Improvement of rate of force development in the quadriceps without increased maximal voluntary contraction through cardiac rehabilitation: A case series of three older patients

Abstract

Cardiac rehabilitation is a comprehensive disease management program for patients with cardiovascular disease, offering established benefits in improving exercise tolerance, quality of life and prognosis.¹ However, with the aging population, there is a growing need for rehabilitation strategies that enhance physical performance and activities of daily living in older patients. The rate of force development (RFD), defined as the velocity of force generation during muscle contraction, has emerged as a key indicator of muscle function, closely linked to daily functional tasks and the risk of falls.^2-4 Despite its potential relevance for designing tailored training regimens, clinical reports on the trajectory of RFD through conventional cardiac rehabilitation remain scarce.

We evaluated RFD in the quadriceps of three patients before and after a 3-month standard cardiac rehabilitation. The program followed guidelines,⁵ and included supervised exercise sessions at the hospital (one session per week: ergometer cycling, treadmill walking and resistance training), physical activity guidance including home exercises and educational classes. The measurement procedure of RFD and its reliability have been previously described.⁶ RFD values were calculated as the mean change in torque per second (Δtorque / Δtime) during intervals of 0–50 ms (RFD₅₀), 0–100 ms (RFD₁₀₀) and 0–200 ms (RFD₂₀₀; unit: Nm/kg [bodyweight]/s). Minimal detectable change (MDC₉₅) for each indicator was calculated using the following formula: MDC₉₅ = standard error of measurement (SEM) × 1.96 × √2. Based on our preliminary data,⁶ the standard deviation (SD) of the two measurements and the interclass correlation coefficient (ICC) for calculating (SEM) were assumed as follows: RFD₅₀, SD 3.3, ICC 0.810; RFD₁₀₀, SD 1.0, ICC 0.918; RFD₂₀₀, SD 0.7, ICC 0.930; maximal voluntary contraction [MVC], SD 0.15, ICC 0.947.

Case 1 was a 76-year-old man who underwent coronary artery bypass grafting suffering from angina pectoris (hypertension, diabetes mellitus, left ventricular ejection fraction 50%, N-terminal pro brain natriuretic peptide 248 pg/mL, no history of heart failure, mean step counts 5197 steps/day at 3 months). Grip strength remained unchanged at 24 kg from baseline to follow up. The Short Physical Performance Battery (SPPB) score improved from 9 to 12 points, indicating a meaningful enhancement in physical performance.⁷ RFD values increased after cardiac rehabilitation, with RFD₁₀₀ and RFD₂₀₀ exceeding MDC₉₅, whereas MVC did not change significantly (Figure 1).

Case 2 was an 81-year-old man hospitalized for heart failure with reduced ejection fraction (dyslipidemia, prior myocardial infarction, left ventricular ejection fraction 20%, prior heart failure hospitalization, mean step counts 2151 steps/day). He experienced no rehospitalizations for heart failure during the follow-up period, and N-terminal pro brain natriuretic peptide levels decreased from 3624 to 1308 pg/mL over 3 months. Grip strength increased slightly from 26 to 28 kg. SPPB scores were stable at 11 points across both time points. RFD values improved over 3 months, with RFD₁₀₀ exceeding MDC₉₅, whereas MVC remained unchanged (Figure 1).

Case 3 was a 75-year-old woman who underwent mitral valve replacement (hypertension, diabetes mellitus, knee osteoarthritis, left ventricular ejection fraction 55%, N-terminal pro brain natriuretic peptide 807 pg/mL, no history of heart failure, mean step counts 4073 steps/day). Although knee pain did not limit muscle strength assessments or SPPB testing, it was reported during daily activities, such as stair climbing. Grip strength slightly declined from 9 to 7 kg. The SPPB score improved from 8 to 11 points, reflecting a clinically meaningful enhancement. Although the increases in RFD₁₀₀ and RFD₂₀₀ did not surpass MDC₉₅, they rose approximately 1.5-fold (150% of baseline) over 3 months (Figure 1).

Two cases with low baseline SPPB scores (case 1 and case 3) showed notable improvements in physical performance despite no significant changes in MVC or grip strength, conventional metrics of muscle strength. Conversely, RFD values showed substantial increases over 3 months post-discharge. RFD also improved, even in a patient with preserved physical function (case 2), unlike MVC. These findings suggest that RFD might serve as a more sensitive measure for assessing improvements in physical performance compared to traditional muscle strength indices. A previous study suggests that the high prevalence of declined muscle function in older patients with cardiovascular disease cannot be explained solely by loss of skeletal muscle mass.⁸ This highlights the multifactorial mechanisms, including neurological factors, contributing to functional deterioration in older patients.⁹ Prior studies have shown that early-phase RFD (≤100 ms) primarily reflects motor unit discharge rate, whereas late-phase RFD (≥200 ms) corresponds to muscle size and muscle-tendon unit stiffness.² Incorporating RFD assessments into rehabilitation programs might facilitate the development of targeted exercise and postural control interventions, thereby enhancing physical performance without necessarily augmenting muscle mass. Future research is warranted to better understand the role of RFD in rehabilitation outcomes, and to explore its potential for guiding tailored interventions within standard cardiac rehabilitation programs.

The authors declare no conflict of interest.

TA contributed to the conception, study design, acquisition, analysis and interpretation of data, and drafted the manuscript for this work. TS and KS contributed to data acquisition, analysis and interpretation. HK coordinated and supervised the project, and interpreted the data. All authors critically revised the manuscript, approved the final version and agreed to be accountable for all aspects of the study, ensuring its integrity and accuracy.

This study was carried out in accordance with the principles of the Declaration of Helsinki and the Japanese Ethical Guidelines for Medical and Biological Research Involving Human Subjects. The study protocol was approved by the Ethics Committee of Nagoya University School of Medicine (approval number: 2023-0223) and the Research Ethics Committee of the School of Health Sciences, Nagoya University (approval number: 22-523). Informed consent was obtained from all participants involved in the study.