PHYSIOLOGY 1 LECTURE 17 SKELETAL MUSCLE MECHANICS

Skeletal Muscle Mechanics n As a skeletal muscle contracts it follows certain mechanical restraints. n Strength verses Speed of contraction n Velocity of shortening to load n Muscle twitch (single twitch, multiple twitch) n Skeletal muscle motor unit n 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 Velocity of Shortening to Load n The maximum velocity with which any muscle fiber can contract is dictated by by the speed of crossbridge cycling. Crossbridge cycling rate is determined primarily by the type of Myosin ATPase expressed by the muscle fiber. Furthermore, as load increases the velocity of contraction must decline until a load is reached which can not be moved.

Skeletal Muscle Mechanics Velocity of Shortening to Load

Skeletal Muscle Mechanics Relationship of Twitch to [Ca++] and Force Development n A single muscle action potential results in a submaximal force development due to the speed of the Ca++ dependent Ca++ATPase which removes Ca++ from the sarcoplasm to the sarcoplasmic reticium.

Skeletal Muscle Mechanics Relationship of Twitch to [Ca++] and Force Development

Skeletal Muscle Mechanics Multi Twitch Force Development n As the frequency of action potentials increases the Ca++ ATPase can not remove cytosolic Ca++ fast enough so the strength of contraction increases to maximum (Tetany).

Skeletal Muscle Mechanics Multi Twitch Force Development

Skeletal Muscle Mechanics Fatigue n In the human there are two types of fatigue. n 1. Skeletal muscle fatigue n 2. Nerve fatigue - In general except for some very unusual circumstances the nerves do not fatigue. Nervous fatigue implies depletion of neurotransmitter, not a normal occurrence except after several thousand high frequency firings.

Skeletal Muscle Mechanics Fatigue n Skeletal Muscle Fatigue n Fatigue in skeletal muscle is not well understood but the most likely scenario is that fatigue is correlated with Glycogen Depletion. In either high intensity exercise or long exercise duration the muscle cells becomes more reliant on the glycolytic pathway (Anaerobic Glycolysis) for energy.

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