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The work loop technique is used in muscle physiology to evaluate the mechanical work and power output of skeletal or cardiac muscle contractions via ''in vitro'' muscle testing of whole muscles, fiber bundles or single muscle fibers. This technique is primarily used for cyclical contractions such as the rhythmic flapping of bird wings〔http://jeb.biologists.org/content/204/21/3587.full〕 or the beating of heart ventricular muscle.〔http://jeb.biologists.org/content/201/19/2723.full.pdf〕 To simulate the rhythmic shortening and lengthening of a muscle (e.g. while moving a limb), a servo motor oscillates the muscle at a given frequency and range of motion observed in natural behavior. Simultaneously, a burst of electrical pulses is applied to the muscle at the beginning of each shortening-lengthening cycle to stimulate the muscle to produce force. Since force and length return to their initial values at the end of each cycle, a plot of force vs. length yields a 'work loop'. Intuitively, the area enclosed by the loop represents the net mechanical work performed by the muscle during a single cycle. ==History== Classical studies from the 1920s through the 1960s characterized the fundamental properties of muscle activation (via action potentials from motor neurons), force development, length change and shortening velocity.〔Hill, A. V. (1970). First and last experiments in muscle mechanics. London: Cambridge University Press.〕 However, each of these parameters were measured while holding other ones constant, making their interactions unclear. For instance, force-velocity and force-length relationships were determined at constant velocities and loads. Yet during locomotion, neither muscle velocity nor muscle force are constant. In running, for example, muscles in each leg experience time-varying forces ''and'' time-varying shortening velocities as the leg decelerates and accelerates from heelstrike to toeoff. In such cases, classical force-length (constant velocity) or force-velocity (constant length) experiments might not be sufficient to fully explain muscle function.〔http://jeb.biologists.org/content/202/23/3377.short〕 In 1960, the work loop method was introduced to explore muscle contractions of both variable speed and variable force. These early work loop experiments characterized the mechanical behavior of asynchronous muscle (a type of insect flight muscle).〔http://rspb.royalsocietypublishing.org/content/152/948/311〕 However, due to the specialized nature of asynchronous muscle, the work loop method was only applicable for insect muscle experiments. In 1985, Robert K. Josephson modernized the technique to evaluate properties of synchronous muscles powering katydid flight〔http://jeb.biologists.org/cgi/content/abstract/114/1/493〕 by stimulating the muscle at regular time intervals during each shortening-lengthening cycle. Josephson's innovation generalized the work loop technique for wide use among both invertebrate and vertebrate muscle types, profoundly advancing the fields of muscle physiology and comparative biomechanics. Work loop experiments also allowed greater appreciation for the role of activation & relaxing kinetics in muscle power and work output. For instance, if a muscle turns on and off more slowly, the shortening and lengthening curves will be shallower and closer together, resulting in decreased work output. "Negative" work loops were also discovered, showing that muscle lengthening at higher force than the shortening curve can result in net energy absorption by the muscle, as in the case of deceleration or constant-speed downhill walking. In 1992, the work loop approach was extended further by the novel use of bone strain measurements to obtain ''in vivo'' force. Combined either with estimates of muscle length changes or with direct methods (e.g. sonomicrometry), ''in vivo'' force technology enabled the first ''in vivo'' work loop measurements.〔http://jeb.biologists.org/content/164/1/1.abstract?sid=106f650c-81c1-4d4e-9cac-4e10039f7131〕 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「Work loop」の詳細全文を読む スポンサード リンク
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