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

Despite persistent motor impairments in the paretic hindlimbs of rats with chronic stroke, key structural muscle properties such as sarcomere length, dispersion, fascicle length, and serial sarcomere number remain preserved, suggesting that adult-onset neural injury alone does not necessarily induce substantial skeletal muscle structural adaptations.

Original authors: Ross, S. A., Dorscher, S. N., Leonard, T. R., Seerattan, R. A., Corbett, D., Herzog, W.

Published 2026-09-15

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CC BY 4.0

Original authors: Ross, S. A., Dorscher, S. N., Leonard, T. R., Seerattan, R. A., Corbett, D., Herzog, W.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

When a person suffers a stroke, the damage occurs in the brain, disrupting the signals that tell muscles how to move. This often leads to weakness, stiffness, and a loss of coordination that can last for years. For decades, scientists have focused on the brain's role in these lasting problems, assuming the muscles themselves are simply passive tools that fail because they are not being commanded correctly. However, muscle is a living tissue that constantly changes its shape and structure in response to how it is used. If a muscle is not stretched or used normally for a long time, it can physically remodel itself, becoming shorter or stiffer. This raises a critical question: does the long-term paralysis caused by a stroke actually change the fundamental building blocks of the muscle, or is the problem purely a matter of the brain's broken wiring?

To answer this, researchers set out to examine the microscopic architecture of muscles in rats that had experienced a stroke. They focused on two specific measurements: the length of the muscle fibers and the length of the tiny, repeating units inside them called sarcomeres. Think of a sarcomere as a single link in a long chain; the number of these links determines how long the muscle can stretch. In conditions like cerebral palsy, where brain injury happens while a child is still growing, these chains often fail to add enough links, leaving the muscle short and tight. The researchers wanted to know if a similar thing happens in adult animals after a stroke, where the body has already finished growing.

The team worked with twenty-four female rats, inducing a stroke in half of them using a precise method that blocked blood flow to a specific part of the brain controlling the hind legs. The other half underwent a similar procedure without the blockage, serving as a control group. Over the next four and a half months, the animals were tested on a narrow, tapered beam to see how well they could walk without falling. The rats with strokes consistently struggled with the task, stumbling far more often with their affected leg than with their healthy one, confirming that the motor impairment was real and persistent.

Once the recovery period ended, the researchers carefully harvested the leg muscles from both sides of the rats. They isolated bundles of muscle fibers and measured their length directly under a microscope. To see the sarcomeres, they used a laser that bounced off the muscle fibers, creating a pattern that revealed the size of each microscopic link. They also counted how many of these links were arranged in a row along the length of the fiber. The results were surprising. Despite the rats' legs being paralyzed and their walking ability remaining impaired for months, the length of the muscle fibers and the size of the individual sarcomeres were exactly the same in the injured legs as they were in the healthy legs. However, the number of sarcomeres in a row was slightly higher in the stroke group compared to the control group, though this difference was present in both the injured and uninjured legs, suggesting it was a general group characteristic rather than a result of the specific paralysis.

The study did find that the rats with strokes had slightly longer muscle fibers and more sarcomeres in a row compared to the control rats, but this difference was present in both the injured and uninjured legs. This suggests the change was likely due to the general condition of the stroke group rather than the specific paralysis of one limb. The researchers concluded that the persistent weakness and poor coordination seen after a stroke are not caused by the muscle fibers themselves shrinking or losing their structural links. Instead, the problem appears to remain rooted in the nervous system's inability to control the muscle, rather than a physical shortening of the muscle tissue. This distinction is important because it implies that the muscle retains its full potential length and structure, even when it is not being used properly, suggesting that the barriers to recovery may be different than previously thought.

Problem
Stroke is a leading cause of long-term motor disability, characterized by persistent impairments in strength, coordination, and locomotor function. While these deficits are commonly attributed to altered central nervous system control, skeletal muscle is an adaptable tissue that remodels in response to changes in neural activation, mechanical loading, and muscle length. Evidence from pediatric conditions like cerebral palsy (CP) suggests that chronic alterations in neural activation and the mechanical environment can induce structural remodeling of skeletal muscle, specifically characterized by increased sarcomere length and fewer sarcomeres in series due to impaired sarcomerogenesis during growth. However, it remains unclear whether similar longitudinal muscle adaptations occur following adult-onset stroke, where the musculoskeletal system is already mature. Previous human studies using ultrasound have reported shorter fascicles and fewer sarcomeres in series in paretic limbs, but these findings are limited by imaging constraints and the difficulty of distinguishing true structural remodeling from measurement artifacts or passive mechanical conditions.

Methodology
This study utilized a photothrombotic stroke model in adult female Sprague-Dawley rats to investigate whether chronic stroke alters sarcomere length, sarcomere length dispersion, fascicle length, or serial sarcomere number.

Subjects and Induction: Twenty-four 14-week-old rats were randomized into stroke ( $n=17$ ) and sham ( $n=7$ ) groups. Stroke was induced via unilateral photothrombosis targeting the hindlimb sensorimotor cortex. Two animals in the stroke group were excluded due to lack of visible lesions, leaving 15 stroke and 7 sham animals for analysis.
Motor Assessment: Limb-specific motor impairment was assessed longitudinally using a tapered beam traversal task. Misstep rates were recorded weekly for 17 weeks (4.5 months) post-surgery to confirm persistent impairment.
Tissue Harvest and Histology: At 18 weeks post-surgery, animals were euthanized. Brains were sectioned and stained with Nissl stain to confirm lesion location. The lateral gastrocnemius (LG) and extensor digitorum longus (EDL) muscles were harvested from both paretic and non-paretic limbs.
Muscle Preparation: Muscles were fixed, treated with nitric acid to digest connective tissue, and transferred to glycerol. Fascicle bundles were teased from the muscles, ensuring selection from consistent anatomical locations.
Measurements:
- Fascicle Length: Measured directly from isolated fascicle bundles using microscopy and custom Mathematica code to digitize fascicle trajectories and calculate arc length.
- Sarcomere Length and Dispersion: Quantified using laser diffraction at five locations along each fascicle bundle. Median sarcomere length was calculated from the diffraction pattern spacing, and dispersion was determined from the width of the first-order peaks.
- Serial Sarcomere Number: Calculated by dividing the measured fascicle length by the median sarcomere length.
Statistical Analysis: Linear mixed-effects models were used to analyze beam traversal performance, sarcomere properties, and fascicle metrics, accounting for group (stroke vs. sham), side (paretic vs. non-paretic), muscle type (LG vs. EDL), and their interactions.

Key Contributions and Results
The study provides direct, quantitative measurements of sarcomere and fascicle architecture in a controlled animal model of chronic stroke, addressing limitations of previous human imaging studies.

Motor Impairment: Stroke animals exhibited persistent, limb-specific motor deficits, with significantly higher misstep rates in the paretic limb compared to the non-paretic limb and sham controls throughout the 4.5-month recovery period.
Sarcomere Structure: Contrary to hypotheses derived from CP literature, chronic stroke did not alter sarcomere length or sarcomere length dispersion. There were no significant differences in these metrics between stroke and sham groups, nor between paretic and non-paretic limbs.
Fascicle Length and Serial Sarcomere Number: The study found that fascicle length and serial sarcomere number were significantly greater in stroke animals compared to sham animals. However, these differences were not limb-specific (i.e., they occurred in both paretic and non-paretic limbs). Consequently, the authors conclude these differences likely reflect a general group effect unrelated to the stroke-induced neural injury, rather than a specific adaptation to paresis.
Muscle Differences: Significant differences were observed between the LG and EDL muscles in all measured parameters (fascicle length, sarcomere length, dispersion, and serial sarcomere number), consistent with known architectural differences and relative muscle lengths at the fixed joint angles.

Significance
The paper claims that persistent neural impairment following adult-onset stroke does not necessarily result in substantial changes in sarcomere length, sarcomere dispersion, fascicle length, or serial sarcomere number in mature skeletal muscle. This finding contrasts with the structural adaptations seen in developmental conditions like CP, suggesting that the timing of neural injury relative to longitudinal muscle growth is a critical factor. The authors propose that impaired sarcomerogenesis may require neural injury to occur during periods of active skeletal growth, whereas adult-onset stroke occurs after the musculoskeletal system has largely matured.

Furthermore, the preservation of sarcomere structure despite persistent motor deficits suggests that stroke-related muscle adaptations may occur at other structural or functional scales not captured by these metrics, such as changes in muscle volume, fiber-type composition, or intramuscular fat. The study highlights the importance of considering the timing, severity, and mechanical context of neural injury when interpreting skeletal muscle adaptations, and suggests that the absence of sarcomere remodeling in adult stroke models may reflect the specific constraints of the adult neuromuscular system rather than a lack of plasticity.

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