Background During isotonic contraction, skeletal muscle shortens against a load. The velocity of muscle shortening is related to the load applied and can be graphically represented by the Hill equation: (F + a)(V + b) = (F o + a)b Equation 1 Where F is the load, V is the velocity of shortening, F o is the maximum force that can be developed by the muscle during an isometric contraction, and a and b are constants. This equation results in a hyperbolic curve. To make it more useful as a tool, this equation can be linearized to produce the following equation: F = b [(F o – F)/V] – a Equation 2 Now if we graph [(F o – F)/V] and load we get a straight line with a slope of b and a y-intercept of –a. This is a useful form of the equation that allows us to use the power of regression to calculate the constants and to predict certain variables from measured parameters. More specifically, it allows us to calculate the velocity of muscle shortening when applying any load. In animal systems, muscles provide locomotor capacity by attaching to the skeletal system and moving the bones in a lever fashion. There are three major classes of levers (1 st class, 2 nd class, and 3 rd class) and most of the lever systems in animals are 3 rd class with the muscle contraction closer to the fulcrum and on the same side as the load. However, other lever systems do exist in animal systems and the one we are going to focus on today is the 2 nd class lever of the gastrocnemius muscle. 2 nd class levers also have the load and muscle contraction on the same side of the fulcrum, but in this case the load is between the fulcrum and the lever. We can use torques (torque = force x moment Muscle Contraction 2 arm) to measure the mechanical advantage of a lever system and to determine the force of contraction necessary in a given musculoskeletal system. We can also look at the animal's velocity attributable to contraction of the gastrocnemius. To do this, we use rotational physics with the fulcrum as the pivot point and the load and muscle contractions as points rotating about the pivot. In a rotating body, all points along a line have the same angular speed (ω), however, not …