NPB 101: Muscle Physiology 2: Mediators of Force

Muscle Cramps

• Involuntary tetanic contraction of skeletal muscle

• Causes unknown

• Related to ion imbalances

• Hypocalcemic Tetany

  • Low extracellular Ca²⁺

How does low extracellular Ca²⁺ cause contraction?

Hypocalcemic Tetany → Low Ca²⁺

• Membrane potential moves closer to the threshold needed to fire an AP, nerves and motor axons begin firing spontaneous, repetitive AP’s.

Length Tension Curve

Active tension depends on filament overlap

• T = thick filament - 1.6 μm

• t = thin filament - 1.0 μm

• 1.0 + 1.6 + 1.0 = 3.6 μm

Based on filament lengths what is the maximum sarcomere length a human muscle can produce active force?

A human skeletal muscle sarcomere can productive active force up to a max length of approx. 3.6 μm, where all physical overlap bw actin and myosin filaments ceases

• T = thick filament - 1.6 μm

• t = thin filament - 1.0 μm

• 1.0 + 1.6 + 1.0 = approx. 3.6 μm

What is total force when stimulated at 140% of Lo?

• Active force = Force you get in response to stimulus

  • Approx. 50 on ex

• Passive Tension = Relaxed fiber

  • Approx. 22 on ex

• Total Force = Active Force + Passive Tension

  • 50 + 22 = approx. 72

Twitch Contraction

Twitch is the result of a single impulse for muscle contraction

• Isometric twitch → Generates muscle tension w/o changing length of the muscle

• Isotonic twitch → Moves a load with a constant muscle tension while the muscle length changes

Isotonic Contractions

• Longer latency periods

• Slower shortening velocity

• Heavier load, longer we have before we move it and slower to move

Isotonic Contractions- Types of Contraction

• Concentric → Muscle gets shorter, positive velocity

• Isometric → Length doesn’t change, velocity is 0

• Eccentric → Forces are higher

Concentric Contraction

Muscle generates tension and shortens, positive velocity

Isometric Contraction

Muscle generates force/builds tension without changing its length, velocity is 0

Eccentric Contraction

Muscle lengthens while under tension bc external resistance or load is greater than the force the muscle generates

Muscle Twitch and Tetanus

• Twitch

  • Force generated from single stimulation (temporal summation)

• Tetanus

  • Twitch series when next stimulus occur after full relaxation

  • Unfused tetanus when next stimulus occurs during relaxation

  • Fused tetanus when next stimulus occurs before any relaxation

Unfused Tetanus

• Moderate stimulation rate

• Muscle has a brief, partial relaxation period bw stimuli

• Force output shows fluctuating waves or oscillations rather than a flat line

Fused Tetanus

• High stimulation rate

• NO relaxation phase bw individual stimuli

• Individual twitches merge completely into a smooth, steady, and continuous extraction

• Maximal possible muscle tension

Which muscle is stronger? Able to produce more active tension?

More compact = more tension, in parallel, summing up all sarcomeres

Which muscle is faster? Able to contract at higher velocity?

More in series, only pulls against neighboring, ability to work over a long range of lengths will be organized in series

Physiological Cross-Sectional Area (PCSA)

• Number of sarcomeres in parallel

• Relates to maximum tension

• Larger PCSA = Doubled muscle force, twice the maximum force with more sarcomeres in parallel

Normalized Fiber Length

• Number of sarcomeres in series

• Relates to maximum velocity

• Relates to muscle excursion

• Long Fibers have a larger muscle length (mm) as it’s spread out.

Do ankle plantarflexors or dorsiflexors generate more maximum tension?

Plantarflexors → More fibers in parallel , makes it generate more tension