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Predictors of physical strain of walking in adults with spastic cerebral palsy - A cross-sectional study

This cross-sectional study of 89 ambulant adults with spastic cerebral palsy identifies older age and greater spasticity as the independent predictors of higher physical strain during walking, suggesting that clinical interventions should prioritize managing spasticity and preserving aerobic capacity to reduce physiological demands.

Original authors: Sandra Linnea Klund-Hansen, Terje Gjøvaag, Arve Opheim, Eivind Lundgaard, Espen Ingvald Bengtson, Linda Rennie

Published 2026-07-06

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CC BY 4.0

Original authors: Sandra Linnea Klund-Hansen, Terje Gjøvaag, Arve Opheim, Eivind Lundgaard, Espen Ingvald Bengtson, Linda Rennie

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: The "Fuel Tank" vs. The "Engine"

Imagine your body is a car. You have a fuel tank (your maximum aerobic capacity, or how much energy you can use) and an engine (your muscles and walking style).

For most people, walking is like driving on a flat highway at a slow speed. It uses very little fuel, leaving plenty of gas in the tank for other things.

But for adults with spastic cerebral palsy (CP), walking is often like driving a heavy truck up a steep, bumpy hill. It requires a huge amount of fuel just to keep moving. This study looked at Physical Strain, which is a fancy way of asking: "How full is the gas tank when you are just walking to the store?"

If you use 90% of your total energy just to walk, you are under high "physical strain." If you only use 30%, you have plenty of energy left over. The researchers wanted to know: What makes the engine work harder than it needs to?

The Experiment: A Lab Full of Walkers

The researchers gathered 89 adults with spastic CP who could walk (with or without a cane). They put them through a rigorous test:

The Max Test: They had the adults run on a treadmill until they were completely out of breath to measure their "total fuel tank" size.
The Walk Test: They had them walk at their normal, comfortable speed while measuring exactly how much oxygen they burned.
The Gait Scan: They used high-tech cameras and force plates to film how the adults walked, looking at their balance, muscle power, and how "straight" their walking pattern was compared to a healthy person.
The Muscle Check: Doctors checked how stiff the muscles were (spasticity) and how well the adults could control specific muscles (like lifting just the big toe without moving the rest of the foot).

The Findings: What Actually Drives the Strain?

The researchers expected many things to cause high strain, such as bad walking patterns, weak ankles, or poor muscle control. They ran the numbers to see what truly mattered.

The Surprising Truth:
When they looked at all the factors together, only two things were the main culprits making walking feel so exhausting:

Getting Older: As adults with CP get older, their "fuel tank" (max energy) naturally gets smaller, just like it does for everyone else. But because their walking is already hard work, this shrinkage makes the strain feel much heavier. It's like trying to run a marathon with a smaller gas tank; you run out of energy much faster.
Muscle Stiffness (Spasticity): The stiffer the muscles are, the harder the engine has to work. Even though the doctors measured stiffness while the patients were lying down (not walking), it turned out to be a strong predictor of how hard they had to work while walking.

The "Red Herring" Factors:
The study found that things like how weird the walking pattern looked (Gait Deviation Index), how much power the ankle pushed off with, and how well they could control their muscles did seem to matter when looked at alone. However, once the researchers accounted for age and stiffness, these factors stopped being independent predictors.

The Analogy:
Think of it like a leaky boat.

Age is the hole in the hull getting bigger.
Stiffness is the water rushing in faster.
Bad walking patterns are like rowing awkwardly.

The study found that even if you row perfectly (good walking pattern), if your hull has a big hole (age) and water is pouring in fast (stiffness), the boat will still sink (high strain). The awkward rowing wasn't the main reason the boat was sinking once the hole and the water were taken into account.

What This Means for Treatment

The paper concludes that if we want to help adults with CP walk with less effort, we should focus on two main levers:

Managing Stiffness: Treatments that reduce muscle tightness could lower the energy cost of walking.
Protecting the Fuel Tank: Keeping the heart and lungs strong (aerobic capacity) is crucial, especially as people get older, to prevent that "fuel tank" from shrinking too fast.

The authors note that while the effect of these factors wasn't massive in numbers, for someone who is already walking on the edge of their limits, even a tiny improvement could make a huge difference in their daily life.

What the Study Did Not Say

It did not say that fixing walking patterns (like making the foot land straighter) is useless, but it found that in this specific group, those patterns weren't the primary independent cause of the strain once age and stiffness were considered.
It did not propose a new surgery or drug. It simply identified which factors are the strongest predictors of fatigue.
It did not test children; it focused strictly on adults.

In short: Age and muscle stiffness are the main reasons walking feels so exhausting for adults with CP, more so than the specific way they move their legs.

Technical Summary: Predictors of Physical Strain of Walking in Adults with Spastic Cerebral Palsy

Problem Statement
Adults with spastic cerebral palsy (CP) frequently experience elevated physiological demands during walking, leading to early fatigue and reduced participation in daily activities. "Physical strain" is defined as the oxygen consumption required to walk at a preferred speed, expressed as a percentage of the individual's maximal aerobic capacity (%VO2peak). While factors influencing the oxygen cost of walking (VO2 per meter) have been studied in children with CP and healthy older adults—specifically impaired selective motor control, increased spasticity, gait deviations, and reduced ankle push-off power—it remains unclear whether these same factors independently predict physical strain in ambulant adults with CP. This distinction is critical because physical strain reflects the relative effort against an individual's maximal capacity, which naturally declines with age.

Methodology
This cross-sectional study analyzed data from 89 ambulant adults (49 female) with spastic CP, classified as Gross Motor Function Classification System (GMFCS) levels I–III. Participants were recruited from a specialized rehabilitation hospital, public announcements, and previous studies.

Physiological Measurements: Participants underwent cardiopulmonary exercise testing (CPET) on a treadmill to measure maximal aerobic capacity (VO2peak) and oxygen consumption at preferred walking speed (VO2walk). Physical strain was calculated as $(VO2_{walk} / VO2_{peak}) \times 100$ .
Biomechanical Analysis: Three-dimensional gait analysis was performed using an eight-camera motion capture system and force plates. Key metrics included the Gait Deviation Index (GDI), peak ankle power, and mean support moment.
Clinical Assessments: Neuromotor impairments were quantified using the Modified Ashworth Scale (MAS) composite score for spasticity and the Selective Control Assessment of the Lower Extremity (SCALE) for selective motor control.
Statistical Analysis: Bivariate linear regressions examined associations between potential predictors and physical strain. A multiple linear regression model was then constructed to identify independent predictors, controlling for sex, age, GMFCS level, and the aforementioned biomechanical and clinical variables.

Key Results

Bivariate Analysis: Unadjusted analyses revealed significant associations between physical strain and age, GMFCS level, spasticity (MAS), GDI, and peak ankle power (all $p < 0.001$ ).
Multiple Regression Model: When controlling for all variables, only age and spasticity remained independent predictors of higher physical strain.
- Age: Showed a positive association ( $B = 0.320$ , $p < 0.001$ ), indicating that physical strain increases with age.
- Spasticity: The MAS composite score was a significant predictor ( $B = 1.475$ , $p = 0.048$ ).
- Non-Predictors: Gait deviations (GDI), peak ankle power, mean support moment, and selective motor control (SCALE) did not independently predict physical strain after adjustment.
Model Performance: The final model explained approximately 30% of the variance in physical strain ( $R^2_{adjusted} = 0.307$ ).

Significance and Claims
The study concludes that in ambulant adults with spastic CP, the determinants of physical strain differ from those of the absolute oxygen cost of walking found in children or healthy elderly populations. While gait biomechanics (such as deviations and ankle power) correlate with strain in isolation, they do not independently predict it when age and spasticity are accounted for.

The authors claim that age and spasticity are the primary independent drivers of increased physical strain. The effect sizes are described as moderate; however, the authors emphasize that in a population where walking already occurs near maximal capacity, even small increases in physical strain are clinically significant.

Consequently, the paper posits that interventions aimed at reducing physical strain should prioritize:

Targeting Spasticity: Addressing spasticity to reduce the neuromuscular constraints on walking.
Preserving Aerobic Capacity: Mitigating age-related declines in VO2peak.

The authors caution that interventions focusing solely on isolated gait features (e.g., correcting specific joint kinematics) may have limited impact on overall physical strain, as these factors do not independently drive the relative physiological demand in this specific adult population. The study also notes the lack of established benchmarks for minimal clinically important differences in physical strain for adults with CP, highlighting a need for future research to define these thresholds.