Summary: For many with incomplete spinal cord injuries (SCI), the ability to walk doesn’t mean a return to normal function. Simple acts like standing still or pushing with steady force remain incredibly difficult. A new study explains why.
Using non-invasive skin sensors, researchers discovered that SCI fundamentally breaks how the nervous system coordinates motor units. At low effort, the brain fails to synchronize muscle fibers, leading to shakiness. At high effort, it overcompensates with “loud,” rigid signals that strip away precision.
Key Facts
- The Coordination Breakdown: In healthy individuals, motor units (the nerve-to-muscle connections) act like an orchestra following a conductor. In SCI patients, this “shared signal” is disrupted.
- Low-Effort Shakiness: At 20% effort, SCI patients showed significantly less coordination between calf muscles compared to healthy controls, resulting in unstable, shaky movements.
- High-Effort “Loudness”: At 50% effort, the nervous system sends “louder,” less refined signals. This creates a rigid, over-synchronized response that lacks the flexibility needed for precise control.
- Loss of Strategic Flexibility: A healthy nervous system changes its control strategy as force demands increase. After an SCI, the nervous system becomes “rigid,” unable to adapt its approach as muscles work harder.
- New Biomarker: This discovery of “neural drive” patterns could serve as a new rehabilitation biomarker, allowing clinicians to design therapies that “re-tune” the spinal cord’s output.
Source: KTH
Even when people with incomplete spinal cord injuries can walk, everyday functions like standing, balancing or producing steady force may remain difficult. A new study shows why.
Using surface skin electrical sensors, a research team in Sweden identified previously unseen changes in motor coordination that result from incomplete spinal cord injuries (SCI). The study is the first to examine how individual motor units—that is, nerve‑to‑muscle connections that create movement—work together in people with SCI.
“Our study reveals, at the cellular level, how the central nervous system adapts to the injury to control movement,” says Ruoli Wang, associate professor in biomechanics at Promobilia MoveAbility lab, KTH Royal Institute of Technology. She says the researchers’ approach was completely non-invasive.
The results were published in the Journal of NeuroEngineering and Rehabilitation.
The study’s lead author, PhD student Zhihao Duan, says the researchers found the nervous system struggles to spread signals smoothly across muscles at low levels of exertion after the injury. And it appears to overcompensate at higher levels of exertion, sending “louder”, less refined signals.
A single muscle moves through hundreds of motor units, each turning on and off precisely to create smooth force. Composed of a single motor neuron and its connecting muscle fibers, these motor units respond to shared signals from the nervous system, much like different sections of musicians led by an orchestra conductor. That shared input is what allows them to act in coordinated patterns.
To explore how well these units coordinate under the control of the central nervous system the team examined 25 people (including 10 control participants). They used high density electromyography (HD-EMG) to measure electrical activity in the functionally similar calf muscles—soleus and gastrocnemius—while volunteers pushed lightly or moderately against a force measurement device.
Duan says that at 20 percent effort, fewer of the motor units in the two calf muscles were working in a shared, coordinated way compared with people without injury. Their movements as a result were shaky and unstable. “They were much less being driven by the same coordinated signal from the nervous system.” he says.
At a higher level of effort—50 percent—the SCI group showed stronger low‑frequency synchronization between the two muscles. The body loses flexibility and precision in control of the movement. “This could be a sign of the nervous system compensating by sending louder, less refined signals,” Duan says.
“One interesting finding is that after spinal cord injury the nervous system becomes more rigid and less able to change its approach as the muscles work harder. A healthy nervous system on the other hand is able to adapt its strategy as force demands, to adjust the shared neural drive level,” Wang says.
Although the study was limited by a small sample size and challenges in identifying enough motor units per muscle from the skin surface, Wang says the results offer unique insight into how SCI reshapes motor control.
“This finding may open the door to a new rehabilitation biomarker, helping clinicians and researchers design new neurorehabilitation strategies to re-tune the spinal cord control and to restore coordinated neural input,” she says.
Funding: The study was a collaboration with Aleris Rehab Station and was funded by the Swedish Research Council and Promobilia Foundation.
Key Questions Answered:
A: Walking uses a lot of momentum and “big” muscle movements. Standing still or balancing requires “low-level” precision. The study found that at low effort (20%), the nervous system can’t get the small motor units to work together smoothly, making fine-tuned stability nearly impossible.
A: Think of it like a radio. A healthy brain can whisper or shout depending on the task. After an SCI, when the brain tries to push harder (50% effort), it loses that volume knob. It essentially blasts a single, unrefined “GO” signal that makes the muscles act like a rigid block rather than a precise tool.
A: Currently, therapy often focuses on building muscle strength. This research suggests we need to focus on “re-tuning” the signal. By using these new electrical biomarkers, doctors can create specific exercises or electrical stimulation programs that help the nervous system learn how to “conduct the orchestra” smoothly again.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by our staff.
About this SCI and neurology research news
Author: David Callahan
Source: KTH
Contact: David Callahan – KTH
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Adaptation of motor unit synergies in the synergetic ankle plantarflexors in ambulatory persons with incomplete spinal cord injury” by Zhihao Duan, Asta Kizyte, Emelie Butler Forslund, Elena M. Gutierrez-Farewik, Pawel Herman & Ruoli Wang. Journal of NeuroEngineering and Rehabilitation
DOI:10.1186/s12984-026-01874-2
Abstract
Adaptation of motor unit synergies in the synergetic ankle plantarflexors in ambulatory persons with incomplete spinal cord injury
Background
Spinal cord injury (SCI) often results in impaired motor control and coordination. Previous studies have highlighted the role of muscle synergies in coordinating motor tasks and their alterations following SCI. However, the adaptation in muscle synergy patterns at the motor unit (MU) level after SCI remains unexplored. This study aimed to investigate MU synergies and clustering in the synergetic soleus and gastrocnemius medialis (GM) muscles and to explore how these patterns are altered in persons with SCI.
Methods
High-density electromyography (HD-EMG) was used to record MU activity in the soleus and GM muscles of fifteen participants with incomplete SCI and ten non-disabled participants during 20% and 50% maximal voluntary isometric contraction tasks. The HD-EMG signals were decomposed into individual MU spike trains. Inter-muscle coherence analysis was employed to evaluate the shared neural drive between the soleus and GM muscles, and factor analysis was performed to identify synergistic clusters of MUs innervating each muscle.
Results
The results showed that both participant groups demonstrated high coherence between the soleus and GM muscles, highlighting a shared neural drive for coordinated function. However, participants with SCI showed altered coherence in the delta frequency band, with significantly higher coherence observed at 50% maximal voluntary contraction (p = 0.047). Additionally, factor analysis revealed that participants with SCI had a reduced proportion of MUs in the shared cluster within the GM muscle at 20% maximal voluntary contraction (p < 0.01).
Conclusions
These findings suggested that SCI may disrupt MU synergies and clustering, potentially impairing motor coordination. This research offered valuable insights into the underlying mechanism of muscle synergies and the neural adaptations following SCI, providing crucial information for the development of future rehabilitation strategies.
