Background: Muscle Length-Tension Relationship

 The muscle length-tension relationship is the relationship between the length of the fiber and the force that the fiber produces at that length. This length is referring to the length of an isolated fiber and is dependent upon the position of the actin and myosin filaments in the sarcomeres. The sarcomere is the contractile unit of a muscle fiber, and a muscle fiber is composed of thousands of sarcomeres. The actin and myosin filaments are proteins that create cross bridges and are responsible for the contractions, or shortening, of a fiber (see figures).

When an isolated muscle fiber is stretched to the point of minimal overlap of actin and myosin and then stimulated by an electrode to contract , the force of contraction measured is minimal. When an isolated muscle fiber is stimulated to contract when there is optimal overlap of actin and myosin the force produced is maximal. When an isolated muscle fiber is stimulated to contract when there is maximal overlap of actin and myosin the force produced is minimal (see figure). The result is a graph that has a parabolic shape (see figure).

The question you will be asking in this lab is can this situation be applied to whole muscle physiology. Does whole muscle have optimal lengths which produce optimal force of contraction as well as positions of stretch and too much overlap as it operates in the body as a system. You will examine this question by using a bicycle ergometer that measures watts and then calculating force produced. You will perform the exercise at different seat heights which puts the knee at different angles simulating the different lengths of muscle.

Figure shows an electron microscope image of a sarcomere (top), and a schematic diagram of a sarcomere with structures identified.

 

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