The study findings supported our hypotheses and demonstrated that individuals with severe KOA displayed a significantly decreased quadriceps RFD. Moreover, the decrease in the quadriceps RFD was greater than that in maximum quadriceps strength in individuals with severe KOA based on a comparison of the ES between the quadriceps RFD and maximum quadriceps strength.
The mechanism of the early (range 0-100 ms) RFD was associated with neural drive, contractile properties, and fiber type composition, and the late (range 0-200 ms) RFD was associated with muscle size, muscle strength, neural drive, and the stiffness of the tendon-aponeurosis complex15. For fiber type, skeletal muscle fibers are broadly classified as “slow-twitch” (type Ⅰ) and “fast-twitch” (type Ⅱ). The RFD is an index that reflects explosive muscle strength and is thus indicative of type Ⅱ muscle fibers16. Type Ⅱ muscle fibers are more affected by age-related atrophy than type Ⅰ muscle fibers17. However, it has been reported that atrophy of type Ⅱ muscle fiber is not observed in patients with severe KOA compared with that in age-matched elderly controls16. Moreover, there was no significantly decreased early RFD in severe KOA in this study (data not shown). Therefore, the involvement of fiber type in this study is considered to be small. For neural drive, neural activation and rapid neuromuscular activation of the quadriceps were impaired in KOA patients18, 19. Moreover, in this study, RFD was adjusted for peak torque; therefore, the effects of muscle size and muscle strength were considered to have been excluded. For this reason, individuals with severe KOA displayed a significantly decreased quadriceps RFD, and the quadriceps RFD exhibited a large ES due to the effect of neural drive and the stiffness of the tendon-aponeurosis complex.
In this study, participants with severe KOA did not exhibit significantly decreased maximum quadriceps strength. A systematic review reported that decreased maximum quadriceps strength was associated with an increased risk of symptomatic and functional deterioration but not radiographic tibiofemoral joint space narrowing (JSN)7. The results of the maximum quadriceps strength in this study support this review. Therefore, in this study, participants with severe KOA did not display decreased maximum quadriceps strength, and maximum quadriceps strength exhibited a small ES.
For these reasons, individuals with severe KOA displayed a significantly decreased quadriceps RFD rather than maximum quadriceps strength, as the ES was larger for the quadriceps RFD than for maximum quadriceps strength.
The clinical implications of our results are described as follows. First, weakness in the quadriceps RFD may be related to the early detection of severe KOA development. Therefore, the development of severe KOA, which can be determined only by radiography or magnetic resonance imaging (MRI), may be easily determined by measuring the quadriceps RFD if causal relationships are identified in future investigations. Second, interventions for RFD of the quadriceps muscle may lead to the prevention of severe KOA. Since the weakened quadriceps muscle strength can lead to a failure of harmful load distribution in the knee joint4, 5, the quadriceps muscle functions seem to be a general shock absorber during knee joint load. Moreover, excessive mechanical stress on knee cartilage due to muscle weakness has been suggested to contribute to degenerative processes20. In the present study, quadriceps RFD was found to be more strongly related to severe KOA than maximal quadriceps strength. Therefore, if a causal relationship between quadriceps RFD and radiographic severity is found, improving the quadriceps RFD may lead to the prevention of severe KOA.
This study has several limitations. First, the participants were motivated individuals given that they actively enrolled by responding to e-mails and were recruited via public relations magazine advertisements; thus, there may have been selection bias. Second, since a power calculation was not performed, the relationship between severe KOA and weakness of maximum quadriceps strength cannot be determined. Third, there was a small number of individuals with severe KOA, which may bias the incidence of severe KOA events. Therefore, in the present study, univariate analysis was performed at the same time as multivariate analysis, and the results of the multivariate analysis were confirmed to be unchanged. Moreover, to reduce the number of independent variables and reduce the effect of bias in the logistic regression analysis, we conducted the logistic regression analyses using a propensity-adjusted model. However, it is necessary to increase the sample size and confirm the results with different analysis methods in the future. Finally, given the cross-sectional nature of this study, causal associations between severe KOA and the quadriceps RFD could not be determined. Further investigations, including prospective studies that clarify causal associations, are required to confirm our results.