In human skeletal muscle, muscle size and strength are inextricably linked (1). A number of hypertrophy-inducing mechanisms are affected by the stress delivered to the working muscle (2), which in turn can be manipulated internally by joint-angle mechanics (i.e. altering moment arm of in-series elastic components). Simultaneously, joint-angle (and hence muscle length) will also determine the amount of stretch the muscle is undergoing. In vivo adaptations following training at different joint-angle specific ranges-of-motion are unknown. Two training groups – Ex (aged 19±2.2 years; n=10) and Flx (21±3.4 years; n=11) undertook 8 weeks of resistance training of the quadriceps, with concentric/eccentric knee extension over either a relatively extended (Ex- i.e. 50degrees-0deg, where 0deg is a straight leg) or flexed (Flx- i.e. 90deg-40deg, where 90deg is a right angle) range of motion, followed by 4 weeks detraining. Quadriceps muscle strength, vastus lateralis architecture, size and subcutaneous fat were measured at weeks 0, 8, 10 and 12 using B-mode ultrasonography, surface electromyography and dynamometry. A control group (aged 23±2.4 years; n=10) was also monitored during this period. Following training, greater relative increases in fascicle length (proximal, central and distal [25%, 50% and 75% of femur length respectively] mean change Flx 29%±4 vs. Ex 14±4%), strength, anatomical cross-sectional area (aCSA) (distal), and subcutaneous fat (central and distal) were present in Flx compared to Ex (p<0.05, see Figure 1).The relative decrease during detraining was greater in Ex than in Flx in strength, aCSA and fascicle length (p<0.05). To our knowledge, this is the first study to demonstrate through manipulating training mechanics, greater functional, architectural and morphological adaptations in Flx vs. Ex training. This effect is likely to be due to the different mechanical stress and stretch experienced by the muscles (3). In practice, where strength, lean mass and fascicle length adaptations could be advantageous in optimizing performance, exercise through a more flexed range-of-motion should be practiced.