Work Loop

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 or the beating of heart ventricular muscle.

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.

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. 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.

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