Major theories of frailty started to emerge in early 1990s. Each of these has particular implications for research and provides insight and structure to different approaches. Where possible, theories are linked to related frailty measurement instruments, as both theory without measurement and measurement without theory can pose barriers to advancement in science.
- Buchner and Wagner’s Vulnerability Theory (1992): Frailty is the loss of adaptability such that new physiologic stressors lead to disability and disease.
- Bortz’s “Use It Or Lose It” Theory (1993): Frailty is the result of accumulated mismatch between energy metabolism and requirements.
- Fried’s Cycle Of Frailty (1998/2001): Decline in physiologic reserves from underlying aging biological processes decreases the capacity to respond to stress, which leads to further damage. Related instrument: The Physical Frailty Phenotype.
- Lipsitz’s Loss of Complexity (2002) and Varadhan’s Dynamical System Theory (2008): Frailty develops when the complex feedback loops that maintain homeostasis lose their adaptability leading to larger perturbations in function that ultimately manifest as dysfunction.
- Ferrucci’s Homeostatic Dysregulation Theory (2005): Frailty is core biological dysregulation developing with aging that becomes manifest as a wide variety of clinical diseases when homeostatic mechanisms become overwhelmed in different organs at different times.
Deficit Accumulation Theory:
- Rockwood’s Cumulative Deficit Theory (2001): Frailty is the result of an aging process in which the accumulation of age-related deficits across domains of health that are not necessarily known as risks for diminished life expectancy individually, the sum of which predict poor outcomes. Related instrument: The Deficit Accumulation Index.
- SPRINTT Pre-Disability (2017): Frailty as a pre-disability condition, measured by physical performance tasks that can identify older persons with increased vulnerability to stressors and exposed to higher risk of negative health-related events such as disability. Related instrument: The SPPB to Determine Frailty.