Muscle Physiology

KINE 4396/5390 Strength and ConditioningChristopher Ray, PhD, ATC, CSCS

Muscle Physiology

KINE 4396/5390 Strength and ConditioningChristopher Ray, PhD, ATC, CSCS

Objectives

Anatomy of Skeletal Muscle

Muscular contraction

Muscle Fiber Types/Recruitment

Muscle action

Force production

Three Types of Connective Tissue:Epimysium, Perimysium, and EndomysiumThree Types of Connective Tissue:Epimysium, Perimysium, and Endomysium

Three Arrangements of Muscle FibersThree Arrangements of Muscle Fibers

Fibers parallel to tendon

Unipennate muscle

Bipennate muscle

Fiber Pennation

• In a pennated muscle not all of the force generated in the muscle fiber is delivered to the tendon.

• Pennated muscles usually compensate for this disadvantage by increasing the cross-sectional area.

• Pennated muscles do not move a joint through as large of ROM as do unipennate muscles.

A Motor UnitA Motor Unit

• All muscle fibers of a motor unit contract together

• Connects via a neuromuscular junction– Each cell has 1

– Motor Neuron has many

The discharge of an action potential from a

motor nerve signals the release of calcium from

the sarcoplasmic reticulum into the myofibril,

causing tension development in muscle.

Contraction of a Myofibril: Stretched MuscleContraction of a Myofibril: Stretched Muscle

•I-bands are Actin in two adjacent sarcomeres

•A-bands are Myosin

•During contraction the H-zone and I-bands decrease.

Contraction of a Myofibril: Partially Contracted MuscleContraction of a Myofibril: Partially Contracted Muscle

Contraction of a Myofibril: Completely Contracted MuscleContraction of a Myofibril: Completely Contracted Muscle

Contraction of a Myofibril: Stretched Muscle

Contraction of a Myofibril: Completely Contracted Muscle

H-Zone and I-band shrink

Sliding Filament Mechanism/Theory

• Resting Phase– Little Ca++ is present so few X-Bridges attached.

• Excitation-Contraction Coupling Phase– Stimulus spreads thru T-Tubule– SR releases Ca++ – Ca++ binds with Troponin exposing bind site on Actin– Myosin binds with Tropomyosin

• Contraction Phase– ATP downgraded to ADP + P– Myosin arm does work on actin

• Recharge Phase– Pick up new ATP– Myosin head rotates backward

• Relaxation Phase– Ca++ is pumped back into SR

Calcium and ATP are necessary

for myosin cross-bridge cycling

with actin filaments.

Type II, or fast-twitch, muscle fibers are

capable of developing higher forces than

Type I, or slow-twitch, muscle fibers—

especially at higher velocities of muscle action.

The number of cross-bridges

that are attached to actin filaments

at any instant in time dictates the

force production of a muscle.

Force Production & Factors influencing Force

• Motor Unit Recruitment• Preloading [holding a wt tightens up the muscles elastic structures].• Cross-Sectional Area

– Increasing the cross-sectional area increases strength• Velocity of Shortening• Angle of Pennation• Sarcomere and Muscle Length• Prestretching (Stretch-Shorten Cycle)

– Eccentric contraction followed by an immediate concentric contraction [Plyometrics]

• Exercise-Induced Muscle Damage (DOMS)• Older Muscle

– Sarcopenia (reduced muscle size & strength with age)• Muscle Fiber Type

– Type I (SO) aerobic fiber, low force, slow rise time– Type IIa (INT) anaerobic fiber, high force, shorter rise time– Type IIb (FT) anaerobic fiber, very high force, fastest rise time

Many factors may affect rate of cross-bridge

cycling and thus force, including neural

activation, calcium concentration, myosin

ATPase activity, preloading, prestretch, muscle

fiber type and ultrastructure, fatigue through a

variety of mechanisms, and number of

contractile components (myosin and actin) in

parallel.

Improving Force Production

• Use preloading during training to develop strength early in ROM.

• Accomodating resistance apparatus (hydraulic, isokinetic) do not load the muscle prior to contraction.

• Increase cross-sectional area of muscle by using moderate resistance (65-80%) for max or near max # of reps.

• When overloading eccentrically use heavy resistance.• When training for explosive concentric use light resistance.• Prestretch a muscle before concentric to enhance force

production.• Incorporate rest days into training cycle to avoid training with

DOMS and allow muscle time to repair.

Questions

• In an attempt to increase his 1 rm bench max; George increased his resistance and added negatives. He complains of increased soreness 48 hrs later. What is the soreness called? What is the cause?

• What is occurring– Resistance is greater than muscle force

– Resistance and force is equal

– Resistance is less than muscle force