# Morphological and Mechanical Symmetry of the Quadriceps and Hamstrings in Athletes
###### Abstract  
Sports performance can be influenced by several multifactorial conditions. Symmetries between sides can interfere with muscle strength, impacting performance. Balance in morphological aspects and muscle strength is desirable between dominant (DO) and non-dominant (ND) lower limbs to improve performance or to prevent injuries. The study aimed to compare quadriceps and hamstring muscle morphology and knee extensor and flexor torque while analyzing whether cross-sectional area (CSA) and echo intensity (EI) can better explain maximal concentric torque in competitive athletes. Sixteen competitive athletes of team sports were evaluated. Quadriceps and hamstring CSA and EI were assessed using a B-mode ultrasound. The maximal torque production capacity of the quadriceps and hamstrings was evaluated using an isokinetic dynamometer protocol. Our backward multiple linear regression models indicated that the quadriceps cross-sectional area (CSA) and echo intensity (EI) of the dominant (DO) and non-dominant (ND) limbs explain 61-79% of concentric torque production at 60\({}^{\circ}\)/s and 300\({}^{\circ}\)/s, with CSA showing a positive relationship and EI a negative one. Likewise, hamstrings CSA and EI account for 70-77% of torque production at the same angular velocities, both exhibiting positive relationships with torque output. Our data present bilateral mechanical and morphological symmetries in lower limbs of amateur athletes and potential predictive concentric torque capacity of CSA and EI for quadriceps and hamstrings. dominant and non-dominant limbs; symmetries/asymmetries; sports performance 202515210202515  
A strength imbalance of 10-15% or more between limbs and/or muscle groups can be considered problematic [6], as it is recognized as a risk factor for hamstrings injuries [7]. Such imbalances may arise from factors like asymmetrical loading patterns, commonly observed in sports that require single-leg jump tasks [8], as well as previous injuries and other intrinsic factors [4; 9; 10]. Discrepancies in muscle force production can lead to altered movement mechanics, increasing asymmetrical stress on musculoskeletal structures and elevating the risk of injuries [11]. The cross-sectional area (CSA) and echo intensity (EI) of muscle can be strong indicators of force-generating capacity [12]. Muscle CSA reflects the amount of parallel muscle tissue. Therefore, higher CSA is generally associated with a greater capacity to generate muscle force [13; 14]. Quadriceps and hamstrings CSA are directly linked to muscle force production and performance in specific tasks and sports [14; 15; 16; 17; 18; 19; 20; 21]. Although CSA is one of the most assessed morphological variables able to predict torque production capacity, it does not provide real measurements of the quantity of pre-sent contractile tissue [12]. Therefore, muscle quality assessed through EI serves as a valuable metric for quantifying muscle function using the gray scale index, where darker (black) areas indicate more contractile tissue, while lighter (white) areas indicate greater amounts of adipose and connective tissue [12; 22]. Unlike CSA data, the present authors did not find any study using only EI as a dependent variable in regression models as a predictor of muscle force. Some correlation models indicate considerable relationships between muscle quality (EI) and force production [23; 24; 25; 26]. In middle-aged and elderly individuals, there is a significant correlation of EI with force production (r = \(-\)0.4), and when combined with muscle thickness, this could justify 29% (r\({}^{2}\) = 0.29) of muscle strength [24]. Furthermore, there are significant correlations between EI and isometric/concentric peaks of torque at 60, 180, and 360\({}^{\circ}\)/s (range from r = \(-\)0.48 to r = \(-\)0.64 and, respectively) [27]. However, [PERSON] et al. [28] found no relationship between upper (elbow flexors) or lower limb (knee extensors) EI and muscle strength.