Skeletal Muscle Structure and Function

Overview of Muscle | Sarcomere Structure | Excitation-Contraction

Muscle Reflexes - Proprioceptors | Skeletal Muscle Fiber Types | Motor Unit Recruitment

Muscle Movements | Muscle Mechanics | Adaptations to Strength Training

Exercise-Induced Muscle Damage and Soreness 

 

 Overview of Muscle 

Three types of muscles 

    1. skeletal - striated
    2. cardiac - striated
    3. smooth - non-striated

All muscles require adenosine triphosphate (ATP) to produce movement thus, muscles are chemomechanical energy transducers. 

This slide is a cross-sectional slice of human skeletal muscle (X40) with shows muscle fibers surrounded by connective tissue.

 

This is a scanning electron myograph of human skeletal muscle (X20,000) showing horizontal rows of myofibrils. The Z-discs are the dark vertical lines in the middle of the wide, light-colored spaces. Pick out the myosin and actin filaments. The numerous small black dots are glycogen granules and there are a few triglycerides stores visible which are the large, round white areas lying between the myofibrils.

 

 Sarcomere Structure 

Thick filament 

Thin filament

This represents a cross-sectional view of the myosin and actin filaments. The actin filaments (smaller dots) are positioned every 60 so that they are aligned with the myosin heads that protrude from the myosin filaments (larger dots). Thus, six actin filaments surround each myosin filament, moreover active sites from an actin filament are available to three different myosin filaments.

Sarcoplasmic reticulum (SR) (skeletal muscle only)

  

Excitation-Contraction

Much of what we currently know about muscle contraction was originally reported simultaneously by H.E. Huxley and A.F. Huxley (not related) in the 1950s and is referred to as Huxleys' sliding filament mechanism.

Muscle action potential

Cross-bridge cycling

    1. In the resting state, the myosin ATPase has partially hydrolyzed ATP. The ADP and Pi do not immediately dissociate, however a complex is formed in which ADP and Pi remain attached to the myosin head along with the stored energy.
    2. When a binding site on the actin filament becomes available, the myosin head binds to the active site and a cross-bridge is formed (called an actomyosin complex).
    3. The ADP and Pi are released to complete the energy release. This causes the myosin head to return to a position of lower energy by a "ratchet"-like movement, and shortens the sarcomere.
    4. The actomyosin complex is severed when ATP binds to the myosin head.

Entire cycle takes 50 ms, although the myosin head is attached only 2 ms. A single myosin head will produce 3-4 pN.

 

Muscle Reflexes - Proprioceptors

Muscle spindles

Golgi tendon organs

 

Skeletal Muscle Fiber Types

Muscle fibers are usually classified based on histochemical criteria.

Differences between fiber types

 

Motor Unit Recruitment

 

This EMG recording of the biceps muscle reflects a subject performing six arm curls. The first three curls were made using an increasing weight and the last four curls were performed with the same weight. The subject paused momentarily between lifting the weight (concentric movement) and returning back to the starting (eccentric movement) position. Notice the difference in EMG activity between the increasing weight, and the difference between the concentric and eccentric movement. Be able to explain these phenomena.

 

Muscle Movements

 

Muscle Mechanics

Force development by a muscle is dependent upon:

Length-tension (force) relationship

Force-velocity relationship (see Figure 2.15, p 41)

Power-load relationship

Summary of force, power, and velocity relationships

After integrating the force-velocity and load-power relationships, power and shortening velocity are maximized at a load of ~20% of maximal.

 

 

Adaptations to Strength Training 

Neural adaptations

Muscular adaptations

Comparison of strength between sexes

Suggested Reading:

SSE #53: Training for Improved Vertical Jump by W.J. Kraemer, 1994. This is one of Gatorade's Sports Science Exchange articles. http://www.gssiweb.com (you must apply for a free, on-line membership with Gatorade to access their publications).

 

Exercise-Induced Muscular Damage and Soreness

Eccentric movement, generating equal force to a concentric movement, utilizes fewer motor units as fibers develop greater tension while lengthening than shortening. Thus, the tension per fiber is greater during eccentric movements.

Unaccustomed exercise, primarily eccentric, causes a sequence of events that:

Stages of Exercise-Induced Muscle Damage and Soreness

1. During exercise:

-sarcolemma damage

-sarcoplasmic reticulum damage

-myofibrillar damage

-free oxygen radical production

-excessive temperature

-decreased pH

2. 0-3 days postexercise:

-swelling

-release of stimulants

-hypersensitivity

-spasm

 

 

Time Frames for Peak Response to Injury

Effects on performance

Ultrastructural damage