Reliability and Validity of the Rate of Force Development for Quadriceps in Older Patients with Cardiovascular Disease

<b>Background/Objectives</b>: The rate of force development (RFD), which is the change in force over a period of time during muscle contraction, quantifies rapid muscle contractions. RFD may serve as a measure of physical rehabilitation in patients with cardiovascular disease (CVD); however, its reliability and validity in older patients remain unclear. This study examined the reliability and validity of quadricep RFD in older patients with CVD. <b>Methods</b>: This prospective study enrolled 30 outpatients undergoing cardiac rehabilitation (median age, 77 years) and 30 inpatients hospitalized for CVD (median age, 76 years). The quadricep RFD values at three time points (RFD₅₀, 0-50 ms; RFD₁₀₀, 0-100 ms; and RFD₂₀₀, 0-200 ms) were calculated from the slope of the force-time curve. Physical performance was assessed using the Short Physical Performance Battery (SPPB). Intra- and inter-rater correlation coefficients were assessed for outpatients. The correlation coefficients between RFD values and physical performance indicators were assessed separately for outpatients and inpatients. <b>Results</b>: The intraclass correlation coefficients (1,1) and (2,1) for RFD₅₀, RFD₁₀₀, and RFD₂₀₀ were 0.742, 0.893, and 0.873 and 0.810, 0.918, and 0.930, respectively. The correlation coefficients for SPPB with RFD₅₀, RFD₁₀₀, and RFD₂₀₀ were 0.553, 0.547, and 0.597 (all <i>p</i> < 0.05), respectively, for inpatients; similar moderate correlations were observed for gait speed and the chair stand test. <b>Conclusions</b>: The test-retest reliability of the RFD was excellent in older patients with CVD. The RFD was positively correlated with physical function indicators, suggesting its validity as a measure of physical rehabilitation.