## 1 Introduction  
Muscle function (i.e., strength and power) is positively correlated with overall health, independence, and quality of life in aging [1]. The rate of decay of muscle function parallels the progressive loss in muscle mass from age 65 (1.4-2.5% per year for strength and \(\sim\)3.5% per year for power) leading to adverse health outcomes and reduced ability to carry out functional tasks of daily living (e.g., recovering balance, walking, sitting and standing from a chair) [2; 3; 4; 5; 6; 7; 8]. This, in turn, triggers a vicious cycle of reduced mobility, frailty, and loss of independence that further amplifies the deterioration of health [9]. To stop this cycle, early identification of individuals at high risk of loss of muscle mass (sarcopeniaa) and muscle function (dynapenia) allows a timely and effective intervention for those most in need. Low-cost/accessible assessment and periodic monitoring of muscle function in older adults are instrumental to this aim. Force-velocity (F-v) and power-velocity (P-v) profiling are typically used to characterize muscle function. Compared to methods that rely on maximal effort (such as one repetition maximum (1 RM) or the estimation of 1 RM with the repetition-based method),this indirect approach is less affected by motivation, less physically demanding, and reduces the risk of acute injury. Moreover, these relationships allow a full characterization of muscle function: maximum force (F0), velocity (V0), power (Pmax), and optimal velocity (velocity eliciting maximum power, Votp) [5, 8]. In turn, these indexes inform us of the prevailing limitation in a given individual (e.g., a loss of maximal or specific strength would suggest a loss in muscle mass or contractile quality; a prevalent loss of velocity would indicate that the prevalent problem resides in the efficacy of neuromuscular activation) [5], and provide the bases for individualized exercise programs [10, 11]. These characteristics make F-v and P-v profiling particularly valuable approaches for assessing maximal strength and power in older adults [8]. Typically, the F-v/P-v relationship is detected either by assessing velocity during a movement executed at different loads (isotonic evaluation) or by collecting force data during a movement executed at different velocities (isokinetic evaluation) [1, 10]. The gold standard method for the determination of the lower-limb F-v relationship consists of a knee-extension test performed on an isokinetic dynamometer (ISO) [1]. However, this method requires expensive equipment and qualified personnel and is poorly correlated with everyday life activities, as it only examines single-joint movements [12]. In the search for more cost-effective and ecological approaches, alternative multi-joint movements (e.g., leg press, Nottingham power rig) with increasing loads have been used for \"field\" F-v profiling in older adults [10, 13]. Still, this requires the use of relatively expensive and/or non-portable machines. Furthermore, the guidance offered by the machine during the movement nullifies the expression of coordination and balance ability. The 5 Sit-to-Stand (SSTS) test is a simple, low-cost testing approach that is widely used in clinical settings to indirectly assess lower limb muscle power [14]. It evaluates a multi-joint, everyday life movement and its results are strongly associated with markers of physical function (i.e., handgrip strength, walking speed, short physical performance battery score) in older adults [15]. These promising features have suggested the use of a modified version of the 5 STS test for the determination of the F-v profiling in a group of predominantly female older adults [12]. While the study did not find a correspondence between indexes derived from 5 STS test F-v profiling and isokinetic F-v profiling, we speculate that the use of non-gold standard instruments and protocol for the 5 STS (i.e., video camera analysis, number of trials, and choice of load condition) may have affected the correspondence between measures.