Is frailty a helpful concept within the rehabilitation context? I asked myself this question after seeing a recent paper on people with multiple sclerosis that concluded that there was “a significant relationship between frailty and history of falls in multiple sclerosis, independent of age, sex, and disease severity.” Frailty is widely used but without a single agreed meaning or measure. In this post, I wish to explore frailty and whether it might be a useful concept within rehabilitation. I conclude that the underlying idea is sound when deciding how best to manage a patient but that it cannot determine a patient’s management.

Table of Contents

What is the meaning of frailty?

The word is derived from the Latin frangere, to break, through fragilis to the English adjective, fragile, which means “easily broken or damaged’ [OED]. When used about a person, the Oxford English Dictionary describes its meaning as “not strong or sturdy; delicate and vulnerable”.

The French word derived from the same root was fraile, leading to the English word frail, which means “weak and delicate” and “easily damaged or broken” [OED]. The noun for frail is frailty, the “condition of being weak and delicate”.

The central feature underlying these meanings is that a slight, possibly insignificant disturbance leads to an object being damaged or broken. It takes very little to cause irreversible harm. In other words, something that is frail is more likely to break or come to harm than other things. The use of the term acts as an implicit predictor. An insurance company would charge a high premium to insure a frail object because its frailty predicts a greater probability of damage.

What is frailty in healthcare?

In the absence of any agreed definition, I will start with the World Health Organisation’s definition: “Frailty is conceptually defined as a clinically recognisable state in which the ability of older people to cope with everyday or acute stressors is compromised by an increased vulnerability brought by age-associated declines in physiological reserve and function across multiple organ systems.” The associated text also states that frailty is simply one end of a spectrum, renders someone more vulnerable to death and disability, and is multifactorial.

The term frailty was first used in the context of human sickness in 1989 in a paper titled “Frailty, sickness, and death: models of morbidity and mortality in historical populations.”. This was looking at historical data. During 1990 it was mentioned more frequently.

In 1993 Schulz and Williamson published a review of “a decade of research on physical frailty in diverse elderly populations” and emphasised “the psychosocial and behavioral aspects of physical frailty.” They assumed “that the psychological representation of illness and disability, as well as the social contexts in which they occur, are important determinants of their impact on patients and family members.”  In other words, they were viewing frailty in the context of the biopsychosocial model of illness.

In 2001 Muriel Gillick wrote, “Frailty is a syndrome in desperate need of description and analysis. It is currently so ill-defined that some gerontologists dispute that it is more than a synonym for disability.” She supported the development of a biological model of frailty to “legitimise the concept”. And since then, this has been the primary approach.

Linda Fried, who has been actively involved in research in frailty for about 30 or more years, recently reviewed frailty in a paper, posing the question,  “Why do some older people die in the face of heatwaves and others do not?” Linda Fried and her colleagues hypothesised that one factor was “what clinicians term ‘frailty’, a state of depleted reserve resulting in increased vulnerability to stressors that emerges during ageing independently of any specific disease.” The paper puts forward two models of frailty.

One model suggests that patients with frailty are at increased risk due to decreased reserves leading to vulnerability to stressors. This state is referred to as phenotypic frailty because it is based on clinical features observed in the person. It is characterised clinically by five items: weakness, slow walking speed, low levels of physical activity, fatigue (or feeling exhausted), and unintentional weight loss.

The alternative is a model based on accumulated deficits, referred to as the accumulated deficits model. As the number of health conditions, a person has increases, their frailty increases. It is characterised using one of many frailty indices that count the number of conditions a person has.

Despite all the research, there is still little agreement on how frailty is defined and measured. For example, in 2016, Dent and her colleagues found 24 measures, and other studies have found even more striking variations. For instance, in a systematic review in 2015, Olga Theou and her colleagues found 262 variations on one key measure, the Friend’s Frailty Phenotype.

Why the difficulty?

If you have read my posts on defining disability, separating disease, disability and sickness, and prolonged disorders of consciousness, you may already have recognised the problem. I will explain here why there is a difficulty in defining frailty.

Words are never precise: the variegated cloud of meanings they carry about with them is their expressive power. But it also generates confusion, ‘cause you know sometimes words have two meanings’

Carlo Rovelli. Helgoland. Information. p.90

A word arises and continues if it serves a purpose, and most words carry a meaning that will influence what another person knows or does. Many words in healthcare summarise a complex state. In so doing, it also enables a better patient management decision because the state’s cause and/or prognosis is known. For example, diagnosing someone with bacterial meningitis summarises their many symptoms and signs. It predicts that they will likely deteriorate and die or be left with disability and that specific anti-bacterial treatment will alter the anticipated outcomes.

The power of a word to categorise is also its weakness. No phenomenon in nature is precise and easily separated from other phenomena. The colour red cannot be defined by a specific wavelength of light or even a range because the surrounding colours affect how it is perceived.

The word frailty arose when people noticed an association between a cluster of clinical phenomena in ageing people and the likelihood of earlier death or needing institutional care. Unsurprisingly, healthcare researchers then wished to identify more precisely what frailty was and lost sight of its purpose, the prediction of morbidity or mortality.

The evidence for my proposition is given above. Frailty was identified in an epidemiological study of morbidity and mortality. After 12 years of research, there was no agreed descriptor, and a quest for a biological cause was recommended. However, frailty is a syndrome just as chronic pain is, and searching for a unique reason (albeit a complex cause) is fruitless – as time has shown.

Published papers confirm the failure to understand the reason for identifying frailty. For example, Shamliyan et al. undertook a systematic review of the association of frailty with survival. In the introduction, they wrote, “How frailty is defined affects its presumed impact on patient-centred outcomes, including quality of life, institutionalisation, and mortality”, and they concluded that “Frailty is a prevalent and important geriatric syndrome associated with decreased survival.” These statements fail to recognise that frailty is defined by its ability to predict the outcome.

In other words, frailty should not be considered an independent specific phenomenon that causes someone to be vulnerable (fragile) and more liable to disability and death. Instead, frailty is a syndrome, a collection of clinical observations that, taken together, indicate that someone is at an increased risk (fragile) of disability and death compared to their peers.

This formulation of frailty immediately highlights three crucial features of frailty. First, it is part of a continuous spectrum and will not have a categorical separation from the rest of the population concerned. Second, frailty is comparative. One patient is more likely than another to die or need institutional care. Last, the outcome will determine the components of frailty; predictors of death may differ from predictors of needing institutional care.

Frailty and ageing.

Everyone dies, and most people will become more dependent on others towards the end of their life. The clinical markers of frailty, whether in the ‘accumulated deficits’ model or the ‘syndrome of phenotypic frailty’ model, become more common as someone ages. This is likely to make the predictive value of frailty less valuable as someone ages.

Peter Hanlon and his colleagues studied 493,737 participants in the UK biobank. They found that features of phenotypic frailty were associated with mortality and that the association was more remarkable in younger people. They also identified five long-term conditions related to phenotypic frailty: multiple sclerosis, chronic fatigue syndrome, chronic obstructive pulmonary disease, connective tissue disease, and diabetes.

The ageing process is subject to much research, and Susan Howlett and colleagues have reviewed this recently in the context of frailty. Their review covers ageing in different body systems and differential rates of ageing within the human population. It also states, “Frailty is not a disease, but it profoundly influences disease expression.” This indicates that people with Alzheimer’s are more likely to show a cognitive loss if they are physically frail. They also make the vital point that the prevalence of frailty is strongly affected by lifestyle and social factors.

Frailty and rehabilitation.

This part considers two questions: does frailty impact rehabilitation, and is rehabilitation helpful for people deemed frail?

I searched PubMed (rehabilitation AND frailty), and no trials investigated prospectively whether frail patients benefited from rehabilitation, such as reducing the rate of subsequent morbidity or institutional care. I assume that, in the absence of any evidence to the contrary, frail patients can and probably do have a better outcome if they receive expert rehabilitation input. This is because other patients with deteriorating conditions or long-standing disabilities nonetheless benefit.

In contrast, many observational studies have investigated the impact of frailty on outcomes after rehabilitation compared to people who were not frail. As one would predict, frail patients tended to have worse outcomes. For example, Schuijt et al. showed that, for patients admitted with a hip fracture,  “The frailty index can be used to predict discharge destination from both the hospital and rehabilitation facility, 90-day mortality, and return to home after rehabilitation.” and Low et al. found premorbid frailty was associated with less good outcomes after hip fracture.

Nonetheless, Low et al. advised against concluding that frail patients should not receive rehabilitation, suggesting instead that “the presence of these factors should be used to enhance discussions with patients and families to manage expectations better and facilitate discussions regarding goals of care and discharge planning.”

Another interesting study by Sabrina Kohler and her colleagues investigated the inter-relationship between resilience, frailty and outcome. They found that frailty and resilience were inversely related (higher resilience, lower frailty) and that frailty was associated with worse outcomes in people with higher frailty, whereas resilience had no association with outcome. This contrasted with another study by Guilia Rebagliaiti and her colleagues, who found that resilience may have moderated the association between frailty and outcome.


In healthcare, frailty means that a person is more fragile, more likely to be precipitated into a crisis by a relatively small situational change and is expected to have a less good outcome than other people in a similar situation. Describing someone as frail implies a prediction of their future either in terms of risk or in terms of outcome.

Frailty is not a categorical state. Everyone lies on a spectrum of risk and the likelihood of a less promising outcome, and frailty encompasses the lower end with more risk and less good prognosis, but there is no boundary. Artificial boundaries can be set, but one can only draw limited clinical conclusions because all research-based findings are probabilistic, with little predictive value.

Third, one must distinguish between risks and outcomes that apply solely to the person’s bodily functioning and those influenced by other contextual factors. For example, the available support network, housing, and poverty will strongly influence institutionalisation but will have a much less strong association with mortality.

Scroll to Top

Subscribe to Blog

Enter your email address to receive an email each time a new blog post is published. 
Then press the black ‘Subscribe’ button.