PHYSIOLOGY 1 LECTURE 22 SKELETAL MUSCLE MECHANICS

Skeletal Muscle Mechanics n I. Introduction: n Muscle mechanics has to do with how skeletal muscle operates in order to perform their function. In general the movement of the body is a complex interaction between the motor cortex of the brain, alpha motor neurons, and skeletal muscle. It involves afferent information from muscle, bone, and connective tissue sent to the brain, integration by the motor cortex and return efferent signals to the contracting skeletal muscle. That signal carried by the alpha motor neurons produces a graded response in the activated skeletal muscle which is dependent on the weight of the load to be lifted.

Skeletal Muscle Mechanics n Nerve stimulation of skeletal muscle can increase almost infinitely but skeletal muscle response maximizes (max tension development)

Skeletal Muscle Mechanics Strength verses Speed n In general the more sarcomeres a skeletal muscle fiber has in parallel the stronger the fiber becomes. (Hypertrophy or Hyperplasia) n The more sarcomeres a skeletal muscle fiber has in series the faster its velocity of contraction becomes. n Note - Velocity of contraction is limited by the type of myosin ATPase expressed by the fiber.

Skeletal Muscle Mechanics Strength verses Speed

Skeletal Muscle Mechanics Muscle Twitch n A. Single Twitch - a single muscle twitch is the skeletal muscle contraction resulting from a single action potential. –1. Relationship of a single muscle twitch to the muscle action potential - cytoplasmic calcium concentrations - and force development –Note - The electrical event must precede the mechanical event

Skeletal Muscle Mechanics Muscle Twitch

n Single twitch is submaximal - Due to the rapid action of the SR Ca++ATPase calcium concentration falls before the skeletal muscle can develop it’s maximum tension.

Skeletal Muscle Mechanics Muscle Twitch n B. Multiple Twitch - Skeletal muscle responds to several (Multiple) action potentials delivered at a high frequency by increasing it’s force of contraction up to the maximum tension development. Therefore, a graded response can be developed such that just the force needed to lift the load is generated. There are two methods of generating this graded response.

Skeletal Muscle Mechanics Muscle Twitch n 1. Temporal summation - A series of action potentials generated at a high enough frequency to increase the tension in a skeletal muscle fiber beyond that produced by a single action potential. n 2. Tetany - Maximal skeletal muscle contraction produced by a series of high frequency action potentials.

Skeletal Muscle Mechanics Muscle Twitch

Skeletal Muscle Mechanics Fatigue n In the human there are two types of fatigue. n 1. Nerve fatigue - This is usually defined as a depletion of neurotransmitter quanta release to the point where the release falls short of a full quanta or becomes completely depleted and fails to produce a response in the effector organ. Normally this will never occur as most nerves store sufficient neurotransmitter for several thousand full quanta releases.

Skeletal Muscle Mechanics Fatigue n 2. Skeletal Muscle Fatigue n Fatigue in skeletal muscle is not well understood but the most likely scenario is the following. Skeletal muscle fatigues at different rates depending on the type of skeletal muscle fiber involved. Fatigue is fairly tightly correlated with glycogen store depletion. In exercise conditions over time aerobic ATP production declines, therefore, a greater degree of reliance must be placed on glycolysis to produce sufficient ATP to maintain exercise. As more and more ATP is produced from glycolysis lactic acid and [H+] begins to build up inhibiting PFK.

Skeletal Muscle Mechanics Motor Units n Def. - The alpha motor neuron plus all of the skeletal muscle fibers it innervates form a skeletal muscle motor unit. n Motor units can be as small as a nerve and one muscle fiber or as large as a nerve and several thousand muscle fibers.

Skeletal Muscle Mechanics Motor Units n 1. Alpha motor neuron n 2. Muscle fibers innervated n 3. Interdigitation of motor unit fibers with other motor unit fibers n 4. Graded response (Recruitment)

Skeletal Muscle Mechanics Motor Units

Skeletal Muscle Mechanics Graded Response (Recruitment) n A skeletal muscle contraction is a graded response starting with the smallest motor units, activated first, larger and larger motor units are added on until the load is lifted. This process is called recruitment.

Skeletal Muscle Mechanics Graded Response (Recruitment)

Skeletal Muscle Mechanics Tension Development n Any skeletal muscle contraction involves both an active tension development due to crossbridge cycling and a passive tension development due to compression and stretch of elastic elements such as tendons (stretch) and endomysium (compression).

Skeletal Muscle Mechanics Tension Development n Active Tension Curve - Sarcomere shortening n Passive Tension Curve - Tendons and endomysium n Total Tension Curve - The addition of the passive tension curve plus the active tension curve equal the total tension curve.

Skeletal Muscle Mechanics Tension Development n Active Tension Curve - n Generated under isometric conditions n Resting sarcomere length in skeletal muscle occurs at the peak of the active tension curve.

Skeletal Muscle Mechanics Tension Development n Passive Tension Curve - n Generated by pulling on muscle fiber n Starts at resting sarcomere length

Skeletal Muscle Mechanics Tension Development n Total Tension Curve n Adding the Active tension curve to the passive tension curve gives the Total tension curve

Skeletal Muscle Types n Skeletal muscle is composed of three different fiber types, Fast Glycolytic (White), Fast Oxidative-Glycolytic (Fast Red), and Slow Oxidative (Red) fiber types. The name is derived from the primary form of energy production which predominates within the fiber.

Skeletal Muscle Types n Fast Glycolytic - (White muscle fiber) - predominately expresses the enzymes involved in glycolysis. These are the power or strength fibers but lack endurance. n Fast Oxidative-Glycolytic - (Fast Red fibers) - Express a balance between glycolytic and Krebs cycle enzymes. Intermediate fibers strong and fast. Most abundant fibers in the human. n Slow Oxidative - (Slow Red fibers) - Express primarily oxidative (Krebs Cycle) enzymes. These are the endurance fibers but lack strength.

Skeletal Muscle Types

n All types of skeletal muscle fibers occur in every skeletal muscle group. Classification is dependent on the muscle fiber type in greatest abundance. Therefore, a muscle group which predominantly expressed the slow oxidative fibers would be classified as an oxidative muscle or sometimes as an endurance muscle group.

Skeletal Muscle Types Comparison of Enzymes n ENZYMES F-G F-O-G S-O n MyosinATPase 1a 1b 1c n Glycolytic High High Low n Krebs Cycle Low Med. High n Mitochondria Low Med. High n Glycogen High High Med. n Myoglobin Low Med. High

Skeletal Muscle Types Affects of Exercise n While skeletal muscle types are genetically determined they can be moved toward other muscle types by exercise training. For example an individual who predominantly expresses the fast-oxidative- glycolytic fiber type if trained as a marathoner will begin to express more oxidative enzymes in his FOG fibers. The FOG fibers are much easier to move than the two extremes, therefore, exercise training focus on the FOG fibers. In humans the FOG fibers are normally the most abundant and humans show better training effects than most other animals.

Skeletal Muscle Types Affects of Exercise n EXERCISE Predominant fiber type % n MarathonSO 42 n SprintsFG 39 n Power LiftFG 37 n RowerFOG 63 n Swimmer FOG 65 n NormalFOG 48 n FG 22 n SO 30