstep rates. Despite this persistent motor impairment, sarcomere length and dispersion did not differ between stroke and sham animals or between limbs. Fascicle length and serial sarcomere number were greater in stroke than sham animals, but these differences were not limb-specific and were therefore unlikely to reflect stroke-related changes. Fascicle length, sarcomere length and dispersion, and serial sarcomere number differed between the lateral gastrocnemius and extensor digitorum longus, consistent with differences in architecture or relative length at the selected joint angles. These findings indicate that persistent neural impairment following adult-onset stroke does not necessarily result in substantial changes in sarcomere length or sarcomeres in series. Instead, sarcomere changes may depend on additional factors, including the timing of neural injury relative to growth and the mechanical environment experienced by the muscle.