Factors affecting muscle force production

1. Factors affecting muscle force production Module 1: Anatomy and Physiology

2. Motor Unit Recruitment Factors • 1. recruit more motor units,2. increase the firing rate of motor units, or3. synchronize the firing of motor units.

3. Motor Unit Recruitment • The more motor units that are recruited, the more force is produced (remember all or none principle) • Trainable? • Yes • Biggest increases in strength come from neural factors • Increased motor unit recruitment is the #1 factor in increased force production (especially in previously untrained individuals)

4. Rate of Motor Unit Firing • Single twitch- small amount of force generated • If muscle is stimulated again before relaxation occurs, the second twitch builds on the first twitch • If this continues to occurs, twitches will continue to build on each other until they fuse (tetanus) • Trainable? • Another neural adaptation • Learn to fire at higher rate early in contraction (increase rate of force production)

5. Figure 1.6

6. Synchronous Firing • Proper sequence of firing between agonist, antagonist and synergist muscles to provide smooth movement and correct direction of force (thereby providing large amount of force in correct direction) • Trainable? • Neural adaptation to strength training, especially early on in training

7. Other Factors in Force Production • Agonist, antagonist, and synergist activation • Reflex involvement • Length of the muscle • Size of the muscle • Application of a pre-stretch • Type of fiber recruited • Speed of contraction • Angle of pennation of the activated muscle

8. Agonist, Antagonist, and Synergist Activation • The amount of force that can be externally applied is equal to the sum of agonist and antagonist force with contribution of synergist muscles • 100 - 50 = 50 • 120 – 50 = 70 • 100 – 30 = 70 • 100 – 30 + 15 = 85 • Trainable? • Antagonist inhibition is a primary neural adaptation

9. Reflex Involvement • CNS can also work unconsciously • Muscle Spindle • Detects muscle length and changes in length • Causes a excitatory response • Trainable? • Stretch-Shortening Cycle • Plyometrics • Golgi Tendon Organ • Stimulated when tension in organ is very high • Causes an inhibition response • Traininable? • Inhibition of GTO is primary neural adaptation

10. Muscle Length • Force-Length Relationship • Traininable • Can muscle length be changed? • Flexibility F L

11. Muscle Size • Increases in muscle size increase the POTENTIAL for increased muscle force production • Increased number of contractile proteins equals more possible actin and myosin bonds • Trainable? • Hypertrophy • Hyperplasia

12. Application of A Pre-Stretch • By pre-stretching a muscle • actin-myosin bonds are already created • elastic energy is created • use stretch-shortening cycle • Trainable? • SSC- yes, others- no

13. Table 1.1

14. Type of Muscle Fiber Recruited • Type II Muscle Fibers produce more force • larger • Size principle • Motor Units are recruited smallest to largest • Trainable • A later neural adaptation is a “bypass” of the size principle

15. Speed of Contraction • Force-Velocity Relationship • As speed increases, force decreases • Trainable? • Yes, but velocity specific

16. Amount of Pennation • The more sarcomeres that are in parallel, the more actin/myosin bonds per area • Greater angle of pennation = more sarcomeres in parallel • Trainable? • Hypertrophy can lead to greater angles of pennation (but only in muscles that are already pennated)

17. Figure 4.11