Skeletal Muscle Structure and Contractile Properties Skeletal Muscle Structure and Contractile Properties The purpose of this chapter is to describe muscle from the perspective of its primary purpose as a motor.

Training has negligible effects on the number of fibers. Basement membrane is freely permeable to metabolites. Plasma membrane: true cell boundary that maintains acid-base balance.

Connective tissue plays a vital role in transmitting the force generated via actin- myosin to the tendon.

Titin connects myosin to the Z disk and transfers the force longitudinally down the fiber.

MHC Isoforms in order of ATPase activity: 1.I 2.IIa 3.IIx 4.IIb Myosin Heavy Chain (MHC): molecular weight 200 kilodaltons Myosin Light Chain: kilodaltons

fast % hybrid fibers slow med

Concentric Actin-Myosin Cycle

Eccentric Actin-Myosin Cycle

Isometric Actin-Myosin Cycle

The capillary to fiber ratio is the most accurate method of quantifying perfusion.

The capillary-to- fiber ratio has been shown to increase 5-20% in humans subjected to 8-12 wks of endurance training.

When sarcomeres are arranged in series the velocities become additive and the overall velocity of shortening measured at the tendons increases Each sarcomere is capable of generating a fixed amount of force, called specific force. Specific force is approx N/cm 2 of muscle fiber cross-sectional area.

Muscles with long fiber lengths generate greater velocity and move segment thru greater distance. Short length with high cross-sectional area yields high force.

Optimal length is defined as the length with the most overlap of actin & myosin.

Relationship Between Total Muscle Force and Length Active muscle tension developed by sarcomeres (CE) Passive muscle tension (PE) Total tension = Passive + Active

Force – Length & Muscle Architecture The Force-Length relationship is effected by the muscle/tendon architecture and the changes in the moment arm.

Factors That Effect The Torque About A Joint Weight easier to lift Weight harder to lift 1. T = F * Perpendicular Distance. 2. Perpendicular Distance Changes. 3. Muscle Length-Tension Changes. 4. Distance and Tension are not optimal at the same angle. 5. Each synergist has its own length- tension, perpendicular distance curve.

Fiber type determines time to peak tension (TPT). A muscle with a high % of IIb, IIa, or IIx will have a TPT of ms, whereas muscle with high % of type I will have a TPT of ms.

Fused tetanus, often called maximum isometric force or (P o ) is 4-5 fold greater than twitch force (P t ). Fast twitch muscles fuse at Hz stimulation and slow twitch muscles fuse at Hz stimulation.

A fast twitch muscle has a force-frequency curve that is shifted to the right of a slow twitch muscle.

This apparatus can be used to generate the force-velocity curve of an isolated muscle.

Explosive training raises the force- velocity curve

Force – Velocity Relationship Eccentric force is greater than isometric. Isometric force is greater than concentric. Concentrically – an increase in velocity results in a decrease in force, due to reduced force per A-M bond (1-2 pN / bond). Concentric: As the velocity of shortening increases the Xbridges turnover quicker, resulting in fewer Xbridges attached. This shortened attachment times reduces the force transmitted by the Xbridge. Eventually the rate of shortening equals the rate of myosin arm rotation and the Xbridges add little, nothing or actually decrease the fiber speed [Merry-Go-Round Effect]. Eccentrically – an increase in velocity results in an increase in force, due to breaking and making more A-M Bonds (n * 3-4 pN / bond). At high velocity of stretch the minimum time to form an A-M bond is exceeded and the force declines, due to A-M bonds slipping. A-M bonds turnover quicker, with less force (1-2 pN) Break & Make more bonds (n*3-4 pN) Exceeds min time to form A-M bond, slipping occurs

Isometric, Concentric, Eccentric & Stretch- Shorten Contractions Notice that the force goes down in the concentric contraction and up in the eccentric contraction, when compared to the isometric contraction. a) Isometric followed by concentric contraction. The area under c of the Force- Length graph represents the work done in the concentric phase. b) Eccentric followed by concentric contraction (Stretch-Shorten Cycle). During the eccentric contraction energy is stored in the muscle-tendon which results in a more powerful concentric contraction. Compare the area under the c portion of the Force-Length graph. This additional work represents the Force Potentiation due to the SSC. Concentric velocity is the same.

Apparatus Used By Bigland-Ritchie to Compare Energy Cost of Eccentric vs Concentric Exercise Subject pedals forward and does concentric exercise. Subject pedals backward and does eccentric exercise. From Bigland-Ritchie #1656

Eccentric exercise is metabolically more efficient than concentric exercise. When working at the same work load subjects burn fewer calories.

Efficiency of Eccentric and Concentric Exercise O 2 uptake ( O 2 ) for 1 subject during final 2 min of baseline exercise at 15 W during heavy-intensity (HC, 330 W), moderate-intensity (MC, 216 W), and low-intensity (LC, 70 W) concentric exercise, and during high- intensity eccentric exercise (HE, 330 W) for 6 min. Breath-by-breath data have been interpolated to 1-s intervals and averaged across 3-4 repetitions for each exercise condition. [From Perrey #2304, J Appl Physiol 91 (2001)] Evolution of integrated electromyogram (iEMG) as a function of time during MC, HC, and HE exercise for the rectus femoris (A) and vastus medialis (B) muscles. Values are means ± SE for 6 subjects obtained in 2 repetitions of each test normalized to the individual values at 1 min under that test condition. *Different from minutes 1, 2, and 3, P < 0.05.

Muscle Tissue Damage Following Eccentric Exercise Muscle membrane is damaged Z lines are torn apart.

Z Disk Streaming Following Eccentric Exercise Normal striation, Z disks are perpendicular to myofibrillar axis. Streaming and smearing of the Z disks following eccentric exercise.