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Architectural gear ratio, also called anatomical gear ratio (AGR) is a feature of pennate muscle defined by the ratio between the longitudinal strain of the muscle and muscle fiber strain. It is sometimes also defined as the ratio between muscle-shortening velocity and fiber-shortening velocity.〔Azizi, E. and Brainerd, E.L. (2007).("Architectural Gear Ratio and Muscle Fiber Strain Homogeneity in Segmented Musculature." ) Journal of Experimental Zoology. 307A:145-155.〕 AGR = εx/εf where εx = longitudinal strain (or muscle-shortening velocity) and εf is fiber strain (or fiber-shortening velocity) In fusiform muscle, the fibers are longitudinal, so longitudinal strain is equal to fiber strain, and AGR is always 1. As the pennate muscle is activated, the fibers rotate as they shorten and pull at an angle. In pennate muscles, fibers are oriented at an angle to the muscle's line of action and rotate as they shorten, becoming more oblique such that the fraction of force directed along the muscle's line of action decreases throughout a contraction. Force output is dependent upon the angle of fiber rotation, so changes in muscle thickness and the vector of change in thickness vary; based upon the force being produced. Due to the rotational motion; pennate muscles operate at low velocities (low shortening distance). The shortening velocity of the pennate muscle as a whole is greater than that of the individual fibers. This gives rise to the property of AGR. Fiber rotation decreases a muscle's output force but increases output velocity by allowing the muscle to function at a higher gear ratio (muscle velocity/fiber velocity). Azizi and Brainerd demonstrated that the gear ratio of pennate muscle can vary; dependent on external load.〔(【引用サイトリンク】title=Variable gearing in pennate muscles )〕 Segmented musculature, like pennate muscle, has fibers aligned at an angle and due to this feature of design, when muscle fibers increase in angle with respect to the medial axis, along with the direction and amount of muscle bulging, the Architectural gear ratio increases.〔〔Brainerd, E.L. and Azizi, E. (2005). Muscle fiber angle, segment bulging and architectural gear ratio in segmented musculature. Journal of Experimental Biology. 208, 3249–3261.〕 A variable gear ratio, based upon different anatomical position, loading and movement conditions, has since been dubbed spatially varying gear ratio. The occurrence of spatially varying gear ratio gives rise to a new insight of muscle biology; “inhomogenous muscle mechanics.〔Shin, David D., Hodgson, John A., Edgerton, V. Reggie, and Shina, Shantanu. (2009). In vivo intramuscular fascile-aponeuroses dynamics of the human medial gastrocnemius during plantarflexion and dorsiflexion of the foot. Journal of Applied Physiology. 107(4): 1276–1284.〕” One feature of the ratio is that there is an optimal gear ratio for each muscle; as the length-tension and force-velocity relationships describe. Length-tension refers to the max tension that can be created over the muscle fiber strain range and force-velocity refers to the power that is possible of the fiber compared to the shortening velocity. These two features of musculature help to define an optimal AGR for a muscle.〔 ==Muscle model== The Architectural gear ratio is explained through the segmented muscle model 3 proposed by Emanuel Azizi, where a muscle segment is shown as a single muscle fiber attached to the myosepta of a ''Siren lacertina'' an aquatic salamander at a certain acute, pennation angle. The model allows segments to bulge out differently in the horizontal, and vertical direction and was used to calculate the Architectural gear ratio of each segment. Preliminary models results show that with muscle bulging, the Architectural gear ratio will increase. Different bulging conditions were studied, and shown in Fig. 2 The model results show the more a muscle bulges in dorsoventral height, the further the muscle fibers shorten, therefore providing a higher Architectural gear ratio.〔 In pennate muscles, segments with higher pennation angles put out less force per shortening muscle fiber. Therefore, the architectural gear ratio of a pennate muscle is higher than the architectural gear ratio of spindle like muscles (e.g. fusiform). A smaller fiber length neutralizes this higher architectural gear ratio if the muscle fibers must be squeezed into the same space.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Architectural gear ratio」の詳細全文を読む スポンサード リンク
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