Application of the principles of exercise training

The principle of specificity was the principle most commonly included in the papers reviewed, although one-third of the 29 studies did not adequately apply this principle to the development of the exercise prescription. Overall, the aerobic exercise interventions consistently included the principle of specificity by choosing a population vulnerable to decreased cardiovascular fitness (ie, during chemotherapy6 ,25 ,31) or selecting aerobic fitness as a primary outcome.19,–,21 ,23 ,28 ,29 Studies were judged as unclear if they did not justify an appropriate intervention for increasing the stated outcomes. For example, in an aerobic intervention consisting of step aerobics or circuit training (which included jumping rope and skate-jumping), multiple outcomes were measured, including upper body strength, which is unlikely to be affected by the intervention.27 Two studies did not apply the specificity principle because they either attempted to decrease fatigue in a population that was not experiencing fatigue26 or used aerobic activity, rather than resistance exercise to improve lean body mass.22 Resistance-only interventions appropriately used the principle of specificity by matching the intervention to the study objective of increasing lean body mass,32 determining incidence of lymphoedema with weight training33 or increasing muscle strength.34 Of the mixed interventions, 9 of 12 studies justified their combined use of aerobic and resistance training (or comparison of) with their choice of both aerobic fitness and muscle strength outcome measures.11 ,35 ,37,–,41 ,43 ,44 However, in three of the mixed intervention studies, it was unclear how specificity was applied because outcome measures specific to resistance training (eg, muscle strength) were not included.36 ,42 ,45 When an exercise prescription does not match the target outcomes, non-significant findings would not be surprising and may lead to false conclusions regarding exercise efficacy. Specificity can be achieved with consistent and appropriate selection of exercise modes, outcomes and populations.

The principle of progression was reported more often in resistance training interventions than aerobic interventions. Aerobic interventions were considered adherent to the principle if they clearly described the increases in frequency, intensity or duration (time) of the training programme over time. For example, Courneya et al19 prescribed an aerobic exercise programme that progressed from 15 to 35 min/week, by increasing exercise time by 5 min every 3 weeks. Studies that stated the intervention was progressive, but provided no detail on how the programme progressed, provide little useful information for clinical application of a training programme.6 ,25 ,30 ,31 For example, one study was only progressive for the first 3 weeks of the 8-week long intervention, with no justification as to why progression did not continue.23 In the resistance-only interventions, two of the three studies clearly described the progression in terms of number of sets and weight lifted.32 ,33 Of the mixed interventions, four studies described progression for resistance but not for aerobic exercise35 ,40 ,41 ,43 and five studies failed to include progression at all.36,–,38 ,42 ,45 It may be more straightforward to prescribe and report progression for resistance exercise than aerobic. As a participant is able to perform an exercise with a specific load correctly for the prescribed number of repetitions, they are often asked to increase resistance the next session. It may be perceived as more difficult to prescribe increases in training intensity for aerobic exercise because monitoring intensity may be more difficult for patients to do (eg, heart rate monitors, walking pace). Progression of the duration of aerobic exercise is easier to administer and will build endurance, but may not provide enough overload to increase maximal aerobic capacity.49 Building progression into prescribed protocols is necessary for long-term exercise benefits and should be incorporated into all protocols. Similarly, achieving the prescribed progression and/or describing modifications to the training programme is important information for others seeking to replicate a specific training programme in the community setting.

If exercise is expected to yield improvements in outcome measures, a training programme must adhere to the principle of overload by ensuring the prescription is greater in terms of frequency, duration or intensity, than what the individual is already doing. If an individual does less exercise during the intervention than she was doing prior, she may actually detrain and experience no fitness improvements. This could be erroneously interpreted as failure of the exercise programme itself. Few of the aerobic interventions correctly used the principle of overload by prescribing intensity based on measured initial fitness.19 ,21 ,23 ,28 Use of the principle was unclear in two aerobic intervention studies that based their prescription on estimates of maximum heart rate and the validity of this approach in breast cancer survivors is unknown.22 ,26 In addition, three aerobic studies did not adequately describe how initial fitness levels were determined.6 ,30 ,31 Two of three resistance interventions based their prescription on initial fitness,32 ,33 but one was unclear because it did not justify the target intensity and limited participants from lifting more than 20 pounds for safety precautions.34 Of the 12 mixed interventions, 3 studies prescribed intensity based on measured initial fitness,11 ,35 ,39 2 studies were unclear as they reported overload in the resistance portion only of the intervention40 ,43 and 7 mixed studies did not report the use of overload.36,–,38 ,41 ,42 ,44 ,45 For example, one study conducted 20 min of group-based activity twice a week, although the inclusion criteria required that participants must not have been exercising vigorously for three times a week, thus providing the opportunity for detraining in some participants.36 To ensure that sufficient overload is achieved, future studies should strongly consider prescribing exercise relative to measured baseline fitness. Symptom-limited cardiopulmonary exercise testing has been shown to be safe in the cancer population.50 However, a recent review by Jones et al51 examined the use of exercise testing during supervised exercise interventions focused on increasing aerobic fitness (measured as peak oxygen consumption or VO_2peak) in cancer survivors and reported that the majority of studies did not adhere to the fundamental principles of exercise testing, as recommended by national and international guidelines for exercise testing. This finding highlights the utility of exercise testing to determine baseline values to aid in the development of effective exercise prescriptions and also acknowledges that even when baseline testing has been used to determine initial values, there may be limitations to the accuracy and reliability of these results. In terms of assessing muscular strength, Schmitz et al33 have demonstrated the safety and efficacy of using one repetition maximal strength testing for both upper and lower extremities in a weight training intervention for breast cancer survivors with lymphoedema.

Recruiting participants who have lower initial values of the outcome of interest at baseline allows more room for improvement and also maximises the potential that overload is sufficient. This principle was typically applied by excluding individuals already physically active at baseline, or including active individuals who were about to begin chemotherapy because they were likely to experience declines in functional capacity during treatment.11 ,23 ,37 Five studies were unclear about application of the initial values principle. For example, Mock et al26 had a primary aim to reduce fatigue with aerobic exercise during treatment and while they excluded physically active women, they did not recruit a sample with high baseline fatigue levels. The initial values principle is important for establishing realistic expected outcomes for the sample. An individual with low levels of aerobic fitness could see greater improvements in aerobic fitness than more fit persons, but even small improvements in fit persons may be clinically relevant. Furthermore, measuring initial values provides information for developing the most effective exercise prescription. Knowing initial values allows the exercise prescription to be tailored to the individual or exercise group and allows the use of other principles of exercise training, such as overload.

Currently, there is little known about diminishing returns or reversibility in the exercise oncology literature, as the majority of studies have not included follow-up periods. Only one study met our criteria for evaluating reversibility. Daley et al20 conducted follow-up measures 24 weeks after completion of an 8-week aerobic exercise intervention. Physical activity levels decreased from end of study to follow-up and aerobic fitness declined towards baseline. Although Mutrie et al42 also conducted follow-up at 6 months after their supervised group aerobic and resistance intervention, physical activity levels over the follow-up period were not reported, leaving it unclear if the observed decline in submaximal aerobic capacity is due to declines in physical activity. Only one trial met our criteria for including the principle of diminishing returns. Schmitz et al32 used an immediate exercise group (intervention 0–6 months) and delayed exercise group (intervention 6–12 months) and compared both at 12 months. Those from the immediate exercise group who continued to exercise continued to improve, but had less improvement during the follow-up period than they did during the intervention – a prime example of the ceiling effect that can occur with long-term exercise training. Evaluating effects of long-term exercise is an important future area of research so that the lasting benefits of exercise can be established and a realistic time course of a training programme can be mapped.

Reporting of the components of the exercise prescription and adherence to prescribed exercise

Of the 29 papers included in this review, one-third failed to report a minimum of one component of the FITT exercise prescription. Without information on the basic components of the exercise prescription, it is impossible for others to replicate the exercise programme and difficult to evaluate whether or not the intervention was appropriate to achieve the desired outcome. Exercise intensity was the least reported component of the FITT prescription. Intensity is a key component of the exercise prescription, and the appropriate intensity needed to elicit the most therapeutic results is largely unknown. Only when researchers begin to report the prescribed intensity can we begin to determine the minimum intensity that elicits clinical benefits and the maximum intensity beyond which risk of injury is increased and the prescription provides little additional benefit.

Furthermore, reporting of adherence to the prescribed exercise protocol is also of great importance moving forward. Simply stating attendance at exercise sessions does not reveal the exact intensity and duration of exercise that was completed which have implications in relation to observed changes or lack thereof. Reporting sufficient detail about the amount of exercise actually performed by participants enables a more accurate interpretation of study outcomes and, in turn, appropriate translation of the programme in a non-research setting.

Recommendations for developing exercise prescriptions for future studies

Based on the findings of this review, we suggest the following recommendations for future exercise intervention trials in breast cancer survivors. When developing the exercise prescription, consider matching the exercise prescription to the outcome of interest (specificity). For example, if muscular endurance is a key outcome, consider a programme of lighter resistance and higher repetitions to achieve this aim. The use of objective baseline testing of the outcome of interest is also recommended to guide the development of an appropriate exercise prescription that will provide a physiological stimulus (overload) and, therefore, will result in training improvements. Building progression into your prescription and outlining how this progression will be achieved should be included in future trials to ensure that the principle of overload is maintained as participants may otherwise experience a plateau in physiological change. Using objective measures of the outcome of interest to determine initial values and taking initial values into account when planning your programme is also recommended and investigators should be ready to customise the programme across a range of baseline fitness levels. If possible, adding a follow-up visit to the research study or community programme will provide information on exercise maintenance in participants, and provide valuable information on the long-term benefits of physical activity. Finally, when reporting the exercise prescription, and adherence to the exercise prescription, include all components of the FITT principle. For adherence, we suggest this type of detail, 'during the intervention participants completed a mean ± SD: 3.5 ± 1.2 days/week, at 70 ± 5% heart rate reserve, for a total of 165 ± 25 min/week of aerobic exercise, which was primarily brisk walking (on treadmill or outside), elliptical machine or cycle ergometer.'