We read with great interest the review by Bourke et al (2014). We agree there is a dearth of evidence that any specific intervention results in improved adherence to physical activity guidelines in cancer patients and survivors but would like, respectfully, to offer some further observations.

The authors, in our view, could distinguish more clearly between ‘Physical Activity’ and ‘Exercise’ and acknowledge that ‘sedentary behaviour’ can be independent of physical activity levels. The terms ‘exercise behaviour’ and ‘physical activity’ are not interchangeable. ‘Physical Activity’ refers to body movement produced by the contraction of skeletal muscles and that increases energy expenditure. ‘Exercise’ refers to planned, structured and repetitive movement to improve or maintain one or more components of physical fitness (Chodzko-Zajko et al, 2009). So a person may take little exercise but be physically active with low levels of sedentary behaviour, whereas another might do structured exercises but be habitually inactive and spend long periods sedentary. Thus, the end points of some of the reviewed trials and the methods used to measure those end points require more critical discussion. For example, the authors acknowledge that aerobic exercise tolerance may not reflect aerobic fitness – but neither of those necessarily translates into improved habitual physical activity. The authors acknowledge the challenge of achieving current physical activity guidelines, but do not mention potential end points at the lower end of the physical activity continuum – for example, breaking up sedentary behaviour with light activity – which may be critical in cancer survivors, given recent recognition of the adverse health consequences of high levels of sedentary behaviour in cancer populations (Lynch et al, 2013).

We would also like to draw particular attention to differences between the reviewed studies in the methods used to measure physical activity end points, such as self-report measures, heart rate monitors and accelerometers. Two studies (Pinto et al, 2005, 2013) included in the analysis showed that self-report measures did not correspond with objectively measured physical activity, which we also found in both an observational (Broderick et al, 2013b) and an intervention study (Broderick et al, 2013a). Unless sedentary behaviour and all physical activity, including exercise, are accurately, consistently and objectively measured across studies, using, for example, accelerometers, we think it will be impossible to answer the ‘million dollar’ question of how best to improve habitual physical activity and adherence to guidelines for health benefits in cancer patients and survivors.