## 1 Introduction  
Muscle performance changes with musculoskeletal injuries or neuromuscular diseases. Therefore, measurements of muscle strength, power, and endurance are crucial for monitoring recovery and evaluating treatment programmes [1]. Similarly, inadequate muscle performance and muscle imbalance are important risk factors for sports injuries [2]. Muscle contraction can be voluntary, which starts in the higher brain centres, or involuntary, induced by a reflex or electrical stimulation [3]. Isokinetic dynamometry is the gold standard for measuring muscle strength and power [4], but its inaccessibility often limits its use. Tensiomyography (TMG) is a non-invasive, cost-effective assessment method of the contractile properties of skeletal muscles [5]. It records time (Tc), delay (Td) and relaxation (Tr) of contraction and transverse muscle displacement (Dm) during electrically induced muscle twitch [6]. TMG parameters, such as maximum Dm and Tc, are associated with muscle stiffness and the rate of force generation [7]. Due to its practicality and affordability, TMG is increasingly being used to analyse muscle performance in athletes and for functional diagnostics in injured athletes. However, the functional validity of TMG parameters for the assessment of voluntary muscle contraction remains unclear. Research shows that certain TMG parameters correlate with athletic performance. In cyclists, Dm and other parameters are mainly related to muscle strength [8], while in football players, they are related to both power and speed in jumps and sprints [9]. However, not all studies have confirmed statistically significant associations. [PERSON] et al. [10] studied female rugby players and found a negative correlation between performance on a cycle ergometer and Dm, Vc and Td of the vastus lateralis, but no significant relationship with the rectus femoris or vastus medialis. Only a few studies have investigated the relationship between TMG and isokinetic torque, with mixed results. [PERSON] et al. [11] reported positive correlations between knee flexor strength (60\({}^{\circ}\)/s) and the Vc of the vastus lateralis and semitendinosus, and between extensor strength and the Vc of the rectus femoris and the Tc of the vastus medialis. Later, they found a negative correlation between a lower Dm of the biceps femoris and greater flexor and extensor strength in women [12], with stronger associations in inactive individuals than in elite athletes. In another study, the strongest correlations were observed in individuals with low strength, which decreased with increasing strength. In the biceps femoris and rectus femoris, an increase in strength was associated with Tc and Dm [13]. These results suggest that TMG may assist in the assessment of contractile properties of muscles, but its relationship to isokinetic measurements remains unclear. In addition to muscle strength, isokinetic dynamometry is commonly used to assess various muscle ratios, including the ratios between flexors and extensors of the knee joint. These ratios are critical for assessing muscular balance and joint stability. An imbalance between the flexors and extensors has been shown to increase the risk of joint injury [14]. Previous studies have shown the importance of both the conventional ratio of maximum concentric torques between flexors and extensors and the functional ratio, calculated as the ratio between the maximum eccentric torque of the knee flexors and the maximum concentric torque of the knee extensors. The latter has become increasingly important for a more accurate prediction of injury risk [15; 16]. The balance between knee flexors and extensors plays a crucial role in the success of rehabilitation and joint function [17].