Fatigable Weakness vs Functional Weakness: Why Hoover’s Sign Can Mislead When Isokinetic Testing Shows Otherwise

31 Mar 2026

I will not go into the detailed mechanics of Hoover's sign; you can read more about it here.

I am, however, more interested in understanding how clinicians determine whether this sign is appropriate in various clinical practices (the right tool for the job). In this post, we will also discuss how isokinetic testing can provide objective insights when evaluating limb weakness.

Case Context

Patient X (a 100m sprinter) initially reported decreased muscle strength. As a result, they enrolled at a high-performance centre (HPC), undergoing frequent isokinetic testing that measured various performance metrics regularly.

A few months later, the patient began struggling to close their eyes at night. Forcibly closing them caused dizziness and vertigo. One morning, the patient woke up completely off balance, resembling a middle ear infection. A GP evaluated the patient, concluded it was not a middle ear infection (despite red eardrums), and referred them to a neurologist.

Due to limited availability of appointments, exacerbated by the early COVID-19 pandemic, the next consultation was several months away. In the meantime, the patient’s balance worsened, prompting them to use a wheelchair and a cane for mobility.

As preparation, previously recorded isokinetic data from the HPC was exported for the neurologist. The data revealed that, despite rehabilitation efforts, the patient exhibited a consistent 20 to 40 percent strength deficit between the left and right sides, with the dominant right side being weaker.

Clinical Considerations

Hoover's sign is typically used when a patient presents with weakness or paralysis in one or more limbs, but involuntary activation demonstrates normal strength. A positive Hoover's sign is often interpreted as functional limb weakness.

In this case, its appropriateness is questionable, a misinterpretation of the patient’s reported symptoms (e.g. fatigability vs primary weakness) could lead to incorrect assumptions about the nature of the deficit and the choice of clinical tests:

The patient arrives in a wheelchair but is able to walk. The wheelchair is used due to balance issues, not weakness.
The patient reports fatigable weakness in the right side and provides objective isokinetic data confirming this claim.

The initial output of both legs looks normal, especially at the slower speed where force production is highest. There’s no obvious failure to generate force at the start (the actual domain where Hoover’s sign operates).

The deficit only becomes apparent once demand increases (higher speeds or repeated repetitions), where the right side drops off faster.

Overall, both legs can generate force, but the right one cannot maintain it under stress.

Actual Clinical Setting

The clinician claimed ninety-nine percent certainty of functional diagnosis before even seeing the patient.
Objective isokinetic data was dismissed as “too technical” and excluded from the assessment.
Hoover's sign was applied regardless.

Analysis

While Hoover’s sign is a commonly used bedside tool to identify functional limb weakness, its application in cases of fatigable weakness appears inappropriate.

The Knutsson & Mårtensson (1985) study used isokinetic measurements in patients with functional (then called “hysterical”) weakness. In those cases, torque during knee movements showed unpredictable patterns: large trial-to-trial variability, paradoxically higher torque at faster speeds than slower ones, and active restraint by opposing muscles. These inconsistent and contradictory features help distinguish functional weakness from true neuromuscular deficits caused by organic lesions.

Applied to this case:

The patient showed a consistent 20–40% side-to-side deficit under repeated or high-speed demands.
Initial slow-speed force was mostly preserved, with no paradoxical higher torque at fast speeds.
There was no evidence of antagonist restraint or extreme trial-to-trial variability.

This patient’s pattern does not match the inconsistent motor performance typical of functional weakness described in the study. Instead, it supports a true fatigable neuromuscular deficit.

Conclusion / Takeaway

When objective quantitative data such as isokinetic testing is available, it should be given proper consideration rather than dismissed. Hoover’s sign evaluates momentary low-demand effort and is not designed to detect endurance-related or velocity-dependent fatigability. Matching the right tool to the clinical presentation is essential to avoid misclassification.

Comments

5 months ago by Anonymous Neurologist Neurologist

Replied to Fatigable Weakness vs Functional Weakness: Why Hoover’s Sign Can Mislead When Isokinetic Testing Shows Otherwise

The reasoning you applied is flawed. The patient told the neurologist they have unilateral weakness, so assessing for Hoover’s sign is appropriate. Just because one study used isokinetic testing in FND does not mean those results have been validated or can be generalized to all patients with FND. What are the sensitivity and specificity of those isokinetic test results in FND? If that information is not available, then the findings are not clinically useful. Therefore, the absence of findings in this patient that match those of the study participants does not tell you much. The isokinetic test results do not carry much weight in this case. If the patient has a positive Hoover’s sign in the clinic, the isokinetic results are largely irrelevant. A positive Hoover’s sign indicates that the leg appears weak when the patient is asked to move it voluntarily, but strength returns when the movement becomes automatic by focusing on contralateral hip flexion against resistance. The isokinetic findings you described do not change the fact that the patient demonstrated weakness with voluntary movement but not with automatic movement during the neurologist’s physical exam. FND is a disorder of voluntary movement, and the more a person focuses on voluntarily moving a limb, the more difficult the movement becomes. It is possible that this explains the decline in strength when demand increased. Proper clinical reasoning and pattern recognition require years of education and experience. It is not a hobby that anyone can simply pick up.

5 months ago by Christoff Trüter Spousal caregiver

Replied to Fatigable Weakness vs Functional Weakness: Why Hoover’s Sign Can Mislead When Isokinetic Testing Shows Otherwise

With all due respect, I believe you may be misreading the case details or missing the nuance. The article never described initial weakness in the affected leg. It emphasised the opposite: preserved initial strength with clear fatigability under sustained or higher demand efforts. Hoover’s sign is a low demand test that only compares brief voluntary versus automatic activation. It is not designed to detect, nor can it rule out, a reproducible side specific fatigable deficit that emerges with repeated or high intensity loading. This is precisely what the isokinetic data demonstrated in this case. This pattern contrasts with the highly variable and inconsistent weakness described in the 1985 Knutsson and Martensson study on hysterical paresis. Dismissing objective quantitative measurements in favour of a single bedside sign, is exactly how misclassification occurs. When better data is available, it should take precedence over a tool that is simply not suited to evaluate endurance related or velocity dependent deficits.

5 months ago by KenV54 Neurologist

Replied to Fatigable Weakness vs Functional Weakness: Why Hoover’s Sign Can Mislead When Isokinetic Testing Shows Otherwise

Nice analysis, FNDNope, of the difference between initial weakness and fatigability of a limb, and the implications for FND.

5 months ago by Christoff Trüter Spousal caregiver

Replied to Fatigable Weakness vs Functional Weakness: Why Hoover’s Sign Can Mislead When Isokinetic Testing Shows Otherwise

Thank you Kenneth, that means a lot! Appreciate you reading it.

5 months ago by Anonymous Neurologist Neurologist

Replied to Fatigable Weakness vs Functional Weakness: Why Hoover’s Sign Can Mislead When Isokinetic Testing Shows Otherwise

You said “Patient X (a 100m sprinter) initially reported decreased muscle strength”, but then you said “The patient reports fatigable weakness in the right side.” You only say the patient had fatiguable weakness after reviewing the isokinetic data. So you were a bit unclear. Regardless, the patient reports unilateral weakness and arrives in a wheelchair. That is enough to warrant testing for Hoover’s sign, but additional information in the history would likely contribute to that decision as well. The history is used to generate hypotheses which guide the physical exam and testing to strengthen or weaken each hypothesis. The Hoover sign result is just one of many pieces of data. It is not interpreted in isolation, just like the isokinetic test results should not be interpreted in isolated. You seem to have missed my point though. If the Hoover’s sign was positive in this case, the patient clearly does have weakness on movement initiation, specifically when asked to move the leg. If they did not, the test would have been negative. There’s no arguing that. Two things can be true at the same time. This patient can have fatiguable weakness evidenced by isokinetic testing and functional weakness evidenced by a positive Hoover’s sign. Those could be two separate types of weaknesses or both can be functional weakness in two different contexts/tasks. You are assuming functional weakness should always lead to weakness at movement initiation. That’s not true though. People can and often do experience functional weakness with repeated or sustained movement. You would know this with the appropriate training and experience. Majority of FND signs have low sensitivity so the absence of the same findings from the 1985 paper do not rule out the diagnosis. Your analysis does not hold up.

5 months ago by Christoff Trüter Spousal caregiver

Replied to Fatigable Weakness vs Functional Weakness: Why Hoover’s Sign Can Mislead When Isokinetic Testing Shows Otherwise

With respect, can we avoid medical clericalism here? The blog post clearly framed fatigable weakness from the start. It also explicitly states that the initial output of both legs looked normal at low demand. This means we are clearly not dealing with a positive Hoover sign. That is the whole point of the post. I deliberately mentioned the wheelchair use in the post to illustrate how easily one can jump to conclusions based on incomplete assumptions. Ironically, that is exactly what happened here. The wheelchair was used for balance issues, not primary leg weakness. In the same way the clinician in the case did, you are assuming generic unilateral weakness without fully listening to the actual reported symptoms of demand-dependent fatigability. Treating the normal initial output as a contradiction would be incorrect, even according to your own analysis. The post highlighted clear, reproducible, load-dependent patterns of fatigability from real performance data produced and interpreted by sport scientists. These patterns are the opposite of the inconsistent and variable patterns described in the 1985 Knutsson & Mårtensson study for functional weakness. They are straightforward and easily understood by anyone who has experienced fatigue under physical load. None of this is esoteric.

5 months ago by FND Nope Moderator

Replied to Fatigable Weakness vs Functional Weakness: Why Hoover’s Sign Can Mislead When Isokinetic Testing Shows Otherwise

"You are assuming functional weakness should always lead to weakness at movement initiation. That's not true though. People can and often do experience functional weakness with repeated or sustained movement" This post is not arguing against functional weakness, or even functional fatigable weakness. The core question is whether Hoover's sign is an appropriate tool for assessing fatigue-dependent weakness. If so, how would it be applied in practice? Are there any references or protocols supporting its use? Would actual repetitions be performed using Hoover's sign, and how would it be physically applied? I am genuinely interested.

5 months ago by Anonymous Neurologist Neurologist

Replied to Fatigable Weakness vs Functional Weakness: Why Hoover’s Sign Can Mislead When Isokinetic Testing Shows Otherwise

I’m not here to play your games. Did the patient in this scenario have a positive Hoover’s sign or not? Yes or no. It’s like you didn’t read what I wrote. You keep doing this…you ignore what I say and just repeat yourself as if I didn’t address it when I did. I’m not going to repeat myself. Actually read what I wrote and don’t repeat the same thing. I’ve already addressed it. Also, this is not how cases are presented. You include minimal information so that you can back track and fill in information to refute any challenges. In real life there will always be much more clinical information available to the clinician than what you have provided. I’ve already told you why it’s reasonable to assess for Hoover’s sign in this case based on the little information you provided.

5 months ago by Christoff Trüter Spousal caregiver

Replied to Fatigable Weakness vs Functional Weakness: Why Hoover’s Sign Can Mislead When Isokinetic Testing Shows Otherwise

"Did the patient in this scenario have a positive Hoover’s sign or not?" - No The article made it clear from the description itself that Hoover’s sign was not positive. The patient had preserved initial low-demand strength, with fatigability only appearing under higher demand. The isokinetic data supported this presentation — it did not create the assumption. I have engaged with the arguments raised. However, it remains unclear why reproducible isokinetic data is rejected in favour of a bedside test that cannot assess fatigable weakness. At no point did I backtrack, introduce new information, or present minimal details to refute challenges. The relevant information was provided consistently. Isokinetic dynamometry is a highly accurate and validated metric in sports medicine for assessing load-dependent strength and fatigability. That is precisely why it was suggested as an additional objective tool.

5 months ago by FND Nope Moderator

Replied to Fatigable Weakness vs Functional Weakness: Why Hoover’s Sign Can Mislead When Isokinetic Testing Shows Otherwise

The discussion was largely unproductive, unfortunately. I don’t see much point in continuing it further. The clinician here simply echoed the same approach as his colleague in the clinical setting: dismissing the isokinetic data showing clear, reproducible fatigable weakness while assuming a functional diagnosis upfront. Hoover’s sign was negative, consistent with the preserved initial low-demand strength, yet the actual fatigue pattern was never addressed. The patient remains without proper investigation or answers for the measurable deficit. This exchange shows exactly why a site like this is necessary.

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Fatigable Weakness vs Functional Weakness: Why Hoover’s Sign Can Mislead When Isokinetic Testing Shows Otherwise

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