1. Skeletal, striated muscles develop tension (force) proportional to the amount of overlap between the myosin head bearing region of the thick filaments and the thin filaments. (Eckert, Fig. 10-8b).

2. The theory of A. V. Hill, from the 1930's, still forms the basis for understanding the biomechanics of muscle contraction.

• A typical experiment on muscle contraction involves either development of tension, holding the muscle length constant, (isometric) or the development of shortening, holding the tension constant (isotonic).

• Let's do an isotonic contraction. We will stimulate the muscle electrically, producing a SINGLE ACTION POTENTIAL, passing down the length of the muscle fibers and producing a "twitch" (Eckert, Fig. 10-18b). A twitch is a transient rise in muscle tension resulting from one action potential! A fixed amount of calcium is released from the sarcoplasmic reticulum for each fixed amount and time course of depolarization caused by a single action potential. We will discuss what happens if, in the laboratory, we depolarize the muscle cell membranes to different amounts than found in a natural action potential (Eckert, Fig. 10-19b).

• Now, let's set up the muscle to measure tension. Noice the results - - that the more load (weight or tension) we place on the muscle, the slower the velocity (length/time) of shortening (Eckert, Fig. 10-13a) (Eckert, Fig. 10-13b).

• The force-velocity relationship shows that the speed of shortening while lifting a light object is higher than when lifting a heavy object. (Eckert, Fig. 10-13c).

• Since power is force times velocity, the power-velocity relationship has a maximum as shown in: (Eckert, Fig. 10-13d).

3. Muscles in real life - Two more factors to consider:

• While there is a delay in the response of muscle contraction caused by the time to release calcium and have it diffuse through the myofibril and activate the biochemistry of muscle - THERE IS ALSO A SIGNIFICANT DELAY DUE TO THE MECHANICAL PROPERTIES OF MUSCLE - the series and parallel elastic elements. (Eckert, Fig. 10-26).

• Secondly, real muscle contractions are usually the result of excitation by a SERIES OF ACTION POTENTIALS, ONE AFTER ANOTHER, that produce a TETANUS. (Eckert, Fig. 10-27). If the action potentials are closely spaced, the tension won't decrease much before the muscle is stimulated again and a FUSED TETANUS will result (Eckert, Fig. 10-28).

4. There is great diversity in muscle fiber types, even within a single muscle (Eckert, Fig. 10-30). In Fig. 10-30, a muscle from a horse, we see slow oxidative fibers (type I), fast oxidative (type IIa), and fast glycolytic (type IIb).

• Tonic fibers are slow muscle cells used for postural adjustment in amphibians, reptiles, and birds (and other places, as well). Myosin cross-bridges attach and detach slowly and there are no twitches because there are no action potentials on the muscle cell membranes.

• Twitch fibers come in three types:
  • Slow-twitch (Type I) - Dark meat of turkey and chicken, for example. Red color due to a lot of myoglobin. Use ATP at a relatively low rate.

  • Fast-twitch oxidative - Flight muscles of migratory birds, for example. Rapid, sustained movements. Many mitochondria produce lots of ATP to sustain this movement. (Type IIa)

  • Fast-twitch glycolytic - White muscles of turkey and chicken, for example. Few mitochondria, depend a lot on glycolysis to produce ATP (Type IIb)

• As we saw in the cross section of horse muscle, most skeletal muscles don't consist of just one fiber type. But, there are some predominant fiber types in certain muscles, as we saw for the domesticated chicken and turkey. Wolves, dogs, and deer have a high concentration of fast oxidative fibers. Fast cruising fish, such as tuna, have a large percentage of fast glycolytic fibers. To complicate matters further, not all fibers are be activated simultaneously. Slow fibers may begin activation early and then fast oxidative fibers, and finally fast glycolytic fibers.

5. Human training increases muscle size by increasing fiber (muscle cell) size. Increased muscle size means more force produced. And, training may be able to increase the number of mitochondria in fast fibers. There is much debate on whether training changes fiber type from fast to slow, for example.

All text and images, not attributed to others, including course examinations and sample questions, are Copyright, 2009, Thomas J. Herbert and may not be used for any commercial purpose without the express written permission of Thomas J. Herbert.