Muscle Anatomy 101

Part II

Muscle Contraction

Huxley Sliding Filament Theory

Briefly- actin slides over myosin causing the z-lines to pull together; thus shortening the sarcomere

Action potential travels to neuromuscular junction

Acetylcholine (Ach) is released

Sarcolema is depolarized

Muscle fiber is excited

Ca++ is released from sarcoplasmic reticulum

Ca++ binds to troponin

Myosin heads bind to actin

Powerstroke occurs

ATP binds to crossbridge

Myosin detaches from actin

If Ca++ and ATP are present cycle occurs again if physically possible

Muscle Contraction

Relaxation occurs when stimulus stops

Ca++ is actively pumped back into SR

ATP is required for excitation and relaxation

Size Principle

“Motor units are recruited from the smallest to the largest based on the force demands placed on the muscle.”

Smaller (lower threshold) units are recruited first (predominately Type I)

Based on force demands larger (higher threshold) units are recruited (predominately Type II)

Size Principle

*Important Training Concept*

Low resistance = smaller units = 12-15RMHigh resistance = larger units = 1-5RM

*Only recruited units benefit from training*

All-or-None Law

“When a specific motor unit reaches its threshold level for action, all of the muscle fibers in that motor unit are activated fully.”

-Refers to individual unit NOT entire muscle

-Variation of force

Muscle Activation & Training

Different loads recruit different types and numbers of motor units

Heavier loads recruit more units

Exception to size principle: High velocity movements

Muscle Activation & Training

Recruiting low threshold first delays muscular fatigue for submax exercise

Neurons of high threshold units ability to recover quickly allows them to be activated quicker in repeated actions

Muscle Activation & Training

-To achieve training effect of Type II there must be high power output demands

-Recruitment order is generally fixed, may change due to body position-exercising at various angles?

Muscle Actions

Concentric: active shortening (m. force > resistance)

Eccentric: lengthening (m. force<resistance)

Isometric: static (m. force=resistance)

Muscle Actions

Ascending Curve: force gets greater throughout ROM

Descending Curve: force greatest at beginning

Bell Curve: force greatest in middle

Length-Tension (Force) CurveOptimal length at which muscle can generate force Optimal length: maximal crossbridge interaction

Force Velocity Curve

“As the velocity of muscular movement increases, the force a muscle is able to develop concentrically decreases”

-The greater the force the slower the movement (concentrically)

*Opposite true for eccentric muscle actions

Force Velocity Curve

High force development is related to tissue damage-Muscle soreness after eccentric exercise

Questions?