According to Leidy’s framework, functional status includes
four dimensions: functional capacity, functional performance,
functional capacity utilization, and functional reserve
(Leidy, 1994). Functional capacity is defined as one’s
maximal capacity to perform a specific task in the physical,
social, psychological or cognitive domains. For example,
functional capacity might be one’s maximal ability
in strength and endurance or in aptitude or memory. The
second dimension, functional performance, refers to activities
that one performs on a day-to-day basis. Functional
performance is an integrated response and is determined
by multiple factors, including personal choice, values, and
motivation. Functional performance could be assessed by
the level of physical activity and energy expended or as
self-reported activities across multiple categories. Alternatively,
functional performance could be measured by daily memory performance. Functional performance also could
be influenced by functional capacity, as in cases when reduced
capacity limits performance of day-to-day activities.
The third dimension, capacity utilization, refers to the percentage
of functional capacity that is used day to day. The
fourth dimension, functional reserve, refers to the difference
between capacity utilization and functional capacity. People
generally do not function at 100% of their capacity on a
day-to-day basis, and people with high capacity might actually
use only a small percentage of their capacity daily.
When functional capacity declines because of health problems,
a person might be required to use a higher percentage
of capacity or to cut back on daily activities. Capacity utilization
is closely related to functional reserve and indicates
the unused potential. People with low capacity and very low
functional performance could have a fairly large functional
reserve.