MUSCLE PHYSIOLOGY

PRIMER

Muscle/Neural Physiology Overview Gross Structure of

the Muscles Micro Structure of

the Muscles Muscle

Contractions Factors Which

Effect Force Production

GROSS STRUCTURE

Gross Structure of the Muscles

Muscle Myotendinous

Junction Tendon Periosteum Bone

From Brooks, Fahey, and White. (1996).

Gross Structure of the Muscles

Epimysium: Covers the muscle

Fasiculi: Bundles of muscle fibers

Perimysium: Covers bundles of muscle fibers

Endomysium: Covers muscle fibers

Gross Structure of the Muscle

From McArdle, Katch, and Katch.

Endomysium

Surrounds each muscle fiber

Basement membrane: glycoproteins and collagen, freely permeable

Satellite cells

From Brooks, Fahey, and White. (1996).

Endomysium, Cont.

Plasma membrane/ sarcolemma: transport, action potential, acid-base balance

Contains small indentations, called caveolae. Provide additional

lengthening during fiber stretching (10-15%)

MICROSTRUCTURE OF THE MUSCLE

Microstructure

Skeletal muscle is:75% water5% inorganic salts20% proteins

○ 12% myofibrillar proteins○ 8% enzymes, membrane proteins, transport

channels, etc.

Microstructure, cont.Sarcomere: Contractile unit of the muscleMyofibrils: Protein filaments in the muscle fiberMitochondriaSarcoplasmic Reticulum: Interconnecting tubular

channelsTerminal Cisternae: Lateral end of SR, stores

calciumT Tubules: Transports action potential into

myofibrils

Microstructure

From McArdle, Katch, and Katch.

Sarcomere

Actin Myosin H zone: center, no

overlap M line: bisects H

zone A band: dark, overlap I band: light, actin-

only Z line: borders

From Brooks, Fahey, and White. (1996).

Sarcomere

From McArdle, Katch, and Katch.

Sarcomere

-Actinin: hold actin in place at Z disc C protein: holds myosin tails in correct

alignment M proteins: hold actin and myosin in

correct alignment Titin: connects myosin to Z disc

Arrangement of Actin and Myosin

From McArdle, Katch, and Katch.

Arrangement of Actin and Myosin, cont.

From McArdle, Katch, and Katch.

Actin

Globular Actin Filament Actin Tropomyosin:

blocks binding sites on actin, calcium must shift

Troponin: where calcium binds, shifts tropomyosin

From Brooks, Fahey, and White. (1996).

Myosin Light meromyosin Heavy meromyosin

Subfragment-1 (head)

Subfragment-2 (hinge)

From Brooks, Fahey, and White. (1996).

Sarcoplasmic Reticulum (SR)

Interconnecting tubular channels

Lateral end terminates in a vesicle that stores calcium

Surrounds A-band and the I-band

From Brooks, Fahey, and White. (1996).

T-Tubules Run into the fibers Transmit the action

potential deep within the muscle fiber

Triad

From Wilmore & Costill. (1994).

HOW MUSCLES CONTRACT

Muscle Contraction

Sliding Filament Theory Sequence of

Events:Acetylcholine

releasedAction potential

depolarizes the T-tubule

Calcium binds to troponin-tropomyosin

From Brooks, Fahey, and White (1996).

Contraction Cycle

Sliding Filament Theory Actin combines

with myosin-ATP Crossbridge

activation continues in the presence of calcium

Calcium concentration decreases as stimulation ceases

Cross Bridges

From McArdle, Katch, and Katch.

Cross Bridges, etc.

Calcium binds with troponin, shifts tropomyson.

Crossbridge attaches to actin and flexes, shortening sarcomere.

More cross bridges = more force!

From McArdle, Katch, and Katch.

Contraction and the Sarcomere

From McArdle, Katch, and Katch.

Types of Contraction

Isometric: external force = force developed

Concentric: external force less than force developed

Eccentric: external force greater than force developed

Experimental Terms

In Vitro: muscle is excised and studied in solution

In Situ: muscle is surgically exposed in the anesthetized animal, stimulated electronically

In Vivo: muscle is studied during normal physical activity

FACTORS WHICH AFFECT FORCE

PRODUCTION

Factors Affecting Force Production Cross Sectional Area Velocity of Shortening Angle of Pennation Sarcomere and Muscle Length Muscle Fiber Type

Cross Sectional Area

Muscles with a larger CSA have the capacity to produce more force than muscles with a smaller CSA

This is due to more sarcomeres in parallel (thus more cross bridges possible)

Komi, P.V., 1979

Velocity of Shortening

Force production is inversely related to velocity of shortening (no time for many cross bridges to form)

From Brooks, Fahey, and White (1996).

Angle of Pennation

Muscles with greater pennation have more sarcomeres running parallel

Muscles with less pennation have more sarcomeres in series

From Brooks, Fahey, and White (1996).

Sarcomere and Muscle Length

Length-tension relationship

Resting length What this means in

terms of flexibility training

Komi, P.V., 1979

From Frog Semitendinosus Fibers

Effects of Sarcomere Length on Force

0

20

40

60

80

100

120

0 1 2 3 4

Sarcomere Length

% o

f M

ax

imu

m T

en

sio

n

Adapted from Edman, K.A.P. (1966).

Muscle Fiber Types

Methods of classifying muscle fiber types (Staron, 1997):Contraction speed (fast or slow)Color - myoglobin and capillary content (red

or white)Enzymatic properties and speed of

contraction (slow oxidative, fast oxidative glycolytic, fast glycolytic)

pH sensitivity of myofibrillar ATPase

Myofibrillar ATPase sensitivity Differences in pH sensitivity are

correlated with myosin heavy chain content and therefore contractile properties.

mATPase-based fiber types:I, Ic, IIc, IIac, IIa, IIab, IIb

Muscle fibers under the microscope 3: Type I 5 (white): Type IIb 1, 2, 4, 7 , 8: Type

IIab

From Staron, R.S. (1997).

Muscle Fiber Type Characteristics· Fast twitch, high force,

fast fatigue (type IIb)· Fast twitch, moderate

force, fatigue resistance (type IIa)

· Slow twitch, low tension, fatigue resistant (type I)

· Trainable or inherited?

From Brooks, Fahey, and White (1996).

Colliander, et al. (1988).

27 male subjects Subjects performed 3x30 maximal

unilateral knee extensions using isokinetic equipment, 1 minute recovery between bouts

Measuring how peak torque decreased between the first and third bout

Colliander, et al. (1988).

Found that peak torque decreased an average of 20% from the first bout to the third.

Those individuals with a greater percentage of fast twitch fibers had the greatest peak torque but also the greatest decline in peak torque.

Colliander, et al. (1988).

Peak Torque, bout I

Peak Torque, bout III

% Decline

FT Group (~71% area)

192 139 28%

ST Group (~57% area)

144 129 10%

Fast twitch or slow twitch fiber area as a percentage of muscle cross sectional area

Ounjian, M., et al. (1991). Excised motor units from the tibialis

anteriors of 7 cats.

1 2 3 4 5 6 7 Type FF FF FF FF FR S S Contraction Time

23.2

17.6

26.9

24.5

24.4

45 55.8

Tension 21.4

15.9

15.4

41.8

10.4

15.4

3

Fatigue Index

.01 .01 .1 .06 1.02

1 1

Fatigue index: ratio of tension after 2 minutes of stimulation to the maximum tension elicited during the test

Bottinelli, R., et al., 1999 Force-velocity

curves for fiber types

Bottinelli, R., cont.

Power-velocity curves for fiber types

Bottinelli, R., cont.

Calcium sensitivity for fiber types

Karlsson, J., et al., 1978

% Slow twitch fibers and maximal oxygen uptake

From Karlsson, et al. (1978).

Karlsson, J., cont.

% Fast twitch fibers and maximal isometric strength

From Karlsson, et al. (1978).

TRAINING AND MUSCLE SIZE

What the texts say about hypertrophy Essentials (2000), pg. 65:

“The process of hypertrophy involves both an increase in the synthesis of the contractile proteins actin and myosin … within the myofibril and an increase in the number of myofibrils within a muscle fiber.”

Hypertrophy vs. Hyperplasia Hyperplasia: increase in the number of

muscle fibers Has been seen in cats

McCall, et al. 1996

Hypertrophy vs. hyperplasia study Studied 15 college-aged men 12 week training study:

8 exercises3x per week3x10-RM weights1 minute rest between sets

Results

Preacher Curl 1-RM went from approximately 36 kg to approximately 44 kg after 12 weeks

Biceps brachii CSA increased by 12.6% Triceps brachii CSA increased by

25.1%

Fiber Types and Hypertrophy

Type II fibers were consistently larger than Type I

Type II and Type I increased area after 12 weeks

Type II increased area more (17.1% vs. 10% increase)

More Results

% of fiber types was unchanged after 12 weeks

No change in estimated number of muscle fibers after 12 weeks

Increase in capillary density after 12 weeks, 12.7% in Type I and 22.6% in Type II

Conclusions

No hyperplasia evident. Could be study wasn’t long enough or difficult enough, however…

No increase in number of Type II fibers Type I and II increased area, Type II

increased more Increase in capillary density

accompanied hypertrophy for both I and II, type II more

Satellite Cells and Hypertrophy

Studies of rats shows that knocking out satellite cells impairs their ability to undergo hypertrophy.

People:Petrella et al (2008)16 weeks of strength trainingDivided their subjects into non-responders,

moderate responders, and extreme responders.

Petrella et al (2008), cont.

Non Responders

Moderate Responders

Extreme Responders

Satellite Cells 0% 50% 200%

Myonuclei per fiber

0% 9% 26%

Fiber CSA 0% 20% 75%

Table shows percent change after 16 weeks of training.

Research and Hypertrophy DeFreitas et al (2011):

25 untrained men, trained for eight weeks. 3x8-12 to failure on leg extension, leg press, and bench press.

Muscle CSA increased by 10%Strength increased by 24%

DeFreitas et al (2012)

The timing of the gains is interesting:Muscle CSA made biggest increases at the

end of weeks 1, 3, 5, and 6. Leveled off at weeks 7 and 8.

Strength made biggest increases in weeks 3, 4, 7, and 8.

Importance of variety for CSA?Importance of CSA for strength?

Matta et al (2011).

40 subjects, 12 weeks of periodized strength training, 3x/week (1 day light, 1 day medium, 1 day heavy).

Bench press, pulldown, triceps extension, biceps curl

Study meant to look at how the biceps and triceps react to training

Matta et al (2011)Biceps Triceps

Proximal (near shoulder) thickness

12% 2.2%

Mid thickness 7.5% 6.7%

Distal (near elbow) thickness

5% 7.1%

Muscles don’t experience hypertrophy uniformlyDifferent muscles respond differently to training

TRAINING AND MUSCLE SHAPE

Kawakami, et al. (1995).

Studied 5 men Subjects performed triceps pushdowns

with the right arm, 3x/week, for 5x8x80% After sixteen weeks, triceps cross

sectional area increased by average of 33.3%

Angle of pennation of triceps fibers increased by average of 29.1 degrees

Kawakami, et al. (1995).

Study suggests that changes in CSA as a result of training is accompanied by an increase in muscle fiber pennation angles.

Follow up studies by Kawakami, et al. have confirmed that this occurs as a result of training.

Kawakami, et al. (2000). Relationship

between muscle size and pennation angle, comparing untrained subjects with bodybuilders.

The Muscles are Very Adaptable

Nimphius et al (2012).

Looking at elite softball players.

Followed training for 20 weeks

Week Weights Other

1-3 General Prep

General Prep

4-11 Strength Conditioning

11-18 Power Speed/Agility

Nimphius et al (2012)

Results, end of study: 1-RM increased by 10% Speed, agility, aerobic capacity

increased Vastus lateralis muscle thickness

increased 3.5% VL angle of pennation decreased by 4% VL fascicle length increased by 10%

Nimphius et al (2012)

Results are deceptive:Muscle lost thickness over first seven

weeks, gained after thatAngle of pennation increased during first

seven weeks, decreased over last sevenFascicle length shortened over first seven

weeks, increased over last seven

Nimphius et al (2012)First 7 Weeks Last 7 Weeks

Weights Strength Focus Power Focus

Other Conditioning Focus Speed/Agility Focus

Muscle Thickness Decrease Increase

Angle of Pennation Increase Decrease

Fascicle Length Decrease Increase