## 2 Materials and Methods
### Dynamometric Data Collection and Process  
To evaluate mechanical efficiency (ME), the participants performed several SSC contractions using a preset energy level to stretch the quadriceps based on the protocol of [PERSON] et al. [18]. This preset energy (20 J or 100 J) was applied by the dynamometer, which controlled the eccentric contraction. Specifically, the eccentric phase halted once the participants generated the determined amount of work (i.e., stretch load) during stretching, after which the knee immediately extended back to the starting joint position (Figure 1). Each SSC contraction began with the knee at a 30\({}^{\circ}\) flexion angle. The participants were instructed to exert force against the dynamometer's lever arm as quickly and powerfully as possible, first stopping within a limited range of motion during the eccentric phase and then extending the knee through the concentric phase. When participants reached 50% or 100% of their previously measured MVC torque at 30\({}^{\circ}\) flexion, the dynamometer initiated the stretching of the quadriceps MTU. During the knee extension phase, the dynamometer motor was turned off, and resistance was generated solely from the lever arm's friction and inertia along with that of the lower leg. Sufficient rest time (at least  
Figure 1: An example dataset from one participant of dynamometry measurements, MTU dynamics, and EMG activity of the quadriceps femoris muscles during SSC contraction with a 20 J stretching load and a 50% preload (**left** panel) and a 100 J stretching load and a 100% preload (**right** panel). The first row panels present the torque–angle signals in the function of time. The horizontal dashed line marks the initial isometric force levels at which the movement started. Abbreviations: VL–vastus lateralis, VI–vastus intermedius, VM—vastus medialis, RF—rectus femoris, BF—biceps femoris long head, ST—semitendinos muscles, MVC—maximal voluntary isometric contraction, MTU—muscle–tendon unit. 5 min) was provided between each trial to minimize the effects of fatigue. Torque and knee angle data were sampled at a rate of 1000 Hz. Each contraction featured an incrementally increasing stretch velocity, ending at the terminal velocity of 30\({}^{\circ}\)-s\({}^{-1}\). The mechanical work in each contraction phase was derived from the torque-angle data using a custom MATLAB script (v2022a, MathWorks, Natick, MA, USA) by integrating the torque-position curve over the range of motion limits [23]. The work calculation followed Equation (1):  
\[W(\mathrm{J})=\int\limits_{\theta_{1}}^{\theta_{n}}M_{(\theta)}\cdot d \tag{1}\]  
where \(M=\) torque, d\(\theta=\) angular displacement, and \(\theta\)1 and \(\theta\)n represent the first and the last knee angle data points, respectively. In this study, the mechanical work performed during the eccentric contraction (knee flexion phase) was considered as negative work, while the work during knee extension (concentric phase) was considered positive. The ME (\(\eta\%\)) of the quadriceps femoris was calculated by dividing the positive work by the sum of the total and negative work. The formula used was as follows:  
\[\eta(\%)=\frac{W_{pos}(J)}{W_{tot}(J)+W_{neg}(J)}\cdot 100 \tag{2}\]  
where Wp is the positive work calculated during the concentric phase and Wtot is the total work which was expressed as the energy stored in the motor when the active muscle stretch was initiated (20 and 100 J) and is represented the amount of mechanical work required to stop the lever of the dynamometer. The tendon force was calculated by dividing the torque with the angle-specific moment arm length (see later). Then, the peak tendon force was extracted from each trial and was used to calculate the tendon stress as it was divided by the mean cross-sectional area of the tendon. The model developed by [PERSON] [24] was applied to estimate changes in the MTU length of the quadriceps femoris muscle heads during contraction.