Today, health science focuses on mortality and morbidity, but there is a third health indicator: functional ability, which is what truly matters to people. Right now, we are in the midst of a scientific revolution in functional ability, and over time, it will become possible for an occupational therapist not only to gain a precise overview of a patient's functional status but also to map specific developmental trajectories of functional ability over time for individual impairments, writes Jerome Bickenbach in this article.

Although it is a technical term – derived from an international classification of the World Health Organization [1] – everyone knows what “functioning” means. Across the lifespan, all of us experience pain, anxiety, fatigue and weakness, tight joints, skin sores, and many other impairments that affect how we move around, sense the world, or think. These experiences are what matters to us regarding our state of health. When we find that we cannot climb stairs painlessly, walk as far as we used to, clean or dress ourselves, read a book, make and keep friends, do all the homework we need to do, do our schoolwork or perform our jobs, these concrete, real-life difficulties are the lived experience of health.

There is nothing mysterious about functioning, although we tend to focus on it only when there is a problem. But problems in functioning are not just common – across our lifespan – they essentially define what it means to be a human being. All of us get hurt, have injuries and develop diseases. Over time, as we age, we all lose our ability to do what we want in the way we want, or as society expects. Everyone's health declines; everyone dies. Yes, everyone knows what functioning means. 

Health professionals, especially occupational therapists, understand the importance of understanding how a person's state of health plays out in their day-to-day lives. Symptoms and impairments are why people seek health care in the first place. Their lives are affected; ill health is getting in the way. Health professionals, especially occupational therapists, also understand that while limits on functioning can be linked to diseases, injuries and other states of ill  health, their impact on people's life also very much depends on the world around them: on the weather, on the built environment, the design of tools and products, other people's attitudes and beliefs, and the requirements and demands of the family, school and work. 

A problem in functioning – or a disability as it is usually called – is the outcome of interaction between the capacities of the body and the demands of the environment. This is why health matters to us and why health is key to our well-being. [2] We can help people with problems in functioning either by increasing their capacity or by changing the demands of the environment through modification or assistive technology. This, of course, is the health strategy called rehabilitation.[3]

Surprising, those who have needed the most convincing about the importance of functioning as a crucial health dimension have been health scientists, and especially epidemiologists. Traditionally, they have insisted that to know everything there is to know about the health of a person or an entire population, you need to know how long they live (mortality) and the diseases or injuries they have (morbidity). In 2001, the WHO informed the world that there is a third indicator of health – functioning.[4] Sure, it is important to know how long people live and what diseases they have, but what truly matters to people is how their health impacts their lives, what they can and cannot do and how it affects their goals and aspirations. What matters is functioning.

Think of Covid-19. During the pandemic, there was enormous concern about the number of people who would contract the disease and how many would die. But countries also needed to know how the infection affected people's daily lives - both during and after illness. Most countries have fairly robust data collection systems for mortality and morbidity. However, while healthcare professionals routinely collect data on functioning using standard instruments, in the urgency to determine how many people died from Covid-19, few countries prioritized understanding how people lived with the disease, how their functioning changed over time, or how it could be improved.

But this is changing. We are told that two major population health trends are expected to dominate our future: population aging and the increasing prevalence of non-communicable diseases – such as dementia, health and lung diseases, diabetes, and depression.[5] What these trends have in common is that people are living longer but experiencing worse health due to long-lasting and debilitating diseases. In other words, the future of health will increasingly be assessed and treated not merely as a matter of premature mortality but as one of functional impairments or disability. It is no coincidence that the WHO has argued that rehabilitation will become the key health strategy of the 21st century.[6]

But if everyone intuitively understands functioning, if rehabilitation therapists address it daily with their patients, and if it is becoming an increasingly important health issue, why do health scientists still focus on mortality and morbidity? Functioning is a complex and dynamic phenomenon. Unlike death or the presence of a disease, it is not a binary issue but rather a matter of degrees. Environments are constantly changing, and people are moving among and between different environments, each modifying their biological health status and lived experience of functioning in various ways. Mobility, sensory, cognitive functioning varies across a spectrum - from full functioning to moderate, poor or total non-functioning. For every area of functioning and every individual person, the overall portrait of a person's functioning may be very different.

Collecting standardized and routine data on functioning has only become possible in the last couple of decades. The WHO's International Classification of Functioning, Disability and Health provides us with a language of functioning in terms of body functions and structures, activities and participation and the environmental factors that help to shape the nature and severity of disabilities. Statistical methodologies have been used to transform the mass of individually collected functioning data into usable and measurable summary scores that can be mapped over time. In the future machine learning and AI will undoubtedly be required to make sense of human functioning as it changes over time in individuals and populations.

The health science of functioning, in short, is still developing. In time, it will be possible for an occupational therapist, for example, not merely to confidently capture a summary of a patient's functioning status, but also track specific trajectories of functioning over time for individual impairments, alone or in combination. Data science of functioning will complement therapists’ intuitive understanding, not merely of patterns of decline or improvement of capacity in some dimension of functioning for an individual over time but also the contribution of changes to the person's environment – such as assistive technologies, environmental modifications, ergonomics, time management, employee education and support – that remove barriers and enhance functioning and decrease disability.

While functioning it simple to understand – it is what it means to live with a health problem – and rehabilitation professionals of various specialties intuitively understand the dimension of functioning in their patients, the basic science of functioning needs to catch up. It took centuries for scientists and societies to routinely collect and analyze data on mortality and morbidity. The practice of healthcare waited for the health sciences to provide the evidence needed for improvement. We are now experiencing a kind of scientific revolution in functioning one which, in time, will likely transform clinical and population health.[7]

Today, health science focuses on mortality and morbidity, but there is a third health indicator: functional ability, which is what truly matters to people. Right now, we are in the midst of a scientific revolution in functional ability, and over time, it will become possible for an occupational therapist not only to gain a precise overview of a patient's functional status but also to map specific developmental trajectories of functional ability over time for individual impairments, writes Jerome Bickenbach in this article.

Although it is a technical term – derived from an international classification of the World Health Organization [1] – everyone knows what “functioning” means. Across the lifespan, all of us experience pain, anxiety, fatigue and weakness, tight joints, skin sores, and many other impairments that affect how we move around, sense the world, or think. These experiences are what matters to us regarding our state of health. When we find that we cannot climb stairs painlessly, walk as far as we used to, clean or dress ourselves, read a book, make and keep friends, do all the homework we need to do, do our schoolwork or perform our jobs, these concrete, real-life difficulties are the lived experience of health.

There is nothing mysterious about functioning, although we tend to focus on it only when there is a problem. But problems in functioning are not just common – across our lifespan – they essentially define what it means to be a human being. All of us get hurt, have injuries and develop diseases. Over time, as we age, we all lose our ability to do what we want in the way we want, or as society expects. Everyone's health declines; everyone dies. Yes, everyone knows what functioning means. 

Health professionals, especially occupational therapists, understand the importance of understanding how a person's state of health plays out in their day-to-day lives. Symptoms and impairments are why people seek health care in the first place. Their lives are affected; ill health is getting in the way. Health professionals, especially occupational therapists, also understand that while limits on functioning can be linked to diseases, injuries and other states of ill  health, their impact on people's life also very much depends on the world around them: on the weather, on the built environment, the design of tools and products, other people's attitudes and beliefs, and the requirements and demands of the family, school and work. 

A problem in functioning – or a disability as it is usually called – is the outcome of interaction between the capacities of the body and the demands of the environment. This is why health matters to us and why health is key to our well-being. [2] We can help people with problems in functioning either by increasing their capacity or by changing the demands of the environment through modification or assistive technology. This, of course, is the health strategy called rehabilitation.[3]

Surprising, those who have needed the most convincing about the importance of functioning as a crucial health dimension have been health scientists, and especially epidemiologists. Traditionally, they have insisted that to know everything there is to know about the health of a person or an entire population, you need to know how long they live (mortality) and the diseases or injuries they have (morbidity). In 2001, the WHO informed the world that there is a third indicator of health – functioning.[4] Sure, it is important to know how long people live and what diseases they have, but what truly matters to people is how their health impacts their lives, what they can and cannot do and how it affects their goals and aspirations. What matters is functioning.

Think of Covid-19. During the pandemic, there was enormous concern about the number of people who would contract the disease and how many would die. But countries also needed to know how the infection affected people's daily lives - both during and after illness. Most countries have fairly robust data collection systems for mortality and morbidity. However, while healthcare professionals routinely collect data on functioning using standard instruments, in the urgency to determine how many people died from Covid-19, few countries prioritized understanding how people lived with the disease, how their functioning changed over time, or how it could be improved.

But this is changing. We are told that two major population health trends are expected to dominate our future: population aging and the increasing prevalence of non-communicable diseases – such as dementia, health and lung diseases, diabetes, and depression.[5] What these trends have in common is that people are living longer but experiencing worse health due to long-lasting and debilitating diseases. In other words, the future of health will increasingly be assessed and treated not merely as a matter of premature mortality but as one of functional impairments or disability. It is no coincidence that the WHO has argued that rehabilitation will become the key health strategy of the 21st century.[6]

But if everyone intuitively understands functioning, if rehabilitation therapists address it daily with their patients, and if it is becoming an increasingly important health issue, why do health scientists still focus on mortality and morbidity? Functioning is a complex and dynamic phenomenon. Unlike death or the presence of a disease, it is not a binary issue but rather a matter of degrees. Environments are constantly changing, and people are moving among and between different environments, each modifying their biological health status and lived experience of functioning in various ways. Mobility, sensory, cognitive functioning varies across a spectrum - from full functioning to moderate, poor or total non-functioning. For every area of functioning and every individual person, the overall portrait of a person's functioning may be very different.

Collecting standardized and routine data on functioning has only become possible in the last couple of decades. The WHO's International Classification of Functioning, Disability and Health provides us with a language of functioning in terms of body functions and structures, activities and participation and the environmental factors that help to shape the nature and severity of disabilities. Statistical methodologies have been used to transform the mass of individually collected functioning data into usable and measurable summary scores that can be mapped over time. In the future machine learning and AI will undoubtedly be required to make sense of human functioning as it changes over time in individuals and populations.

The health science of functioning, in short, is still developing. In time, it will be possible for an occupational therapist, for example, not merely to confidently capture a summary of a patient's functioning status, but also track specific trajectories of functioning over time for individual impairments, alone or in combination. Data science of functioning will complement therapists’ intuitive understanding, not merely of patterns of decline or improvement of capacity in some dimension of functioning for an individual over time but also the contribution of changes to the person's environment – such as assistive technologies, environmental modifications, ergonomics, time management, employee education and support – that remove barriers and enhance functioning and decrease disability.

While functioning it simple to understand – it is what it means to live with a health problem – and rehabilitation professionals of various specialties intuitively understand the dimension of functioning in their patients, the basic science of functioning needs to catch up. It took centuries for scientists and societies to routinely collect and analyze data on mortality and morbidity. The practice of healthcare waited for the health sciences to provide the evidence needed for improvement. We are now experiencing a kind of scientific revolution in functioning one which, in time, will likely transform clinical and population health.[7]