Michael H. Dryzer, Bardia Nourbakhsh, Jennifer Keller, Vikram S. Chib
5 min
Individuals with multiple sclerosis (MS) frequently report fatigue and motor impairments that impact their daily lives, yet the underlying mechanisms of how they perceive and assess physical effort remain poorly understood. This study investigated whether increased motor variability in MS patients contributes to distorted judgments of physical exertion.
The researchers recruited 29 individuals with MS and 22 age-matched healthy controls. Participants performed a grip-force task using a hand-clench dynamometer. In an initial association phase, they learned to map specific force levels to effort units. In the subsequent assessment phase, participants exerted varying levels of force and were then asked to retrospectively estimate the effort they had just produced. The researchers used hierarchical linear modeling to analyze the relationship between trial-to-trial exertion variability, actual mean exertion, and the accuracy of effort assessments, while also accounting for disease severity using the Expanded Disability Status Scale (EDSS).
The study found that individuals with MS exhibited significantly higher exertion variability than healthy controls. This increased variability was directly associated with greater errors in effort assessment. Specifically, MS patients tended to overreport their levels of exertion. This effect was more pronounced in patients with higher disease severity, particularly those with greater deficits in the pyramidal functional system, which is associated with muscle weakness and limb mobility. These results suggest that motor variability acts as a mechanism that inflates the perceived cost of physical activity in MS.
These findings provide a potential explanation for why physical effort often feels disproportionately costly to individuals with MS. By identifying a link between motor performance and subjective effort perception, this research highlights a specific target for rehabilitation. Clinical interventions that focus on improving motor consistency or enhancing interoceptive awareness—the ability to accurately perceive internal bodily states—could help patients form more accurate assessments of their physical capabilities, potentially improving their engagement in daily physical activities.
Accurate assessment of exertion is crucial for determining whether to continue or rest during physical activity. Individuals with multiple sclerosis (MS) often report fatigue and motor impairments, yet the mechanisms underlying their assessments of effortful exertion remain poorly understood. Recent work with healthy individuals suggests that motor variability can distort judgments of effort; however, this relationship has not been explored in individuals with MS. In this study, we investigated how variability in physical exertion affects effort assessment in individuals with MS compared to healthy participants. We had participants exert varying levels of physical effort and retrospectively assess the effort they exerted. Individuals with MS exhibited increased exertion variability and tended to overreport their levels of exertion compared to healthy participants. Increased exertion variability was associated with less accurate effort assessment, and this effect was more pronounced in individuals with more advanced MS. These results suggest a possible mechanism through which motor variability may be associated with inflated perceptions of effort in MS, highlighting a potential account of why efforts feel particularly costly to those individuals living with MS and identifying a promising target for treatment and rehabilitation.
Alex: [reflective] So it's not that MS patients are globally worse at self-assessment — it's that damage to a specific motor pathway degrades the signal quality the brain is working from.
Sam: [quiet conviction] That's the argument. And it reframes what fatigue actually is in this population. The "tax" on daily activity isn't just the physical work — it's the cognitive overhead of managing a noisy signal. The brain is constantly trying to reconcile what it intended to do with what it actually did, and when that feedback is unreliable, it errs on the side of overestimating cost. [[RP_SECTION:rehabilitation-and-future-research|Rehabilitation and Future Research]]
Alex: [leaning in] Which has real implications for how you'd design a rehabilitation target. If the problem is perceptual bias rooted in motor instability, then improving metabolic capacity alone won't fix it.
Sam: [grounded] Exactly. The paper points toward motor precision as the intervention lever — if you can tighten the variability of force output, you may directly reduce the perceived effort cost, independent of any change in actual muscle strength or endurance. That's a meaningful shift from the standard metabolic framing. [[RP_SECTION:limitations-and-critique|Limitations and Critique]]
Alex: [thoughtful] Where would a careful referee push back on this?
Sam: [candid] A few places. The task is a simple isometric hand clench — it's well-controlled, but it's a long way from the kinds of whole-body, sustained activities that MS patients actually report as fatiguing. Whether the coefficient of variation on a grip task generalizes to walking, dressing, or climbing stairs is an open question. The sample is also relatively small, and the cross-sectional design means you can't rule out that patients with higher variability also have other correlated deficits — attentional load, for instance — that are doing some of the explanatory work. The EDSS is also a fairly coarse clinical measure; the pyramidal subscale has better resolution, but it's still an imperfect proxy for tract integrity.
Alex: [nodding] So the mechanism is plausible and the within-study evidence is internally consistent, but the causal chain from motor noise to perceived fatigue in ecologically valid settings still needs longitudinal and intervention work to close.
Sam: [steady] That's a fair read of where it stands. What makes the paper worth attention is the precision of the hypothesis and the fact that the pyramidal specificity finding gives you a testable, anatomically grounded target. It's not just "MS patients feel more tired" — it's a specific claim about why, with a mechanism that points toward what to measure and potentially what to treat. That's a more tractable starting point than the metabolic account has offered.
Alex: [settling] A signal-processing framing for a condition that's spent decades being described in energy terms. Thanks for walking through it.
Sam: Thanks for listening to ResearchPod.