Our prospective study using objectively measured physical function with 20 years of follow-up data revealed that breast cancer survivors aged 65 and over at baseline demonstrated worse decline in physical function compared to women without cancer and such changes varied depending on the physical function component examined. As expected, physical function significantly declined over time across all components measured for both cases and controls during the pre-diagnosis period. After cancer diagnosis, there was a significantly greater decline for grip strength and rapid walking speed among breast cancer survivors relative to cancer-free controls. For quadriceps strength, there was a sharp, short-term loss after breast cancer diagnosis followed by a gradual improvement over time. We did not observe significant differences in the rate of change for other measures (including timed chair stand, grip speed at regular pace or stride length) between cases and controls.
Our study is the first to prospectively examine the impact of breast cancer on objectively measured physical function. When pre-/post- diagnosis measures from two time points closest to breast cancer diagnosis were analyzed, declines were significantly worse in breast cancer survivors than controls for gait speed and quadriceps strength. While not measured in this study, the short-term physical function declines between pre- and post-diagnosis may likely be due to the direct and indirect effects of cancer treatment. Indeed, such changes can precipitate sedentary physical activity patterns that can exacerbate mobility difficulties  and systemic deconditioning . Moreover, breast cancer survivors are often treated with multiple aggressive therapies that are known to contribute to functional declines in older adults [23–26].
The short-term physical function decline we observed in older breast cancer survivors is consistent with findings from some studies [11, 12, 26, 27], while other studies reported non-significant differences in older breast cancer survivors compared with their cancer-free peers [9, 28–30]. However, all the previous studies were based on self-reported physical function and most did not include an appropriate cancer-free control group to represent ‘usual aging,’ such that it is hard to determine whether physical function changes are attributable to breast cancer and its treatments. This is especially problematic in studies of older women, given the high prevalence of aging symptoms.
Among several studies that included pre-/post-breast cancer measures and cancer-free women, Satariano et al.  found that at 3 months after diagnosis, cases were approximately twice as likely as age-matched controls to report upper-body limitation, especially for women younger than 75 years. At one year after diagnosis, younger breast cancer patients showed the greatest improvement . Kroenke et al.  reported a similar finding in the Nurses’ Health Study: younger (< 65 years) but not older (65 or older) breast cancer survivors experienced worse functional limitations compared with women without cancer. However, both studies were based on assessments conducted at two points in time without examining the long-term impacts of the disease on trajectories of functional aging. One longitudinal study based on 15 years of follow-up of self-reported physical function on both pre-/post-diagnosis measurements observed that breast cancer survivors had dramatic physical function declines one-year post-diagnosis and then began to return to levels of physical function similar to their pre- diagnosis status . These findings are consistent with the long-term trends we observed for quadriceps strength.
Although self-reported physical function has been used widely to monitor treatment-related symptoms and physical function decrements in cancer survivors, it is subjective and may be prone to bias. Furthermore, it cannot easily quantify and distinguish various physical components and thus it is hard to evaluate which aspects of function may be most affected by the cancer diagnosis and therefore which components are targets for interventions. Using objective measurements of physical function can overcome these limitations. However, we only identified one small cross-sectional study that examined objective measures of physical function in breast cancer survivors , and one cohort study based on older adults with any cancer . The small cross-section study with one-time measurement reported that breast cancer survivors had significantly lower short Physical Performance Battery score, longer chair stand times, and lower handgrip strength than controls, but similar walk speed . The cohort study observed that a steeper decline in gait speed prior to a cancer diagnosis and an accelerated declines in appendicular lean mass after a cancer diagnosis, compared with cancer-free controls .
Our study observed a worse decline in hand grip strength and rapid walking speed but not regular pace of gait speed in breast cancer survivors. This suggests that the maximum gait speed (walk as fast as safely possible) may be more sensitive to disease-related function decline than regular pace. Gait speed is a well-known indicator of functional decline and predictor of mortality in older adults and is frequently used in geriatric settings as a quick and reliable way of monitoring the functional capacity of older adults including cognitive ability [34–36]. Gait speed depends on the function and coordination of the musculoskeletal, visual, central nervous, and peripheral nervous systems . It reflects mobility and dynamic balance, and quadriceps strength reflects lower body muscle function. Lower-limb muscle strength is linked to walking speed. Gait speed should be considered not only as a motor function but an integrative measure of health. Our findings indicate that the impacts of breast cancer on physical function may include both upper and lower body muscle function and mobility and dynamic balance. Our findings suggest that interventions to build skeletal muscle strength may improve physical function thereby positively affecting mobility among breast cancer survivors. Studies have suggested that structured training programs for older adults including resistance exercise can increase muscle strength and physical functioning . Our study also highlights a need for routine assessment of physical function including grip strength, gait speed and leg strength by primary care physicians to help monitor and prevent physical function decline. As most individuals complete the intensive phase of cancer treatment during the first year after diagnosis, interventions to help them return to their pre-cancer status more quickly could be beneficial.
Strengths of our study include use of age-matched cancer-free comparison groups, objective assessment of physical function and using multiple assessments of physical function over time to assess long term impact of breast cancer. Several limitations also deserve mention. First, we examined incident breast cancer during the follow-up; thus, the length of measurement of physical function between pre and post- diagnosis varied, which may affect assessment of physical function. Second, we were unable to adjust for information on cancer treatment. Third, our sample includes only older, non-Hispanic white women and thus may not generalize to other populations. Another limitation is there were no direct measures of cardiorespiratory fitness which is known to be linked to mobility and physical activity.
In conclusion, our prospective study using objectively measured physical function with 20 years of follow-up data revealed that older breast cancer survivors had significantly worse declines in grip strength and rapid walking speed and a sharp short-term drop followed by gradual improvement over time in quadriceps strength compared to women without cancer. Our findings suggest that targeted exercise training among older breast cancer survivors focused on developing body muscle strength and mobility would improve physical functioning and thereby improve the healthy life span of this growing population. Further work is needed to understand how cancer treatments may impact the trajectories of specific physical function components in this growing population.