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Optimal muscle fiber length is defined as the number of sarcomeres in series, and has been shown to be a major component of maximal velocity of shortening during a contraction [26]. Muscle belly fiber lengths can be determined by methods described by Veeger et al [82], where the distance between the most proximal and most distal musculotendinous conjunctions are measured in situ then removed, macerated, and measured again via calibrated microscopic examination.

Tendon slack length is typically measured in situ prior to dissection and after muscular tissue separation. Tendon slack length represents the noncontractile element of the musculotendinous unit and each bundle's tendon slack length is usually quantified (cm) via calibrated microscopic examination.

Pennation angle of muscle fibers represents the angle or direction of pull between the insertion and origin of the muscles. These angles are noted in situ and prior to dissection and the angle of pull can be measured with a goniometer. Of note, how researchers determine individual muscle bundles within each broad fan shaped muscle is subject to much debate. For instance, most hip anatomic studies have divided the gluteus medius into at least three separate bundles based on the broad anatomic insertion sites across the pelvic-iliac crest. Similarly, some authors have combined the illiacus and psoas; while others separate their functions.

Physiologic cross-sectional area of muscle is defined as the number of sarcomeres in parallel and is reported to be directly related to the amount of tension a muscle can produce [26] (muscle mass + fiber length) / pennation angle).

Data from Wickiewicz TL, Roy RR, Powell PL, et al. Muscle architecture of the human lower limb. Clin Orthop 1983;179:275-83; Brand RA, Pedersen DR, Friederich JA. The sensitivity of muscle force predictions to changes in physiologic cross-sectional area. J Biomech 1986;19(8):589-96; Friederich JA, Brand RA. Muscle fiber architecture in the human lower limb. J Biomech 1990;23(1):91-5.

Because muscle moment arms and fiber length may be different within the resting geometry of a muscle, or may change over a given range of motion for a specific muscle, using single lines of action to represent these actions may over-or underestimate each muscle's force generating capacity given a dynamic movement [31,33,34]. Moreover, Herzog and Keurs [36] have shown that lumped parameter models do not accurately predict in vivo force-velocity behaviors for muscles with complex geometries. To illustrate this point further, Blemker and Delp [32] developed a mathematical model of the hip joint in which the complex geometries of the major muscles of the hip over a specified range of hip flexion and extension were estimated from an MRI of a single subject. This technique allowed the researchers to reconstruct and characterize the complex 3D geometries of the hip musculature and to represent each muscle with multiple muscle fibers with varying fiber lengths and with each fiber possessing its own moment arm. This 3D model highlighted the diverse behaviors (please see Figs. 6A-L and 7A-L in Blemker and Delp, Aunals Biomedical Engineering, 2005, pp. 668-9) among individual muscle fibers within a specific hip muscle as well as illustrated the changing roles specific fibers of a particular hip muscle may have while undergoing flexion and extension [31,33,34]. The considerable change in fiber moment arms within each muscle indicates that the force generating capacity of a muscle may in fact change with different femoral, pelvic, or lumbar motions. This is also evident from the work of Arnold et al [37], who suggested that during upright standing with normal femoral anteversion, the medial hamstrings, adductor brevis, adductor longus, pectineus, and ischiocondylar portion of the adductor magnus produce internal rotation via hip internal rotation moments; the gracilis and proximal portion of the adductor magnus produce external hip rotation moments; and, the middle and distal portions of the adductor magnus have negligible rotation moments. When the hip is rotated more than 20°, or when the knee is flexed more than 30°, the rotational moment arms of the semimembranosus and semitendinosus switch from internal to external [37]. The gracilis also becomes more external with hip internal rotation and knee flexion and the moment arm of the ischiocondylar portion of the adductor magnus becomes less internal with internal hip rotation.

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