Physical activity versus exercise in cancer survivors: incorrect definitions
In their response to Fong et al. (1), Spruit and Wouters (2) have promulgated the use of poor definitions that supposedly distinguish between "physical activity" (body movement that is produced by the contraction of skeletal muscles and that increases energy expenditure) and "exercise" (planned, structured, and repetitive movement to improve or maintain one or more components of physical fitness). There is no need to make the distinction (3). The definitions can be traced to Caspersen et al. (4) but they are flawed. They do not acknowledge isometric muscle actions in which either intentionally or unavoidably, no movement occurs. Such actions characterise performance in competitive sport e.g. balance and statics in gymnastics and activities of everyday living e.g. threading the eye of a needle and undoing screw-top jars. Moreover, as Swammerdam identified more than 300 years ago (5), when muscle is active, it does not reduce in volume i.e. "contract". Knuttgen (6) identified the use of inappropriate definitions and Winter and Fowler (3) proposed a definition both of exercise and physical activity that applies to all situations: "A potential disruption to homeostasis by muscle activity that is either exclusively or in combination, concentric, eccentric or isometric". Fong et al. (1) got it right.
1. Fong DY, Ho JW, Hui BP et al. Physical activity of cancer survivors: meta-analysis of randomised controlled trials. Bmj 2012;344:e70.
2. Spruit MA and Wouters EFM Physical activity versus exercise in cancer survivors: it is a matter of definition! BMJ 2012;344:e70.
3. Winter EM and Fowler N Exercise defined and quantified according to the Système International d'Unités. Journal of Sports Sciences 2009;27:447-460.
4. Caspersen CJ, Powell KE and Christenson GM Physical activity, exercise and physical fitness: Definitions and distinctions for health-related research. Public Health Reports 1985;100:126-131.
5. Needham DM Machina Carnis. Cambridge: Cambridge University Press.
6. Knuttgen HG Force, work, power and exercise. Medicine and Science in Sports 1978;10:227-228.

Competing interests: None declared