Training Adaptations in Skeletal Muscle

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Training Adaptations in Skeletal Muscle

Introduction

Adaptability is a fundamental characteristic of skeletal muscle (and the body in general). The nature of this adaptation can be summarized using the following principle: cells will adapt in a manner that tends to minimize any movement away from homeostasis, or resting conditions. In exercise physiology we refer to the acute changes that occur in a sytem, organ, or cell during exercise as responses. An example is the increase in heart rate that occurs when we jump up from our chair and start jogging. The long-term changes that occur as a result of repeated bouts of exercise are called adaptations. Cellular adaptations generally involve an increase or decrease in the rate of synthesis of a specific cellular protein. All muscle cells are in a constant state of synthesis and degradation. If synthesis rate exceeds degradation rate, an increase in the cellular component occurs. A change in protein synthesis requires a cellular signal. Biologists and physiologists continue to explore the communiction process by which different forms of muscular work induce cellular changes. At the cellular level, there are some theories, but no complete understanding. However, we do know quite a bit about what adaptations do occur, even if all the details regardinf how remain unclear just yet.

Contrast Between Maximal Strength and Maximal Endurance

If we could build a skeletal muscle for the purpose of endurance, what would the recipe be? Since the heart is the supreme endurance muscle, let’s cheat by taking alook at it first.

Characteristics of Fatigue Resistant Muscle Cells

  • Heart cells are smaller in diameter than skeletal muscle cells. This results in very short diffusion distance between oxygen molecules coming from capillaries and the mitochondria where they are used.
  • The surrounding network of capillaries is extremely well developed. This characteristic also facilitates even and rapid oxygen distribution to all myocardial cells.
  • The mitochondrial density of heart cells is extremely high, 20-25% of cell volume in adults. Mitochondria use oxygen to metabolise carbohydrate and fat and produce ATP.
  • The cytoplasmic enzymes responsible for breaking down fatty acid molecules into 2 carbon fragments that can enter the mitochondria are present in high concentrations.
  • Contractile protein makes up about 60% of cell volume. The ATPase subtype found in heart is slower than that seen in skeletal muscle. Consequently, the rate of force development is slower, although absolute tension/cell diameter is the same.
  • Heart lactate dehydrogenase, the enzyme that converts pyruvate to lactic acid competes poorly with pyruvate dehydrogenase. This contributes to the very low lactate production in heart cells despite high metabolic flux. So, heart cells display almost zero fatiguability due to the tremendous capacity they have to receive and consume oxygen. Fatigue resistance is traded for anaerobic capacity. This is why the heart has little tolerance for oxygen deprivation, the dreaded a heart attack. If we want to build a skeletal muscle that is highly fatigue resistant, it must resemble heart muscle in its basic features.
    Now let’s build a muscle that is optimized for brief efforts and maximum force production. Here are the characteristics needed.Characteristics of Maximal Strength Muscle Cells
    • Each muscle cell should contain a high volume of contractile protein. Since oxygen diffusion is not a concern, making the cell diameter larger will help it hold more contractile protein (actin and myosin).
    • To make more room for actin and myosin, mitochondrial density should be minimized to that necessary to maintain resting cell function.
    • Since fat can only be metabolized aerobically, high levels of fat- cleaving enzymes in the cytosol are also unnecessary.
    • The capacity for anaerobic glycolysis should be high to allow brief but high capacity energy production without oxygen. The capacity for lactic acid production should be high.
  • What you should notice is that these two lists are exactly opposite. The optimal muscle for endurance CAN NEVER be maximally strong or powerful. And the muscle fiber that produces the most force CANNOT be optimally developed for endurance as well. The two conditions are mutually exclusive. This is one of the most important concepts to understand when designing a training program.Three Points to Remember:
    • There are identifiable proteins in the muscle that contribute to its ability to produce high force at high rates (strength and power respectively).
    • There are also identifiable proteins and structural characteristics that confer high fatigue resistance (endurance).
    • There is no identifiable specific protein or structure that confers the quality “Strength-Endurance”. When we train for strength-endurance, what we are really doing is training in a way that fails to stimulate either strength or endurance adaptations optimally. An example of this “best of neither worlds” approach is circuit training.
  • As a coach/athlete, your sucess begins with your ability to accurately understand the muscular demands of your sport. Then, a training program can be designed that will result in muscular development suited to the combination of strength and endurance that your sport requires. 
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