Skeletal muscle powers joint movements via force production that is length-dependent. The force-length (FL) relationship is often measured from maximally activated muscle preparations, but muscles are rarely maximally recruited in-vivo. Increased joint range-of-motion and stride frequency that accompany increases in movement speed likely require muscle to produce greater force over a broader range of muscle-tendon unit lengths. Accordingly, we hypothesized that increasing activation duration, from a twitch (single stimulus) to a tetanic (train of stimuli) contraction, would broaden the plateau of the FL curve: the range of operating lengths where muscle reaches near-maximal force. We tested this hypothesis in two mouse hindlimb muscles, soleus and tibialis anterior, which differ in fiber type composition (slow vs fast), fiber architecture (parallel vs pennate), mechanical function (plantar vs dorsiflexor), and size (small vs large). To broaden our comparison, we also included muscles from other species (rat and duck). Our hypothesis was supported by evidence of increased FL plateau width from twitch to tetanic contractions in mouse soleus (n = 5, 38.1 ± 10.6%; mean ± S.E.M.) and tibialis anterior (n = 5, 22.7 ± 12.1%), as well as the rat medial gastrocnemius (n = 7, 17.9 ± 3.8%) and by preliminary results from duck lateral gastrocnemius (n = 1, 29.8%), both of which are large, mixed-fibered, and pennate plantar flexors. We anticipate these data from muscles with varying form and function to help determine if muscle structure, size, or function is the better predictor of FL plateau width.