Factors Affecting the Magnitude of Muscle Force

1. Factors Affecting the Magnitude of Muscle Force Velocity of muscle shortening Length of the muscle when it is stimulated Time interval between muscle stimulation and tension development (electromechanical delay)

2. Force-Velocity Relationship • Concentric Contraction • As concentric muscle force output increases the rate at which it is capable of shortening (velocity of shortening) decreases • IN OTHER WORDS: A muscle shortens more slowly against increasing resistance (assuming the recruitment of the same number of motor units in each case)

3. Force-Velocity Relationship (continued) • Eccentric Contraction • As resistance (and therefore force output) increases beyond the isometric limit the muscle begins to lengthen • As resistance increases the rate of lengthening also increases • Effects of Eccentric Training • More effective increase in muscle size and strength than with concentric training • More soreness and structural damage than with concentric or isometric training

4. Force-Velocity Relationship Isometric Maximum Eccentric Force Concentric 0 Velocity

5. Length-Tension Relationship • Maximum potential isometric force is partly dependent on muscle length • Active Tension (produced by activation of the motor units) is greatest when the muscle is at its resting length • Active tension decreases as muscle length goes above or below resting length

6. Active Tension – Length Curve Active tension is greatest when the muscle is at its resting length.

7. Length-Tension Relationship (continued) • Passive Tension (produced by the parallel elastic component or PEC and the series elastic component or SEC) increases as the muscle is stretched beyond its resting length • This is primarily due to the SEC

8. Passive Tension - Length Curve

9. Length-Tension Relationship (continued) • Total muscle tension is a result of the addition of Active and Passive Tension • Parallel fibered muscles generate maximum tension when stretched to slightly more than resting length • Pennate fibered muscles generate maximum tension at 120% to 130% of resting length

10. Total Tension – Length Curve

11. Electromechanical Delay (EMD) • The lapse in time between electrical activation of a muscle and the development of tension • Believed to be the time required to stretch the SEC, therefore removing laxity from the muscle

12. Electromechanical Delay

13. Electromechanical Delay (EMD) (continued) • Length of EMD may range from 20 msec to 100 msec • Shorter EMD is related to higher percentage of fast twitch fibers • Higher contraction forces are associated with shorter EMDs

14. Electromechanical Delay (EMD) (continued) • Muscle length, type of contraction (concentric, eccentric, etc), velocity of contraction, and fatigue don’t seem to affect EMD • It may take a full second for a muscle to develop maximum isometric tension following EMD • A higher percentage of fast twitch fibers is associated with a decrease in the time required to develop maximum isometric tension following EMD • EMD in children is significantly longer than in adults