1. Muscle Excitation – Contraction Coupling Chapter 16 KINE 3301 Biomechanics of Human Movement

2. A single muscle fiber contains hundreds of myofilaments (Actin & Myosin) which are bundled together and surrounded by the muscle membrane (sarcolemma). A muscle fiber has a diameter of 10 – 60 μm and a length of 10 – 300 mm.

3. Troponin and tropomyosin are regulatory proteins. Under resting conditions they a crossbridge from forming between actin and myosin. In the presence of Ca ++ stops inhibiting muscular contraction by rotating away from it’s resting position where it covers the actin binding site.

4. Sarcomere: The Functional Unit of the Muscle Cell

7. Concentric Cross-bridge Cycle Ca ++ released from SR Ca ++ binds with troponin Tropomyosin moves away from binding site. An Actin-Myosin cross- bridge is formed. ATP downgraded to ADP + Pi. Myosin does mechanical work on Actin, Myosin arm rotates shortening the muscle fiber.

8. Eccentric Cross-bridge Cycle Ca ++ released from SR Ca ++ binds with troponin Tropomyosin moves away from binding site. An Actin-Myosin cross-bridge is formed. ATP downgraded to ADP + Pi. Myosin attempts to rotate and shorten fiber. External force causes the fiber to lengthen, storing elastic energy in myosin arm. The cross-bridge is broken (1-4 pN) while it is still in the actively charged state, it immediately forms another cross-bridge at the next available Actin site without the need for any additional ATP. This “break & make” cross-bridges will continue until the length of the active state is exceeded.

9. Isometric Cross-bridge Cycle Ca ++ released from SR Ca ++ binds with troponin Tropomyosin moves away from binding site. An Actin-Myosin cross-bridge is formed. ATP downgraded to ADP + Pi. Myosin attempts to rotate and shorten fiber. External force causes the fiber to lengthen, storing elastic energy in myosin arm. From this lengthened position the myosin arm rotates and shortens the fiber.

10. Contraction Type Energy Utilized Force Generated Change in Fiber Length Effect of Velocity on Force Concentric 1 ATP1 – 3 pNShortened ~5 nm Force decreases with increasing velocity: 1 pN for fast velocity 3 pN for slow velocity Eccentric1 ATPn × 1-4 pN, n is # of make & break A-M bonds Lengthened ~5-20 nm Force increases with increasing velocity: 3 pN for slow velocity 4 pN for fast velocity Isometric1 ATP3 pNRelatively Constant none Summary of Actin-Myosin Crossbridge Cycle