Exercise Physiology

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Force Length tension relationship part 1
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.

Force length tension relationship part 2
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 mysoin filaments.
Force-velocity Relationship
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.
Skeletal Muscle Structure
Skeletal muscle comprises the largest single organ of the body. It is highly compartmentalized, and we often think of each compartment as a separate entity (such as the biceps muscle). Each of these individual muscles is composed of single cells or fibers embedded in a matrix of collagen. At either end of the muscle belly, this matrix becomes the tendon that connects the muscle to bone. Muscle cells contain most of the structures common to all cells. Each cell is enclosed by a cell membrane or plasmalemma; they contain mitochondria for the oxidative metabolism of nutrients; and all the machinery necessary for protein synthesis. Skeletal muscle fibers are multinucleated and can be as much as two centimeters long. The principal force generating components are actin and myosin molecules. These myofilaments are arranged in interdigitating matrices capable of sliding across each other. To produce force, crossbridges from the myosin filaments associate with the actin filament, then rotate slightly to pull the filaments across each other (much like the oars of a rowboat pull across the water). Muscle fibers, though, are just the building blocks for whole muscles. The precise way in which fibers are arranged into muscle is referred to as architecture.
The Cross-bridge Cycle
In its simplest form, biochemical experiments on muscle contractile proteins have shown that, during the cross-bridge cycle, actin (A) combines with myosin (M) and ATP to produce force, adenosine diphosphate (ADP) and inorganic phosphate, Pi This can be represented as a chemical reaction in the form A + M + ATP -> A + M + ADP + Pi + Force (Equation 1) However, we also know that upon the death of a muscle, a rigor state is entered whereby actin and myosin interact to form a very stiff connection. This can be represented as A + M -> A.M "rigor" complex (Equation 2) If actin and myosin can interact by themselves, where does ATP come into the picture during contraction? Experiments have demonstrated that the myosin molecule can hydrolyze ATP into ADP and Pi. In other words, M + ATP -> M + ADP + Pi (Equation 3) Scientists now agree that ATP serves at least two functions in skeletal muscle systems: First, ATP disconnects actin from myosin, and second, ATP is hydrolyzed by the myosin molecule to produce the energy required for muscle contraction. This description of the different biochemical steps involved in muscle contraction is referred to as the Lymn-Taylor actomyosin ATPase hydrolysis mechanism. (Webb and Trentham, 83) The relationship between the Lymn-Taylor kinetic scheme and the mechanical cross-bridge cycle is not fully known.
The four-step cross-bridge cycle Lymn and Taylor proposed that their biochemical data could be incorporated into a four-step cross-bridge cycle:
  1. The actin-myosin bridge very rapidly dissociates due to ATP binding to myosin.
  2. The free myosin bridge moves into position to attach to actin, during which ATP is hydrolyzed. (Eq. 3)
  3. The free myosin bridge along with its hydrolysis products rebinds to the actin filament. (Eq. 2)
  4. The cross-bridge generates force, and actin displaces the reaction products (ADP and Pi) from the myosin cross-bridge. This is the rate-limiting step of contraction. The actin-myosin cross-bridge is now ready for the ATP binding of step 1.

a protein forming the thin filaments in muscle fibers that are pulled on by myosin cross-bridges to cause a muscle contraction.
Mysoin a protein that makes up close to one half of the total protein in muscle tissue. The interaction between myosin and another protein, actin, is essential for muscle contraction

Thin filaments
Thin filament Actin The main protein of actin that interacts with myosin during excitation – contraction coupling Tropomyosin Tranduces the conformational change of the tropin complex to actin Troponin Binds Ca2+ and affects tropomyosin; represents the “switch” that transforms for the Ca2+ signal into a molecular signal that induces crossbridge cycling Nebulin Present adjacent to the actin and believed to control the number of actin monomers joined to each other in a thin filament

Thick Filaments
Thick filament Myosin Helps hold thick filaments in a regular array C stripes C protein Holds the myosin thick filaments in a regular array; may hold the H protein of adjacent thick filaments at an even distance during force generation; may also control the number of myosin molecules in a thick filament M line M protein Helps hold thick filament in a regular array Myomesin Provides a strong ancvhoring point for the protein titin M-CK Provides ATP form phosphocreatine; located proximal to the myosin heads

Z line
Z line α-actinin Hold the thin filaments in the place spatially Desmin Forms the connection between adjacent Z lines from different myofibrils; help to keep the sarcomeres in register so they maintain their striated appearance
Elastic filaments Elastic filament Titin Helps keep the thick filament centred between two Z lines during contraction; believed to control the number of myosin molecules contained in the thick filament