For much of 2018, I had the good fortune to work with an international group of experts in exercise science who were reviewing and assessing the application of the American College of Sports Medicine's FITT (Frequency, Intensity, Time, Type) model to the oncology population.1 I was specifically charged with examining the responsiveness of measures of physical function to FITT-based exercise prescriptions. Painter et al2 define physical function as the ability to perform basic actions (ie, mobility, strength, and endurance) that are essential for maintaining an individual's independence and carrying out more complex activities. Physical function is predicated on muscle strength and aerobic capacity and is therefore presumably responsive to interventions that improve these fundamental physiologic properties. This not to say that clinical assessment of physical function simply provide surrogate measure of strength and aerobic capacity. Rather, measures of physical function reflect the integration of these fundamental physiologic properties with higher-level activities (balance, sensory integration, motor planning, etc) required for an individual to be maximally active and participatory. The Timed Up and Go Test, for example, assesses not only strength but also balance, mobility, and fall risk. The distance covered in the 6-Minute Walk Test is frequently used as a surrogate measure of aerobic capacity, but an increase in this distance can also be, and perhaps more correctly so, interpreted as an increase in general functional capacity.3 The stair climb test assesses one's ability to ascend stairs but is predicated on the power-generating capacity of the involved muscles.4 So, assessing the impact of the FITT model on clinical measures of physical function seemed appropriate and reasonable.
So, what have I learned from my efforts? First, few studies using exercise training as a treatment intervention for cancer survivors utilized physical function tests to assess the adaptive responses to exercise training. The tests most frequently reported included repeated sit-to-stands (5 times and 30 seconds), Timed Up and Go Test, 6- and 12-Minute Walk Tests, 400-m Walk Test, and gait speed—usual and fast. Different testing protocols were used for the same test, making comparisons across different studies difficult. Often, when functional outcome measures were reported, they were reported only as secondary outcomes, with assessments of aerobic capacity and strength being the primary outcome measures. Summary conclusion in many studies often extended the findings from a single physical functional test to support a general conclusion that exercise training had changed overall physical function. Such conclusions implicitly suggest that the results from a 6-Minute Walk Test and a Timed Up and Go Test are equivalent, giving results from individual tests an inappropriate global perspective of physical function. Studies combining aerobic and strengthening interventions were more consistent in bringing about improvement in physical functional capacity than were studies focusing on these interventions individually.
What was not reported in the reviewed studies is interesting as well. Minimally important clinical differences, the smallest change in an outcome measure that is of value to the patient, were not reported in any of the reviewed studies, further limiting the clinical utility of outcomes from FITT-based interventions.5 Many of the reviewed studies used a treatment intervention that lasted 12 or more weeks and included regular progression of the exercise prescription as defined by the FITT model. Results from physical function tests were only reported for baseline and conclusion of study time-points, missing changes in physical function that may have occurred over the course of the intervention. Reporting intermediate changes would have broadened our understanding of the relationship between exercise prescription and physical function, thus informing clinical decision-making FITT prescriptions, clinical practice guidelines, and health policy downstream. Such information can provide important insight into the optimal FITT prescription needed to positively change physical functional capacity. Presenting intermediate physical, functional, and physiologic outcomes in a time-based graph would have provided additional insight into temporal relationships between FITT model prescriptions and changes in physical function outcomes. Presenting the results of correlational analysis between physiologic outcomes and physical function outcomes would have provided additional insights into this relationship.
Strength and aerobic capacity are often described in terms of their maximal capacity, that is and 1 repetition max (1-RM). These measurements provide valuable information that can be used to define exercise intensity and demonstrate physiologic improvement, but rarely do clinicians ask their patients to perform at their maximum capacity. Most of life's activities involve a continuum of events performed over time at a submaximal exertional level and not as a series of maximal exertional events. Intuitively, endurance and strength are needed for one to successfully be active and participate, thus making assessments of muscle endurance seem like a more reasonable outcome measure of an exercise intervention. As such, assessment of muscle endurance—the length of time a resistance can be moved before fatigue sets in—may be of more clinical interest than assessment of maximal physiologic capacity. This is particularly true of a clinical population in which fatigue and its effects on activity and participation are well-recognized adverse effects. So, perhaps, assessment of muscle endurance should be used as an outcome measure in exercise training studies and clinical settings.
Several of the reviewed studies included the results of a stair climb test, the time required to ascend a set number of stairs.5 Clinicians immediately recognize this test as assessing an important functional task; physiologists recognize these results as an assessment of power, the rate at which work is done. Any clinical assessment tool that incorporates a time component is a measure of power so that gait speed, Timed Up and Go Test results, and the shuttle walk test are fundamentally tests of power. When we seek to increase gait speed, we are seeking to increase the power output of an individual, which, in turn, reflects changes in fundamental physiologic properties. Assessment of power was relatively infrequent, but when reported, this measure provided a reminder that (1) results from such tests can provide insight into the effectiveness of FITT-based exercise prescriptions and (2) clinicians might want to consider using tests of power as an outcome measure when assessing the physical function of cancer survivors.
In summary, the work I have done over the past several months points out that much remains to be learned in terms of linking the fundamentals of FITT-based exercise prescription to the practical arena of physical function in the cancer survivor population. Equally important, this work reinforces the notion that cancer survivors will derive significant benefit from greater collaboration between exercise scientists and rehabilitation professionals, a notion discussed elsewhere.6
I hope everyone is having a wonderful spring.
G. Stephen Morris, PT, PhD, FACSM
1. American College of Sports Medicine. ACSM's Guidelines for Exercise Testing and Prescription. 10th ed. Philadelphia, PA: Wolters Kluwer; 2018.
2. Painter P, Stewart AL, Carey S. Physical functioning: definitions, measurement, and expectations. Adv Ren Replace Ther. 1999;6(2):110–123.
3. Carter R, Holiday DB, Stocks J, Grothues C, Tiep B. Predicting oxygen uptake for men and women with moderate to severe chronic obstructive pulmonary disease. Arch Phys Med Rehabil. 2003;84(8):1158–1164.
4. Bean JF, Kiely DK, Herman S, et al The relationship between leg power and physical performance in mobility-limited older people. J Am Geriatr Soc. 2002;50(3):461–467.
5. McGlothlin AE, Lewis RJ. Minimal clinically important difference: defining what really matters to patients. JAMA. 2014;312(13):1342–1343.
6. Morris GS. Who is the best exercise professional to provide an exercise training program in an oncology rehabilitation setting-It depends! Rehab Oncol. 2018;36:185–187.