Correlation coefficients between self-reported scores and quadriceps asymmetries were higher for KOS-ADLS total score (r = −0.45) compared with KOS-ADLS subscores (r = −0.41), and subjective rating score (r = −0.32). Total KOS-ADLS score correlated (p range = 0.02-0.001) with all RFD asymmetry variables but did not correlate (p = 0.08) with MVC strength asymmetry (Fig. 3). As an observation, all correlation coefficients were greater for women (r = −0.69) than for men (r = −0.06). Similar findings were obtained when using KOS-ADLS subscores instead of total score.
Quadriceps muscle strength is an important predictor of functional abilities in patients having TKAs. However, because several activities of daily life are characterized by a limited time to exert force, quadriceps RFD could better predict functional difficulties than maximal strength [1, 6, 13, 30, 31]. We presumed quadriceps RFD asymmetry is an important functional outcome measure for patients having TKAs. Therefore, we hypothesized that (1) RFD would show larger side-to-side differences compared with MVC strength (construct validity), and (2) RFD asymmetry would correlate with subjective symptoms and/or functional limitations during activities of daily living.
One limitation of this study is related to the use of single-joint isometric contractions, which are rare in daily life. However, measurement of isometric strength in these conditions bears a strong predictive relation with functional capacity , and offers the possibility to assess RFD with valid procedures . It is possible the relation between quadriceps RFD asymmetry and subjective knee function could be improved substantially by using short and rapid concentric and/or eccentric actions of the lower limb muscles performed, for example, on a seated leg press machine . Another second limitation is that we made only side-to-side comparisons and no attempt to compare the outcomes of the subjects undergoing TKA with those of healthy matched control subjects. Although this approach might be criticized , because approximately 40% of patients with unilateral TKA progress to a TKA in the nonoperated knee by 10 years , we believe the approach is valid  for several reasons. First, in addition to involved muscle function assessment, strength of the noninvolved quadriceps deserves consideration because it plays an important role in patients' functional ability [20, 34]. Second, side-to-side comparisons (ie, asymmetries) do not require data normalization by body weight  or BMI [20-22], which enormously simplifies between-study comparisons (Table 3). Third, we quantified the osteoarthritis level of the noninvolved knee, which provided validity to our asymmetry index.
We found side-to-side differences were larger for RFD in the 0 to 200-ms interval (approximately 36%) than for MVC strength (approximately 24%), which confirms the construct validity of quadriceps RFD asymmetry in patients who have TKAs. According to the benchmarks proposed by Sapega , the extent of these side-to-side differences must be considered as “almost certainly abnormal”, more particularly for RFD. Our asymmetries in MVC strength are consistent with those at the same postoperative time reported in several studies [10, 16, 19] (Table 3). Although there have been few studies attempting to quantify neural [3, 10] and muscular [21, 25] impairments after TKA, such quadriceps MVC strength asymmetries are likely associated with reduced neuromuscular activation, reduced muscle size, and probably altered fiber-type composition of the involved quadriceps. According to Mizner et al. , diminished neuromuscular activation is by far the largest impairment after TKA. It has been suggested that the most important factor underlying RFD is fast neuromuscular activation . The observation that RFD asymmetries (0-200 ms) were larger than MVC strength asymmetries in our cohort suggests the occurrence of neural impairment following TKA. We believe it is likely that discharge rate of the activated motor units, particularly at the beginning of maximal actions , was suboptimal for the involved quadriceps, and this inevitably resulted in lower RFD compared with the uninvolved side. The influence of muscle atrophy  and stiffness of the tendon-aponeurosis complex  on RFD impairments, however, cannot be ruled out.
Many daily actions for elderly and weakened individuals such as balance recovery after tripping , fast walking  or stair descending  are characterized by a limited time to develop force (50 to 200 ms, depending on the action), which is considerably less time than it takes to achieve maximal strength (approximately 400-600 ms) [31, 32]. Despite that, quadriceps MVC strength assessment is used as the gold standard to identify muscle weakness in patients having TKAs [10, 16, 19-22, 34]. Our asymmetry values were similar to those previously reported (Table 3). Gapeyeva et al.  observed large side-to-side differences in quadriceps RFD before and 6 months after TKA. However, they did not investigate the possible link between RFD and daily function. Additionally, because RFD was quantified exclusively at 50% of the MVC strength (ie, 200-300 ms after the onset of the contraction), they probably overlooked the most crucial phase for rapid RFD.
We found the subjective knee function of patients who had TKAs was negatively related to all quadriceps RFD asymmetry variables but not to MVC strength asymmetry, and this was particularly true for women. This finding is similar to the positive correlation reported by Suetta et al.  between maximal walking speed and quadriceps RFD in patients who had THAs, while walking speed was not related to MVC strength. More importantly, they observed the postoperative increase in maximal walking speed was correlated to RFD increase, but not to MVC strength improvement . Although we acknowledge correlation does not imply causality, these findings, taken as a whole, suggest quadriceps RFD could be considered as a valid functional outcome measure complementary to MVC strength.
We suggest quadriceps RFD provides an alternative functional outcome measure for individuals undergoing TKA. The ability of rapid force production, which relies on the capacity of fast neuromuscular activation at contraction onset, needs more attention (eg, in TKA rehabilitation programs) because of its relation to symptoms and functional limitation during daily tasks. Specifically, we believe more emphasis is needed on rapid (0-200 ms) rather than on maximal force development following TKA. For example, exercise training programs involving a rapid production of force (ie, ballistic contractions) , particularly during the concentric movement phase , or visual feedback of muscle activation and strength  could be recommended for patients undergoing TKA to promote RFD improvements and minimize side-to-side asymmetries. In addition to maximal quadriceps strength, a rapid RFD is required for good quadriceps function after TKA.
We thank the subjects for participation in the study; Kristina Gabel and Sandra Schatt-Spiss for technical assistance; and the following orthopaedic surgeons who performed total knee replacements: Drs. Tomas Drobny and Stefan Preiss.
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