The goals of the present study were (1) to measure the previously unstudied isometric forces of activated human Gracilis (G) muscle as a function of knee joint angle and (2) to test whether length history effects are important also for human muscle. Experiments were conducted intraoperatively during anterior cruciate ligament (ACL) reconstruction surgery (n = 8). Mean peak G muscle force, mean peak G tendon stress and mean optimal knee angle equals 178.5 +/- 270.3 N, 24.4 +/- 20.6 MPa and 67.5 +/- 41.7 degrees, respectively. The substantial inter-subject variability found (e.g., peak G force ranges between 17.2 and 490.5 N) indicate that the contribution of the G muscle to knee flexion moment may vary considerably among subjects. Moreover, typical subject anthropometrics did not appear to provide a sound estimate of the peak G force: only a limited insignificant correlation was found between peak G force and subject mass as well as mid-thigh perimeter and no correlation was found between peak G force and thigh length. The functional joint range of motion for human G muscle was determined to be at least as wide as full knee extension to 120 degrees of knee flexion. However: the portion of the knee angle-muscle force relationship operationalized is not unique but individual specific: our data suggest for most subjects that G muscle operates in both ascending and descending limbs of its length-force characteristics whereas, for the remainder of the subjects, its function is limited to the descending limb, exclusively. Previous activity of G muscle at high muscle length attained during collection of a complete set of knee angle-force data showed for the first time that such length history effects are important also for human muscles: a significant correlation was found between optimal knee angle and absolute value of % force change. Except for two of the subjects, G muscle force measured at low length was lower than that measured during collection of knee joint-force data (maximally by 42.3%). (C) 2010 Elsevier Ltd. All rights reserved.