Ex phys 2/19

Muscle Fiber Types and Force Production

• Fiber Types: Different muscle fiber types contribute to muscle function and strength.

  • Fast-Twitch Fibers: Specialized for rapid contraction; produce more force.

  • Slow-Twitch Fibers: More endurance-oriented, less forceful.

Factors Influencing Force Production

• Motor Unit Activation

  • The number of fibers within an activated motor unit directly affects force production.

  • Size of muscle fibers: Larger muscle fibers produce more strength than smaller fibers.

  • Speed of Contraction: Influences the ability to generate force, discussed in lab.

• Stimulation and Inhibition: Relationship between stimulation frequency and inhibition impacts muscle force.

  • GABA: Main inhibitory neurotransmitter competing with ACh at the synapse.

Frequency of Stimulation

• Impact on Force Production

  • Increasing frequency leads to increased force, up to a limit known as tetanus.

  • Initial stimulation leads to a forced deflection, after which recovery happens.

  • As frequency increases, muscle tension reaches a peak and plateaus.

Motor Unit Activation Strategies

• Strategies to Modulate Force

  • Recruitment: Adding more motor units to increase force.

  • Rate Coding: Changing firing frequency of activated motor units affects strength.

  • Effective actions involve both recruitment of additional motor units and increasing their firing rates.

Concomitant Recruitment and Rate Coding

• In lower-intensity contractions, both recruitment and rate coding increase; this remains effective until about 80-90% of max effort.

  • For small muscles, like in the hand, this threshold is around 50%. Recruitment contributes less to force production at higher intensities.

Fatigue and Force Production

• Implications for Muscle Function

  • Fatigue inhibits force production compared to the fresh state; significance in sports performance.

Task Dependency Model of Fatigue

• Fatigue mechanisms vary based on the task:

  • Accumulation Hypothesis: Buildup of metabolic byproducts (lactate, inorganic phosphate, ammonia) causes fatigue in higher-intensity short bursts (20-30 seconds).

  • Depletion Hypothesis: Depletion of energy substrates—like ATP and glycogen—impacts performance over longer durations.

Accumulation Hypothesis Explained

• Lactate buildup interferes with:

  • Calcium release from the sarcoplasmic reticulum (SR) during muscle contraction.

  • Actin-myosin binding affinity during sliding filament processes.

  • Overall ATP metabolism effectiveness is impaired, decreasing energy available for muscle contraction.

Inorganic Phosphate's Role in Fatigue

• Inorganic Phosphate: Buildup impacts muscle function by:

  • Impairing both calcium release and reuptake by the SR.

  • Affecting actin-myosin binding affinity and troponin binding during contraction cycles.

Summary

• Effective force production in muscles relies on various factors including fiber types, stimulation frequency, motor unit recruitment, and coping with fatigue through different metabolic pathways. Understanding these mechanisms can clarify training approaches and performance in sports.