Incidence and Factors Associated with Suboptimal Exercise Workload Progression in Cardiac Rehabilitation: A Retrospective Cohort Study

Background: Improving cardiorespiratory fitness is a central goal of cardiac rehabilitation (CR), yet exercise workload progression varies substantially. We aimed to characterize changes in treadmill workload during CR and determine the frequency of, and the factors associated with, suboptimal progression. Methods: This retrospective cohort study included 180 patients enrolled in an outpatient CR program. Treadmill exercise workload was quantified in metabolic equivalents (METs) from recorded speed and grade using estimated oxygen-consumption equations. Using prespecified exploratory thresholds, suboptimal progression was defined as an absolute increase of <1 MET or a relative increase of <30%, determined by comparing mean workloads achieved during the initial and final four sessions. Factors associated with suboptimal progression were assessed using multivariable logistic regression. Results: Median workload increased from 3.07 METs (IQR 2.40–4.01) during the initial four sessions to 3.90 METs (IQR 2.93–5.56) during the final four sessions (p < 0.001), with improvements observed across baseline exercise capacity, age, and sex strata. Nevertheless, 104 patients (57.8%) achieved <1 MET improvement and 97 (53.9%) achieved <30% relative improvement. In the primary multivariable analyses, female sex and higher body mass index were associated with suboptimal progression under both definitions; a history of coronary artery disease was additionally associated with suboptimal absolute progression, and recent smoking with suboptimal relative progression. Among concurrent training-related factors, lower session attendance was associated with both definitions, whereas higher mean perceived exertion was associated with suboptimal relative progression. Conclusions: More than half of participants demonstrated suboptimal treadmill workload progression despite significant overall improvement during CR. Routine monitoring of session-level workload may facilitate identification of patients with limited progression and support individualized adjustment of exercise prescriptions.