The language of rehabilitation: confusion over intensity, duration, and frequency

Documenting the treatment dose provided to a particular patient is important in both clinical research and clinical practice. How do you describe how much treatment you gave someone? The terms that are frequently tossed around in the realm of rehabilitation dosing are intensity, duration, and frequency. However, what is meant by these terms often differs. In the August 2012 Issue of the Archives of Physical Medicine and Rehabilitation, this was the topic of a special communication by the American Congress of Rehabilitation Medicine (ACRM) Stroke Movement Interventions Subcommittee (Arch Phys Med Rehab, 2012:93:1395-1399).

Motivated by inconsistencies in the literature, the subcommittee discusses and proposes operational definitions for: intensity, duration, and frequency. The goal is to facilitate communication across disciplines and across research and clinical domains.

I applaud the subcommittee’s endeavor to unite the field of stroke rehabilitation and agree that developing consistent nomenclature may facilitate interdisciplinary communication. However, there are some potential pitfalls with the proposed definitions.

Let’s start with the big one, the most confusing term: Intensity.
The Merriam-Webster online dictionary (medical) defines intensity in 2 ways:

1. The quality or state of being intense; especially: extreme degree of strength, force, energy, or feeling
2. The magnitude of a quantity (as force or energy) per unit (as of surface, charge, mass, or time)

The ACRM Stroke Movement Interventions Subcommittee recommends the following definition for intensity, within the scope of stroke rehabilitation:

“The amount of physical activity or mental work put forth by the client during a particular movement or series of movements, exercise, or activity during a defined period of time” (page 1396)

This definition encapsulates both aspects of the lay definition – the feeling of intense effort (“mental work”), and the magnitude of quantity (“amount of physical activity”) per unit (“time”).

However, mental effort is a subjective phenomenon, and is not only difficult to measure, but near impossible to prescribe. As a physiotherapy researcher, I prescribe the treatment dose to be tested in an experimental intervention. I can’t imagine prescribing a standardized level of mental effort to be achieved in each session; in contrast, a specific number of repetitions within a pre-determined period of time can be operationalized and controlled. The problem with the definition proposed by the ACRM subcommittee is that mental effort may differ for the same amount of physical activity between two patients. If you define intensity by amount of physical activity, and two patients performed the same number of repetitions in a defined period of time, it is easy to say the patients had the same intensity of treatment. However, if you define intensity by the “mental work put forth by the client” and the reported mental effort exerted by two patients to complete the prescribed amount of physical activity is different, then the intensities for the two patients were not the same. Thus, the two versions of the subcommittee’s definition of intensity can lead to contrasting classifications of intensity. As illustrated by this example, the recommended definition is unlikely to help achieve the subcommittee’s goal of streamlining interdisciplinary communication.

Since mental effort is subjective, and is likely influenced by individual characteristics (such as comfort threshold or even personality traits), it would seem that intensity may be more concretely and consistently defined solely by the amount of physical activity, expressed in terms of number of repetitions (and sets) in a fixed period of time.

There is also an issue with the subcommittee’s proposed definition of “duration.” The problem is that the subcommittee, again, supports two different definitions of “duration” of treatment – the duration of a single session, and the duration of an intervention period. The first definition – duration of a single session – is part of intensity. Specifically, intensity, by the subcommittee’s own definition, is defined as amount of physical activity or mental work within a “defined period of time.” That is, the duration of the session. If one appropriately specifies the treatment intensity (i.e., includes the defined period of time for the session), then the logical definition for treatment duration is the length of time over which the intervention is provided. This information is critical for consumers of research literature.

Frequency refers to how often the intervention is administered. There is little to debate over this term.

So while I applaud the effort of the subcommittee to “optimize terminology for stroke motor rehabilitation” and support the need for a common language, I fear the recommended definitions are too imprecise to achieve this goal, and may contribute to further confusion.