Fundamental Functional Properties of Skeletal Muscle
The isometric length-tension curve represents the force a muscle is capable of generating while held at a series of discrete lengths. When tension at each length is plotted against length, a relationship such as that shown below is obtained.
While a general description of this relationship was established early in the history of biologic science, the precise structural basis for the length-tension relationship in skeletal muscle was not elucidated until the sophisticated mechanical experiments of the early 1960s were performed (Gordon et al. 1966). In its most basic form, the length-tension relationship states that isometric tension generation in skeletal muscle is a function of the magnitude of overlap between actin and myosin filaments.
The force generated by a muscle is a function of its velocity. Historically, the force-velocity relationship has been used to define the dynamic properties of the cross-bridges which cycle during muscle contraction.
The force-velocity relationship, like the length-tension relationship, is a curve that actually represents the results of many experiments plotted on the same graph. Experimentally, a muscle is allowed to shorten against a constant load. The muscle velocity during shortening is measured and then plotted against the resistive force. The general form of this relationship is shown in the graph below. On the horizontal axis is plotted muscle velocity relative to maximum velocity (Vmax) while on the vertical axis is plotted muscle force relative to maximum isometric force (Po).
What is the physiologic basis of the force-velocity relationship? The force generated by a muscle depends on the total number of cross-bridges attached. Because it takes a finite amount of time for cross-bridges to attach, as filaments slide past one another faster and faster (i.e., as the muscle shortens with increasing velocity), force decreases due to the lower number of cross-bridges attached. Conversely, as the relative filament velocity decreases (i.e., as muscle velocity decreases), more cross-bridges have time to attach and to generate force, and thus force increases. This discussion is not meant to provide a detailed description of the basis for the force-velocity relationship, only to provide insight as to how cross-bridge rate constants can affect muscle force generation as a function of velocity.
Muscles are strengthened based on the force placed across the muscle. Higher forces produce greater strengthening. Therefore, exercises performed with muscle activated in a way that allows them to contract at high velocities, necessarily imply that they are also contracting with relatively low force. This is intuitively obvious as you lift a light load compared to a heavy load—the light load can be moved much more quickly. However, these rapid movements would have very small strengthening effects since the muscle forces are so low.
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