9 Neuromuscular Factors Affecting Muscle Force Production Fiber Type Number of Muscle Fibers Rate of Stimulation % Fiber Recruitment Force-velocity Relationship Fiber Architecture Muscle Temperature Elastic Properties Angle of Pull

Sliding Filament Theory

1. Fiber Type Fast twitch muscle fiber (Type IIb) Contraction speed is fast – high force production Fatigues relatively fast Slow twitch muscle fiber (Type I) Contraction speed is slow – lower force production Slow to fatigue Intermediate muscle fiber (Type IIa) Population distribution is bell-shaped curve

2. Number of Muscle Fibers The more muscle fibers one possesses the more force production they create Largely genetically pre-disposed Increase cross-sectional area of muscle fiber will increase force production

3. Rate of Stimulation The faster the rate of stimulation the more force production Based on the “All or None Principle” twitch → summation → tetanus Electromechanical Delay (EMD) – due to the elasticity of tendons and muscle

4. % Fiber Recruitment Largely dependent on the resistive forces that the muscle deals with – slow twitch (ST) fibers have a lower activation threshold than fast twitch (FT) fibers As the speed and force requirement of the muscle increases more FT fibers are activated

5. Force-Velocity Relationship As speed of contraction increases force production decreases and vice-versa Force-Velocity Curve

Force-Velocity Curve

6. Fiber Architecture Fusiform or Parallel (parallel tendons) – specializes in greater ROM and speed of ROM Pennate – specializes in greater force production (contains greater # of fibers w/n a cross sectional area)

7. Muscle Temperature Warm muscles will result in: ↑ elasticity of muscles ↑ conduction velocity (↑ rate of stimulation) ↓ internal friction

8. Elastic Properties Tissue Elastic Properties Series Elastic Component (SEC) Parallel Elastic Component (PEC) Proportional Length of Muscle Under isolated conditions the muscle fiber produces greatest force when at resting length Inclusion of SEC and PEC than greatest force production is produced when slightly stretched Stretch-reflex (Stretch-shortening Cycle)

Musculotendon Unit Model

Proportional Length of Muscle

9. Angle of Pull Two important functions of muscle contraction: Rotation of the bone segment Stabilization of the bone segment Near 90 degrees of pull the muscle work best to rotate the joint. As it goes away from 90 degrees it works more towards joint stability