Length-tension relations come in two types--the classical type with linearly descending limb, and the "flat," higher type. The classical type, now confirmed in several laboratories, is obtained when sarcomeres are servo-controlled to maintain constant length. The flat type, also confirmed in several laboratories, is obtained in fixed-end contractions, where some sarcomeres have the opportunity to shorten at least slightly. We find that the higher tensions seen in the flat type are indeed the result of very small shortening: when isometric sarcomeres shorten to a slightly shorter (e.g., by 40 nm) length, they go on to produce considerably more isometric tension than if they had remained at the shorter length throughout contraction. We term this phenomenon "shortening-induced tension enhancement." The phenomenon accounts not only for the higher, flatter length-tension relation seen in fixed-end contractions but can explain the creep of tension that occurs in extended tetani. Thus, several issues concerning the interpretation of length-tension relations are resolved by this newly discovered phenomenon.