## 1 Introduction  
Training-induced muscle adaptations are one of the core elements in training strategies for players, coaches, sports teams, sports federations or non-athletes. The number of studies focusing on muscle architecture has increased due to increasing access to technology for non-invasive muscle visualisation methods, e.g., magnetic resonance imaging (MRI) and ultrasound measurements. For example, investigating relationships between muscle architectural parameters and sports performance, muscle strength or sports injuries, and adaptations resulting from training, detraining, bed rest, or micro-gravity has received attention from researchers. Approximately 65% of PubMed database records containing the term \"muscle architecture\" have been published in the last decade (Supplementary Table S1). The term muscle architecture has a broad definition in the literature and includes the anatomical cross-sectional area (ACSA) and physiological cross-sectional areas (PCSA) of muscles, fascicle length (FL), muscle thickness (MT), muscle length and pennation angle (PA) [1]. These skeletal muscle architectural parameters identify the functional traitsof a muscle [2]. Studies revealed that muscle architectural parameters are predictors of strength [3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14], athletic performance [15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26] and athletic injuries [27; 28; 29; 30; 31; 32; 33; 34]. The upper extremity muscles include muscles involving shoulder joint movements, e.g., rotator cuff muscles, the pectoralis major muscle, and the deltoid muscle; arm muscles, e.g., biceps brachii and triceps brachii; forearm muscles, e.g., flexor and extensor carpi ulnaris, flexor and extensor carpi radialis; and hand and wrist muscles, such as palmaris brevis, lumbrical muscles, hypothenar and thenar muscles [35]. Muscle size parameters of the upper extremity muscles are strongly correlated with better lifting (r = 0.77-0.91) [17], swimming (r = \(-\)0.56) [36], rowing (r2 = 0.195) [37], and shot put performances (r = 0.68) [38]. Additionally, the upper extremity muscle sizes are significantly correlated with the upper extremity strength parameters such as elbow joint torque (r = 0.705-0.945) [39], elbow flexion maximal power (r = 0.81) [40], elbow extensor strength (r = 0.7-0.78) [41], finger extension force (r\({}_{8}\) = 0.85) [42], bench press strength (r = 0.866) and bench throw peak power (r = 0.821) [43], and shoulder external rotation strength (r = 0.287) [44]. Regarding the upper extremity muscles' fascicle geometry, the triceps brachii FL is one of the best predictors of better 200 -m front crawl swimming time (r2 = 0.392) [36] and significantly correlated with better swimming (r = \(-\)0.64) [36] and lifting performances (r = 0.45-0.52) [17]. The triceps brachii PA was significantly correlating with elbow extension strength parameters (r = 0.471-0.563) [45]. Training-induced muscle architectural changes may depend on the exercise's contraction type. Eccentric (lengthening) and concentric (shortening), and isometric training can lead to comparable hypertrophic responses in skeletal muscles [46; 47]. [PERSON] et al. [48] noted muscle size increments are accompanied by pennation angle increases in hypertrophic muscles. By comparison, [PERSON], [PERSON], and [PERSON] [46] highlighted that the underlying myogenic and molecular responses may be different in eccentric and concentric muscle actions because the eccentric training is considered to favour increases in fascicle length, and concentric training to favour higher increments of pennation angle [46]. A recent study by [PERSON] et al. [49] showed that eccentric training can increase fascicle length by increasing sarcomere lengths.