The mechanical efficiency of the quadriceps femoris muscle-tendon unit likely depends on its structural and mechanical properties. Therefore, the purpose of this study was to assess the mechanical efficiency in vivo under various stretch-shortening cycle conditions and to investigate how the morphological and mechanical properties of the quadriceps femoris influence mechanical efficiency. We used MRI to measure quadriceps femoris muscle and tendon morphological properties in young females (n 5 9), and we determined mechanical efficiency during stretch-shortening cycle contractions using computer-controlled dynamometer. Testing protocol included contractions with moderate and maximal pretension level and stretching loads of 20 and 100J. Greater mechanical efficiency was associated with larger knee flexion angles and increased positive work under moderate pretension levels with both 20 J (r 5 0.67, P 5 0.045; r 5 0.82, P 5 0.007) and 100 J stretch loads (r 5 0.87, P 5 0.006; r 5 0.82, P 5 0.007). These findings suggest that lower stretching loads enhance muscle-tendon interaction efficiency by favoring tendon elongation during muscle-tendon unit lengthening, resulting in higher mechanical efficiency. No morphological or mechanical parameter of the muscle-tendon unit were linked to mechanical efficiency, suggesting that efficiency may depend more on muscle activation patterns than on structure.  Kulcsszavak: MRI, dynamometric, physiological cross-sectional area, knee extensor muscles, ultrasound