###### Abstract  
Currently, limited information is available on the influence of quadriceps femoris muscle-tendon unit morphological parameters on mechanical efficiency; therefore, the aim of this study was to investigate the interaction between these variables. The morphological characteristics of the quadriceps femoris muscle-tendon unit of nine healthy females (\(22\pm 0.9\) years) were assessed using magnetic resonance imaging. The mechanical efficiency was assessed using a dynamometer. During contractions, the electromyographic activity of the vastus lateralis and rectus femoris was recorded. The highest mechanical efficiency was achieved with moderate pretension and a 20 J stretch load, resulting in an efficiency of 54.14 \(\pm\) 2.24%. A large correlation was found between the patella tendon cross-sectional area and mechanical efficiency (r = 0.93; \(p<\) 0.01). No correlation was observed between the muscle-to-tendon cross-sectional ratio and mechanical efficiency. The association between the patella tendon cross-sectional area and mechanical efficiency aligns with previous research, suggesting that a larger cross-sectional area can reduce tendon stress under a given force, potentially lowering the risk of strain injuries. The lack of association between the muscle-to-tendon cross-sectional ratio and mechanical efficiency indicates that the relative cross-sectional dimensions of muscle and tendon do not influence muscle-tendon dynamics. MRI; ultrasound; dynamometric 2025 15 3047 2 are arranged parallel to each other, this will increase the thickness (i.e., the cross-sectional area) of the tendon [3,4]. A larger cross-sectional area will reduce the stress on the tendon when a given force is applied, potentially decreasing the risk of strain injuries [5]. This idea has been proposed in earlier studies, suggesting that there is an optimal range for the tendon cross-sectional area (CSA) relative to the muscle (1:34) that could reduce injury risk [6]. Habitual loading may result in an increased patellar tendon CSA [1], indicating that collagen synthesis in the patellar tendon can be facilitated by regular exercise [4]. Since tendon morphological adaptation is slower than that experienced in the muscle, intervention studies often fail to observe increments in tendon CSA; instead, evidence suggests substantial increases in tendon stiffness [7,8]. These inconsistent results could be explained by the insufficient duration of the intervention to produce a robust tendon hypertrophy response, or the region of the observed tendon may not be responsive to training-associated adaptation [1]. As mentioned earlier, muscles can adapt more quickly to mechanical loading by increasing muscle mass (the cross-sectional area) [9]. The size of a muscle is related to its tendon size [10,11], suggesting that the magnitude of loading may also influence tendon size. A larger muscle can produce greater force [12,13]; hence, the tendons of a larger muscle can be subjected to a greater load. Region-specific increases in the patellar tendon CSA can be achieved through 12 weeks of resistance training [1,14], indicating that the relationship between the muscle and tendon area might play a role in efficient movement execution. In addition to the tendon CSA, the tendon length and the magnitude of length changes affects the amount of strain energy that can be stored and recoiled in the tendon, which consequently determines the efficiency of muscle and joint work. However, tendon stress, strain, and stiffness are interrelated [1]; therefore, the tendon CSA relative to muscle size could influence mechanical work efficiency. The elastic properties of the tendon can be utilized most in the stretch-shortening cycle (SSC) contraction, where tendons can store elastic strain energy during lengthening and return a significant amount of the stored energy during the subsequent shortening phase [15,16].