Summary: For many people with incomplete spinal cord injuries (SCI), being able to walk again does not restore normal movement control. Everyday tasks such as standing quietly, keeping balance, or producing a steady push can remain difficult. A new non‑invasive study explains why by showing how SCI alters the way motor units are coordinated by the nervous system.
Using high‑resolution surface sensors on the skin, researchers found that SCI changes the spinal and central nervous system’s ability to share and shape the neural drive that coordinates individual motor units. At low effort, this disrupted coordination makes muscles unstable and shaky. At higher effort, the nervous system appears to overcompensate by sending large, less refined signals that produce rigid, imprecise muscle responses.
Key findings
- Coordination breakdown: In healthy people, motor units—each a motor neuron and the muscle fibers it activates—receive a shared control signal that lets them work together smoothly. After incomplete SCI this shared signal is disrupted, so motor units no longer act like a well‑conducted orchestra.
- Shaky low‑force control: At about 20% of maximal voluntary effort, people with SCI showed reduced coordinated activation between the calf muscles (soleus and gastrocnemius) compared with controls. This weaker shared drive makes small, steady tasks such as quiet standing unstable.
- Loud, rigid high‑force signals: At roughly 50% effort, the SCI group displayed stronger low‑frequency synchronization between muscles. This reflects a loss of precision: the nervous system uses a “louder” signal that forces muscles to act in a more locked, less flexible way.
- Loss of adaptive strategy: A healthy nervous system adjusts how it coordinates motor units as demands change. After SCI those adaptive adjustments become limited, producing a more rigid control strategy that cannot scale smoothly with force.
- Potential new biomarker: Patterns of neural drive and motor unit synchronization revealed by surface electromyography could become biomarkers for rehabilitation. Clinicians may be able to use these signals to guide therapies that retrain the spinal cord’s coordinated output.
Source: KTH Royal Institute of Technology
Even when people with incomplete spinal cord injuries can walk, fine motor control often remains impaired.
A Swedish research team used high‑density electromyography (HD‑EMG) applied to the skin to reveal previously unseen changes in how individual motor units coordinate after incomplete SCI. This was the first study to examine motor unit synergies between functionally similar calf muscles at the level of individual motor units in ambulatory people with incomplete spinal cord injury.
“Our work shows at the cellular and motor‑unit level how the central nervous system adapts to injury when controlling movement,” says Ruoli Wang, associate professor in biomechanics at the Promobilia MoveAbility lab, KTH. The entire measurement approach was non‑invasive, using surface sensors to record the electrical activity of motor units during controlled force tasks.
The study, published in the Journal of NeuroEngineering and Rehabilitation, was led by PhD student Zhihao Duan. The team recorded motor unit activity from the soleus and gastrocnemius medialis muscles while participants produced isometric plantarflexion at two force levels: about 20% and 50% of their maximal voluntary contraction.
Motor units are the basic elements of muscle activation. Each motor neuron and the specific muscle fibers it excites fire in carefully timed patterns so that a muscle produces smooth, graded force. In healthy control participants, many motor units across the two calf muscles share common neural input, enabling coordinated, adaptable action. After incomplete SCI, that shared input was altered.
At low force (20%), fewer motor units across the two muscles were driven by the same neural input in the SCI group than in non‑injured controls. The result is reduced coordination and greater shakiness during small, steady tasks. At higher force (50%), the SCI group displayed stronger low‑frequency synchronization between muscles — a sign that the nervous system is sending broader, less differentiated commands that sacrifice finesse for brute drive.
“One striking outcome is that the injured nervous system becomes more rigid,” Wang explains. “Healthy control systems adapt their shared neural drive as force demands change. After SCI, that adaptability is diminished, limiting the nervous system’s ability to change strategy as muscles work harder.”
The authors caution that the study had a modest sample size and that surface recordings limit how many motor units can be reliably identified per muscle. Still, the findings provide unique insight into how SCI reshapes motor control and suggest measurable neural signatures that could guide rehabilitation.
“These neural‑drive patterns could serve as practical biomarkers,” Wang adds. “They point to rehabilitation strategies that go beyond building muscle strength and toward retraining the nervous system to ‘conduct the orchestra’ of motor units with more precision.”
Funding: The study was a collaboration with Aleris Rehab Station and was funded by the Swedish Research Council and the Promobilia Foundation.
Key questions answered
A: Walking relies on momentum and larger, rhythmic muscle patterns. Quiet standing and balance require precise, low‑level coordination of many small motor units. The study found that at low effort (around 20%) the shared neural drive that normally synchronizes those small motor units is weakened after SCI, making fine stability difficult.
A: It means the nervous system shifts from subtle, differentiated commands to broader, high‑amplitude signals. At higher effort levels after SCI, the system appears to “shout” one generalized command so muscles contract more uniformly and rigidly, reducing the capacity for fine adjustments.
A: Rehabilitation could expand beyond strength training to include interventions that restore coordinated neural drive. Using HD‑EMG biomarkers, clinicians might design targeted exercises or electrical stimulation protocols that help the nervous system relearn how to distribute and modulate shared signals across motor units.
Editorial notes
- This article was edited by a Neuroscience News editor.
- The journal paper was reviewed in full.
- Additional context was added by staff to clarify clinical relevance.
About this research
Author: David Callahan
Source: KTH Royal Institute of Technology
Contact: David Callahan – KTH
Image credit: Neuroscience News
Original research: Open access. Study title: “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. 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 commonly impairs motor control and coordination. Prior research has shown that muscle synergies coordinate complex motor tasks and that these synergies change after SCI. However, how patterns at the motor unit level adapt after SCI was previously unexplored. This study investigated motor unit synergies and clustering in the soleus and gastrocnemius medialis muscles and assessed how these patterns differ in people with incomplete SCI.
Methods
The research used high‑density electromyography to record motor unit activity from the soleus and gastrocnemius medialis in 15 participants with incomplete SCI and 10 non‑disabled controls. Participants performed isometric plantarflexion at approximately 20% and 50% of maximal voluntary contraction. HD‑EMG signals were decomposed into individual motor unit spike trains. Inter‑muscle coherence analysis assessed shared neural drive, and factor analysis identified clusters of synergistic motor units within each muscle.
Results
Both groups showed evidence of shared neural drive between the soleus and gastrocnemius medialis, but participants with SCI exhibited altered coherence in the low‑frequency (delta) band. At 50% contraction, coherence in that band was significantly higher in the SCI group (p = 0.047). Factor analysis also revealed a reduced share of motor units in the common cluster for the gastrocnemius medialis at 20% contraction in the SCI group (p < 0.01).
Conclusions
The findings indicate that incomplete SCI can disrupt motor unit synergies and clustering, which likely contributes to impaired motor coordination. These results provide new insights into the neural adaptations after SCI and suggest measurable neural signatures that could inform future targeted rehabilitation strategies aimed at restoring coordinated neural input.