Motor actions are governed by coordinated activation of mono- and biarticular muscles. This study considered differences in mono- and biarticular knee extensors between runners and cyclists in the context of adaptations to task specific movement requirements. Two hypotheses were tested: 1) the length-at-use hypothesis, muscle adapts to have it operate around optimal length; and 2) the contraction-mode hypothesis, eccentrically active muscles prefer to operate on the ascending limb of the length-force curve. Ten runners and ten cyclists performed maximal, isometric knee extensions on a dynamometer at five knee and four hip joint angles. This approach allowed separating the contribution of mono- and biarticular extensors. Three major differences occurred: 1) compared to runners monoarticular extensors of cyclists reach optimal length at larger muscle length; 2) in runners optimal length of the biarticular extensor is shifted to larger lengths; 3) the moment generated by monoarticular extensor was larger in cyclists. Mono- and biarticular extensors respond to different adaptation-triggers in runners and cyclists. Monoarticular muscles seem to adapt to the length-at-use, whereas biarticular muscles were found to be sensitive to the contraction-mode.