## 4 Discussion
Other authors, who used a hyperbolic or hybrid fitting of the F-v profile, found higher values (\(\sim\)10 and \(\sim\)4.5 rad \(\times\) s\({}^{-1}\), respectively) than ours [5; 26]. This difference is likely due to the fitting model. Even though hyperbolic fitting is likely more appropriate for single-joint movement, we decided to use a homogeneous linear fitting for both tests and chose the linear model that is preferable for the multi-joint sit-to-stand action [21]. This is clearly an arbitrary decision. Interestingly, if we use a hyperbolic fitting to build the F-v relationship in ISO tests, our data align well with the latter authors (V0 ~10 and Vopt ~4.25 rad \(\times\) s\({}^{-1}\)). Regarding V0 (0.9 \(\pm\) 0.2 m \(\times\) s\({}^{-1}\)) and Vopt (0.4 \(\pm\) 0.1 m \(\times\) s\({}^{-1}\)) results derived from the 5 STS test, we found lower values from those found in the literature (6 \(\pm\) 7 rad \(\times\) s\({}^{-1}\) that approximatively correspond to 1.9 \(\pm\) 2.2 m \(\times\) s\({}^{-1}\)) [12]. However, the V0 values are similar to those found for a more comparable leg press exercise in older adults (~0.9 m \(\times\) s\({}^{-1}\)) [10]. In order to compare movement velocity in the two tests, the angular velocity of the ISO test was transformed into linear velocity. Maximum and optimal velocity results were different and poorly correlated (r = 0.23, \(p\) < 0.001) between ISO and 5 STS. Bland-Altman analysis reported a significant bias (V0, bias = \(-\)1.6 m \(\times\) s\({}^{-1}\), \(p\) < 0.001; Vopt, bias = \(-\)0.8 m \(\times\) s\({}^{-1}\), \(p\) < 0.001) with high limits of agreement (V0, L.O.A. = [\(-\)2.3 to \(-\)1.0 m \(\times\) s\({}^{-1}\)]; Vopt, L.O.A. = [\(-\)1.1 to \(-\)0.5 m \(\times\) s\({}^{-1}\)]). Our results confirm that velocity parameters extrapolated from the F-v relationship seem to have less concurrent validity and precision than other indexes [21]. Therefore, these parameters could have less relevance in medical screening and clinical assessment. The second aim of this study was to investigate the relationship between maximum force and power with markers of muscle mass and strength. In our study, F0 and Pmax were highly correlated with all body and specific lean muscle mass for both tests (r from 0.65 to 0.82). These results are comparable to those found by [PERSON] et al. [7] between STS power and leg muscle mass (r = 0.80). It is well known that grip strength is a strong predictor of mortality, disability, complications, and length of stay [30]. Furthermore, this index represents the main reference value of the general strength for the diagnosis of sarcopenia [31]. The force values for the handgrip test (27.3 \(\pm\) 5.6 kg for females; 41.2 \(\pm\) 6.9 kg for males; 34.8 \(\pm\) 9.4 kg tot) are consistent with those presented in the literature for healthy older adults of both genders (~26 kg for females; ~40 kg for males) [32; 33]. Furthermore, Pmax was highly correlated with handgrip strength for both tests (r = 0.75), as in [PERSON] et al. (r = 0.67) [34]. Recently, it has been asserted that handgrip strength alone would be insufficient as a measure of overall muscle strength in clinical practice [35]. In fact, muscle power would appear to be a better indicator of loss of muscle function than muscle strength alone [36]. In addition, [PERSON] et al. found that lower limb muscle power (from a jump test) was approximately 2-fold more correlated with all physical performance tests than handgrip strength [36; 37]. Furthermore, a better association was found in this study between lower limb strength/power and appendicular lean mass indices with respect to handgrip strength. This suggests that lower-limb strength tests could better reflect both the condition of physical function and appendicular lean mass in older adults. The present study has some limitations. Due to logistic limitations, it was not possible to divide the subjects into two groups to randomize the order of the tests.