Significance Muscle-force production and energy consumption are highly dependent on stiffness of the connecting tendinous tissues (tendon and aponeurosis). Although reduced tendinous tissue stiffness favors greater elastic energy recovery, it permits muscle fiber shortening during fixed-end contractions, which is economically unfavorable for force production. Here, we provide in vivo evidence that the longitudinal central aponeurosis stiffness of human tibialis anterior increases not only with force but also with muscle-tendon unit length. Such a mechanism is likely to be beneficial for different movement scenarios for a range of lower limb muscles. These findings are important for interpreting and modeling muscle-force production and energy consumption during movement and understanding muscle and tendon injury mechanics.