9: Skeletal muscle 2

• Isometric contractions are when the muscle fibers involved are fixed in length. Measuring these involves measuring tension, or force, of the contraction.
• Isotonic concentric contractions are when the muscle fibers shorten, which occurs when tension becomes greater than the load.
• Isotonic eccentric contractions are when muscle fibers lengthen, which occurs when load is greater than tension. Cross-bridge cycling is still occuring in these cases.
• The load-velocity relationship uses a graph of shortening distance vs. time to demonstrate that shortening velocity decreases as load increases. This is because it takes longer to perform the power stroke step of cross-bridge cycling.
• The force-frequency relationship demonstrates that higher action potential frequency in the fiber membrane results in increased force due to Ca2+ buildup.
• The length-tension relationship shows that shortening or lengthening the muscle past optimal length results in reduced force. This is either because fewer cross-bridges are able to cycle or there is overlap of filaments inhibiting binding..
• The first available source of ATP is creatine phosphate. It is utilized at the onset of any increased activity and is rapidly deleted (within 8 to 10 seconds).
• Oxidative phosphorylation provides most of the ATP for low to moderate intensity activity. The specific fuel source used for this changes over long-term activity:

  • Glyocgen in muscle for first 5-10 minutes
  • Glucose and fatty acids in plasma equally for next 30 minutes
  • Fatty acid breakdown increases steadily after 35 to 40 minutes while glucose breakdown decreases
• Glycolysis provides most of the ATP during high intensity exercise, due to the limiting factors of adequate blood flow and O2 delivery (performs fermentation). It provides energy for about 1.3 to 1.6 minutes, and utilizes both glycogen in the muscle and blood glucose.
• Oxidative fibers have a high concentration of myoglobin, which binds, transfers, and stores O2.
• Glycolytic fibers do not only perform glycolysis. Under relaxed conditions, these fibers make ATP using oxidative phosphorylation; only during high activity will they primarily perform glycolysis.
• Oxidative fibers also perform glycolysis to break down glucose into important substrates for oxidative phosphorylation.
• What protein within the muscle fiber would determine rate of force production?

  Myosin ATPase