Why do people with pain report differently on questionnaires than they do in physical performance testing?


One of the topics thrown around by people who don’t have an up-to-date understanding of pain is why people say one thing on a questionnaire, for example, what they can and can’t do, and perform quite differently when asked to do the same task in a clinic. It’s a good question, on the face of it: after all, people should know what they can and can’t do, and be consistent. If there is a difference, well obviously the physical performance test is far more objective than self-report – the therapist is right there watching, so there’s no room for doubt about which measure is The Most Accurate.

The main reason, according to these clinicians, for someone doing differently in the clinic compared with self-reporting, has to be because they’re biased. The person wants to misrepresent what they can and can’t do. Of course.

Superficially, and if you’re not knowledgeable about pain, behaviour, context and human interaction, you could be forgiven for accepting the idea that what you see in clinic is consistent with what’s being done in every day life. The physical movements are pretty much the same and the person is just being asked to do something they do all the time.

BUT – and it’s an enormous exception – humans are not robots. Not body bits that move when they’re pulled like a puppet on a string. People are meaning making, interpreting, social creatures with rapidly responding body systems that represent contexts in relation to memories, predictions and current demands.

I wrote a talk recently on some research that made my heart sing a bit. As an occupational therapist, my profession has long recognised that doing activities (occupations) that hold meaning is quite a different thing from doing a-contextual, meaningless movements. This is why occupational therapists are known to ask about what matters to you, and to use meaningful activities/occupations both as therapy and as outcome (Hitch & Pepin, 2021). The research I referred to was a proposal for an “ecologically grounded functional practice” within rehabilitation (Vaz, Silva, Mancini, Carello & Kinsella-Shaw, 2017). In this paper, the authors point out that “improvements at one level of functioning might not transfer immediately to other levels” and by this they mean that elbow flexion/extension improvements may not transfer into a person being able to feed themselves. They also pointed out that when people perform well-rehearsed activities in the context of goal pursuit – such as getting dressed, ready for work; catching a ball as part of a fast-moving game of netball; hammering a nail – the movements are not just about motor control, they’re about goal-directed behaviour in a context with an interaction between the person, the environment, any tools, the purpose of the activity and so on.

For example, if I want to eat soup, I not only need to have sufficient elbow flexion/extension, I also need to know where the soup bowl is (tried eating soup while lying down?), the texture of the soup (is it thick, thin, lumpy?), the heat of the soup (hot, cold, spicy) and even the social context – I might be OK slurping when I’m on my own, but I’m less inclined to slurp when in polite company. The way in which I carry out the flexion/extension will be very different with each contextual variation.

OK. So context matters, and both the what and why of movement will influence how I go about my performance.
What else?

Well, with a painful condition and especially when I’m not confident I can do it without increasing my pain, I’m much more likely to attempt a difficult movement task in the presence of someone who can monitor what I’m doing. Firstly that person might stop me if they think I’m doing something harmful (professional liability insurance offers some protection!). Secondly, it’s a lot harder to say “no” to someone who is right there in the room! This is called “demand characteristics” and has been associated with problems of the rubber hand illusion (Lush, Vazier & Holcombe, 2020). If someone expects you to do something, you’ll probably do it – because we social creatures don’t like to offend, because the person may inadvertently signal the response they want (see link).

There are other reasons people don’t report the same level of disability on a questionnaire and in physical performance testing: they don’t measure the same things, people forget (especially if they haven’t tried in a while), the day of physical performance testing could be a bad day (or a good day), in physical performance testing the person is usually asked to do it maybe once or twice – in daily life that same activity might be carried out many times across a day, week, month. The environment in a clinical testing environment is typically well-lit, the space around the person is clear, the noise level is usually reasonably low, the floor surface is flat and usually hard lino and free of rugs or pets, there’s minimal distraction, the only thing the person has to think of is this one movement – they’re not picking up the washing off the floor while rushing to put it in the washing machine before dashing out the door to pick the kids up from school.

Even the questions are different – “does pain interfere with…?” is a different question from “can you step up onto this step using a hand rail?”

And don’t let me even start on the meaning of performance either way – for example, if the person is really keen on getting knee surgery, might “good” performance mean they, without even knowing it, alter how they do a movement? What if the person is apprehensive about how the results of this testing might affect their rehabilitation and return to work – again without even knowing it, might this not have some influence on how the person performs?

Testing and measurement is a core skill and research area in psychology. Dang, King & Inzlicht (2020) offer some really good insights into the reasons responses differ between self-report and performance, and to be fair, they don’t even consider the influence of pain and physical capability as I have above. Pain-related performance is a specialty area of its own, nonetheless we can still draw from their paper because many of the problems they recount are absolutely part of pain and disability self-report and physical performance.

They describe the reliability paradox (that reliability = variance between individuals divided by variance between individuals + error variance) – in other words, we need low levels of between-person variability so that any experimental manipulation is maximised. But in real life, we almost always exhibit variability in our performance – so the reliability of two measures limits the correlations that can be observed between then, with lower reliability leading to weaker observed correlations.

The authors also describe the very different response processes involved in self report and performance – as I mentioned above, self-report measures ask people to reflect on what they do in real life in many different contexts that are unstructured. Performance measures take a snapshot based on performance in specific and highly structured situations. Self-report measures capture a person’s perception of their capabilities whereas physical performance reflects the observations of someone else. And performance assessments generally tap into peak performance, not daily performance – tapping into some of the discrepancies we see between “can do” and “will do” (competence-performance discrepancy).

So, when you read arguments on social media from well-known physiotherapists suggesting that the person who reports a difference between what they perceive they can do, and what they have done in a physical performance test is “biased”: know that we have absolutely NO WAY to determine “bias”, “malingering”, “faking bad”, “faking good” – and that there are many well-understood reasons for the difference in performance. Read this paper for more on why we can’t detect “malingering” in people with pain: Tuck, N. L., Johnson, M. H., & Bean, D. J. (2019, Feb). You’d Better Believe It: The Conceptual and Practical Challenges of Assessing Malingering in Patients With Chronic Pain. J Pain, 20(2), 133-145. https://doi.org/10.1016/j.jpain.2018.07.002

Dang, J., King, K. M., & Inzlicht, M. (2020). Why are self-report and behavioral measures weakly correlated?. Trends in cognitive sciences, 24(4), 267-269.

Hitch, D & Pepin, G. (2021) Doing, being, becoming and belonging at the heart of occupational therapy: An analysis of theoretical ways of knowing, Scandinavian Journal of Occupational Therapy, 28:1, 13-25, DOI: 10.1080/11038128.2020.1726454

Lush, P., Vazire, S., & Holcombe, A. (2020). Demand characteristics confound the rubber hand illusion. Collabra: Psychology, 6(1).

https://methods.sagepub.com/reference/the-sage-encyclopedia-of-communication-research-methods/i4252.xml

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