While recent research has evaluated the three energy systems in breast cancer survivors [19], this study has expanded that research into women with metastatic breast cancer. Additionally, it is the first to use a separate assessment to target each energy system, allowing development of initial reference values related to performance in each test. Findings of this research provide further support of the need to individualize exercise programs beyond general guidelines [28]. Research by Yee et al [29] reported aerobic fitness in women with metastatic breast cancer as 21% lower than healthy controls and impacting functioning, but no data were found assessing anaerobic fitness. Given the symptom burden of fatigue and reduced physical functioning reported by metastatic patients [30–32], and the positive associations between physical activity participation and fatigue reduction as well as improved physical functioning, research is needed that further dissects the link of both aerobic and anaerobic fitness to such symptoms. Central to enabling this is a more comprehensive baseline assessment [11, 33], with this research providing support for scope to include energy system assessments for a more complete cardiorespiratory fitness measurement. Particularly, including both objective and subjective measures as part of these assessments, such as HR and RPE alongside fatigue and functionality self-report outcomes, can provide health professionals working with oncology patients a more patient-centered approach to assessment and prescription.
While maximal VO2 tests are gold standard for cardiorespiratory capacity determination [34], they require extensive equipment, may be uncomfortable for the participant and have limited use in women with metastatic breast cancer [9]. However, studies in non-metastatic breast cancer survivors provide support for the use of submaximal, HR-based tests in this population, with an example study finding a 90% VO2 corresponds with 89.1% age-predicted maximum HR [22]. The findings from this current study, alongside previous research, suggest HR can be used as a marker of more individualized intensity [16, 19, 35, 36]. Additionally, quantifying intensity via an objective (HR) and subjective (RPE) measurement may provide a more balanced and clinically feasible approach to prescribing exercise, as done in the present study and supported by findings that higher HR was accompanied by increased RPE.
Worth noting is that anaerobic alactic task performance resulted in a lower average maximum HR (119.5 [15.3] bpm) and RPE (5.8 [1.4]) than those measured during the higher-intensity anaerobic lactic test (164.7 [4.0] bpm; 9.6 [0.6]). Such findings, however, may have resulted from only 5, compared to 70, women completing the adapted burpees versus 30-STS, and these 5 completed it after the submaximal aerobic test and 30-STS. However, a higher HR and RPE was expected with adapted burpees compared to 30-STS due to longer time and combined upper- and lower-limb movement. Average % maximum HR also reflected this, with participants in the adapted burpees actually exceeding 85% maximum HR (94.0% [3.3]), although not accompanied by other symptoms warranting test cessation. Despite these expectations though, there exists potential influences of testing order and fatigue, as all tests were performed the same day. As such, future research could conduct each task on three separate days. Additionally, as women completing the modified burpees test exceeded the 85% maximum HR but were not symptom-limited, this further supports the necessity to further research on suitable exercise intensities in this population, particularly from an individualized basis. For consistency in clinical translation, health professionals conducting these energy system assessments could still ensure 85% maximum HR is used as a cut-off to maintain consistency with other submaximal testing.
A key translation from this research is the benefit of using measurements that patients can use to self-monitor exercise performance and intensity. As findings support an objective measure (HR) correlating with a subjective measure (RPE), health professionals can provide education on using RPE to both gage exercise intensity and monitor progression. Additionally, the anaerobic tasks provide a baseline repetitions number, which can be used to guide more personalized exercise prescription and give patients progression towards self-management. For example, women could monitor the number of repetitions they are able to perform before reaching a certain RPE on various days and develop an exercise routine for ‘high’ versus ‘low’ days. This would also allow patients to better self-monitor their overall functioning ability and potential worsening of health that may warrant medical follow-up.
As done in the present study, a more physically comprehensive patient assessment can allow better establishment of activity tolerance and baseline by progression determined by individual response. For example, lack of walking balance or inability to walk unaided on a treadmill during the aerobic test stopped a few participants from being able to undertake this task, but they were still able to complete at least one of the anaerobic tasks. As such, these patients may require a cycling-based aerobic assessment to determine whether to prioritize aerobic or anaerobic fitness. Additionally, while the energy system demands for the aerobic assessment are less than those for the anaerobic assessments, this research found the perceived physical demand was reversed for some women. That is, patients reported that the unilateral loading required with walking posed a greater challenge than the anaerobic task or tasks they undertook. As such, health practitioners should take into account such individual factors and design assessments accordingly, attempting to get a baseline for each of the energy systems where possible for program integration.
Additionally, this research also supported potential benefit of integrating subjective and objective assessments for a more patient-centered approach. Research developing an assessment method for cancer-related fatigue using an integrated subjective (PFS-R) and objective (30-STS) measure highlights the scope of exercise testing in better monitoring patient symptoms and allowing personalization [37]. All 70 participants undertook the 30-STS test (Fig. 1), supported by a relatively high functionality reported (lower-limb: 68.4% [26.4] and upper-limb 70.8% [20.3]). However, only five of these individuals met criteria to progress to the modified burpees, which potentially is reflected by a moderate level of CRF (4.5 [2.5] out of 10) [38]. Future research should aim to look at correlations between self-reported measures such as the PFS-R and energy system assessment results to further examine ways of providing a more comprehensive patient assessment.
As the main limitations, care must be taken in interpreting these results, as all participants were female metastatic breast cancer patients with moderate functioning but also moderate fatigue. Regardless, findings support incorporating objective assessment of the three key energy systems with such self-report measures, as only five individuals were able to undertake all three assessments. Additionally, on average participants were classified as ‘overweight’ based on BMI (26.8 [5.2] kg/m2), with recent research suggesting a task-dependent increase in fatigability with increased BMI [39]. Future research should aim to incorporate lower-functioning participants, further investigate the relationship between body composition and functional ability and examine reliability and validity of these tests in metastatic breast cancer. Additionally, although women reported a moderate level of lower-limb functionality, six were unable to undertake the treadmill test due to physical limitations and multiple others reported it posed a significant biomechanical challenge compared to the anaerobic task(s) that used bilateral leg functioning. As such, future research should examine a cycling-based or other seated aerobic task and participant progression ability as such. There also remains a need to undertake longer-term training at various intensities, purposely targeting each system, to observe more chronic effects on both objective and subjective measures. Important to note is no adverse events were associated with completion of this testing, with testing progression carefully based on symptom-free participation and return to baseline HR during and following the previous task. For both health professionals and patients, these findings support integration of both objective and subjective measurements of effort to improve exercise prescription and monitoring, as well as providing a more personalized assessment and prescription that considers all three energy systems as tolerated.